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Title: The Principles of Leather Manufacture

Author: H. R. Procter

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Language: English

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*** START OF THE PROJECT GUTENBERG EBOOK THE PRINCIPLES
OF LEATHER MANUFACTURE ***

Please see the Transcriber’s Notes at the
end of this text.

The cover image has been created for this
doucument, and is placed in the public
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THE PRINCIPLES

OF

LEATHER MANUFACTURE

Frontispiece.

PLATE I.

Section of Calf-skin. (For key, see Fig. 9.)

THE PRINCIPLES

OF

Leather Manufacture

BY

H. R. PROCTER, F.I.C. F.C.S.
PROFESSOR OF LEATHER INDUSTRIES AT THE YORKSHIRE COLLEGE, LEEDS;
PAST PRESIDENT OF THE INTERNATIONAL ASSOCIATION
OF LEATHER TRADES CHEMISTS

London:
E. & F. N. SPON, Limited, 125 STRAND

New York:
SPON & CHAMBERLAIN, 123 LIBERTY STREET

1903

Dedicated to

PROFESSOR F. L. KNAPP
GEHEIMEN HOFRATH, DR. PHIL. AND DR. ING.

THE PIONEER OF SCIENTIFIC RESEARCH

IN LEATHER MANUFACTURE

P R E FA C E .

The origin of the present work was an attempt to prepare a second edition of
the little Text-Book of Tanning which the Author published in 1885, and which
has been long out of print. Though persevered in for years, the work was never
brought to completion, partly owing to the constant pressure of other duties,
but still more to the rapid advances which have been made in our knowledge of
the subject, and in the scientific thought which has been devoted to it. For his
share in the initiation of this work, much credit is due to Wilhelm Eitner,
Director of the Imperial Royal Research Institute for Leather Industries in
Vienna, but the advance he began has been energetically carried forward not
only in Vienna, but in the Tanning Schools and Research Institutes of Freiberg,
Leeds, London, Liège, Copenhagen, Berlin and elsewhere, and to a less extent
in private laboratories.
Under the pressure of this rapid growth, as it was impossible to complete the
work as a whole, the Author published an instalment dealing with the purely
chemical side of the subject in 1898, under the title of the ‘Leather Industries
Laboratory Book’; which has been translated into German, French and Italian,
and of which the English edition is rapidly approaching exhaustion.
The present work, which should by right have preceded the Laboratory
Book (and which frequently refers to it as “L.I.L.B.”), attempts to deal with the
general scientific principles of the industry, without describing in detail its
practical methods (though incidentally many practical points are discussed). To
complete the subject, a third volume ought to be written, giving working
details of the various methods of manufacture; but apart from the difficulty of
the subject, and the weariness of “making many books,” the methods of trade

are so fluctuating, and dependent on temporary conditions that they have not
the same permanent value as the record of scientific advance.
As the present volume is intended to appeal both to the chemist and to the
practical tanner, it must to a certain extent fail in both, since many matters are
included which are already familiar to the former, and it is to be feared, some,
which may prove difficult to the latter. For these and other imperfections the
Author claims the indulgence of his Readers.
The Author must here acknowledge his indebtedness to Dr. Tom Guthrie
and to Mr. A. B. Searle for assistance in writing several of the chapters; to Dr.
A. Turnbull and Mr. F. A. Blockey for much help in reading proofs and
preparing the MS. for the press; and to the many gentlemen who have
furnished or allowed him to use their blocks and drawings in illustration.

The Yorkshire College,

Leeds.

CONTENTS.

CHAPTER I.

INTRODUCTORY AND HISTORICAL.

Primitive methods of leather manufacture — Use of leather by the ancients — Progress of leather
manufacture in England — Methods of production of leather — Vegetable tannages — Combination
tannages — Use of aluminium, iron and chromium — Oil- and fat-leathers — Difficulties of
scientific treatment PAGE 1

CHAPTER II.

INTRODUCTORY SKETCH OF LEATHER MANUFACTURE.

The object of tanning — Washing and soaking — Removal of hair by liming — Unhairing by
putrefaction — Unhairing and fleshing — Deliming — Bating, puering and drenching — The
vegetable tanning process — Currying — Alum, chrome and chamois leathers PAGE 7

CHAPTER III.

THE LIVING CELL.

The structure of cells — White blood-corpuscles — The yeast-cell — Epidermis cells — The building up
of plants PAGE 10

CHAPTER IV.

PUTREFACTION AND FERMENTATION.

The nature of ferments — Organised and unorganised ferments — Classification of organised ferments
— General properties of ferments — The alcoholic fermentation — The action of enzymes or
unorganised ferments — The destruction of ferments by heat and antiseptics — The products of
fermentation — The fermentations of the tannery — Fermentation in bating and puering —
Fermentation in the tanning liquors — Moulds and mildews — Control of fermentation PAGE 15

CHAPTER V.

ANTISEPTICS AND DISINFECTANTS.

Distinction of antiseptics and disinfectants — Lime — Sulphur dioxide — Manufacture of sulphuric acid
— Bisulphites and metabisulphites — Boric acid and borates — Mercuric chloride — Mercuric
iodide — Copper sulphate — Zinc salts — Arsenic — Fluorides — Phenol — Use of carbolic acid —
Eudermin — Creasote — Creolin — Salicylic acid — Benzoic acid — Cresotinic acid —
Anticalcium — “C.T.” bate — Naphthalene sulphonic acid — Naphthols — Hydronaphthol —
Oxynaphthoic acid — Carbon disulphide — Formaldehyde — Triformol — Camphor and essential
oils PAGE 21

CHAPTER VI.

THE ORIGIN AND CURING OF HIDES AND SKINS.

Marking of hides — Fellmongering of sheep-skins — The use of salt — Salting of packer hides —
Brining — Dry-salting — Indian plaster cures — Analysis of salt-earths — Salt- and iron-stains —
Drying of hides and skins — Damage by insects — The warble-fly — Damage by branding —
Cockle PAGE 33

CHAPTER VII.

STRUCTURE AND GROWTH OF SKIN.

Similarity of Mammalian skins — Development of skin — Structure of calf-skin — The epidermis —
The structure of hair — The sebaceous glands — The development of hair — The hair-sheath — The
hair-muscle — The hyaline layer — The corium — Connective tissue — Fat cells — Striped muscle
— Elastic fibres — The unhairing process — The sweating process PAGE 46

CHAPTER VIII.

THE CHEMICAL CONSTITUENTS OF SKIN.

The keratin tissues — Production of gelatine from connective tissue — Analyses of hide and gelatine —
Constitution of gelatine — Analysis and Reactions of gelatine — Decomposition of gelatine —
Reactions of gelatine — Chondrin — Coriin — Hide-albumin — “Acid” and “alkali” albumins —
Egg-albumin — Vitellin — Casein — Keratins — Elastic fibres — Analytical methods — Kjeldahl
process PAGE 56

CHAPTER IX.

THE PHYSICAL CHEMISTRY OF THE HIDE-FIBRE.

Causes of swelling and contraction — The essentials of the tanning process — The constitution of matter
— The nature of molecules — Vapour-pressure — Surface-tension — Solution-pressures — Jellies
— Crystals — Osmotic pressure — Electrolytic dissociation — Electrolysis — Reactions of ions —
Absorption of water by gelatine — Dehydration by alcohol — Action of acids, alkalies and salts on
gelatinous fibre — Physical explanation of swelling — Action of acids on gelatine — Action of
alkalies on gelatine — Effect of salt — The pickling process PAGE 73

CHAPTER X.

WATER AS USED IN THE TANNERY.

Impurities of natural water — Hardness — Soap test — Temporary hardness — Clark’s softening process
— Archbutt and Deeley’s softening apparatus — Other appliances — Effect of temporary hardness in
tanning and dyeing — Permanent hardness — Boiler scale — Mud — Iron — Alumina — Soda —
Copper, lead, etc. — Sulphuric acid — Nitrates and Nitrites — Chlorine — Carbonic acid — Silicic
acid — Effect of hardness on plumping — Peaty waters PAGE 93

CHAPTER XI.

SOAKING AND SOFTENING OF HIDES AND SKINS.

Washing of fresh hides — Danger of putrefaction — Soaking of salted hides and skins — Soaking and
softening of dry and dry-salted hides — American wash-wheel — Chemical methods — Difficulty of
softening hides dried at high temperature PAGE 108

CHAPTER XII.

DEPILATION.

Methods of depilation — Sweating process — Liming — Sources of lime — Quicklime — Slaking of
lime — Solubility of lime in water — Analysis of lime — “Available” lime — Action of lime on hide
— Liming in pits — Suspension limes — Effect of warming limes — Quantity of lime required —
The Buffalo method — Action of old limes — Solution of hide substance by limes — Sodium and
potassium hydrates — Payne and Pullman’s process — Alkaline carbonates — Alkaline sulphides —
Sodium sulphide — Calcium Sulphydrate — Gas-lime — Tank-waste — Lufkin’s liming preparation
— Barium sulphydrate — Realgar, or red sulphide of arsenic — “Inoffensive” unhairing solution —
Earp’s patent — Unhairing on the beam — Unhairing machines — Vaughn machine — Leidgen
machine — Unhairing in stocks and wash-wheel — Jones machine — Fleshing — Vaughn fleshing
machine — Rounding PAGE 119

CHAPTER XIII.

DELIMING, BATING, PUERING AND DRENCHING.

Methods of removing lime and reducing swelling — Use of acids — Lactic, acetic and formic acids —
Boral — Sodium bisulphate — Boric (boracic) acid — Borax — “Pulling down” process — Use of
ammonium chloride and sulphate — Pickling solutions — Drenching with lactic acid —
Metabisulphite of soda — Washing out lime, French process — Nesbitt’s process — Use of carbonic
acid — Carbolic acid — Cresotinic acid — Oxynaphthoic acid — “Anticalcium” — “Acrilene bating
acid” — “C.T. Bate” — Use of sulphides and polysulphides — Babool pods — Bran-drenching —
Bating and puering — Causes of bating effect — Pepsin — Trypsin, or Pancreatin — Wood’s
researches — Erodin — Palmer’s experiments — Other artificial bates — Relative effect of dog- and
pigeon-dung bates — Analysis of dungs — “Scudding,” or “fine hairing” — Preservation and use of
dung PAGE 152

CHAPTER XIV.

ALUM TANNAGE, OR TAWING.

Nature of leather — Mineral tanning substances — Salts of aluminium — Alums — Aluminium sulphate
— Effect of salt in tawing — Basic alumina solutions — Tawing of skins for rugs — Calf-kid
manufacture — Glove-kid — Green leather and other combination tannages PAGE 184

CHAPTER XV.

IRON AND CHROME TANNAGES.

Iron tannages — Chrome tannages — Chemistry of chromium compounds — Knapp’s method of chrome
tannage — Cavallin — Swan — Heinzerling — Hummel’s improvement — Schultz’s method —
Theory of the two-bath process — Practical management of the two-bath process — Dennis’s chrome
tanning liquor — Procter’s liquors — Theory of basic process — Practical use of basic liquors —
Washing and neutralisation — Effect of sulphur on chrome leather — Bluebacking — Fat-liquoring
— Dyeing of chrome leather — Glazing and finishing PAGE 198

CHAPTER XVI.

PRINCIPLES OF THE VEGETABLE TANNING PROCESSES.

Methods of sole-leather tanning — Finishing of sole-leather — Theory of vegetable tannage — Deliming
of sole-leather — “Mellowness” of liquors — Penetration of tannage — Drying of sole-leather —
Tanning of dressing leathers — Preparation for tannage — Avoidance of “bloom” — Tannage of
moroccos and other skins PAGE 220

CHAPTER XVII.

COMBINATION OF VEGETABLE AND MINERAL TANNAGES.

Early combination tannages — Respective effect of mineral and vegetable tannages — Use of fat-liquor
— Action of mineral and vegetable tanning materials on each other — Danish and Swedish glove
leathers — Green leathers — Making of fat-liquors — Chrome combinations PAGE 236

CHAPTER XVIII.

VEGETABLE TANNING MATERIALS.

Distribution of tannin in plants — Structure of barks — Botanical list of important tanning materials
PAGE 242

CHAPTER XIX.

THE CHEMISTRY OF THE TANNINS.

Sources of tannins — General qualities of tannins — Chemical constitution — Catechol- and pyrogallol
tannins — Catechins — Tendency of Catechol tannins to darken with light — “Physiological” and
“pathological” tannins — Presence of mordant colouring matters PAGE 294

CHAPTER XX.

THE SAMPLING AND ANALYSIS OF TANNING MATERIALS.

The International Association of Leather Trades Chemists — The American Official Association of
Agricultural Chemists — The sampling of material — Preparation of solution for analysis —
Extraction of solid materials — Total soluble matter — Evaporations of solutions — The weighing of
residues — The determination of non-tannins — The hide-powder filter method — The hide-powder
shake method — Determination of moisture — Colour-measurement PAGE 300

CHAPTER XXI.

THE GRINDING OF TANNING MATERIALS.

Primitive methods of grinding — The bell mill or coffee mill — Disc mills — Disintegrators — Carr’s
disintegrator — Carter’s disintegrator — Adjustment of disintegrators — The Williams pulveriser —
Myrobalans and Valonia crushers — Sawing mills — Shaving mills — Dyewood cutting machines —
Screening of ground materials — Hatching of bark — Disintegrators and fire insurance — Dust from
disintegrators — Chain conveyors — Belt conveyors — Vibrating conveyors PAGE 316

CHAPTER XXII.

THE EXTRACTION OF TANNING MATERIALS, AND THE MAKING OF
EXTRACTS.

Leaching — Early forms of leaches — The press-leach system — Handling of liquors — Distributing
troughs and valves — Construction of leaches — Influence of temperature — Use of silent boiling jet
— Closed extractors — Sprinkling leaches — Manufacture of extracts — Decolorisation of extracts
— Soluble extracts — Concentration of extracts — Yaryan evaporator — Multiple effects — The use
of extracts in the tannery — Effect of temperature on extraction and colour PAGE 328

CHAPTER XXIII.

FATS, SOAPS, OILS AND WAXES.

Characteristics of fats and oils — Chemical constitution — Nature and production of soaps — Insoluble
soaps — Distillation of fats — Solvents of oils — Drying oils — Saturated fatty acids — Non-drying
liquid fatty acids — Less-saturated liquid fatty acids — Castor oil — Tallow — Neatsfoot oil —
Wool fat — Holden fat — Distilled wool grease — Distilled stearine — Olive oil — Castor oil —
Turkey-red oil — Linseed oil — Boiled oils — Japan for leather — Cottonseed oil — Sesame oil —
Cod oil — Shark liver oil — Whale oil — Seal oil — Menhaden oil — Fish oils — Fish tallow —
Dégras and Sod oil — Waxes — Sperm oil — Beeswax — Carnauba wax — Japan wax — Volatile
or essential oils — Birch oil — Wintergreen oil — Mineral oils and waxes — Vaseline and vaseline
oil — Paraffin wax — Ozokerit — Resin oils — Resin PAGE 350

CHAPTER XXIV.

OIL TANNAGES, AND THE USE OF OILS AND FATS IN CURRYING.

Primitive use of oil in leather manufacture — Chamoising and the production of washleather —
Manufacture of Moellon, or Dégras — Sod oil — Formaldehyde leathers — “Crown” and “Helvetia”
leathers — Theory of oil leathers — Processes of currying — Theory of the stuffing process — Hand-
stuffing — Drum-stuffing — Stuffing of dry leather — “Spueing” and its causes — Fat-liquoring
PAGE 378

CHAPTER XXV.

DYES AND DYEING.

Coal-tar colours — Acid and basic colours — Theories of dyeing — Fixation of colours on leather —
Mordants and mordant colours — Curriers’ inks — Glazes and finishes — “Assistants” in dyeing —
Bronzing — Fading of colours — Practical methods of leather dyeing — Use of dyewoods — Iron
“strikers” — Tannin blacks — Staining — Theory of colour-mixtures — Finishing dyed leathers —
Testing of dyes — Injurious effects of metals in dyeing PAGE 394

CHAPTER XXVI.

EVAPORATION, HEATING AND DRYING.

Theory of evaporation — Boiling point and vapour-pressure — Consumption of heat in evaporation —
Heat-units — Mechanical energy of heat — Evaporation by “multiple effect” — Vapour-pressure of
atmospheric moisture — Wet and dry bulb thermometers — Heat and air required in leather-drying

— Loss of heat by buildings — Quantity of heat given by steam and hot-water pipes — Screw-fans
for drying — Centrifugal fans — “Turret” dryer — Downward ventilation — Arrangement of steam-
pipes — Hot water pipes PAGE 420

CHAPTER XXVII.

CONSTRUCTION AND MAINTENANCE OF TANNERIES.

Selection of site — Arrangement of buildings — Fire insurance — Automatic sprinklers — Possibility of
extension — Production and distribution of power — Electric motors — Shafts, pulleys and belting
— Balancing of machinery — Fire-risk from bark mills — Chain-conveyors — Lubricating oils —
Construction of pits — Underground pipes and overhead troughs — Pumps and pumping appliances
PAGE 444

CHAPTER XXVIII.

WASTE PRODUCTS AND THEIR DISPOSAL.

Hair — Fleshings and glue-stuff — Fat — Bate-shavings — Horns — Spent tan — Tan-furnaces —
Sewage and other waste liquids — Chemical purification of sewage — Settling tanks — Filter-
presses — Bacterial purification of sewage — Tannery waste-liquors PAGE 460

APPENDIX A.

METHOD OF THE INTERNATIONAL ASSOCIATION OF LEATHER-
TRADES CHEMISTS FOR THE ANALYSIS OF TANNING MATERIALS:
Corrected to 1901.

Sampling from bulk — Preparation for analysis — Preparation of infusion — Determination of tanning
matters and non-tannins — Colour-measurement — Analysis of used liquors PAGE 475

APPENDIX B.

THE DECIMAL SYSTEM.

Metrical weights and measures — Centigrade thermometer PAGE 481

APPENDIX C.

METHOD OF ANALYSIS OF TANNING MATERIALS OF THE AMERICAN
ASSOCIATION OF OFFICIAL AGRICULTURAL CHEMISTS: Corrected to
1901.

Preparation of sample — Quantity of material — Moisture — Total solids — Soluble solids — Non-
tannins — Tannins — Testing of hide-powder — Testing non-tannin filtrate PAGE 482

APPENDIX D.

LISTS OF COAL-TAR DYES SUITABLE FOR DYEING AND STAINING
LEATHER, furnished by Mr. M. C. LAMB.

Colours for staining leather — Colours for dyeing vegetable-tanned leather — Dyeing and finishing
chrome-leather — List of colours suitable for chrome-leather PAGE 485

INDEX PAGE 499

PRINCIPLES

OF

LEATHER MANUFACTURE.

CHAPTER I.
INTRODUCTORY AND HISTORICAL.

The origin of leather manufacture dates far back in the prehistoric ages, and
was probably one of the earliest arts practised by mankind. The relics which
have come down to us from palæolithic times, and the experience of the
modern explorer, alike tell us that agriculture is a later and a higher stage of
development than the life of the hunter; and since, in the colder regions,
clothing of some kind must always have been a necessity, we may conclude
that it was first furnished by the skins of animals.[1]
[1] See also Gen. iii. 21.

While wet skins putrefy and decay, dry ones are hard and horny; and nothing
could be more natural to the hunter than to try to remedy this by rubbing the
drying skin with the fat of the animal, of which he must have noticed the
softening effect on his own skin. By this means a soft and durable leather may
be produced, and this process of rubbing and kneading with greasy and
albuminous matters, such as fat, brains, milk, butter and egg-yolks, is in use to
this day, alike by the Tartars on Asiatic steppes and the Indians on American
prairies; and not only so, but we ourselves still use the same principle in the
dressing of our finest furs, and in the manufacture of chamois, and many sorts
of lace- and belt-leathers.
Such a process is described in the Iliad (xvii. 389-393) in the account of the
struggle over the body of Patroclus:
“As when a man
A huge ox-hide drunken with slippery lard
Gives to be stretched, his servants all around
Disposed, just intervals between, the task
Ply strenuous, and while many straining hard
Extend it equal on all sides, it sweats
The moisture out and drinks the unction in.”

It must also have been early noticed that wood smoke, which in those days
was inseparable from the use of fire, had an antiseptic and preservative effect
on skins which were dried in it, and smoked leathers are still made in America,
both by the Indians and by more civilised leather manufacturers. To this
method the Psalmist refers[2] when he says, “I am become like a bottle in the

smoke;” and such bottles, made of the entire skin of the goat, are still familiar
to travellers in the East.
[2] Ps. cxix. 83.

The use of vegetable tanning materials, though prehistoric, is probably less
ancient than the methods I have described, and may possibly have been
discovered in early attempts at dyeing; an art which perhaps had its origin even
before the use of clothing! The tannins are very widely distributed in the
vegetable kingdom, and most barks, and many fruits, are capable of making
leather.
The employment of alum and salt in tanning was probably of still later
introduction, and must have originated in countries where alum is found as a
natural product. The art was lost or unknown in Europe till introduced into
Spain by the Moors.
Leather manufacture reached considerable perfection in ancient Egypt. A
granite carving, probably at least 4000 years old, is preserved in the Berlin
Museum, in which leather-dressers are represented. One is taking a tiger-skin
from a tub or pit, a second is employed at another tub, while a third is working
a skin upon a table. Embossed and gilt leather straps have been found on a
mummy of the ninth century B.C., and an Egyptian boat-cover of embossed
goat leather, as well as shoes of dyed and painted morocco, are still in
comparatively good preservation. The art is of very early date in China, and
was well understood by the Greeks and Romans. In the Grosvenor Museum at
Chester is the sole of a Roman caliga, studded with bronze nails, which is yet
pretty flexible. After the fall of the Roman empire many arts were lost to
Europe, and it was not until the Moorish invasion of Spain that the art of
dyeing and finishing the finer kinds of leather was reintroduced.
England was very backward in this manufacture up to the end of the last
century, owing to the fossilising influence of much paternal legislation, and of
certain excise-duties, which were only repealed in 1830. Since this time the art
has made rapid strides, especially in the use of labour-saving machinery, and
England may at the present moment be considered fairly abreast of any other
country as a whole; though in some special manufactures we are surpassed by
the Continent and by America. In making comparisons of this kind, it must,
however, be remembered that, especially in sole-leather tannage, the most
rapid progress has been made during the last few years in those countries
which were more backward, and that therefore our superiority is much less
pronounced than formerly, and in a few years will probably cease to exist
unless marked improvements are introduced in the methods of production.

In the sketch of the development of leather manufacture which has just been
given, it has been implied that its object is to convert the putrescible animal
skin into a material which is permanent, and not readily subject to decay, while
retaining sufficient softness or flexibility for the purposes for which it is
intended. As these range from boot-soles to kid-gloves, there are wide
divergences, not only in the processes employed, but also in the materials used
and in the principles of their application.
The most important method of producing leather is by the use of vegetable
tanning materials, and this is perhaps the only one which is really entitled to be
called “tanning,” though the distinction is not very strictly adhered to. It
includes the whole range—from sole leather, through strap, harness and
dressing leather, to calf and goat skins, and the various sumach tannages which
yield morocco and its imitations. All of these products but the first and the last
undergo, after tanning, the further processes of “currying,” of which the most
important operation consists in “stuffing” with oily and fatty matters, both to
increase the flexibility and to confer a certain amount of resistance to water.
Sumach-tanned skins are not strictly “curried” but usually receive a certain
amount of oil in the process of “finishing.”
Next in importance to the vegetable tannages are the “tawed” leathers
produced by the agency of alum and salt, including the “white leathers” for
belt laces and aprons, and calf- and glove-kid. A connecting link between
tanning and tawing is found in the “green leather,” “Dongola,” and
“combination” tannages, in which alum and salt are employed in conjunction
with vegetable tanning materials, and especially with gambier.
Salts of several of the metals, and particularly those of aluminium, iron, and
chromium, have the power of converting skin into leather; and processes in
which salts of chromium are used have recently attained very considerable
commercial importance.
In the production of calf- and glove-kid, in addition to alum and salt,
albuminous and fatty matters, such as egg-yolk, olive oil and the gluten of
flour, play a considerable part, and are thus linked both to the primitive
methods in use by the Indians and Kalmucks, and to those by which “crown”
and “Helvetia” leather, and many other forms of belt- and lace-leathers are
now produced by treatment with fats and albumens.
From these again the step is a short one to the “chamois” and “buff”
leathers, and the German “fettgar” leathers, in which oils and fats only are
used; and these are probably again related chemically to leather produced by
the aid of formaldehyde and other aldehydes.

In an attempt to view all these complex processes from the scientific
standpoint, the reader should constantly realise that the present methods of
leather manufacture are the results of tens of centuries of experience, and of
innumerable forgotten failures, and must not therefore expect that they can be
easily superseded. Science must follow before it can lead, and its first duty is
to try to understand the reasons and principles of our present practice, for we
can only build the new on the foundation of what has been already learned.
Another fact, which is scarcely understood by the practical man in his demands
on science, is that in leather manufacture every question which is raised seems
to rest on the most recondite problems of chemistry and physics; the chemistry
of some of the most complex of organic compounds, and the physics of
solution, of osmose, and of the structure of colloid bodies—problems which
are yet far from completely conquered by the highest science of the day.
It may seem bold to attempt the scientific treatment of such a subject at all;
and, indeed, it must be admitted that our knowledge is still far from adequate
for its complete accomplishment, but enough has been done to lay a foundation
for future work, and this can at least be summarised and arranged in an
available form. The subject falls naturally into two sections, in the first of
which the processes of manufacture would only be described in general terms,
and with sufficient fulness to enable the reader to understand the scientific
considerations on which they are based, and the methods of investigation
which can be applied to them; while in the second an effort should be made to
give working details of the various processes sufficient to enable those with a
general knowledge of the trade to experiment successfully in its various
branches. It was at first intended that these two sections should be published in
one book as a second edition to the Author’s ‘Text-book of Tanning,’ but
owing to the long delay in its publication, it was decided to publish the first
section under the present title ‘Principles of Leather Manufacture,’ leaving the
latter section ‘Processes of Leather Manufacture’ to a later, and I fear,
somewhat uncertain date; while the strictly chemical portion has already
appeared in the ‘Leather Industries Laboratory Book,’ frequently referred to in
the following pages under the abbreviation “L.I.L.B.” Where quantities and
details are given, they must not be taken as recipes to be blindly followed; or
even, in every case, as the best known methods; but rather as mere guides to
experiment, which must be modified to suit varying conditions and
requirements. It is the special virtue of the scientific, as opposed to the merely
traditional way of looking at such questions, that knowing the cause and effect
of each part of the process, it can so adjust them as to get over difficulties, and

to suit novel conditions. It is needless to add that many methods are jealously
preserved as trade secrets, and full details are frequently unattainable.
After what has just been said, it may be well to emphasise the great
importance of practical knowledge and experience to the leather manufacturer.
Even in trades which have reached the highest scientific development, such,
for instance, as the manufacture of the coal-tar colours, the small experiments
of the laboratory are not transformed into manufacturing operations without
experience and sometimes even failure; and this must still more often be the
case in a trade like that of leather-making, where our knowledge of the actual
changes involved is still so incomplete. On the other hand, the cost of
experiments on a manufacturing scale is usually so heavy that the least
scientific must admit the advantage of learning all which the laboratory can
teach before venturing on anything more; while even our present imperfect
knowledge of the chemical changes involved will often warn us off hopeless
experiments, and give us hints of the directions in which success may be
attained. A knowledge of chemistry will probably prove at least as important to
the future of our trade as that of mechanics has been in the past.

CHAPTER II.
INTRODUCTORY SKETCH OF LEATHER MANUFACTURE.

The object of tanning has been stated to be the rendering of animal skin
imputrescible and pliable, but as we now rarely require leather with the hair
on, preliminary processes are needed to remove it, and to fit the skin for
tanning, and the nature of these processes has great influence on the
subsequent character of the leather produced.
The first step is usually a washing of the skin to remove blood and dirt;
while, where it has been salted or dried, a more thorough soaking is needed to
remove the salt, and to restore the skin to its original soft and permeable
condition.
The hair is then loosened by softening and partial solution of the epidermis
structures (see p. 47) in which it is rooted. This is most generally accomplished
by soaking for some days in milk of lime, which is occasionally assisted by the
addition of caustic alkalies or of sulphides. When the latter are used in
concentrated solution, the hair itself, as well as the epidermis tissues, is
softened and destroyed in the course of a few hours. The lime not only serves
to loosen the hair, but swells and splits up the fibre-bundles of which the hide
tissue is composed, and so fits it to receive the tannage (cp. p. 125).
For some purposes a regulated putrefactive process is substituted for the
liming; the hides or skins being hung in a moist and warm chamber (see p.
119), when the soft mucous layer which forms the inner part of the epidermis
is disintegrated, partly by direct putrefaction, partly by the action of the
ammonia evolved, so that the hair can be scraped off. In this case the hide-fibre
is not swollen, and the necessary swelling has to be obtained by subsequent
processes.
In whatever way the hair has been loosened, it is scraped off with a blunt
and somewhat curved two-handled knife on a sloping rounded “beam” of
wood or metal; this operation being termed “unhairing” (see p. 144).
This is generally followed by “fleshing,” which is performed on the same
beam with a somewhat similar knife, which, however, is two-edged and sharp.
In this operation, portions of flesh, and the fat and loose tissue which underlie
the true skin (see p. 147) are removed by scraping and cutting. Machines for
fleshing are also largely in use for certain purposes (see p. 148).

For sole leather, the hide, after some washing in soft water to cleanse from
lime, is then ready for the actual tanning process; but for the softer leathers
more thorough treatment is needed to remove the lime, and to still further
soften the skin by solution and removal of a portion of the cementing
substance of the fibres.
This treatment is generally of a fermentive or putrefactive nature, and the
most common form is that known as “bating,” which consists in steeping in a
fermenting infusion of pigeon- or hen-dung. The theory of its action is not yet
thoroughly understood, but the effect is largely due to the unorganised
hydrolysing ferments produced by the bacteria present; while at the same time
the lime is neutralised and removed by the weak organic acids and salts of
ammonia which are produced; and the fibre which had been plump and
swollen with lime, becomes extremely relaxed and flaccid.
In the lightest leathers, such as kid- and lamb-skins for gloves, and goat and
sheep for moroccos and the like, dog-dung is substituted for that of fowls, and
the process is then called “puering” (see p. 170).
These processes are often followed by “drenching,” which sometimes
indeed takes their place, the skins being soaked in a fermenting bran infusion.
In this, the small quantities of acetic and lactic acid formed by fermentation are
the active agents, neutralising and dissolving the lime, and cleansing and
slightly plumping the pelt (see p. 166).
The tanning process which follows consists in soaking the pelt in infusions
of various vegetable products containing bodies of the class known as
“tannins,” which have the power of combining with skin-fibre and converting
it into leather.
If at first strong infusions were used, they would act too violently on the
surface of the skin, hardening and contracting it so that the subsequent tannage
of the interior would be impeded, and the “grain” or outer surface would be
“drawn” and wrinkled. This is avoided by the use at first of very weak
infusions which have already been used on goods in a more advanced stage. In
the later part of the process much stronger solutions are employed, and the
hides are frequently “dusted” in them with ground tanning material.
In the case of sole leather, these processes may require from two to twelve
months for completion; after which the leather is dried, smoothed, and
compressed by mechanical means, and is then ready for use.
Dressing-leathers, ranging from calf-skins to harness-hides, receive a much
shorter tannage, and the subsequent treatment with fats and oils, which,
together with mechanical manipulations, constitute “currying.” The thin film of

grease distributed over the surface of the fibres renders them supple, and to
some extent waterproof.
The lighter fancy leathers, such as morocco, are dyed, and undergo many
complex processes to fit them for their required purposes and improve their
appearance.
Many skins such as calf, glove, and glacé kid, are not tanned, but “tawed”
by a solution of alum and salt, which is often supplemented with mixtures of
flour and egg-yolk to fill and soften the leather.
Salts of chromium are also employed in place of alum and salt, and produce
an equally soft, but more permanent and enduring leather.
Lastly, wash-leather, or so-called “chamois,” and buff-leather are produced
by fulling the prepared pelt with fish or whale oil, which converts the skin into
leather by subsequent oxidation, during which aldehydes are evolved.

CHAPTER III.
THE LIVING CELL.

The larger part of the materials employed in leather manufacture are organic
in their origin, and the skin itself is an organised structure, while the life-
processes of putrefaction and fermentation play a large part in the tannery.
Some knowledge, therefore, of biological structures and processes is necessary
to a full understanding of much which follows, and a few words are not out of
place with regard to the foundations of life itself.
The bricks of which all living structures are built are the living “cells” and
their products, and these first elements differ little, if at all, whether the life is
animal or vegetable, the distinction being produced rather by the way in which
they are put together, than by differences in the cells themselves. This is so
much the case that it is often difficult to decide in which of the two classes to
place the simplest organisms, since most of these forms are capable of active
movement, and their modes of nutrition and reproduction are common to both
kingdoms.
In its simplest form, the cell, whether animal or vegetable, is strictly
speaking not a cell at all, but consists merely of a minute mass of living jelly or
protoplasm. Such is the amœba found in water and damp soil, such are the
lymph-cells and white blood-corpuscles of our bodies, and such also some
stages at least of the lowest forms of fungi, like the Æthalium septicum which
is sometimes found on old tan-heaps as a crawling mass of yellow slime. If a
drop of saliva be examined with the microscope under a cover-glass, with one-
sixth objective and small opening of diaphragm,[3] a few scattered semi-
transparent objects will be found, of the apparent size of a lentil or small pea,
and of rounded form. These are lymph-corpuscles (Fig. 1). Their contents are
full of small granules, and if they be observed quickly, or if the slide be kept at
about the warmth of the body, it will be noticed that these are in constant
streaming motion. If the warmth can be kept constant, which is difficult
without special apparatus, and the cells can be observed from time to time, it
may be seen that they lose their circular form, and put out protuberances
(pseudopodia, “false feet”) one of which will gradually increase in bulk, till it
absorbs the whole cell, which thus crawls about. It will now readily be
understood how these cells wander through all the tissues of the body, passing

through the smallest pores like the fairy who put her finger through a keyhole,
and grew on the other side till she was all through! This independent vitality, in
a warm and suitable nutrient liquid, may continue for more than a week, and,
in the case of amœba, quite indefinitely.
[3] For details of microscopic manipulation in this and the following chapter see L.I.L.B., p.
234 et seq.

Fig. 1.—Lymph-corpuscle of frog, showing gradual change of form.
(Ranvier.)

It is possible that by close attention, a rounded or elongated body, somewhat
like an oil-globule, may be seen within the cell, though it is generally more
obvious when the latter has been killed and stained with a weak solution of

iodine. This is the nucleus, and within it is a still smaller speck called the
nucleolus, which bears an important, and as yet little understood, part in the
life-history of the cell. After a period, it undergoes certain somewhat
complicated changes, and divides into two, the nucleus elongates, and also
divides, each half carrying with it a portion of the living protoplasmic jelly,
and thus forming two complete and independent cells. This is the life-history,
not only of the lymph-cell, but with more or less modification, of every living
cell or tissue.

Fig. 2.—Yeast-cells, much magnified.

These cells, like all living things, feed on the nutriment which surrounds
them, and even enclose small particles of solid food, which are gradually
dissolved and disappear. In this way the white blood-corpuscles are said to
feed upon and destroy the still smaller organisms which gain access to the
blood, and which might otherwise cause disease. The matter which cells
consume is not, of course, destroyed, but simply converted into other forms,
some of which are useless, or even poisonous to the cells, and which, like the
secretions of higher animals, are discharged into the surrounding fluids; while
others are retained, and contribute to the growth of the cell. Thus most
vegetable cells secrete cellulose, or plant-tissue, which forms a wall enclosing
the protoplasm, and so justifies the name of cell. If to warm water and a little
sugar we add enough yeast to render it slightly milky, and examine it like the

saliva, we shall have before us typical vegetable cells of the simplest form
(Fig. 2). There is the same granular protoplasm, and there is the nucleus,
though it cannot be seen without special preparation, the rounded spaces which
look like one, being simply filled with transparent fluid, and called vacuoles.
There is, however, no motion, as in the case of amœba, for the cells are
enclosed in a tough skin of cellulose, which will be evident if they are crushed
by putting some folds of blotting paper on the cover-glass, and pressing it with
the handle of a needle or a rounded glass rod, when the protoplasm will be
forced out and the skin remain like a burst bladder. This will be more obvious
if the cells are previously stained with iodine or magenta, which will stain the
protoplasm, but not the membrane. It is easy to observe the multiplication of
the yeast-cells, which is somewhat different to that of the corpuscles. Instead
of enlarging as a whole, and dividing into two equal cells, a small bud appears
on the side of the parent-cell, and enlarges till it becomes itself a parent-cell
with buds of its own. These do not break away at once, and hence chains and
groups of attached cells are formed which are easily noticed in growing yeast
if a microscope be employed. The principal nutriment of yeast is grape-sugar
or glucose; and much more of this is consumed than is needed to produce the
cellulose wall and the substance of new cells; just as in the animal, sugar,
starch and fat are consumed to give heat and energy. In the yeast, this extra
sugar is split up into carbon dioxide, which escapes as gas, and to which yeast
owes its power of raising bread; and into alcohol, which in too large proportion
is poisonous to the yeast itself.

Fig. 3.—Epithelium-cells. Ranvier.
p, pressure-marks; g, granular protoplasm.

In examining the saliva for lymph-cells, it is probable that some much larger
objects may have been noticed of irregular polygonal outline and with a well-
marked nucleus. These are cells from the lining epithelium of the mouth, and
only differ from those of the epidermis of skin in their form and size (Fig. 3).
Note the markings caused by the pressure of overlapping cells. In these cells
the wall is formed of keratin or horny tissue, which takes the place of the
cellulose of the yeast.

Fig. 4.—Penicillium glaucum, a common
green mould.

Other simple forms of cell are those of Saccharomyces mycoderma or torula
which forms a skin on the surface of old liquors, and which much resembles a
small yeast; and of the various ferments which are found in liquors, bates and
drenches, which will be more fully described in the chapter following.

Many of these, such as the acetic and lactic ferments, which, like all other
bacteria, multiply by division, do not separate, but remain connected in chains
or chaplets, like a string of beads. From these, the step is not a long one to the
hyphæ or stems of the higher moulds, which are too frequently found on
leather which has been slowly dried, and which consist simply of tubular cells
which elongate and divide by the formation of septa or cross-partitions, and
thus build up a complicated plant-structure (Fig. 4). As we proceed higher in
the scale of plant and animal life, the forms and products of the cells become
more varied, and instead of one single cell, fulfilling all the functions of the
plant or animal, each class of cell has its own peculiar duties and properties,
while all work together for the maintenance of the complex structure of which
they form a part.

CHAPTER IV.
PUTREFACTION AND FERMENTATION.

The chemical changes produced by the unicellular plants, such as yeasts and
bacteria, to which allusion has been made in the last chapter, are known as
fermentation and putrefaction, and are of such importance to the tanner, both
for good and evil, that the subject must be treated in some detail. No scientific
distinction exists between fermentation and putrefaction, though it is
customary to restrict the latter term to those decompositions of nitrogenous
animal matter which yield products of disagreeable smell and taste.
The organisms which are the cause of both fermentation and putrefaction are
known by the general term of “ferments.” This term has also been extended in
recent years so as to include the so-called “unorganised ferments” (enzymes,
zymases) which are active products secreted by the “organised ferments” or
living organisms.
These latter are again divided into three classes:—
1. Moulds.
2. Yeasts (Saccharomycetes).
3. Bacteria.

The members of one class are distinguished from those of another by their
form, and, more especially, by the substances they produce during their life-
history. All three classes are now considered to be fungi.
All ferments possess the following three properties:—

1. They are nitrogenous bodies.
2. They are unstable, i.e. they are destroyed by heat, chemicals, etc.
3. A relatively small quantity of the ferment is capable of producing great
changes in the substances upon which it acts, especially if the products of the
change can be removed as they are formed.

The general character of fermentation will be best understood by a closer
study of the yeast cell, which has already been described (p. 12), and its life-
history briefly sketched. It has been shown that it is a growing plant of a very
simple type, belonging to the fungi. These are devoid of the green colouring
matter which enables the higher plants to utilise the energy of sunlight to

assimilate the carbonic acid of the atmosphere, exhaling its oxygen, and
employing its carbon for the building up of tissue; and they must therefore, like
animals, have their nutriment ready formed, and capable of supplying energy
by its oxidation. For yeast, as has been stated, the appropriate nourishment is
glucose, or “grape-sugar.” This is broken down, in the main, into the simpler
compounds, alcohol and carbonic acid, while a small portion is utilised for the
building up of the cell and the formation of secondary products. The main
reaction is represented by the following equation:
C6H12O6 = 2C2H6O + 2CO2
Glucose Alcohol Carbon
dioxide

Yeast cannot directly ferment ordinary cane-sugar (C12H22O11), but secretes a
substance called invertase, which so acts on the sugar as to break it up, with
absorption of one molecule of water, into two molecules of fermentable
glucose (dextrose and levulose) which serve as nourishment for the yeast.[4]
This invertase is the type of the series of bodies which are known as
“unorganised ferments,” enzymes, or zymases, differing from the organised
ferments in being simply chemical products without life or power of
reproduction, but capable of breaking up an unlimited quantity of the bodies on
which they act, without themselves suffering change. The way in which this is
done is not clearly understood, but some parallel may be found to it in the
action of sulphuric acid on alcohol, of which it will convert an unlimited
quantity into ether, without itself suffering any permanent change. The action
of enzymes is limited to breaking down complex bodies into simpler forms,
often with absorption of water, as in the case of sugar, while some of the
products of living ferments are often complex, a part of their nutriment being
broken down into simple products such as carbonic acid, marsh gas and
ammonia, to supply the necessary energy to elaborate the remainder.
[4] Compare O’Sullivan and Thompson, Jour. Chem. Soc., 1890, p. 834; 1891, p. 46.

Very many different unorganised ferments are known to exist, as they are
not only produced by yeasts and bacteria, but are formed by the cells of higher
plants and animals; thus the digestive principles, pepsin, trypsin, ptyalin, are of
this character—ptyalin, like diastase, converting starch into sugar; and such
bodies fulfil many functions both in animal and vegetable economy. In
fermentation, as in disease, it is often difficult to distinguish what is due to the
direct action of bacteria, and what to the unorganised ferments which they
produce, and the question is further complicated by the fact that in most natural

fermentations more than one ferment-organism is present. Sometimes the
action of the unorganised ferments may be distinguished by the fact that the
addition of chloroform has little effect on their activity while it paralyses that
of the living organism. By exposure to high temperature both are destroyed,
the bacteria, yeasts and moulds being killed and the unorganised ferments
coagulated like white of egg, and so rendered inoperative. Many antiseptics
also destroy the activity of both organisms and enzymes; but others, like
chloroform, have no action on the latter. In some cases, as in that of invertase,
the actual zymase can be precipitated by alcohol from its aqueous solution,
filtered off, and restored to activity by transference into water. Since both
classes of ferments are destroyed by high temperatures, all fermentation-
processes are completely and permanently arrested by exposure to sufficient
heat, and subsequent preservation in vessels so closed that no new ferment-
germs can gain access. A familiar instance is that of tinned meats. All fully
developed bacteria are destroyed by a very short exposure to a boiling
temperature, and most by 60° to 70° C., but many species produce spores
which are extremely difficult to destroy. The thermophilic bacteria discovered
by Globig and further investigated by Rabinowitsch,[5] thrive at a temperature
of 60° C. About eight species are known, and they take part in the heating of
hay and similar fermentations where high temperatures are involved, and are
therefore presumably present in spent tan.
[5] Centr. Blatt für Bakt., II. Abth. vol. i. p. 585.

For absolute sterilisation it is therefore necessary either to boil under
pressure so as to raise the temperature to, say 110° C., or to heat repeatedly for
a short time to temperatures of 80°-100° C. at successive intervals of 24 hours,
in order to allow the spores to develop. This process is frequently performed
for bacteriological observation in flasks or test-tubes merely stopped with a
plug of sterilised cotton-wool, which has been found to efficiently filter the
germs from the air which enters through it (see L.I.L.B., p. 270).
The ferment-organisms cannot thrive and multiply unless they have proper
nourishment and conditions of growth, the amount of moisture and the
temperature being two of the most important of the latter. Use is made of this
in the preservation of many articles of food, etc., since by ensuring that at least
one of the conditions necessary for growth shall be absent, these substances are
prevented from decomposing. For instance, hides are preserved by drying
them; the absence of sufficient moisture hindering the growth of any organisms
in them so long as they are dry, but as soon as they become somewhat damp,
putrefaction commences at once.

The waste products of organisms are often poisonous to themselves, and for
this reason fermentations frequently come to an end before the whole of the
substance is fermented. Thus neither beer nor vinegar can be obtained of more
than a certain strength by direct fermentation, the alcohol or acetic acid
checking the growth of their respective ferments. A solution of glucose “set”
with the lactic ferment of sour milk will only produce lactic acid to the extent
of about half a per cent.; but if chalk be added, the lactic acid will be
neutralised as produced, and the fermentation will go on till the whole of the
glucose is converted into insoluble calcium lactate.[6] When this is
accomplished the lactic ferment dies from want of nutriment, and its place is
taken by another organism, of which some germs are sure to be present, which
ferments the calcium lactate into calcium butyrate. If the nourishment fails, or
the conditions become less favourable for one ferment than for some other
which exists even in small quantity in a liquid, the former is quickly
overgrown and killed, and the latter takes its place. Thus the ordinary ferment
of the bran drench will die out rapidly unless constantly transferred to fresh
bran infusions.
[6] For the practical preparation of lactic acid, the solution may contain 71⁄ -11 per cent. of
2
glucose, and some nitrogenous nourishment. The solution should be slightly acid. See
Journ. Soc. Ch. Ind., 1897, p. 516.
Many of the products of bacteria (like those of some of the higher plants) are
intensely poisonous both to animals and man. Many of the severe symptoms of
disease are caused by these poisons produced in the body. Thus the tetanus-
bacteria produce a poison similar in its effects to strychnine, and quite as
virulent. Not only are such poisons produced by disease-bacteria in the body,
but frequently also in the earlier stages of putrefactive fermentation. The latter
are known as ptomaines, and when present in cheese and preserved foods are
liable to cause poisoning. Such putrefactions are often unaccompanied by any
disagreeable odour or flavour.
The fermentations which are most important in the tannery are, firstly, the
ordinary putrefaction which attacks hides as well as other animal matter, and
which is usually a complicated process carried on by many sorts of bacteria
and other micro-organisms. This may be regarded as generally injurious to the
tanner; but it is utilised in the “sweating” process for depilation and in the
“staling” of sheepskins, in both of which advantage is taken of the fact that the
soft mucous layer of the epidermis, which contains the hair-roots, putrefies
more rapidly than the fibrous structure of the hide itself. In soaking also, use is
made of the power of putrefactive ferments to dissolve the cementing
substance of the hide, though in this case with doubtful advantage to the

tanner. In the liming process putrefaction makes itself felt when the limes are
allowed to become stale and charged with animal matter, softening the hide
and finally rendering the leather loose, empty and inclined to “pipe.” Here the
effect is in many cases useful if not carried too far.
In bating and puering, the action is almost entirely due to the enzymes and
other products of bacterial activity, the original chemical constituents of the
dung being apparently of minor importance. Naturally the liquid is adapted to
the growth of many other organisms beside those acting most advantageously
on the hide, and injury in the bates from wrong forms of putrefaction is very
common, if indeed it is not always present in greater or less degree.
In drenching, the effect is, at first, entirely due to the weak acids produced
by bacterial fermentation of the bran, but becomes complicated in its later
stages by putrefactive and other fermentations which may be desirable or
otherwise.
In the tanning liquors, fermentation is not so marked, but is of great
importance owing to the production of acids by bacterial action from the
sugars present in the material. The acids themselves are apt to be fermented
and destroyed, principally by the oxidising action of Saccharomyces
mycoderma and the higher moulds (see p. 14), which also act destructively on
the tannins.
The effect of these acids on the hides is to swell them and to neutralise any
lime they may contain. They also give to the liquors a characteristic sour taste,
as a consequence of which, liquors containing acetic and lactic acids are
usually known in the tannery as “sour liquors.”
It is doubtful whether the action of fungi is completely stayed even by the
drying process. The heating of leather in the sheds is due to bacteria and the
higher moulds, and Eitner considers their growth one of the causes of the
“spueing” or “gumming” of curried leathers.
From what has been said, it is obvious that, with regard to fermentations, a
double problem is presented to the leather manufacturer, since he desires to
utilise those which make for his advantage, while controlling or destroying
those which are injurious. The first step to a solution of these problems is a
more complete knowledge of the organisms which serve or injure us, that we
may, as it were, discriminate friends and enemies. We may then approach the
question in two ways. Taking the drenching process as an example, we may on
the one hand introduce a “pure cultivation” of the right ferment into a sterilised
bran infusion, and so induce only the one fermentation which we require; or,
on the other hand, as different ferments are affected in varying degrees by
antiseptics, we may perhaps choose such as permit the growth of the organism

we want, while killing or discouraging the rest. We may also arrange the
nutriment, temperature, degree of acidity and other conditions, so as to favour
one organism rather than another. All three methods have been applied in
brewing with good results.

CHAPTER V.
ANTISEPTICS AND DISINFECTANTS.

“Antiseptics” are often defined as substances which check putrefaction
without necessarily destroying bacteria and their spores, while “disinfectants”
are poisonous to ferment-organisms, and actually destroy them; great
differences exist in the extent of their sterilising power, and the whole
distinction is one rather of degree than of kind, and has little practical value.
Thus common salt is incapable of killing most bacteria, even in concentrated
solution, though it holds putrefaction in check both by withdrawing water from
the hide and by directly preventing the multiplication of bacteria. If the salt be
washed out of the hide, putrefaction is at once resumed by the organisms
present. Hides, on the other hand, which have once been sterilised by powerful
disinfectants, such as phenol (“carbolic acid”) or mercuric chloride, do not
again putrefy till the organisms which are killed are replaced by fresh ones
from outside. The action of sodium sulphate, and many other salts, is similar to
common salt in this respect, while a large proportion of the aromatic
compounds are permanently disinfectant, though their efficiency varies with
the species of bacteria involved.
Biernacki and others have shown that some disinfectants when extremely
diluted actually stimulate alcoholic fermentation, and probably the growth of
other ferments, e.g. mercuric chloride 1 in 300,000, salicylic acid 1 in 6000,
and boric acid 1 in 8000, and in many cases organisms become habituated to
antiseptics in doses which would at first have proved fatal.
The number of antiseptics available is now so great that it is impossible to
give a detailed account of all, but the following are among those which are best
known and have been practically employed.
Lime possesses some antiseptic properties, and is largely used in the
preservation of fleshings before they are sent off to the glue factory. They are
most conveniently stored in a large vat filled with a strong milk of lime. Dilute
solutions of caustic alkalies have an effect similar to that of lime.
Common salt, sodium chloride, NaCl, acts to a certain extent by its
solubility and a dehydrating effect on animal tissues common to chlorides,
which removes water from hides and other materials which it is used to
preserve. Probably the latter characteristic has a good deal to do with its effect

in checking the development of bacteria, since many species thrive quite well
in weak salt solutions, and some even in brine, and the dehydrating effect of
the salt enables it to harden many animal tissues if used in sufficient quantity,
the water they contain running away in the form of brine.
Ordinary rock salt frequently contains ferric chloride, and this, either
originally present in the salt, or in some cases derived from the action of the
latter upon the iron contained in the blood, is the cause of what is known as
“salt-stains.” These show but little during the liming of the hides, unless
sulphides are used, when stains appear of a greenish black, from the formation
of sulphide of iron; when, however, the hides come into the tanning liquors,
black or blue stains are produced by the action of the tannin, which are
partially removed by the acids of the liquors during the tanning process, but
generally show to some extent in the finished hide. There is another species of
salt-stain, not apparently due to iron, but to the colouring matter produced by
some fungoid or bacterial growth, which it is practically impossible to remove,
and which is stated to be sometimes caused by the use of old salt with which
hides have been previously salted. Iron stains are most readily recognised by
the use of a solution of potassium ferrocyanide or thiocyanate slightly acidified
by hydrochloric acid. If this be applied to the leather, the stains will be changed
from a blackish to a blue, if the former, or a red colour if the latter salt has been
used. A more absolutely conclusive proof is to lay a piece of filter paper
soaked in dilute hydrochloric acid upon the stain, and then to test for iron upon
the paper with ferrocyanide or thiocyanate. The freedom of the paper itself
from iron must be ascertained before use. Iron-stains produced in the salted
state are more difficult to discharge than those which are caused later in the
tanning process, since iron salts have distinct tanning power, and attach
themselves firmly to the untanned fibre. On the Continent, where common salt
is heavily taxed, alum, carbolic acid, naphthalene and other materials are
frequently added to it to “denaturise,” or render it incapable of being used as
food, and these additions are often the cause of trouble to the tanner.
Sodium sulphate, Na2SO4, has little if any disinfectant power in dilute
solution, but if used in the calcined form (anhydrous sodium sulphate) as
proposed by Eitner[7] as a substitute for common salt in preserving hides, it
withdraws water from the hide and crystallises with 10 Aq (about 56 per cent.).
This does not run away like brine, but remains in the hide, which retains its
weight, and remains plump and swells well in the limes and liquors, which
chlorides have a great tendency to prevent; 10-15 per cent. on the weight of the
hide is sufficient, while salt must be used in nearly double this quantity. Care
must be taken that the sulphate used is free from bisulphate, NaHSO4, which

has a powerful swelling effect upon the hide-fibre, like sulphuric acid. The
neutral sulphate does not redden methyl orange or litmus. Pickled skivers may
be in part preserved by the sodium sulphate formed by the action of sulphuric
acid upon the salt employed in the pickling bath (see p. 90).
[7] Gerber, 1880, p. 185.

The stronger mineral acids have considerable antiseptic power, and are of
course especially fatal to such ferments as thrive best in alkaline solutions. The
use of sulphuric acid in pickling skivers has already been alluded to, and a very
dilute solution applied without salt to raw hides prevents putrefaction, though
the principal object in using it is to plump the hides and produce a fictitious
weight and substance which disappear on tanning. Such hides of course have a
powerful acid reaction to litmus. Sulphuric acid in small quantities has been
used with advantage in soaking E.I. kips. A very small excess of hydrochloric
acid will sterilise putrid effluents, and no doubt nitric or sulphuric acid would
have the same effect. The powerful effect of mineral acids on animal fibre, and
their solvent action on cements and iron, preclude however, their general use as
antiseptics.
More important is the use of sulphurous acid and sulphur dioxide, which,
from their mild acidity and great antiseptic powers, are capable of a variety of
useful applications. Considerable doubt has been raised as to the germicide
power of sulphur dioxide, and it is certain that the dry gas is less effective on
dry objects than when applied in solution, or to moist materials, as is almost
invariably the case in the tannery. It may possibly be more efficient in its
action on some moulds and putrefaction-ferments than on the pathogenic
bacteria which have been most frequently used to test the power of
disinfectants; but in practice it is found extremely useful in the brewery and in
gelatine manufacture, and there is no reason that it should be less so in the
tannery.
The gas is most conveniently produced by burning sulphur, which produces
double its weight of sulphur dioxide. If used for “stoving” drying rooms and
other places infested with moulds, care must be taken to avoid risk of fire. A
shallow cast-iron pot set on bricks or sand is generally the most suitable vessel,
and the sulphur may be ignited by a piece of red-hot iron or a rag which has
been previously dipped in melted sulphur. It is corrosive to metalwork, and
bleaches many colours, but does not produce any marked injurious effect on
leather, though the sulphuric acid formed by oxidation may, if not removed,
ultimately make it tender.

For many purposes a solution of the gas is required, and this is most easily
made by burning the sulphur in a small metal or firebrick stove from which the
fumes are sucked through a “scrubber,” which, on a small scale, is
conveniently made of large glazed sanitary pipes, packed with coke or broken
earthenware, over which water is allowed to trickle. The lowest pipe has an
opening for a branch pipe, which is connected with the stove and rests on three
bricks in a tub, which collects the acid solution and forms a water-seal to
prevent the escape of gas. Above the inlet for the gases is fixed a wooden
grating on which the coke rests. The scrubber may be 10-15 feet in height and
connected at the top with a chimney or steam ejector to produce the draught.
The arrangement is illustrated in Fig. 5. Another method is to burn the sulphur
in a closed cylinder and to force the products through water with an air-
compressor or steam-jet injector.
In place of using a scrubber, the fumes may be blown by a steam ejector
direct into a tank. This is a very good arrangement for washing and bleaching
hair, etc., but where large quantities of solution are required is inferior to the
scrubber. Ejectors of hard lead or regulus metal should be used, and are less
acted on by the dry gases than by the very dilute moist exhaust from the
scrubber (see p. 335).

Fig. 5—Sulphurous acid apparatus.

Bisulphites have also strong antiseptic properties. “Bisulphite of soda”
(hydric sodic sulphite) solution may be made by supplying the scrubber with
solution of soda-ash or washing soda; bisulphite of lime, by using milk of lime
or packing the scrubber with chalk or limestone (free from much iron) in place
of the coke. In either case a much stronger solution is obtained than with water
alone.
Boakes’ “metabisulphite of soda”[8] is a very convenient source of
sulphurous acid when the latter is wanted in small quantities. It is an
anhydrosulphite, Na2O.2(SO2), and contains 67·4 per cent. of its weight of
SO2. One molecule of the salt (= 190) requires one molecule of H2SO4 (= 98)

to set free the whole of the sulphurous acid. For many purposes the sulphate of
soda formed may be neglected and the acidified solution used direct.
[8] Patented by Boakes, Ltd., Stratford, London, E.

For analysis of sulphites and sulphurous acid solution, see L.I.L.B., pp. 16
and 37.
Boric acid, borax and other borates are not very powerful disinfectants.
They have no injurious action upon the skin, but to be effective require to be
employed in pretty strong solutions, say 1 per cent., and their comparatively
high cost unfits them for general use as antiseptics in the tannery, though boric
(boracic) acid is very useful as a drenching and deliming agent (see pp. 156,
229, and L.I.L.B., p. 37).
Mercuric chloride, corrosive sublimate, HgCl2, is an extremely powerful
antiseptic, preventing the growth of some species of bacteria in solutions so
dilute as 1 in 300,000 (Koch). 1 in 14,000 is disinfectant (Miquel), but its
power varies very much upon different organisms (Jörgensen states that 1 in
400 is required to kill Penicillium glaucum), and it is unsuited for most
purposes in leather manufacture, both from its extremely poisonous character,
and because it is rendered inactive by various substances present in the
materials used.
Mercuric iodide dissolved in iodide of potassium solution was patented by
Messrs. Collin and Benoist as an antiseptic in tanning, but it is ineffective for
the same reasons as mercuric chloride; although under favourable
circumstances it is even more powerful than the latter.
Copper sulphate, zinc chloride and sulphate, and many other metallic salts
are powerful antiseptics, but have only a limited application in leather
industries, and do not usually actually sterilise. Arsenic (arsenious acid), which
has been used in curing hides, is an excellent insecticide, but not particularly
effective as an antiseptic; and sulphide of arsenic (realgar) when used in limes
(see p. 139) seems to have but little antiseptic effect. Arsenious acid is easily
soluble in alkaline solutions.
Fluorides have been suggested as antiseptics in the tannery, but do not seem
of much practical value.
The most important antiseptics at present are those derived from coal tar,
and belonging to the aromatic series. Of these, the phenols (carbolic acid,
cresol, etc.) are the most used.
Pure phenol, “pure crystallised carbolic acid,” is hydroxybenzene
C6H5(OH), but the crude forms which are generally employed contain cresols
and higher members of the series in which one or more of the atoms of

hydrogen are substituted by CH3 groups. These are oily bodies scarcely soluble
in water, and even pure phenol is only soluble in cold water to the extent of
some 7 per cent. Crude carbolic acid should not be employed in the tannery,
since the insoluble oily particles stain the hide, and render it unsusceptible of
tanning. Suitable carbolic acid should be of a pale yellow colour when fresh
(though it will darken on exposure to air and light), and it should be wholly
soluble in a sufficient quantity of water. Its specific gravity should be 1·050 to
1·065. For methods of chemical examination, see L.I.L.B., p. 40. A saturated
solution of carbolic acid sterilises hide completely against most putrefactive
organisms, but has a sort of tanning effect, adhering obstinately to the fibre so
that it cannot be removed by washing; and hides which have been cured with it
cannot be unhaired by sweating, though they may be limed in the usual
manner, if somewhat more slowly. Care should be taken in mixing with water
or liquor, as undissolved drops will produce the same effects as those of the
crude acid. Hides are occasionally stained, as has just been described, by salt
which has been denaturised with common sorts of carbolic acid. Eitner
recommends the use of a solution of carbolic acid in an equal weight of crude
glycerine, which readily dissolves in water, and seems to prevent any injurious
effect on the hide.
An aqueous solution containing 1 per cent. of carbolic acid is sufficient for
mere sterilising of hides, but if it be desired to preserve them for a long period,
stronger solutions (up to 4 per cent.) may be employed.[9]
[9] Gerber, 1889, p. 98.

Quantities so small as 1 part per 1000 control the fermentation of liquors,
and prevent the formation of moulds on the surface, economising tannin, and
preserving vegetable acids already present, but at the same time lessening their
production by fermentation, and therefore sometimes leading to difficulties in
the early stages of tanning. Carbolic acid is not, strictly speaking, an acid, but
rather of the nature of an alcohol, although it forms weak combinations with
bases. It is a powerful narcotic poison, and if dropped on the skin in a
concentrated form it produces severe burns; these are best treated with oil,
while in cases of poisoning, oil and chalk must be administered internally, but
if the quantity of carbolic acid taken has been large, are not likely to be
effective. From its cheapness and efficiency, carbolic acid is likely to be
increasingly used, although for special uses some of the newer antiseptics have
great advantages.
Eudermin is a tar-oil manufactured by Speyer and Grund, of Frankfort-on-
Main, which is intended as an antiseptic addition to stuffing greases to prevent

mould and spueing. It is recommended for the purpose by Eitner[10] and can be
used in proportions such as 10 per cent. of the grease. Creasotes and cresols
can be dissolved in oils and stuffing greases, and act as antiseptics, though less
powerfully than in aqueous solution. Rosin oils and turpentine have also
antiseptic properties.
[10] Gerber, 1893, p. 41.

Creasote, “heavy coal oil,” or “dead oil,” is a complex mixture of
hydrocarbons, phenols and cresols, obtained by distillation of coal tar, heavier
than water, and almost insoluble in it. It is largely used as a preservative for
timber. Carbolineum is an oil of this class, boiling at over 300° C., and
intended for application to wood. One or more coats are applied to the dry
wood at a temperature of 80° C. The workman’s hands must be protected by
gloves, as the hot creasote raises painful blisters. Eitner[11] recommends its use
for preserving pits, posts and other woodwork in tanneries. Wood-creasote is a
somewhat similar product obtained from wood-tar.
[11] Gerber, 1889, p. 183.

The heavier cresols are so little soluble in water as to be valueless in their
ordinary form as antiseptics, but several preparations are made under the
names of “Creolin,” “Jeye’s fluid,” “Lysol,” “Izal,” “Soluble phenyl,” etc., in
which they are treated with additions of soap or alkalies, which cause them to
emulsify or dissolve in water, generally as milky liquids. These are powerful
germicides and have the advantage over phenol of being non-poisonous. 0·1 to
0·5 per cent. solution of creolin will sterilise hides after bating so that no
putrefaction takes place in the liquors. Mr. J. T. Wood specially recommends
creolin for the general purposes of the tannery, disinfecting pits and tubs, and
for checking the action of puers and drenches on goods which have gone a
little too far, by throwing them into a 0·2 per cent. solution.
Salicylic acid, orthohydroxybenzoic acid, C6H4OH(COOH), is now
artificially prepared from phenol. It is much less poisonous than the latter and
has no smell, which makes it valuable for certain purposes, but is too dear for
most technical applications. Many bacteria appear to become gradually
habituated to its action, and the same is true of phenol to a less degree.
Salicylic acid is closely related to protocatechuic and gallic acids, and, like
these, gives a blackish colour with iron salts. It is freely soluble in hot water,
but very sparingly in cold. The addition of 1-21⁄2 parts of sodium phosphate,
sulphate, or potassium nitrate to each part of salicylic acid greatly increases its
solubility. It seems much more powerful in preventing the development of

bacteria than carbolic acid; a solution of 1 part of salicylic acid in 666 of water
is said to be equal in this respect to 1 part of carbolic in 200.
Benzoic acid, C6H5COOH, though not much employed, except in medicine,
is a still more powerful disinfectant, and has the advantage of being non-
poisonous to human beings.
“Cresotinic acid,” which is derived from the cresols as salicylic acid is
derived from phenol, is more soluble than salicylic acid. It is not very
poisonous, and a powerful disinfectant. In a crude form it has been introduced
by Hauff, of Feuerbach, for bating or removing lime from hides. This it does
very well, though without the softening action of a true bate. It has a tendency
to produce a pinkish stain, and in some degree a sort of tanning of the fibre. Its
price, moreover, is rather high for extensive technical use. (See also p. 162.)
“Anticalcium” is a more recent preparation introduced as a bate by the same
firm.[12] It is a solution of mixed sulphonic acids derived from cresols, and has
considerable disinfectant powers. It removes lime very effectively, but from its
acid character somewhat swells the skin. It is used very successfully as a
drench for thin skins (p. 163).
[12] Gerber, 1895, p. 133.

“C.T.” (coal-tar) bate is a grey crystalline pasty mass, with a tarry smell, and
is chemically very similar to anticalcium if not identical with it.
Naphthalene sulphonic acid has strong antiseptic properties. Its use in bating
has been patented by Burns and Cross. (See p. 163.)
Naphthols, C10H7(OH).—These bodies, which have the same relation to
naphthalene as the phenols to benzene, are powerful antiseptics; and
naphthalene itself appears to have antiseptic power, and is occasionally used
for denaturising salt. There are two naphthols, varying in the position of the
OH group in the molecule, and denominated α and β, of which α naphthol is
the more powerful antiseptic and the less poisonous, though β, being cheaper,
is the common commercial article. It is said that quantities so small as 0·1-0·4
grams of α naphthol per liter are sufficient to prevent the development of
microbes, while of β naphthol about ten times that quantity is required.
Naphthols are not very expensive, but their value is diminished by the fact
that they are insoluble in water. They are soluble in alkaline solutions, but their
compounds with bases are of much lower antiseptic value, and the same is true
of their alcoholic solutions; when an alcoholic solution is added to water the
naphthol is precipitated, but if an addition of soap or camphor be made to the
alcoholic solution, the naphthol remains in a very finely divided condition, if
not dissolved.

Adopting Eitner’s suggestion with regard to oxynaphthoic acid (see below),
hides may no doubt be sterilised by treatment first with an alkaline naphthol
solution, and then with a very dilute acid to set the naphthol free.
“Hydronaphthol,” β tetra-hydro-naphthol, C10H12O, is obtained by the
reduction of β naphthol by sodium (Rideal). It seems to be an excellent
disinfectant.
Oxynaphthoic acid, α hydroxynaphthoic acid, C10H6(OH)COOH, which
bears the same relation to naphthol as salicylic acid does to phenol, is cheaper
than salicylic acid, and said to be a more powerful antiseptic. Its salts have no
antiseptic power. In its commercial form it is a reddish crystalline powder,
inodorous, but with a burning taste, and its dust causes violent sneezing. It is
scarcely soluble in water, and is said to undergo some change on keeping
which lessens its germicide power; it is readily soluble in alcohol, and the
solution produces a milky fluid on mixture with water. Such a solution
containing 15 grams of the acid in 4 liters of water, will sterilise a hide. Eitner
recommends[13] that it should be dissolved in dilute soda solution, and the
hides, after soaking in it, passed through water slightly acidified with
hydrochloric acid, as has been suggested in the case of naphthol; the method is
also applicable to creosotinic acid, the hides being permanently sterilised so
that they cannot be unhaired by sweating, though they will lime in the usual
manner.
[13] Gerber, 1888, p. 101; 1889, pp. 99 et seq. See also p. 163.

Carbon disulphide.—Moret has suggested an aqueous solution of this
compound as an antiseptic, and it seems to have considerable sterilising
powers, but from its inflammability, poisonous character, and unpleasant smell,
it is not likely to come largely into use.
Formaldehyde, COH2, has recently been introduced as an antiseptic in
aqueous solution containing 40 per cent. of formaldehyde together with a little
formic acid, under the names of “formalin,” “formol,” etc. It seems to have
great disinfectant powers, and may possibly be valuable in various processes of
leather manufacture as it becomes cheaper, but has a curious hardening tanning
effect on hide fibre and gelatinous matters, so that in very dilute solution it will
produce leather.[14] The vapour of formaldehyde, or of its condensation-
product paraform, may be employed to harden microscopic preparations. 1 part
of formaldehyde, and consequently 21⁄2 parts of “formalin” in 12,000 parts of
water, is said to sterilise, and this proportion would form a good disinfectant
solution. Even in considerably larger proportion than the above, it does not
appear to be poisonous, and thus possesses the bactericidal power of sublimate

without the latter’s poisonous properties. Formaldehyde has another advantage
over most, if not all other antiseptics, in that it may be used as well in the
gaseous as in the liquid state, and on that account it is largely employed in the
disinfection of rooms or of articles which would be spoiled if they were to be
wetted, as the gaseous formaldehyde, though thoroughly disinfecting them,
will not injure the colours of materials of the most delicate fabrics.
[14] Gerber, 1897, p. 67; ibid., 1899, pp. 101, 205, 218.

On account of its capability of rendering gelatinous matters hard and
insoluble in water, formaldehyde requires to be employed with great care, but
0·2-0·3 per cent. may be successfully used in admixture with egg-albumen in
the preparation of “seasoning” in the finishing of morocco leather. It is also
used commercially to produce different varieties of white leather for soldiers’
accoutrements and similar purposes (p. 380).
Triformol (tri-oxymethylene, “paraform”) is a product of the polymerisation
of formaldehyde, and is prepared by evaporating a solution of the latter to
dryness on the water-bath. It is said to be more powerful than formalin in its
antiseptic properties, but has not entered very largely into use as a disinfectant,
though considerable use is made of it to “fix” bacteria in gelatin for
bacteriological purposes.
Camphor and essential oils, as well as oil of turpentine, have considerable
antiseptic powers, and the cheaper essential oils such as those of winter-green,
black birch, sassafras and aniseed are frequently employed, especially in
America, in preserving pastes, finishes and seasonings, and at the same time
covering offensive odours. The odour of essential oils becomes much more
powerful as they are diluted, and very small quantities suffice for the purposes
mentioned. Birch-tar oil, such as is used to give the scent to Russian leather (p.
372), has considerable antiseptic effect.

CHAPTER VI.
THE ORIGIN AND CURING OF HIDES AND SKINS.

A considerable proportion of the hides and skins used in leather manufacture
are those of animals killed by the butcher for food, and these are frequently
employed by the tanner without any preliminary curing. Domestic hides and
skins are now generally sold by auction in weekly markets in the principal
towns, after sorting and classification in weight and quality.[15] This is in many
respects an improvement on the old method of purchase direct from the
butcher, but it often leads to delay in delivery, and in hot weather hides suffer
from putrefaction. In most cases, the damage is not sufficient seriously to
affect the durability of the leather, but the delicate membrane of the “grain” is
injured, and the hide or skin unfitted for coloured leather, or any purpose
where small damages to appearance are important. Butchers are adverse to the
use of salt, because it withdraws water from the hide in the form of brine, and
so causes it to lose weight; but much injury would be saved by a light salting,
and all hides or skins on which the hair is “slipping” should be regarded as
damaged for fine leather manufacture.
[15] The weight of English market-hides as credited to the butcher is usually marked on the
edge of the butt near the tail, by cuts with a knife, the mode of numeration being
sufficiently explained by Fig. 6, in which cuts crossing the horizontal line each represent
20 lb., that above it 10 lb., while less amounts are expressed in Roman figures.
On the Continent weights are usually given in pounds of half a kilogramme (50 kilos = 110
lb. English). In Paris the marking is on the tail, and is also shown on Fig. 6.

Fig. 6.—Method of marking weight on hides; 97 lb.

Sheep-skins are not usually bought direct by the tanner, but by the
fellmonger, who removes the wool; and as this is usually of much greater value
than the skin, the latter is frequently handled very carelessly, and its quality
sacrificed for the sake of real or fancied improvement to the wool. In very
many cases the skin is “sweated” or “staled” by hanging in a warm and moist
chamber, heavily charged with ammonia derived from the putrefaction of the
skin, until the wool is sufficiently loosened to be “pulled.” If this treatment is
conducted with extreme care the skin may escape serious injury, but in most
cases the grain is weakened, and the foundation is laid of damage, which
makes itself felt throughout the tanning process. For the purposes of the tanner,
a much better way is to lime the skins by painting with thick limewash on the
flesh-side, and after folding the skins down the back, flesh-side in, to prevent
as much as possible the access of the lime to the wool, to place them in a pit,
and cover them with water, till the wool is loosened by the penetration of the
lime through the skin. A still more satisfactory method, and one which is in
general use in the American stockyards, and to some extent also in Europe, is
to wash the skins in water to free them from blood and dirt, and then, laying
them in a wet condition, flesh side up, to paint them with a solution containing
about 25 per cent. of sulphide of sodium, thickened with lime. The skins, as
they are painted, are doubled down the back, flesh-side in, and laid on a floor,
overlapping each other like tiles on a roof, for some hours, or overnight, till the
wool is sufficiently loosened to pull, after which the pelts are limed and treated
in the ordinary way. As a general rule the English fellmonger keeps his skins in
lime till they are sold to the tanner, and as in small yards some time is taken to
accumulate a parcel, the earlier skins may suffer great injury from overliming.
Even sweet fresh limes dissolve the cementing substance of the fibre, and
increase the naturally loose texture of the sheep-skin, but the injury is much
more considerable when old and stale limes, charged with ammonia and
bacterial products, are employed, as is frequently the case. In the American
stockyards the skins are generally limed only for the necessary time to act
upon the grease, and to swell and differentiate the fibres, and are then at once
puered, drenched and preserved by “pickling.” For details of “pickling” see p.
89. It is very probable that the Pullman process of liming (p. 137) would
answer well for fellmongered skins, as goods will keep for a considerable
length of time uninjured after treatment with calcium chloride.
Where hides or skins cannot be used at once in the fresh state, there is
probably no better method of preserving them than the use of salt. Although
salt is not fatal to bacteria, it so slows bacterial growth, partly by its direct
antiseptic effect on many organisms, and partly by withdrawing water from the

skin, that well-salted skins can be kept in good condition for almost an
unlimited time. Where it is only required to preserve goods for a week or two,
a moderate sprinkling on the flesh side is efficient, but if they are to be
preserved for any length of time, more thorough treatment is necessary. It is
said that however carefully hides are salted they deteriorate if kept in this
condition above twelve months.
The method of salting employed in the Chicago stockyards for “packer”
hides may be taken as a good type of a thorough salting. The hides are first
trimmed from useless “switches,” and any large portions of adhering fat are
removed. The curing takes place in large and cool cellars, with concrete floors.
The detail is well given in the following extract from the ‘Shoe and Leather
Reporter’:—
“Great care is taken to make the sides of a pack higher than the middle, so
that the brine which is made by the juices of the hide coming in contact with
the salt will be retained. The brine can only escape by percolation and hence
the fibre of the hides is thoroughly cured. The floor of a hide cellar is usually
of concrete, and a pack is from 15 to 20 feet long and as wide as the space
between the posts which support the floor above. The sides of a pack are built
first to a height of from 4 to 6 inches; the cross layers are then put on,
generally three on each side, two being inside and one having the butts drawn
out to the edge. In a pack 20 feet long, the side layers will contain about 25
medium-sized hides each, and a cross-layer 12 or 14. To begin a pack a truck-
load of hides is run along to the front of the place selected, one spreader grasps
the butt and his partner the head of a hide, and together they carry it to what is
to be the rear of the bed. The hide is then dropped, so that the folded back is
parallel to and from 15 to 20 inches from the inside line of the posts, the head a
trifle closer than the butt. The front man takes the dewlap and front shank in
his left hand, and extends his right along the belly of the hide as far as is
necessary to raise the edge, the rear man holding the flank with one hand and
the hind shank with the other. They keep their legs well out of the way of the
salt thrower, who with a single throw covers the whole hide, being particular
that enough salt strikes against the edges held by the men to make a
pronounced ridge when they are lapped down. A little salt is thrown on the hair
surface and the butt folded over about a foot. The folded edge is then drawn
out even with the outer line of the pack. More hides are placed the same way
until the corner is high enough. After this, each hide is put further forward to
make a level surface from rear to front, the heads at the front corner being
folded back as the butts were at the starting place. The other side is built the
same way, and then the cross layers are put on alternately until the pack is

level, when sides are again built as before. In putting on the first hides of the
cross layers, they are thrown over the edge, to lap back again when the salt is
thrown on; the layer is then continued on to the front. The spreader who holds
the butt does the guiding in every case. He drops the butt down at exactly the
proper place, takes the upper flank and shank in each hand, sets one foot on the
lower shank to keep it firm, and throws the one in his hands from him with
considerable force. The man at the head watches his partner, keeps the folded
hide taut, and drops it at the same time as the latter. He takes the fore-shank at
the knee in the one hand and the upper head-piece in the other, and setting his
foot on the lower side, throws the upper side forward simultaneously with the
rear man. Two expert spreaders, accustomed to working together, spread a hide
at a single throw, but some little straightening has to be done by hand before
the hide is ready for the salt. A gang composed of two spreaders, one salt
thrower and a salt trucker put down forty hides an hour. When gangs are
doubled, two men do all the spreading; the other two place the hides where
they can be got at conveniently. A double gang put down eighty hides an hour.
The salt trucker brings the salt to the pack in box-trucks open at one end to
permit the entrance of a shovel. The salt thrower keeps the edges and corners
of the pack full of salt. He must see that every part of the flesh-side is well
covered. Each hide takes two scoop shovelfuls of ground rock or coarse white
salt, mixed with an equal quantity of old or second salt. The salt thrower
throws the shovel forward and to one side and back again with a peculiar
swinging jerk, causing the salt to fall regularly over the entire surface of the
hide. The ease and rapidity with which a gang operates depends greatly upon
the efficiency of the salt thrower. When the pack gets too high to be
comfortable for the men, it is brought to a dead level and covered over with
clean salt. It then presents a very neat and workmanlike appearance. Spreaders
and salt throwers receive 20 cents an hour, and truckers get 171⁄2 cents. When
the temperature is kept at an even average, two weeks is ample time to cure the
hides.
“In ‘taking up,’ two men strip the hides from the pack. As they were put
down from the rear to the front, they are taken up in the reverse direction. No
matter how much loose salt is lying on the top, the man knows exactly where
to place his hand on a shank; as the hides are moved forward, the loose salt is
thrown off toward the front. One man takes away the salt as it accumulates and
trucks it to the salt bins, where it is mixed with new, to be used again. A ‘horse’
made of a network of scantling about 31⁄2 feet wide by 6 feet long, and standing
21⁄2 feet from the floor, is placed in front of the pack, on this the hides, flesh
side down, are shaken to remove the salt that is clinging to them. This process

requires four men, one at each corner. The hide is brought down heavily on the
horse twice, and then spread on the floor flesh side up for examination by the
inspectors, of which there are two, one representing the house and the other the
buyer of the hides. They sweep off any salt that may be left, and examine for
cuts, sores, brands, manure and grubs. They also see that the hide is properly
weighed and classified. If the contract calls for a special trim it is now done.
Two men then roll the hide, beginning by lapping over the shanks, head and
neck. Then the sides are folded over and lapped again, leaving the roll 15 to 18
inches wide. The ends are thrown inward, slightly overlapping each other; a
final fold is then given, and the hide is ready to be tied. Rope the size of
clothesline is used for tying, and is cut into lengths of about seven feet. It takes
three men to tie for a gang such as we have described. After tying, the neat
bundles are weighed and loaded on the cars for shipment. A small tare is
allowed the buyer. Ordinary workmen in hide cellars get 171⁄2 cents an hour,
and inspectors 25 cents an hour.”
About 25 per cent. of salt on the green weight of the hide is required for
thorough curing. Rock salt merely crushed is frequently employed, but this is
very liable to contain iron in the form of oxide and chloride, which causes the
peculiar marbled markings known as “salt-stains.” It is therefore much better
to use a white crystallised salt, though it is possible even in this case that stains
may arise from the iron present in the blood. Some salt-stains appear also to be
due to the action of pigment-bacteria, and not to contain iron. A reddening of
the flesh side is often noticed in hides which have been kept in salt long or
under unsatisfactory conditions, and is very frequent in wet-salted South
American hides. Such hides are said never to produce so firm a leather as those
which are sound.
Hides are not unfrequently cured by steeping in salt brine, instead of
strewing with dry salt. This method is principally resorted to in order to give
fictitious weight. Brined hides do not plump well in tanning, the leather is not
so good in quality as from those salted with dry salt, and the cure is much less
efficient.
Many hides are not only salted but also dried in order to preserve them. Not
much detail has been published with regard to the methods used, which no
doubt vary much in different places, but probably in some cases the hides are
salted in pile and in others by brining, and then hung up to dry. The principal
object of this drying is to economise weight and cost of transport, but it makes
the hides much more difficult to wash and soften for tanning, and probably the
crystallisation of the salt has a weakening effect on the fibre. Hides cured in
this way are styled “dry salted.”

A large number of the hides of the small native cattle of India are imported
into this country in a dry-salted condition. The following particulars of their
cure are taken from a paper by the Author and Mr. W. Towse.[16]
[16] Journ. Soc. Ch. Ind., 1895, p. 1025.

Dry-salted, or, as they are commonly called “plaster cures,” such as those of
Dacca and Mehapore, are thickly coated with a white material, which in the
first instance is merely the insoluble portion of a saline earth used in the cure;
though in many cases it is applied in larger quantities than necessary, with the
simple object of giving weight. The salting is thus described by Mr. W. G.
Evans, who some years since had considerable experience as a tanner at
Cawnpore:—
“The salt used by the natives is a salt-earth; and is so called by them. It is
found extensively in the districts of Cawnpore, Agra, Delhi, Lucknow, Patna,
etc., and has no doubt something to do with the localisation of the hide-curing
and kindred industries in these places. The mode of procedure used is pretty
much as follows:—the salt-earth is mixed into a very thin paste, and this is
lightly brushed on to the flesh side one day, and the hide allowed to remain
over night under cover. Next day, for best hides, the same solution is again
spread on the flesh side of the outstretched hide and rubbed into it with a
porous brick, and then for legitimate salting, the hide is allowed to dry under
cover. If for export, the saltings may be three or four, and the hides are treated
out in the open, subject to the intense heat of the sun; which accounts for the
number of hides which go back in the soaks in England and elsewhere.”
“We had a clause in our agreement with hide-factors, that any hides which
did not come down to natural suppleness in two days in clean water were to be
returned. Of arsenic curing I know nothing, and it is not so much in vogue as
formerly. There is quite a trade in Cawnpore, Lucknow, Allahabad, etc., in
treating old and inferior hides with new for export, and great efforts are made
by native holders to get their stocks down before the rains commence, as they
say, and rightly I think, that hides are not worth so much after the rains by 30
per cent. The peculiar latent moisture of the rains affects them very
detrimentally.”
Under certain circumstances this mode of cure gives rise to extensive iron-
staining of the skins, and analyses of the material scraped off Dacca and
Mehapore kips were undertaken with a view to elucidating the causes of this
injury. The following are the results of the analyses referred to, which were
made upon the residue after the rather considerable quantity of fibrous organic

matter, which had been scraped off with the cure, had been destroyed by
ignition, together no doubt with traces of ammoniacal salts:—
— Dacca. Mehapore.
Entire Cure. Entire Cure.
Sand and silica 20·55 27·38
Fe2O3 2·77 1·86
Al2O3 2·48 2·74
Mn3O4 0·60 0·40
CaO 2·60 3·70
MgO 3·38 3·69
Na2O 28·97 26·80
SO3 38·90 33·75
Cl 0·22 0·18
H3PO4 and CO2 Traces Traces
100·47 100·50

The soluble salts of the Dacca cure were also analysed separately with the
following result:—
CaO 0·70
MgO 0·60
Na2O 29·00
SO3 37·90
Cl ·22
Insoluble 32·12
100·54

It thus consisted exclusively of sulphates, with the exception of a trace of
chloride. The cures, after ignition, were both neutral to phenolphthalein, but
before ignition the Dacca was distinctly alkaline, in consequence probably of
the presence of ammonium salts, and both showed considerably larger traces of
carbonates before than after.
The most striking feature of these analyses is the absence of more than the
smallest traces of chlorides. The cures are thus practically free from common
salt, and owe their antiseptic power to the sodium sulphate which they contain,
and which indeed forms their principal constituent. Nitrates appear to be
entirely absent. Sodium sulphate sometimes forms large crystals in pits used
for soaking these kips.
The iron-staining of hides which has been mentioned appears to result only
when the hides after cure are exposed for a lengthened period to a moist
atmosphere, in which the carbonic acid present probably also plays its part, the
iron passing into solution as hydric carbonate.

The analyses show a striking resemblance to those of the soda deposits of
Wyoming, given by Dr. Attfield,[17] except that their percentage of sodium
carbonate is smaller, which is quite intelligible in the light of Mr. Brunner’s
abstract on the ‘Probable origin of natural deposits of sodium carbonate,’[18]
which supports the view that the sodium carbonate is derived from sodium
sulphate by the reducing and carbonating action of low organisms.
[17] Journ. Soc. Ch. Ind., 1895, p. 4.
[18] Ibid., 1893, p. 116.

It may be noted here that the preservative properties of sodium sulphate are
well known, and the anhydrous sulphate has been recommended as a substitute
for common salt (see p. 23).
Drying is a very common method of preserving hides as well as other
putrescible matters. It has no effect in killing bacteria, but putrefaction can
only go on in presence of a considerable amount of moisture. As applied to
hides, it is, to the tanner, one of the least satisfactory modes of cure, involving
very considerable difficulties in bringing hides back to the moist and swollen
condition which is necessary at the outset of his operations, but it is the only
practical method in districts far from the coast and with primitive modes of
transit, both on account of the cost of salt, and the lessened weight of the dried
hide. Great differences are found in the ease with which dried hides soften,
according to the way in which the drying has been accomplished, the difficulty
being greater the higher the temperature which has been used (see p. 111). The
best mode of drying is to hang in the shade in a good draught of cool air, with
the flesh side out. Hides or skins dried in a tropical sun are not only difficult to
soften, but are liable to damaged portions, which either refuse to soften, or
blister and go to pieces in liming, owing to the structure of the hide being
destroyed by heat, the outer surface drying first and forming an impervious
layer which hinders evaporation from the inside, so that the moist interior
becomes melted, while the outside appears quite sound. Such injuries are often
only to be discovered by soaking and liming. Very similar damage may occur
from putrefaction of the interior after the outside has become dry, and to get
good results, the drying must be gradual, but rapid, especially in hot climates.
South American hides are mostly dried in the sun, suspended by head and tail
from stakes, with the hair side out.
The risk of injury by putrefaction during drying is diminished by the use of
antiseptics. Solutions of arsenic have been frequently used for this purpose,
and many of the dried Indian kips are of what are known as “arsenic cures,”
although the writer has never been able to detect arsenic in any which he has

examined, and its use seems by no means general. The arsenious acid is
usually dissolved in soda solutions. Unless used pretty freely it has little
antiseptic effect, but is useful in preventing the attacks of insects, which are
often very destructive. The larva of a small beetle, Dermestes vulpinus,
frequently devours the whole tissue of patches of the hide, leaving only the
epidermis.
It may be well here to say a few words about the injuries and defects to
which hides and skins are liable, although some of them are not strictly due to
the cure. The most serious, and yet preventable injury is that due to butchers’
cuts. As the value of the hide bears only a small proportion to that of the meat,
many butchers do their work extremely carelessly, and this is encouraged by
the loose classification of “damaged hides” in some markets. There is also an
idea that the appearance of the meat is improved by a thin layer of the white
skin-tissue being left on it, and for this reason as well as mere carelessness,
butchers frequently score the flanks of the hide with shallow cuts which greatly
diminish its value. The “packer hides” of the United States, and the products of
the large saladeros or slaughtering (“salting”) establishments of South
America, such as Liebig’s, show what can be done by skilled work in this
respect. In the United States, much of the flaying is done by means of a
wooden cleaver, instead of a sharp knife. Another method to some extent in
use, and which may be recommended for calf and sheep skins, is to inflate the
carcase before skinning, with air from a compressing syringe, which tears the
connecting tissue between the skin and the body, and renders flaying much
easier.
Brands are a great source of damage to hides, but where cattle roam at large
on unfenced plains, as on the prairies of Texas and the Pampas of South
America, it seems indispensable for the recognition of ownership; no other
mode of marking being sufficiently permanent and conspicuous. It is
unfortunate, that as the animals crowd together, and cannot be closely
approached, it is necessary that the brands should not only be large, but placed
on the most valuable part of the hide. Generally on the Pampas an effort is
made to keep them on one side only, so that in South American hides it is
possible to select clear and branded sides. In the United States much land is
now fenced with barbed wire, which while it obviates the necessity of
branding, introduces another evil in the form of “barbed wire scratches,” which
are frequently troublesome in “packer hides.”

Fig. 7.—Hypoderma bovis. 1, egg; 2, maggot; 4, chrysalis case; 6, fly, magnified (Brauer); 3, 5,
chrysalis and fly, natural size (B. Clark).

In countries where cattle are used for
draught purposes, goadmarks are a
frequent source of injury, and some of
the large cattle-ticks do considerable
damage to the hides of Spain and South
America. From the tanners’ point of
view, however, the most injurious
insects are the “bot-flies” or “warble-
Fig. 8.—Sac of warble, showing growth of flies” (Hypoderma bovis and allied
epidermis round aperture. species, Fig. 7). There is still some
controversy as to how the eggs of these
insects are deposited. In the horse-bot fly it is known that the eggs, first
deposited on the skin, are licked off and swallowed by the animal, and develop
in the stomach, where they pass their larval and pupal life hanging on to its
interior coats, and only drop off and are passed out with the dung before their
final change to the complete fly. Fortified by this, and by some direct
observation, some American naturalists are of opinion that the American
species at least, hatches in the stomach, and as a minute larva wanders through
all the intervening tissues till it reaches the skin, where it undergoes its further

development. The late Miss Ormerod, who has made a careful study of the
English species,[19] states that the egg hatches on the hair, and that the larva
simply eats its way below the skin, leaving a minute red puncture which it
subsequently enlarges to obtain air for its spiracles, which are in the tail. As it
grows it continues to irritate the lower part of the cavity with hooked
mandibles, and lives on the pus and matter so produced. It grows to a length of
fully 3⁄4 inch, and the cavity, Fig. 8, situated between the skin and the
subcutaneous tissue is often as large as half a walnut. It remains in the sac not
only during its larval, but its pupal stage, which do not differ much in
appearance, and falls out on the ground before complete development. In small
numbers, the warble seems to do little injury to the general health of the
animal, but cases have been known where animals have actually died of the
inflammation produced. Some idea of the extent of the plague may be realised
from the statement that an Indian kip in the possession of the writer has not
less than 680 warble holes, and that almost equal numbers have been counted
in English hides. Preventive measures are the sheltering of the cattle during the
summer months when the fly is most prevalent; the application of mixtures of
oil or grease with tar-oil and sulphur to the hair, to prevent egg-laying; and the
destruction of the larva in its early stages, in autumn and winter by smearing
the breathing aperture with grease, or better, with mercurial ointment. When
this is done sufficiently early, the hole heals up without permanent injury, but
when it is allowed to remain open during the period of growth, its sides
become partially coated by the growth of epidermis, and this permanently
prevents their proper union by skin-tissue. It is believed that if the larvæ were
systematically destroyed in a district, they would soon become extinct, as they
are not supposed to travel far.

[19] ‘Some Observations on the Œstridæ,’ E. A. Ormerod, Simpkin and Marshall, London,
1884, price 4d.
A very troublesome injury to the skins of lambs and sheep is the disease
known as “cockle,” in which the skin becomes thickly dotted with spots of
thickened tissue, which bear some fanciful resemblance in form to a
cockleshell. The affection is prevalent during the spring while the wool is
thick, and disappears almost immediately on shearing, but little is known of its
causes or mode of prevention.
Climate and breed have a considerable effect on the quality of hides and
skins. As a rule the less highly bred races, and those which are most exposed to
the extremes of weather, have the thickest hides, and in most cases highly bred
animals have had their meat-producing, or in the case of sheep, their wool-
bearing qualities developed at the expense of the characteristics most valued
by the tanner.

CHAPTER VII.
STRUCTURE AND GROWTH OF SKIN.

Although, at first sight, the skins of different animals appear to have little in
common, a closer examination shows that all the Mammalia possess skins
which have the same general structure, and thus an anatomical description of
the skin of an ox applies almost equally to that of a sheep, goat, or calf, though
on account of the difference in texture and thickness the practical uses of these
various materials may differ widely. The skins of lizards, alligators, fishes and
serpents differ from those of the higher animals, chiefly in having considerable
modifications in the epidermis, so that it becomes harder and forms “scales,”
and the arrangement of the fibres presents considerable difference. In many
fish-skins for instance, the fibres are in successive layers, at right angles to
each other and diagonal to the skin, but not interlaced.
In its natural condition, the skin is not merely a covering for the animal, but
at the same time an organ of sense and of secretion, and hence its structure is
somewhat complicated. It consists of two principal layers, the epidermis
(epithelium, cuticle) and the corium (derma, cutis or true skin). These are
totally distinct, not only in structure and functions, but in their origin. In the
egg of a bird and the ovum of a higher animal, the living germ consists of a
single cell, which, as soon as fertilised, begins to multiply by repeated division.
The mass of cells thus formed early differentiates into three distinct layers,
from the upper of which the epithelium arises, while the true skin, together
with the bones and cartilages, is derived from the middle one.
This distinction of origin corresponds with a wide difference of both
anatomical and chemical characteristics. A diagrammatic section of calf-skin is
shown in Fig. 9, and a more correct representation of its actual appearance is
given in Plate I. (Frontispiece). The epidermis is very thin as compared with
the true skin which it covers, and is entirely removed preparatory to tanning; it
nevertheless possesses important functions. It is shown in Fig. 10 at a and b,
more highly magnified. Its inner mucous layer b, the rete malpighi, which rests
upon the true skin c, is soft, and composed of living nucleated cells, which
multiply by division and form cell-walls of keratin. These are elongated in the
deeper layers, and gradually become flattened as they approach the surface,
where they dry up, and form the horny layer a. This last is being constantly

worn away, thrown off as dead scales of skin, and as constantly renewed from
below, by the multiplication of the cells. It is from the epithelial layer that the
hair, as well as the sweat and fat-glands, are developed.

Fig. 9.—Vertical section of calf-skin, magnified about 50 diameters. a, epidermis; b,
grain or papillary layer; c, fibrous layer of skin; d, hairs; e, fat-glands; f, sweat-
glands; g, opening of ducts of sweat-glands; h, hair-muscles.

Each hair is surrounded by a sheath which is continuous with the epidermis,
and into which the young hair usually grows as the old one falls out. The hair
itself is covered with a layer of overlapping scales, like the slates on a roof, but
of irregular form. These give it a serrated outline at the sides, and when
strongly developed as in wool and some furs, confer the property of felting.

Within these scales, which are called the “hair cuticle,” is a fibrous substance
which forms the body of the hair; and sometimes but not always, there is also a
central and cellular pith, which under the microscope frequently appears black
and opaque, from the optical effect of imprisoned air. On boiling or long
soaking in water, alcohol, or turpentine, the air-spaces become saturated with
the liquid, and then appear transparent.

Fig. 10.—Epidermis layer.

The fibrous part of the hair is made up of long spindle-shaped cells, and
contains the pigment which gives the hair its colour. The hair of the deer
differs from that of most other animals in being almost wholly formed of
polygonal cells, which, in white hairs, are usually filled with air. In dark hairs,
both the hair and sheath are strongly pigmented, but the hair is much the most
so, and hence the bulb has usually a distinct dark form. The dark-haired
portions of a hide from which the hair has been removed by liming still remain
coloured by the pigmented cells of the hair-sheaths, which can only be
completely removed by “bating and scudding.”

Fig. 11.—a, sebaceous gland; b, hair; c, erector
muscle. Mag. 200.

Near the opening of the hair-sheath to the surface of the skin the ducts of the
sebaceous or fat-glands pass into the sheath and secrete a sort of oil to lubricate
the hair. The glands themselves are formed of large nucleated cells arranged
somewhat like a bunch of grapes; the upper and more central ones being highly
charged with fatty matter. Their appearance is shown in Fig. 11. The base of
the hair is a bulb, enclosing the hair papilla h (Fig. 12), which is a projecting
knob of the true skin and which by means of the blood-vessels contained in it
supplies nourishment to the hair. The hair-bulb is composed of round soft cells,
which multiply rapidly, and pressing upwards through the hair-sheath, become
hardened, thus increasing the length of the hair.
The cells outside the bulb, shown at f in Fig. 12, pass upwards as they grow,
and form a coating around the hair, known as the “inner root-sheath.”
In embryonic development, a small knob of cells forms on the under side of
the epidermis, over a knot of capillary blood-vessels in the corium, and
enlarges and sinks deeper into the latter, while the root-bulb of the young hair
is formed within it, surrounding the capillaries from which it derives
nourishment, and which form the hair-papilla, Fig. 13. In the renewal of hair in
the adult animal the process is very similar. The bulb of the old hair withers,
and the hair falls out, and in the meantime a thickening takes place in the
epidermal coating of the bottom of the sheath, and the young hair is formed
below, and usually to one side of the old one, growing into the sheath, and
taking the place of the old hair. This is one cause of the difficulty of removing

ground-hairs in the process of unhairing, since they
are not only short, but deeper seated than the old
ones.
The process of development of the sudoriferous
or sweat-glands is very similar to that of the hairs.
They consist of more or less convoluted tubes with
walls formed of longitudinal fibres of connective
tissue of the corium, lined with a single layer of
large nucleated cells, which secrete the perspiration.
The ducts, which are exceedingly narrow, and with
walls of nucleated cells like those of the outer hair-
sheaths, sometimes open directly through the
epidermis, but more frequently into the orifice of a
hair-sheath, just at the surface of the skin. Each hair
is provided with a slanting muscle called the
arrector or erector pili (see Fig. 11), which is
contracted by cold or fear, and causes the hair to
“bristle,” or stand on end; by forcing up the attached
skin, it produces the effect known as “goose-skin.”
The muscle, which is of the unstriped or involuntary
kind, passes from near the hair-bulb to the
epidermis, and just under the sebaceous glands,
which it compresses when it contracts.
Beside the hair, and hair-sheaths, and the Fig. 12.—a, hair; b, hair
cuticle; c, inner root-sheath;
sebaceous and sudoriferous glands, the epidermis d, outer root-sheath; e,
layer produces other structures of a horny character, dermic coat of hair-sheath;
including horns, hoofs, claws and finger-nails; f, origin of inner sheath; g,
bulb; h, hair-papilla.
which both chemically and anatomically are
analogous to exaggerated hairs, such as the quills of the porcupine.
The whole of the epidermis, together with the hairs, is separated from the
corium by an exceedingly fine membrane, called the hyaline or glassy layer.
This forms the very thin buff-coloured “grain-” surface of tanned leather,
which is evidently of different structure from the rest of the corium, since, if it
gets scraped off before tanning, the exposed portion of the underlying skin
remains nearly white, instead of colouring. The whole of the hair-sheath is
enclosed in a coating of elastic and connective-tissue fibres, which are supplied
with nerves and blood-vessels, and form part of the corium.

Fig. 13.—Development of young hair.

Fig. 14.—Connective-tissue fibres. (Ranvier.)

The structure of the corium or true skin is quite different from that of the
epidermis which has just been described, as it is principally composed of
interlacing bundles of white fibres, of the kind known as “connective tissue”
(see Fig. 14); these are composed of fibrils of extreme fineness, cemented
together by a substance somewhat more soluble than the fibres themselves.

The fibres are not themselves living cells, but are apparently produced by
narrow spindle-shaped cells lying against them. The felted fibre-bundles are
more loosely interwoven in the middle portion of the skin, but become
compacter again near the flesh. In the case of sheep-skins this is especially
marked, the middle part being full of fat-cells and very loose. Any ill treatment
of the pelt during the wet-work is liable to still further loosen this middle layer
so that grain and flesh may sometimes be torn apart. The flesh-splits of sheep-
skins must have this loose fatty layer frized off before chamoising, and
American “waxed fleshes” from ox-hides are levelled by splitting away this
portion, and finished on the flesh. The outermost layer, just beneath the
epidermis, is exceedingly close and compact, the fibre-bundles that run into it
being separated into their elementary fibrils, which are so interlaced that they
can scarcely be recognised. This is the pars papillaris, and forms the lighter-
coloured layer, called (together with its very fine outer coating) the “grain” of
leather. It is in this part that the fat-glands are embedded, while the hair-roots
and sweat-glands pass through it into the looser tissue beneath. It receives its
name from the small projections or papillæ, with which its outer surface is
studded, and which form the characteristic grain of the various kinds of skin.
[20] (See Fig. 9 and Plate I.)
[20] It will be noted that the word “grain” is used by the tanner in at least three different
senses, which are productive of much confusion. The extremely thin hyaline layer forms a
natural glaze to the skin, and might well be spoken of as such; the form and arrangement
of the papillæ and hair-pores might be called the “pattern” of the grain, leaving the use of
the word “grain” itself restricted to the pars papillaris.

Fig. 15.—Fat-cells in connective tissue. a, fat-globule; p,
protoplasm; n, nucleus; m, cell-wall. (Ranvier.)

The study of the structure of the grain, and especially of the arrangement of
the hair-pores is very important, as it is usually the readiest means of
identifying the kind of skin of which a leather is made, which in finished skins
with artificially printed grain is often very difficult. (Plate II.) The examination
is facilitated by wetting and stretching the skin, and by the use of a good lens,
or a low power of the microscope.[21]
[21] Under the microscope, the skin is of course lighted from above by direct light from a
window, or by that of a lamp concentrated by a “bullseye” condenser. The reversal of the
image in the microscope often causes a pseudoscopic effect very puzzling to the beginner,
prominences appearing as hollows, and vice versa till the real direction of the lighting is
considered.

PLATE II.

Photo-micrographs of Grain of various Skins (A. Seymour-Jones).

1. Cow-hide; 2. Calf-skin; 3. East India Goat; 4. Pig-skin; 5. East India Sheep; 6. Welsh Sheep.

[Face p. 52.

As stated above, the surface of skin which is next
to the flesh is firmer than that in the centre, and as
the fibres run nearly parallel with the surface it has
a more or less membranous character. The skin is
united to the body of the animal by a network of
connective tissue (panniculus adiposus), which is
frequently full of fat-cells and is then called adipose
tissue. This constitutes the whitish layer which is
removed, together with portions of actual flesh, in
the operation of “fleshing.” If a minute portion of
adipose tissue be examined microscopically, it will
appear to consist of a mere mass of fat-globules
entangled in connective tissue. If, however, it be
stained with carmine or logwood it may be at once
observed that each globule is contained in a cell, of
which the nucleated protoplasm, by which the fat
was secreted, is pressed closely against the wall
(Fig. 15). Similar cells are contained in considerable
quantities throughout the hide, and especially in the
Fig. 16.—Striped, or voluntary
loose tissue of the central part; hence in leather
muscular fibre. (Ranvier.) manufacture it is impossible to expel or wash out
the fat until the cells have been broken down by
“liming” or in some other way.
Many animals (ox, horse, etc.) possess a thin layer of voluntary muscle (red
flesh) spread over the inner side of the skin, and used for twitching to drive off
flies. In rough fleshing this is sometimes left on and may be a cause of dark
flesh in sole leather. Even in the finished leather its striped structure may be
detected microscopically (Fig. 16).
Besides the connective-tissue fibres, the skin contains a small proportion of
fine yellow “elastic” fibres. If a thin section of hide be soaked for a few
minutes in a mixture of equal parts of water, glycerine, and strong acetic acid,
and then examined under the microscope, the white connective-tissue fibres
become swollen and transparent, and the yellow “elastic” fibres may be seen,
as they are scarcely affected by the acid. The hair-bulbs and sweat- and fat-
glands are also rendered distinctly visible by this treatment. On the other hand,
the white gelatinous fibres are most easily seen by examining the section in a
strong solution of common salt, or in one of ammonium sulphate; or by
staining with some aniline dyes such as safranine. Sections are most readily cut

for these purposes by the use of the freezing microtome, or after previous
hardening in alcohol. For further details see L.I.L.B., p. 254.
Ordinarily in the production of leather only the corium, or true skin is used,
and in order to obtain it in a suitable condition for the various tanning
processes, the hair or wool, together with the epithelium, must be completely
removed without damaging the skin itself; and especial care must be taken that
the grain, or portion next to the epidermis, does not suffer any injury during the
treatment. All the methods employed depend upon the fact that the epidermis
cells, especially the soft growing ones next to the corium, and those of the
epidermis layer which surround the hair-roots, are more easily destroyed than
the corium itself owing to their different chemical character. The “unhairing”
process consists essentially in breaking down these cells by chemical or
putrefactive agents, and removing the hair together with the rest of the
epidermis by mechanical means. Of the various substances which may be used
for this purpose, lime is one of the most convenient, as its solubility in water is
so slight, that a solution of such a strength as to injure the hide cannot be easily
made. Caustic alkalies, on the other hand, are much more soluble, and unless
care be taken to use only the proper quantity, a dangerously strong solution
may be made with consequent damage to the skin. The addition of small
amounts of sulphides to the lime-solution accelerates the unhairing owing to
their special solvent action on the epidermis-structures, and also in the case of
alkaline sulphides, by the caustic alkali which is produced by their reaction
with the lime. Even if used alone, strong solutions of alkaline sulphides rapidly
destroy both hair and epidermis, converting them into a mass which may be
swept off the skin like wet pulp, and yet they have practically no injurious
action on the true skin.
In the “sweating” process the epidermic cells are broken down by
putrefactive organisms and their products, so that the hair becomes loose and
may then be either rubbed or scraped off. Ammonia, which is produced during
the putrefaction, has also an important solvent action, and its presence
doubtless tends to quicken the processes both of unhairing and of destruction.
To obtain useful knowledge of the structure of any particular skin, it is not
necessary to have a very elaborate or expensive microscope, and it is quite
possible to obtain useful information merely by the use of a good pocket lens,
as for instance, in the examination of various forms of “grain,” and the
embossing of one skin to imitate another.
For further details of the manipulation and selection of the microscope, the
reader must consult L.I.L.B., pp. 234 et seq.

CHAPTER VIII.
THE CHEMICAL CONSTITUENTS OF SKIN.

The chemistry of the various constituents of skin is still very imperfectly
understood, but Beilstein, in his great handbook of organic chemistry, places
gelatin, albumens and keratins in the “aromatic” series, and implies therefore
that they contain the “benzene” ring. It is at least certain that all are very
complex.
The epidermis structures belong to the class of keratins, which are closely
related to coagulated albumin; while the white fibres of the corium (or true
skin) are either identical with gelatin, or only differ from it in their molecular
condition or degree of hydration. This gelatinous tissue constitutes the bulk of
the corium, but it also contains albumen as a constituent of the lymph and
blood which supply its nourishment, keratins in the epithelial structures of the
blood and lymph vessels, and “yellow fibres,” which are perhaps allied to the
keratins, but which cannot well be isolated for analysis.
The white connective tissue of the corium is converted into gelatin (glutin)
by boiling with water. Owing to the impossibility of obtaining unaltered hide-
fibre free from the other constituents, and still more to that of deciding to what
point it should be dried to remove uncombined water, it is impossible to prove
by analysis whether its composition is identical with that of glutin; but as the
white fibre constitutes by far the largest part of the corium, and the other
constituents do not differ largely from it in their percentage composition, an
analysis of carefully purified corium is practically identical with that of the
actual fibre. The following analyses of hide and gelatin are therefore of
interest.
The analyses of Von Schroeder and Paessler[22] are of special importance as
being the average of a large number of separate determinations. Their nitrogen
determinations are by Kjeldahl’s method. Small amounts of ash and traces of
sulphur are neglected, and probably included in the O, which is obtained by
difference.
[22] Ding. Polyt. Journ., 1893, cclxxxvii. pp. 258, 283, 300.

Analyses of Purified Corium.

Analyst. Material. C H N O S

Stohmann and Langbein .. 49·9 5·8 18·0 26·0 0·3
Müntz Ox-hide 51·8 6·7 18·3 23·2 ..
Von Schroeder and Paessler Ox, calf, horse, camel, pig, rhinoceros 50·2 6·4 17·8 25·4 ..
„ Goat and deer 50·3 6·4 17·4 25·9 ..
„ Sheep and dog 50·2 6·5 17·0 26·3 ..
„ Cat 51·1 6·5 17·1 25·3 ..

Analyses of Gelatin (free from Ash).

Analyst. C H N O
Von Schroeder and Paessler 51·2 6·5 18·1 24·2
Mulder 50·1 6·6 18·3 25·0
Fremy 50·0 6·5 17·5 26·0
Schützenberger 50·0 6·7 18·3 25·0
Chittenden and Solly[23] 49·4 6·8 18·0 25·1
[23] Contained also 0·7 sulphur. Journ. Physiol., xii. p. 23.

It will be noted that the above analyses of skin differ more widely among
themselves than their average does from that of the gelatin analyses, though on
the whole the nitrogen is somewhat higher in the latter. The molecular weight
of gelatin must be very high,[24] and any empirical formula founded on
ultimate analysis therefore quite hypothetical. Bleunard,[25] Schützenberger
and Bourgois,[26] and Hofmeister agree on the formula C76H124N24O29, which
leads to the following percentage composition:—
per cent.
C76 = 912 = 49·7
H124 = 124 = 6·8
N24 = 336 = 18·3
O29 = 464 = 25·2
1836 100·0
[24] Paal (Berichte D. Ch. Ges., xxv. (1892) pp. 1202-36, and Ch. Soc. Abst., 1892, pp. 895-
7) calculates a molecular weight of about 900 from physical (freezing, boiling point)
methods.
[25] Annales de Chimie [5] xxvi. p. 18.
[26] Compt. Rend., lxxxii. pp. 262-4.

The addition of a molecule of water would make a difference in the
percentage composition indicated by these formulæ which would be less than
their probable experimental error, and the change may therefore be one of
hydration.
Gelatin certainly contains both carboxyl and amido-groups, and is capable
of combining with both acids and alkalies (see p. 84).

Reimer[27] obtained what he supposed to be pure unaltered fibre-substance
by digestion of purified hide with 1⁄2 per cent. acetic acid for many days and
subsequent neutralisation. His analysis showed C = 48·45 per cent., H = 6·66
per cent., N = 18·45 per cent., O = 26·44 per cent., thus deviating considerably
from direct analysis of unaltered skin. It is obvious that little weight can be
placed on this result, Reimer’s precipitate being probably a mere
decomposition product.
[27] Ding. Polyt., ccv. p. 164.

Hofmeister[28] notes that on heating gelatin it loses water and forms an
anhydride which he considers identical with collagen or hide-fibre. When
gelatin is dried at a temperature of 130° C. it becomes incapable of solution in
water, even at boiling temperature, and can only be dissolved by heating under
pressure. It is certain that collagen (hide-fibre, ossein) is less easily soluble in
hot water than ordinary gelatin.
[28] Bied. Centr., 1880, p. 772.

So far as our present knowledge goes, we may regard hide-fibre as merely
an organised and perhaps dehydrated gelatin.
Gelatin or glutin (not to be confounded with the gluten of cereals), when
pure and dry is a colourless, transparent solid of horny toughness and of sp. gr.
1·3. It begins to melt about 140° C., at the same time undergoing
decomposition. It is insoluble in hydrocarbons, in ether, or in strong alcohol. In
cold water it swells to a transparent jelly, absorbing several times its weight of
water, but does not dissolve. In hot water it is soluble, but a solution containing
even 1 per cent. of good gelatin sets to a weak jelly on cooling. Gelatin jellies
melt at temperatures which vary considerably with the quality or freedom from
degradation products, but which within pretty wide limits (5-10 per cent.) are
little affected by the concentration. A 10 per cent. solution of best hard gelatin
melts about 38° C., while low glue may fail to set at 15° C. A useful technical
test for the setting power of gelatin, based on this fact, consists in placing an
angular fragment of the jelly in a small tube attached to a thermometer, and
stirring in a beaker of water, which is slowly heated till the jelly melts, when
the temperature is noted. The exact point is perhaps more easily seen if the
tube is drawn to a conical point. The jelly may also be allowed to set in
capillary tubes open at the bottom, and the moment noted when water rises into
the tube. The temperature of fusion is raised by the addition of formaldehyde,
salts of chromium, alumina and ferric salts, which produce a tanning effect,
and in a less degree by sulphates, tartrates, acetates, some other salts, and
diminished by iodides, bromides, chlorides and nitrates.[29] Solutions of gelatin

too weak or too warm to gelatinise possess considerable viscosity. Gelatin may
therefore be estimated, in the absence of other viscous matters, by the
viscosimeter, an instrument which measures the time taken by a liquid in
flowing through a capillary tube.[30] The firmness of a jelly, which is often
important for commercial purposes, is frequently measured by Lipowitz’s
method, in which a slightly convex disc, conveniently of exactly 1 cm.
diameter, and cemented to the bottom of a thistle-head funnel tube, is loaded
gradually with mercury till it sinks in the jelly. The jelly (5 or 10 per cent.)
should be allowed to set some hours before the test is made.
[29] See Pascheles, ‘Versuche über Quellung,’ Archiv für ges. Path., Bd. 71.
[30] See Prollius, Ding. Polyt. Journ., ccxlix. p. 425, who employs a 1 per cent. solution;
also Stützer, Zeit. Ann. Ch., xxxi. pp. 501-15.
Solutions of gelatin from skin and bone are powerfully lævorotatory to
polarised light. At 30° C. (A)D = -130°, but temperature and the reaction of the
solution have much influence on the value found.
Gelatin is precipitated from aqueous solution by the addition of strong
alcohol and concentrated solutions of ammonium sulphate and some other
salts. Many other colloid bodies such as dextrin and gums behave similarly. In
the absence of these substances, precipitation by alcohol may be utilised for
the technical analysis of gelatins and glues, printers’ roller compositions and
gelatin confectionery. 25 c.c. of the gelatinous solution, which is preferably of
about 10 per cent., is placed in a small beaker tared together with a glass
stirring rod, and thrice its volume of absolute alcohol added. On stirring, the
gelatin sets firmly on the rod and sides of the beaker, and may be washed with
dilute alcohol or even with cold water, dried and weighed. A very pure French
gelatin gave 98·6 per cent., while a common bone-glue only yielded about 60
per cent. precipitate. Absolute alcohol withdraws water from gelatin-jelly,
leaving a horny mass. Gelatin may also be precipitated completely by
saturating its solution with sodium chloride, and then acidifying slightly with
sulphuric or hydrochloric acid; and masses of jelly become hardened in
acidified salt solution as in alcohol, though a neutral solution has little effect.
The cause of this is difficult of explanation, but its bearing on the pickling of
sheep-skins (p. 89) and the production of white leather (p. 186) is obvious.
Decompositions.—When aqueous solutions of gelatin are heated under
pressure, or in presence of glycerin and other bodies which raise the boiling-
point, or more slowly at lower temperatures, they gradually lose the power of
gelatinising on cooling, the gelatin being converted into modifications soluble
in cold water, but still capable of being precipitated by tannin. Hofmeister[31]
states that the gelatin takes up 3 molecules water and is split up into

hemicollin, soluble in alcohol and not precipitated by platinic chloride
solution; and semiglutin, insoluble in alcohol and precipitated by platinic
chloride solution. Both are precipitated by mercuric chloride. Dry gelatin is
soluble in glycerin at high temperatures, but probably suffers a similar change.
Hence high temperatures and long-continued heating must be avoided in
gelatin manufacture; and in making printers’ roller compositions, which are
mixtures of gelatin and glycerin, the gelatin must be swollen with water and
melted at a low temperature with the glycerin.
[31] Bied. Centr., 1880, p. 772, and Ch. Soc. Abs., 1881, p. 294.

Gelatin is also converted into soluble forms (peptones), perhaps identical
with the above, by the action of heat in presence of dilute acids and alkalies.
These, like gelatin, are precipitated by tannin and by metaphosphoric acid.[32]
Heated for longer periods or to higher temperatures with aqueous solutions of
the caustic alkalies, baryta, or lime, gelatin is gradually broken down into
simpler and simpler products, ending in nitrogen or ammonia, water and
carbonic acid. Among the intermediate products may be mentioned various
acids of the amido-acetic series, as amido-acetic (glycocine, glycocoll), amido-
propionic (alanine), and amido-caproic (leucine); and of the amido-succinic
series (amido-succinic = aspartic acid).[33]
[32] Lorenz, Pflüger’s Arch., xlvii. pp. 189-95; Journ. Chem. Soc., 1891, A. p. 477.
[33] Compare Schützenberger, Comptes Rend., cii. pp. 1296-9; Journ. Chem. Soc., 1886, A.
p. 818.

Treatment with acids produces very similar effects. The first products are
soluble peptones. Paal[34] on treating 100 parts of gelatin on the water-bath
with 160 parts water and 40 parts concentrated HCl till the product was soluble
in absolute alcohol, obtained, on purification, a white hygroscopic mass of
peptone salts containing 10-12 per cent. of hydrochloric acid.[35]
[34] Berichte, xxv. pp. 1202-36; Journ. Chem. Soc., 1892, A. p. 895.
[35] See also Buchner and Curtius, Ber., xix. pp. 850-9; Journ. Chem. Soc., 1886, A. p. 635.

The products of digestion of gelatin with gastric and pancreatic juice are
peptones which do not differ materially from gelatin in ultimate composition,
and the action is probably mainly hydrolytic.[36]
[36] Chittenden and Solly, Journ. Chem. Soc., 1891, A. p. 849.

The earlier products of putrefaction are very similar. Many bacteria have the
power of liquefying gelatin-jelly. This has been shown by Brunton and
McFadyen[37] to be due not to the direct action of the bacteria, but to a soluble
zymase secreted by them which peptonises the gelatin. Its action is favoured

by an alkaline condition, and destroyed by a temperature of 100° C.[38] As
putrefaction progresses, the solution becomes very acid from the formation of
butyric acid, and later on ammonia and amido-acids are formed.
[37] R. S. Proc., xlvi. pp. 542-53.
[38] Compare pp. 17, 171; also Ch. Zeit., 1895, p. 1487.

Fahrion,[39] starting with the idea that albuminoids and gelatin were
condensation products of a lactone character (L.I.L.B. p. 185), and that they
might, like lactones, be depolymerised by saponification, digested these bodies
with alcoholic soda till they were dissolved, and on neutralising the solution
with hydrochloric acid, of which the excess was driven off by repeated
evaporation, and removing the sodium chloride by treatment with alcohol,
obtained in each case bodies of acid reaction, which from their composition he
supposed to be identical with Schützenberger’s proteic acid, C8H14N2O4, which
is soluble in water and alcohol, insoluble in ether and petroleum,
uncrystallisable, and forming uncrystallisable salts. Fahrion suggested that the
nitrogenous character which Eitner attributed to his “dégras-former” (p. 370)
was probably due to contamination by this body; and that its formation might
be utilised in the analysis of leather and other proteid bodies. These products
have since been further investigated by Prof. Paal and Dr. Schilling,[40] who
show that they contain hydrochloric acid, to which their acid reaction is due,
and that they are identical with the peptone salts previously obtained by Prof.
Paal (v. s.) by digestion of proteids with hydrochloric acid. The free peptones
are strongly basic.
[39] Ch. Zeit., 1895, p. 1000.
[40] Ch. Zeit., 1895, p. 1487.

By dry distillation of gelatin a mixture of pyrrol and pyridin bases are
produced. This is commercially obtained by the distillation of bones, and is
known as “bone oil,” or “Dippel’s animal oil.” Pyrrol, C4H5N, resembles
phloroglucol in giving a purple-red colour to fir wood moistened with
hydrochloric acid (p. 299).
Reactions of Gelatin.—Gelatin is precipitated by mercuric chloride, in this
respect resembling peptones, but not by potassium ferrocyanide, by which it is
distinguished from albuminoids, and it differs from albumin in not being
coagulated by heat. Solution of gelatin dissolves considerable quantities of
calcium phosphate; hence this is always present in bone-glues. Gelatin and
some of its decomposition products are precipitated by metaphosphoric acid.
[41] The precipitate contains about 7 per cent. P2O5, but gradually loses it on
washing. Various salts diminish the solubility of gelatin in hot water, and

especially those of the alum type. Chrome alum and basic chrome salts are
especially powerful, rendering it practically insoluble. The addition of about 3
per cent. ammonium or potassium dichromate causes glue or gelatin to become
insoluble by the action of light with the formation of basic salts of chromium,
and has been utilised in photography and as a waterproof cement. Other
colloids besides gelatin are similarly affected.
[41] Lorenz, Pflüger’s Archiv, xlvii. pp. 189-195.

Gelatin is precipitated by all tannins, even from very dilute solutions; one
containing only 0·2 grm. per liter is rendered distinctly turbid by gallotannic
acid or infusion of gall-nuts; but some other tannins give a less sensitive
reaction. The precipitate is soluble to a considerable extent in excess of gelatin,
so that in using the latter as a test for traces of tannin care must be taken to add
a very small quantity only. The addition of a little alum renders the reaction
more delicate. Whether the precipitate is a definite chemical compound has
been disputed, as its composition varies according to whether gelatin or tannin
is in excess. Böttinger[42] states that the precipitate produced by adding gelatin
to excess of gallotannic acid contains 10·7 per cent. of nitrogen, indicating the
presence of 66 per cent. of gelatin on the assumption that gelatin contains 16·5
per cent. N (see p. 57). Digested with water at 130° C., the precipitate is
decomposed, yielding a solution which precipitates tannin, and probably
indicating the formation of a more acid compound. Gelatin with excess of oak-
bark tannin gives a precipitate containing 9·5 per cent. of nitrogen,
corresponding to 57·5 per cent. of gelatin. Treated with water at 150° C., this
precipitate yielded three products: one soluble in cold water, another in hot
only and one insoluble. On addition of a solution of formaldehyde (formalin)
to one of gelatin no visible action takes place in the cold, unless the solution of
gelatin be very concentrated and alkaline, but on heating, the gelatin is
rendered insoluble owing to the formation of a compound with the
formaldehyde. From the very small amount of formalin which is required to
produce formo-gelatin it is very doubtful if this is a definite compound.
[42] Liebig’s Ann. der Ch., ccxliv. pp. 227-32.

Weiske[43] states that bone-gelatin, carefully freed from all mineral matter, is
not precipitated by tannin till a trace of a salt (e. g. sodium chloride) is added.
So far as is known, bone-gelatin is identical with that of skin.
[43] Bied. Centr., 1883, p. 673.

Chondrin is the gelatinous body produced by the digestion of cartilage with
water at 120° C. for three hours. In most of its physical properties it is identical

with gelatin, but differs from the latter in being precipitated from its solution in
water by acetic acid, lead acetate, alum, and the mineral acids when the latter
are not present in excess. Chondrin also differs from gelatin in producing a
substance capable of easily reducing cupric oxide when it is boiled for some
time with dilute mineral acids. It is extremely probable that chondrin is merely
an impure gelatine.[44]
[44] Cp. Petri, Berichte, xii. p. 267; Mörner, Skand. Archiv f. Physiol., i. pp. 210-243; and
Journ. Chem. Soc., 1889, A. p. 736 and Zeit. Physiol. Chem., 1895, xx. pp. 357-364; and
Journ. Chem. Soc., 1895, A. i. p. 254. See also Richter, Org. Chem., i. p. 559.
Coriin.—Rollet[45] has shown that when hide and other forms of connective
tissue are soaked in lime- or baryta-water, the fibres become split up into finer
fibrils, and as the action proceeds, these again separate into still finer ones, till
the ultimate fibrils are so fine as to be only distinguished under a powerful
microscope. At the same time, the alkaline solution dissolves the substance
which cemented the fibres together, and this may be recovered by neutralising
the solution with acetic acid, when the substance is thrown down as a
flocculent precipitate. This was considered by Rollet to be an albuminoid
substance; but Reimer[46] has shown that it is much more closely allied to the
gelatinous fibres, and, indeed, is probably produced from them by the action of
the alkaline solution. Reimer used limed calf-skin for his experiments, and
subjected it to prolonged cleansing with distilled water, so that all soluble parts
must have been pretty thoroughly removed beforehand. He then digested it in
closed glasses with lime-water for 7-8 days, and precipitated the clear solution
with dilute acetic acid. He found that the same portion of hide might be used
again and again, without becoming exhausted, which strongly supports the
supposition that the substance is merely a product of a partial decomposition of
the hide-fibre, and indeed that there is no distinct “cementing substance,” but
merely a difference in the hydration or physical condition of the fibre
substance which causes it to split more readily in certain directions. The
dissolved substance, which he called “coriin,” was purified by repeated
solution in lime-water and reprecipitation by acetic acid. It was readily soluble
in alkaline solutions but not in dilute acids, though in some cases it became so
swollen and finely divided as to appear almost as if dissolved. It was, however,
very soluble in common salt solution of about 10 per cent., from which it was
precipitated both by the addition of much water and by saturating the solution
with salt. Reimer found that a 10 per cent. salt solution was equally effective
with lime-water in extracting coriin from the hide, and that it was partially
precipitated on the addition of acid, and completely so on saturating the
acidified solution with salt. Other salts of the alkalies and alkaline earths acted

in a similar manner, so that Reimer was at first deceived when experimenting
with baryta-water, because, being more concentrated than lime-water, the
coriin remained dissolved in the barium salt formed on neutralising with acid,
and it was necessary to dilute before a precipitate could be obtained. The
slightly acid solution of coriin gave no precipitate either in the cold or on
boiling with potassium ferrocyanide, being thus distinguished from
albuminoids. The neutral or alkaline solution showed no precipitate with iron
or mercuric chloride, copper sulphate, or with neutral lead acetate; but with
basic lead acetate, basic iron sulphate, or an excess of tannin a precipitate was
produced. Reimer’s analysis showed: Carbon, 45·91; hydrogen, 6·57; nitrogen,
17·82; oxygen, 29·60; and he gives a formula showing its relation to the
original fibre, which does not seem supported by sufficient evidence. In all
probability coriin is merely an impure degradation-product of hide-fibre or
gelatin.
[45] Sitz. Wiener Akad., xxxix. p. 305.
[46] Ding. Polyt. Journ., ccv. p. 153.

Hide Albumin.—The fresh hide contains a portion of actual albumin, viz.
that of the blood-serum and of the lymph, which is not only contained in the
abundant blood-vessels, but saturates the fibrous connective tissue, of which it
forms the nourishment. This albumin is mostly removed from the skin by the
liming and working on the beam, which is preparatory to tanning. Probably for
sole-leather, the albumin itself would be rather advantageous if left in the hide,
as it combines with tannin, and would assist in giving firmness and weight to
the leather. It is, however, for reasons which will be seen hereafter, absolutely
necessary to get rid of any lime which may be in combination with it. The
blood must also be thoroughly cleansed from the hide before tanning, as its
colouring matter contains iron, which, by combination with the tannin,
produces a bad colour.
The albumins form a class of closely allied bodies of which white of egg
may be taken as a type. They are also related to the casein of milk, to fibrin,
and more distantly to gelatin. A good deal of information on the class may be
found in Watt’s Dict. of Chem., 2nd ed., article ‘Proteids,’ and Beilstein’s
article ‘Albuminaten,’ and in Allen’s ‘Commercial Organic Analysis,’ vol. iv.
The most characteristic property of albumins is that of coagulation by heat.
The temperature at which this takes place differs somewhat in different
members of the group, egg and serum albumin coagulating at 72-73° C. Dry
albumins become insoluble if heated to 110° C. for some time. Traces of acid
tend slightly to lower, and traces of alkali to raise the temperature of

coagulation. Sodium chloride and some other neutral salts favour coagulation.
Solutions of albumin become opalescent at a temperature slightly below that at
which flakes form.
Albumins are also coagulated by alcohol and by strong mineral acids.
Coagulated albumin is only soluble in strong acids and alkalies by aid of heat,
and strongly resembles keratin (pp. 56, 68).
Solutions of albumin are lævorotatory to polarised light.
“Acid” and “Alkali” Albumins are formed by the action, in the cold, of dilute
acids (such as acetic, hydrochloric) and alkalies on albumin solution. They are
uncoagulable by heat, and are precipitated by careful neutralisation, but are
soluble in excess of either acid or alkali, or alkaline carbonates. They are
thrown out of solution by saturation with sodium chloride or magnesium
sulphate. It is doubtful whether albumins combine with either acids or bases,
and it is probable that the “acid” or “alkali” albumins are identical with the
parapeptones formed in the first stage of peptic digestion.
On putrefaction, or on more severe treatment with acids and alkalies,
albumins break down in a way similar to gelatin, and yield almost identical
products (see p. 57); amido-acids of the acetic series, and tyrosin (para-oxy-α-
amido-phenyl-propionic acid) and aspartic (amido-succinic) acid, being the
most important.
Treatment with alcoholic soda (see p. 62) yields peptones similar to those of
gelatin.[47]
[47] Paal, Ch. Zeit., 1895, p. 1487.

Heated for some days with dilute nitric acid (1 : 2) all proteids, including
albumins, gelatin and keratins, yield yellow flocks of “xantho-proteic acid,” a
substance of somewhat indefinite composition, soluble in ammonia and in
fixed caustic alkalies with production of an orange-red or brownish-red colour.
Millon’s reagent gives an intense red coloration when heated with albumins,
keratins, or gelatin. The reagent is made by dissolving 2·5 grm. of mercury in
20 c.c. of concentrated nitric acid, adding 50 c.c. of water, allowing to settle
and then decanting the clear liquid.
Albumins, previously purified by boiling with alcohol and washing with
ether, when dissolved in concentrated hydrochloric acid (sp. gr. 1·196) by aid
of heat, give a violet-blue coloration, but the reaction is often somewhat
indefinite. Gelatin, chondrin and keratins do not give this reaction.
Treated with a trace of cupric sulphate and excess of caustic potash solution,
albumins give a violet, and gelatin and peptones a pink solution (biuret
reaction).

Dissolved in glacial acetic acid and treated with concentrated sulphuric acid,
albumins and peptones give a violet and feebly fluorescent solution. A
somewhat similar reaction is obtained if sugar solution be substituted for acetic
acid.
A solution of albumin rendered strongly acid with acetic acid is precipitated
by potassium ferrocyanide, salt, sodium sulphate, lead acetate, mercuric
chloride, tannin and picric and tungstic acids.
Egg-Albumin is contained in the whites of eggs in membranes which are
broken up by beating with water and can then be removed by filtration. When
fresh its reaction is slightly alkaline, and it is lævorotatory.
According to Lehmann, white of egg contains 87 per cent. of water, and 13
per cent. of solid matter, the latter being almost entirely composed of egg-
albumin. This latter coagulates and becomes insoluble in water on heating to
60° C.
Vitellin (the albumin or globulin[48] of the yolk) is insoluble in water, and is
obtained as a white granular residue on extracting undried egg-yolk with large
quantities of ether. It closely resembles myosin, the chief globulin of muscle,
but differs from other globulins in being soluble in a saturated solution of
common salt. A neutral solution of vitellin in very dilute brine coagulates at
70-75° C.
[48] Globulin is an albumin soluble in dilute salt solutions, but insoluble in water.

Yolks of eggs, preserved by the addition of salt, borax, or formalin, are used
for dressing skins in the process of “tawing” (see p. 191). For the analysis of
such yolks, see L.I.L.B., p. 159. Their most important constituent for the
leather-dresser is egg-oil of which they contain about 30 per cent.
Casein, the principal proteid of milk, may be mentioned here in connection
with the albumins to which it is closely related, since, though in no way
connected with the animal skin, since it is used to some extent as a “seasoning”
or glaze for leather, for which it is well adapted, and it is now to a considerable
extent a waste product of butter manufacture. It differs from albumins in being
very incompletely if at all coagulated by boiling, but separates at once in curdy
flakes on the addition of acids (hydrochloric, acetic, butyric), and by the action
of rennet. The curd is easily soluble in small quantities of dilute alkalies, lime-
water, and salts of alkaline reaction, such as sodium carbonate and borax. If no
more than the necessary quantity of alkali is employed for solution, the
compound has an acid reaction to phenolphthalein, and like the original milk,
is curdled by rennet and dilute acids. Casein may also be dissolved by
digestion with diluted mineral or organic acids.

Hair, Epidermis and Glands.—These are all derived from the epithelial
layer, and hence, as might be inferred, have much in common in their chemical
constitution. They are all classed by chemists under one name, “keratin,” or
horny tissue, and their ultimate analysis shows that in elementary composition
they closely resemble the albumins. It is evident, however, that the horny
tissues are a class rather than a single compound.
The keratins are gradually loosened by prolonged soaking in water, and, by
continued boiling in a Papin’s digester at 160° C., evolve sulphuretted
hydrogen, at the same time dissolving to a turbid solution which does not
gelatinise on cooling. Keratin is dissolved by caustic alkalies; the epidermis
and the softer horny tissues are easily attacked, while hair and horn require
strong solutions and the aid of heat to effect complete solution. The caustic
alkaline earths act in the same manner as dilute alkaline solutions; hence lime
easily attacks the epidermis, and loosens the hair, but does not readily destroy
the latter. Alkaline sulphides, on the other hand, seem to attack the harder
tissues with at least the same facility as the soft ones, the hair being often
completely disintegrated, while the epidermis is still almost intact; hence their
applicability to unhairing by destruction of the hair. Keratins give the
xanthoproteic reaction with nitric acid, and a red coloration with Millon’s
reagent, and also resemble albumin, in the fact that they are precipitated from
their solution in sulphuric acid by potassium ferrocyanide. By fusion with
potash, or prolonged boiling with dilute sulphuric acid, keratin is decomposed,
yielding leucin, tyrosin, ammonia, etc. The precipitate produced by the
addition of acids to alkaline solution of keratin (hair, horns, etc.), mixed with
oil and barium sulphate, has been employed by Dr. Putz as a filling material for
leather, for which purpose it acts in the same way as the egg-yolks and flour
used in kid-leather manufacture. Eitner attempted to use it for the same
purpose with bark-tanned leather, but without much success. Putz has also
proposed to precipitate the material after first working its solution into the
pores of the leather.
Elastic Fibres.—The elastic or yellow fibres of the hide are of a very stable
character. They are not completely dissolved even by prolonged boiling, and
acetic acid and hot solutions of caustic alkalies scarcely attack them. They do
not appear to combine with tannin, and are very little changed in the tanning
process. They are present in hide and skin to the extent of less than one per
cent.
Analytical Methods.—The reactions distinguishing the principal skin
constituents are summarised in the following table:—

Reagent. Gelatin. Albumins. Keratins.
Cold water Swells only Soluble Insoluble.
Heated in water Soluble Coagulate at 72° Soluble only at
to 75° C. temp. over
100° C.
Acetic acid and potassium ferrocyanide to No precipitate Precipitate Precipitate
aqueous solution
Millon’s reagent No reaction Red coloration Red coloration.
Hot concentrated hydrochloric acid No coloration Violet blue No coloration.

There is no simple method for the quantitative separation of the different
constituents of skin. It is, therefore, customary to simply determine the amount
of nitrogen which any particular portion of the material may contain, and, as
gelatinous fibre, which constitutes by far the greater portion of the true skin,
contains 17·8 per cent. of nitrogen, to base the estimation of the amount of skin
present upon this figure (see p. 57).
The most convenient process for the determination of the nitrogen is that
devised by Kjeldahl, which is most easily carried out as follows:—
A known weight of the substance which contains about 0·1 gram of nitrogen
(0·5 gram of skin, or a corresponding quantity of liquor) is placed in a flask of
Jena glass, capable of holding 500-700 c.c. together with 15 c.c. of
concentrated sulphuric acid. The contents of the flask are then boiled over a
small Bunsen flame for 15 minutes, or more, until all the water has been driven
off and the material is quite disintegrated; and are then allowed to cool below
100°. 10 grams of dry powdered potassium persulphate is now added, and the
boiling continued till the liquid has become colourless. The operation of
boiling should be conducted in a good draught, or in the open air. Before the
substance has begun to char it is advisable to place a small funnel in the neck
of the flask to prevent, as far as possible, spirting and loss of sulphuric acid.

Fig. 18.—Kjeldahl Apparatus.

The colourless liquid is allowed to cool thoroughly, and the flask is then
fitted with a tapped funnel and tube, as shown in Fig. 18. This tube must not be
less than 4 mm. in diameter, and with the end in the flask cut diagonally to
facilitate drops of liquid falling back again into the flask. It rises obliquely for
a height of 12 to 15 inches, is then bent over as shown in the figure and
connected by a rubber tube[49] to a 100 c.c. pipette, or similarly shaped tube,
the other end of which dips just below the surface of a volume of exactly 50
c.c. of “normal” hydrochloric acid contained in a second flask. About 50 c.c. of
distilled water is introduced into the flask containing the treated sample, and
after this 100 c.c. of a solution of 50 grams of caustic soda in 100 c.c. of water
is carefully and slowly run into the flask by means of the tapped funnel with
which it is provided. The contents of the flask are now boiled for about half an
hour,[50] the normal acid in the receiving flask being kept cool by immersing
the latter in cold water. The liquid in this second flask is then titrated with

normal sodium carbonate, using methyl orange as indicator. The difference in
c.c. between 50 c.c., the volume of acid used, and the quantity of normal
sodium carbonate required to neutralise it, when multiplied by 0·014
represents the amount of nitrogen (in grams) in the weight of the substance
used for the determination; or if multiplied by 0·0786 shows the weight of
hide-fibre in the same quantity of material. Some chemists add copper
sulphate, or a drop of mercury before boiling up the substance with the strong
sulphuric acid, but the use of such substances introduces complications in the
process without, in the case of gelatinous matter, securing more accurate
results. It is absolutely necessary that the acids and alkali used should be free
from ammonia, and a blank experiment should be made using pure sugar
which contains no nitrogen, and a correction applied if necessary for the
ammonia they contain.[51]
[49] The ends of the glass tubes should fit closely together, so as to expose the rubber as
little as possible to the action of ammoniacal vapour.
[50] “Bumping” is often very troublesome at this stage, and may be prevented by passing a
current of steam from another flask, or ammonia-free air through a tube with a capillary
opening into the boiling liquid; fragments of pure zinc, of platinum, or broken tobacco-
pipe are much less efficient. It is an additional safeguard against the escape of ammonia
to fix a small absorption-tube containing fragments of glass to the absorption-flask. The
normal acid is run through this tube into the flask, so as to wet the broken glass, and is
finally rinsed into the absorption flask before titrating its contents.
[51] Cp. Procter and Turnbull, Jour. Soc. Chem. Ind., 1900, p. 130; also Nihoul,
Composition des Cuirs Belges, p. 14 (Bourse aux Cuirs de Liège, Sept. 1901), who
advocates the use of potassium permanganate in the oxidation; and Law (Jour. Soc. Ch.
Ind., 1902, p. 847).
In place of using 10 grm. of potassium persulphate as described, 10 grm. of ordinary
potassium sulphate may be used, and potassium persulphate added in small quantities
towards the end of the operation till a perfectly colourless solution is obtained.

CHAPTER IX.
THE PHYSICAL CHEMISTRY OF THE HIDE-FIBRE.

The nature of the changes which take place in the conversion of raw hide
into leather, and the causes of swelling and “falling” in the various stages of
the wet-work and tannage are among the most difficult problems with which
we have to deal, and no intelligible explanation can be given without taking
into account facts which are among the most recent discoveries of physical
chemistry; and of which even yet our knowledge is by no means complete.
We know from our study of the structure of hide, that it consists in its
natural state of gelatinous fibres which are soft and swollen with water, and
easy putrescible. When these are dried, they contract and adhere to each other,
forming a hard and almost homogeneous mass, resembling in degree, a sheet
of glue or gelatine. After the tanning process, the fibres are changed in
character, though not in form; they no longer absorb water so freely, and in
drying they do not adhere together, but remain detached and capable of
independent movement. The leather is therefore porous, flexible, and opaque
on account of the scattering of light from the surfaces of the fibres, although
the individual fibres are translucent. At the same time, chemical changes have
taken place which render the fibres incapable of ordinary putrefaction. Our
first necessity, therefore, in the conversion of skin into leather is to dry the
fibres without allowing them to adhere. This is accomplished in the most
primitive mode of leather dressing, by mechanically working fatty substances
into the skin as it slowly dries, so as to coat and isolate the fibres, which are
loosened by kneading and stretching; while at the same time the fat forms a
waterproof coating which prevents them from again absorbing the water which
is necessary to putrefaction. Similar results may be produced by causing
chemical changes in the fibres themselves, which render them insoluble in
water, and consequently non-adhesive; and a sort of leather may even be made
by merely replacing the water between the fibres with strong alcohol, in which
they are insoluble, and which absorbs and withdraws the water from them,
allowing them to shrink and harden, while preventing their adhesion. The merit
of having first clearly seen and expressed these cardinal principles in leather
production belongs to the now venerable Professor Knapp, who published in
1858 a short paper (Natur und Wesen der Gerberei und des Leders) which is a

model of clear explanation and practical experiment. Knapp, however, deals
mainly with the changes in the condition of the fibre which are necessary to
convert it into leather, and not with their physical causes; and before we can
explain the means by which these changes are brought about, we must be
acquainted with certain facts and theories about solutions which have become
much clearer since he wrote.
The particles (molecules) of all substances are drawn together by attractive
forces somewhat of the same character as the attraction of gravitation which
holds together the solar system, and which is the cause of weight. It is indeed
even possible that these forces are identical. Like gravitation, these molecular
attractions increase rapidly in intensity as the distance of the attracting bodies
diminishes, so that in solids and liquids, where the molecules are near together,
they are immensely powerful, while in gases and vapours they are barely
perceptible. These attractions are opposed by the motion of heat, which takes
the same part in molecular physics which the energy of planetary motion does
in the solar system. In solids, the attractive forces hold the molecules rigidly in
position, the motion of heat being limited to short vibrations round a fixed
point, the effects of which are visible in the expansion caused by rising
temperature. If the temperature is increased, most substances become liquid, a
condition in which the particles can roll round each other, but are still held
together by their mutual attractions, as the sun holds the earth from flying off
into space. If the temperature goes on rising, the orbits of the molecules
become greater, the liquid expands, and finally molecules fly off at a tangent
out of reach of the attractions of the mass of liquid, and are only diverted from
their course by colliding with solids or with other flying molecules, from
which they rebound. This constitutes the state of vapour or gas.
The molecules usually consist of groups of atoms. Thus in the vapour of
water, each molecule contains one atom of oxygen combined with two of
hydrogen, and it is only at immense temperatures that this inner grouping is
broken up. Naturally, the more complicated and heavier the molecular group,
the more easily it is broken up by outside causes into simpler groupings, and
molecules may exist in liquids or solids, which break up before they reach the
gaseous form. Of such substances the chemist says that they “cannot be
volatilised without decomposition.” In very rare instances does the gaseous
molecule consist of a single atom; even those of the most perfect gases, such as
hydrogen, oxygen and nitrogen consist of pairs which are not broken up at any
known temperature. The pressure of a gas, and its tendency to expand is due
simply to the motion and impact of its flying molecules, and it may be noted
that at the same temperature and pressure equal volumes of all gases have the

same number of molecules, the lighter molecules making up for their want of
weight by their greater velocity. The average velocity of a molecule of oxygen
(O2) at freezing point is 461 meters per second or about that of a rifle-bullet. It
must not be taken however, that in any given solid, liquid, or gas, all the
molecules at any temperature move at a uniform velocity, but that each
individual molecule may vary from moment to moment from rest up to a very
high speed, while the temperature of the mass only represents the average.
Thus it happens that in all liquids, and even in solids, a certain proportion of
the molecules at any temperature will have a speed sufficient to enable them to
leave the surface, and take the form of vapour, while a certain proportion will
fall back and be caught and retained. Thus every liquid, and theoretically every
solid, has a “vapour-pressure,” rising with the temperature, and depending on it
only, and at the boiling temperature of the liquid equal to that of the
atmosphere, or about 15 lb. per square inch, and therefore able to form bubbles
in the interior of the liquid. If a little of a liquid is confined in a flask, the flask
will become filled with its vapour, and so long as any of the liquid is present,
the pressure of the vapour will depend only on the temperature and not at all
on the respective quantities of liquid or vapour. Neither will it be affected by
the pressure of other vapours or gases present in the flask, the total pressure in
which will be the sum of the “partial” pressures of all the gases and vapours
present.[52]
[52] Cp. p. 421.

The behaviour of gases and vapours has been described in some detail
because it possesses very close analogy to that of substances in solution. The
molecules of liquids are held together by attractions which are very powerful
over the short distances which separate them, amounting in most cases to many
tons per square centimeter of sectional area, but the range over which they act
is very small. In the interior of the liquid the attractions on one side of a
molecule are of course exactly balanced by those on the opposite side, so that
it is free to move within the liquid without hindrance, but at the surface a very
small part of the force due to the attractions of the surface-layer is unbalanced
and acts as a sort of elastic skin holding the liquid together, and is called
“surface-tension.” Familiar examples of this are found in the force which
supports a drop on the end of a tube, the possibility of laying a slightly oily
needle on the surface of water without sinking, and the ability of some flies to
walk on water as if it were covered with a sheet of india-rubber. Many liquids
will mix or dissolve in each other in any proportions, e.g. water and alcohol;
the attraction of the alcohol for the water-molecule being as great or greater

than that of alcohol for alcohol, or water for water. In other cases, such as
water and oil, or water and petroleum spirit, practically no mixture takes place,
their mutual attraction being small; and each retains a considerable surface-
tension at the points of contact, though less than that of the free surfaces, since
each exerts an attraction on the other. There are also many intermediate cases,
such as water with chloroform, carbolic acid, or ether, in which each solvent
dissolves a portion of the other, but the two solutions do not mix, but form
separate layers. In these cases an equilibrium is attained, in which there is just
as much tendency for either of the liquids to pass into as out of the other layer.
In this there is an extraordinary resemblance to what has been said of vapour-
pressures; and the tendency to pass into solution is often called solution-
pressure; and it may be noted that when equilibrium has been reached, not only
is the solution-pressure, but the vapour-pressure of each constituent equal in
both solutions. Like vapour-pressures, the solution-pressures usually increase
with rise of temperature, more of each constituent passing into the other, till at
last the composition of the two layers becomes identical, their surface-tensions
disappear, and complete mixture takes place. With phenol (carbolic acid) and
water this takes place at about 70° C.
Most of what has been said of the mutual solution of liquids is also true of
the solution of solids, but the latter may be divided into two very distinct
classes, colloids and crystalloids (which, however, shade off into each other).
The colloid or gluey bodies are mostly miscible in any proportion with liquids
in which they dissolve, and there is no such thing as a definite point of
saturation. There are however some which form jellies which have great
analogy to the partially miscible liquids; there is a mutual solubility, a portion
of the solid dissolving to a liquid solution, while the remainder of the liquid
dissolves in the solid, increasing its volume, but still retaining the
characteristics of the solid state. As the temperature is raised, this mutual
solubility generally increases, till at a given point the jelly melts, and complete
solution takes place, as in the case of partially miscible liquids. These
phenomena are of prime importance in the theory of tanning, but their further
consideration must be deferred till a few words have been said about the
crystalloids. These are characterised by regular crystalline form, indicating that
the attractive forces of their molecules are exerted in definite directions, giving
them a tendency to attach themselves together in definite geometrical
arrangements. They dissolve in themselves no part of the solvent, but are
dissolved by it till an equilibrium is reached in which the tendency of further
particles of the solid to pass into the solvent is balanced by that of those
already dissolved to attach themselves to the remaining solid, or “crystallise

out.” Such a solution is “saturated” with respect to the solid residue, but the
word has no meaning unless solid crystals are present, and where a body has,
as sometimes happens, more than one crystalline form, a solution may be
saturated with regard to one of them, and more or less than saturated with
regard to another. In “supersaturated” solutions, crystallisation is at once
started by the addition of a “seed” crystal of the proper form.
If a crystalloid substance, such, for instance, as copper sulphate, be placed in
a solvent (e. g. water), the dissolved salt will gradually spread itself through
the whole body of the solvent, though in the complete absence of currents in
the liquid, the motion is extremely slow, and years may be taken for the
diffusion to rise through a few feet. In many cases salts diffuse through
aqueous jellies at the same speed as they would through still water. Colloid
substances on the other hand have little or no power of diffusion and mostly
cannot pass through jellies at all. This is the reason why tannage with mineral
salts is so much more rapid than with vegetable tannins which are of colloidal
character, and diffuse through the gelatinous fibres of the hide with extreme
slowness.
All dissolved crystalloids do not pass through gelatinous membranes with
equal ease, and substances are known, mostly gelatinous precipitates, which do
not permit the diffusion of dissolved salts, though they allow water to pass
freely. Thin layers of such precipitates form what are called “semipermeable
membranes.” The existence of such membranes affords us the possibility of
direct measurement of the tendency to diffusion, or as it is generally called the
“osmotic”[53] pressure of dissolved bodies. Thus a porous earthenware battery-
cell may be immersed in a solution of copper sulphate, and filled with one of
potassium ferrocyanide. In this way its pores will be filled with a gelatinous
precipitate of copper ferrocyanide, which is pervious to water, but impervious
to most dissolved substances. If now the cell be filled with a dilute solution of
some crystalloid, say sugar, and its top closed by a perforated cork fitted with a
vertical tube, and the cell be plunged in water, the latter will pass into the cell,
and the dilute solution will rise in the tube to a height of many feet above the
water outside. By substituting a mercury pressure gauge for the vertical tube,
exact measures of the pressure in the cell can be made, which is the osmotic
pressure of the dissolved substance. At first sight it is paradoxical that the
water should flow into the solution, apparently against a heavy pressure, but
the explanation is simple. Mention has already been made of the enormous
internal pressures of liquids produced by the attractions of their molecules. In
the solution a portion of this is borne by the dissolved substance, and the water
flows in from the outside till an internal mechanical pressure is produced,

equal in amount to the osmotic pressure of the dissolved substance. The
resemblance of the phenomena of solution to those of vapour-pressure has
already been mentioned, and it is found to be even quantitative, since the
measured osmotic pressures are exactly equal in amount to those which the
dissolved body would produce if it were in the state of vapour at the same
temperature and occupying the same volume as the solution. It acts, in fact,
precisely as the “partial pressure” of a vapour. There are several indirect ways
of measuring the osmotic pressure of dissolved bodies, as for instance, from
the lowering of the freezing point, or the raising of the boiling point of the
solution as compared to those of the pure solvent, all of which confirm the
direct measurements, and show that in a given volume at the same temperature,
the same number of molecules will produce the same osmotic pressure
whatever their nature, or conversely, that at the same osmotic pressure and
temperature equal volumes of any solution must contain the same number of
molecules. The use of these facts in determining molecular weight is obvious.
[53] Solution-pressure and osmotic pressure are really two names for the same force; the
former being employed to signify the tendency of a solid to dissolve, and the latter the
pressure produced by the dissolved body which tends to prevent further solution. Thus, in
a saturated solution in contact with its solid, the two pressures are always equal, but
exerted in opposite directions.
A curious apparent deviation from this law is however noticed in solutions
of salts, acids, and alkalies, and indeed of electrolytes generally; thus a dilute
solution of sodium chloride produces an osmotic pressure nearly double that
corresponding to the number of molecules of NaCl present; and in fact behaves
as if it were a solution of Na and Cl existing separately. Such a solution
conducts a current of electricity very readily, while at the same time the
chlorine is carried to the positive, and the sodium to the negative pole, where
they separate as Na2 and Cl2 (the Na decomposing the water present and
forming NaOH). In fact, the modern theory of electrolysis asserts that these
dissociated atoms are not separated from each other by electricity, but that they
exist already separated in the solution of the electrolyte, and merely act as
carriers for the electricity, and that the work done by the latter is not that of
breaking up the salt-molecule, but of giving its dissociated atoms fresh charges
of electricity which enable them to combine as new molecules, and escape
from the electrolyte. Complex salts do not always break up into single atoms,
thus calcium sulphate dissociates into Ca and SO4, hydrogen sulphate
(sulphuric acid) into 2H and SO4, and so on. These dissociated atoms and
atom-groups are called “ions,” and may be monovalent, divalent, and so on;
the divalent ion carrying double the electrical quantity or charge of the

monovalent. Without discussing the ultimate nature of electricity itself, the
matter is most easily pictured by assuming that the molecule of the undissolved
salt is made up of an ion with a + charge (“kation,” e.g. Na), and an ion with a
- charge (“anion,” e.g. Cl), by the electrical attraction of which charges they
are held together. In the solution these attractions are balanced by those of
other ions, so that they can wander freely within the liquid, but in order to take
the molecular form of free elements and escape, say as Na2 and Cl2, the pair of
kations must go to the - pole and give up one + charge, and at the same time a
pair of anions must go to the + pole and receive a + charge. Thus the Na and all
other kations separate at the - pole, and the Cl and all other anions at the +
pole.
From what has been said, it will be obvious that free ions can only exist in
solution, and can neither evaporate, nor separate as solids; but that in the liquid
they act much like other dissolved molecules, exerting their own osmotic
pressure independently of each other or of the dissolved salt, but with the
limitation that a solution must always contain at the same time equal numbers
of + and - ions. As a solution is diluted, more ions are liberated; as it is
concentrated, more recombine to form undissociated salt. This will be made
clearer by an example. In a saturated solution of sodium chloride with solid
salt present, we have dissolved salt at the solution-pressure of the crystallised
salt, and Na and Cl ions at the dissociation-pressure of the saturated salt
solution, and neither affect the others. If we now add hydrochloric acid, it has
no effect directly on the solubility of the salt, but as HCl dissociates largely
into H and Cl, it increases the pressure of the Cl ions, and so compels the salt
to recombine till the Cl pressure is reduced to its normal amount. This
increases the concentration of the undissociated salt-solution, and thus salt is
precipitated or crystallises out till the solution is no longer super-saturated with
respect to the salt-crystals.
Most chemical reactions, and especially those between acids and bases, are
really reactions of the ions. Thus NaOH in dilute solution is mostly ionised
into Na and OH, while HCl is similarly ionised into H and Cl. On the other
hand, water ionises only very slightly. Hence, on mixture, the H and OH
combine and form water, with evolution of heat, while no actual combination
occurs between the Na and Cl, so long as they remain in dilute solution. For
this reason, the heat of neutralisation of all strong acids and bases is the same,
independent of their nature, since strong acids, bases and salts are almost
completely ionised. The rapidity of action, and consequently what we call the
“strength” or “avidity” of an acid or base depends on the number of its free
ions in solution; very weak acids and bases are very little ionised, though their

salts ionise almost completely in dilute solution. On this depends the
explanation of a fact of great practical importance. Hydrochloric acid, a strong
acid, is almost completely ionised in solution; acetic, a weak one, very little;
while sodium acetate and sodium chloride as salts are both almost completely
ionised. If we add hydrochloric acid to a solution of sodium acetate, we shall
have sodium-ions, acet-ions, chlorine-ions and hydrogen-ions in the solution.
As the pressure of the acet-ions and the hydrogen-ions will be greater than the
dissociation-pressure of acetic acid, they will combine to form it, till the
pressure is equalised, and we shall have in the solution, free acetic acid slightly
ionised, the sodium- and chlorine-ions of sodium chloride, and the sodium-
and acet-ions of any excess of sodium acetate left. If the hydrochloric acid
were just sufficient to combine with the whole of the sodium, we should have
an equilibrium containing much (ionised) sodium chloride and little sodium
acetate, together with much free acetic acid, and little hydrochloric. Thus the
“strong” acid would displace the weak one.
Taking another example, we add sodium acetate to a solution of acetic acid.
As the ionisation-pressure of the acetic acid is much less than that of sodium
acetate, and both have a common acet-ion, the ionisation of the acetic acid will
diminish, and more undissociated acetic acid will form, till by its concentration
the two pressures are equalised. The total quantity of free acetic acid will be
unchanged, but a less proportion of it will be ionised, and it will act like a
weaker acid. This reduction of the activity of a weak acid by the addition of its
neutral salt is often made use of by chemists. Instances in tanning practice are
the use of excess of potassium dichromate with chromic acid in the chrome
tanning process, the effect of neutral salts in “mellowing” the action of tanning
liquors, and the use of salt in “pickling.”
Let us now try to apply these facts to the physics of tanning, taking first the
simplest cases, where electrolytic dissociation does not take place. We may
consider the wet hide as made up of a mass of fibres of gelatine-jelly, with
interspaces which are filled with water. In fact, for many purposes of
experiment we may substitute for hide, mere sheets of swollen gelatine, so as
to avoid the complications introduced by the water or solution mechanically
retained between the fibres.
If we place a sheet of dry gelatine in water, it swells, absorbing perhaps
seven or eight times its weight of water, but does not appreciably dissolve. A
condition of equilibrium is reached when the attraction of the water-molecules
for the gelatine is equal to the sum of the cohesive attraction of the gelatine for
itself and the internal attraction of the water outside. An increase of the
cohesion of the gelatine would tend to make it contract and expel part of the

water, and this contraction would tend further to increase both the cohesion of
the gelatine, and its attraction on the diminished number of water molecules it
contained, and clearly these causes would act in opposing directions. The
equilibrium is therefore a very unstable one, and slight causes might be
expected to produce great changes in the degree of swelling, which is indeed
the case. If we increase the temperature we diminish the cohesion of the
gelatine, till at a point it becomes less than its attraction for the water, and the
jelly suddenly loses its solid condition and dissolves.
The absorption of water by colloids (including gelatine) is accompanied by
contraction of volume (compression) of the water absorbed, and by evolution
of heat, and, as has been pointed out by Koerner,[54] it is opposed (and swelling
decreased) by increase of temperature. Solution, on the other hand, absorbs
heat, and is therefore favoured by rise of temperature.
[54] Beiträge zur wissenschaftlichen Grundlage der Gerberei, Freiberg, 1899.

If we place the swollen jelly in alcohol, it parts with water and contracts.
The gelatine and alcohol are not mutually soluble, the sum of the attraction of
water for alcohol, and the cohesive attraction of the gelatine is greater than the
attraction of the latter for water, and as the alcohol cannot pass into the
gelatine, the water passes out, and the jelly contracts. The greater the
concentration of the alcohol, the more completely is the jelly dehydrated, and
in strong alcohol it may become quite hard and solid. If we like to express the
same facts in language more familiar to the modern chemist, but perhaps less
clear to the non-chemical reader, we may say that the alcohol exerts an osmotic
pressure outside the gelatine, but little or none inside it, and therefore the water
is squeezed out. It would be equally true to say that the water passes out of the
jelly till its osmotic pressure is equal in both the jelly and the alcohol. The jelly
is a true “solid solution” of water in gelatine, and in a solution we may regard
either of the two constituents as the solvent. Exact parallels may be found in
the distribution of a third substance between two immiscible solvents (see p.
76), say alcohol between water and ether.
The osmotic pressure of water into alcohol may be demonstrated in a very
simple way, taking advantage of the fact that a film of jelly is permeable for
water but not for alcohol. If the experiment described on p. 78 be made by
placing alcohol in a cell previously washed out with a gelatine solution, and
the cell be placed in water, the water will pass into the cell, and the alcoholic
solution will rise many feet in the vertical tube. The insolubility of gelatine in
alcohol may be made use of for its estimation. If three times its volume of
absolute alcohol be added to a solution containing gelatine, the latter will

separate as a solid mass on a stirring rod, or on the sides of the beaker, and
may be washed with further portions of alcohol. The method is useful in the
analysis of gelatine lozenges and “jelly squares,” roller compositions,
hectograph masses, and the like; and for the determination of true unaltered
gelatine in glues, and commercial gelatines (see page 60). Many other colloids
are however also precipitated by alcohol.
If hide be treated with alcohol, as in Knapp’s experiment (p. 74), the action
is precisely the same as has been described with gelatine-jelly. The water is
withdrawn, first from the spaces between the fibres, and then from the fibres
themselves, and the skin dries with the fibres isolated and non-adherent, and is
in fact converted into a sort of leather, which, however, returns to raw pelt on
soaking in water.
The action of solutions of sugars, glycerine, and the like is in principle
similar to that of alcohol, but more complex, since in general these bodies are
soluble not only in the water, but in the gelatine or hide-fibre, so that their
effect cannot be foretold, though usually it tends towards contraction rather
than swelling. In general terms the equilibrium is a balance of the attraction of
the water and the sugar for the gelatine, against the sum of their mutual
attraction in the solution outside and the resisting cohesive force of the
gelatine; and will depend not only on the nature of the substances, but on
temperature and concentration.
The action of acids, alkalies and salts on gelatinous fibre is yet more
complex, since not only electrolytic dissociation, but most probably actual
chemical combination comes into the question. The chemical constitution of
gelatine is as yet quite uncertain, but it is known that the molecule contains
both amido-groups capable of linking with acids, and carboxyls which will
combine with bases (see p. 58). Hence hide-fibre absorbs both acids and bases
with great avidity, so much so that the sulphuric acid of a decinormal solution
may be completely removed by hide, leaving only water without a trace of acid
recognisable by litmus. Alkalies are absorbed in a similar way, and in both
cases the gelatine or gelatinous fibre acquires a greatly increased power of
absorbing water, and consequently of swelling. Familiar cases of this are the
swelling of hide by acid, and by lime, and in neither case can the added
substance be removed in any reasonable time by mere washing with water.
Hence to free hides from lime or acids it is necessary to neutralise the alkali
with acids (see p. 153) or the acid with chalk or alkalies (p. 91). No accurate
determination has yet been made of the amount of acid or alkali with which
gelatine or hide-fibre will combine, since the matter is complicated by the
volume of acid or alkaline solution which is absorbed mechanically, and by the

tendency of the compound to partially decompose on washing with water.
Experiments made by the author lead to the conclusion that 1 grm. of air-dried
gelatine will combine with about 0·025 grm. of actual hydrochloric acid (HCl)
when placed in a very dilute solution of the latter, and this compound will
absorb 40 or 45 grm. of water while still retaining the jelly state. The
maximum swelling, with both acids and alkalies, is obtained with dilute
solutions; and with the stronger acids, the outside solution must be almost
neutral when equilibrium is attained, increasing quantities of acid diminishing
the amount of water absorbed. The same statement is true of the strong
alkalies. Thus in both cases, where swelling is desired, the object is defeated
by the use of too strong solutions, and the quantity of acid or alkali should be
rigidly adjusted to the weight of pelt, and not to the volume of solution.
As regards a physical explanation of the effect of acid and alkaline solutions
upon gelatine, anything which can yet be said must be regarded rather as
speculation than as actual scientific knowledge. It must also be admitted that
while the view that actual chemical combination takes place between the
gelatine and acids (or alkalies) seems much the most probable, difficulties arise
from the fact that different acids apparently do not always combine in
proportion to their equivalents, though it is probable that these will prove only
apparent anomalies when more accurate means are known of determining how
much acid is really combined, and how much merely mechanically absorbed.
Leaving out of account for the moment the question of swelling, a few
words must be said about a property of these acid- (and alkali-) gelatine
compounds, a knowledge of which is essential to understanding the swelling
process. If a mass of acid-gelatine be suspended in pure water, a certain portion
of it will be decomposed into neutral gelatine and free acid, and the latter will
diffuse into the water. Thus acid-gelatine can only exist in presence of a certain
amount of free acid. This dissociation by water is a common property of all
salts, and necessarily follows from what has been said of ionisation; but it is
only where the combining affinity of the constituents is weak, that it becomes
practically perceptible. Water to a very small extent ionises to H and OH. If we
imagine a salt dissolved in it, such as NaCl, which ionises almost completely
to Na and Cl, we see that a certain proportion of NaOH and HCl must be
formed by combination with the water-ions. In the case named the quantity is
absolutely negligible, since both sodium hydrate and hydrochloric acid are
almost completely ionised themselves, but if either the acid or the base is weak
(that is little ionised), the process of combination must go on till the acid or
basic solution is strong enough to have an ionisation-pressure equal to that of
the salt. As this acid or base is no longer in an ionised condition, it may be

removed from the solution by volatilisation or diffusion. For instance, if a
solution of ferric chloride be confined in a tray of parchment paper, through
which it has little power of diffusion, and this tray floated upon water which is
frequently changed, the dissociated acid will diffuse through the membrane
into the water, and in this way the whole of it may be ultimately removed,
leaving nothing but a colloid solution of hydrated ferric oxide in the tray.
Actions of this sort, in which the gelatinous fibre of the hide plays the same
part as the parchment-paper membrane, have an important share in many of the
phenomena of tanning.[55] Thus, in the case of hide swollen with acid, the acid
compound with the fibre is somewhat dissociated, and if the hide be hung in
water which is constantly changed, the acid diffuses into it, and the whole may
be ultimately, though slowly removed. A similar effect is produced in the
familiar operation of removing acid from pelt or chromed leathers by paddling
with “whitening” (calcium carbonate). The latter is insoluble in water, and
therefore cannot penetrate into the hide, but as it instantly combines with any
acid which diffuses out, the acid-gelatine compound is rapidly decomposed,
since it is only permanent in a solution containing enough free acid to have an
ionisation-pressure equal to that of the compound. Similar statements are true
of the alkali-gelatine and lime-gelatine compounds.
[55] Probably chrome, aluminium and iron salts are decomposed in this way in mineral
tanning, and thus fixed in the hide as insoluble basic salts. Cp. pp. 186, 215.
It will be easier to follow the results of what has been said if we take a
concrete case which has been carefully investigated by the author and others;
that of the action of hydrochloric acid solutions on gelatine. If a weighed sheet
of gelatine be placed in a very dilute solution of the acid, it swells much more
considerably than it does in water, a maximum swelling being attained with a
concentration of the outer solution of 0·1 to 0·2 grm. of HCl per litre. The
swollen jelly has then a volume of about 45 c.c. per gram of the air-dried
gelatine, and a concentration equal to about 0·75 grm. of HCl per litre of
swollen jelly, or at least about five times that of the outer solution. As the
concentration of the latter is increased, the concentration in the jelly also
increases, but in a much smaller ratio, while the volume of the jelly diminishes,
till, with a concentration of 5 grm. of HCl per litre in the outer solution, the
volume of the jelly is only about 18·5 c.c., and its concentration not quite 6
grm. per litre. These facts cannot be accounted for by any theory of simple
solution of the HCl in the jelly, since the law of such solutions is that the
concentration in each maintains a constant ratio, unless chemical change takes
place. It is possible that they might be explained by adsorption (surface

attraction), but as it is known that gelatine contains both amido-groups capable
of combining with acids, and carboxyl-groups which can combine with bases,
it is much more likely that actual chemical combination takes place, and that
the apparent irregularities in the amount of acid fixed are due to partial
hydrolysis of the compound.[56]
[56] Cp., however, Walker and Appleyard on the ‘Absorption of Acids by Silk,’ Chem. Soc.
Trans. 1896, p. 1334.
The following may be suggested as a working hypothesis. As both water and
hydrochloric acid can pass freely in and out of the jelly, it must be in osmotic
equilibrium with the outer solution in every respect, and neither the un-ionised
hydrochloric acid of the solution, nor the small amount which may be formed
by hydrolysis of the gelatine compound can have any effect on the swelling. So
long as the outer solution is very dilute, by far the greater part of the acid
present is absorbed and fixed by the gelatine, and almost the whole of the outer
acid will be ionised, as well as a portion of that in combination with the
gelatine. In the latter case, however, the ions will be unable to pass out of the
jelly, and will therefore cause an internal osmotic pressure, and the gelatine
will swell till the Cl-ions are in osmotic equilibrium with those of the outer
solution. At the same time, this internal pressure of Cl-ions will oppose the
entry of the Cl-ions (and therefore also of their associated H-ions) from the
outer solution, and the acid solution absorbed mechanically will be somewhat
less concentrated than that outside. As the concentration of the outer solution is
increased, the pressure of the outer Cl-ions will repress the ionisation of the
gelatine-chloride, and at the same time its tendency to hydrolyse. Thus the acid
actually combined with the gelatine should somewhat increase, but the
swelling should diminish, as is actually the case.[57] It is impossible to carry
the concentration of the hydrochloric acid much above 5 grm. per liter without
causing solution of the gelatine, but the addition of common salt to the outer
solution should equally increase the pressure of its Cl-ions, and cause further
diminution of swelling, the Na-ions in this case increasing the outside pressure
in the same way as the hydrogen ions. In fact the addition of salt in sufficient
quantity will reduce the swelling till the gelatine becomes quite solid, and
retains only about its own weight of water, while at the same time the
apparently combined acid largely increases. This cannot be attributed to any
direct dehydrating action of the salt, since concentrated sodium chloride
solutions have no dehydrating, but rather a swelling effect on gelatine in the
absence of acid, and the concentration of the salt in the outer solution and in
the jelly proves precisely the same within the limits of experimental error.

Several other facts may be noted, tending to support the explanation which has
been given. The tendency to swell gelatine is common to all acids of
appreciable strength, and in all cases where the concentration of the acid could
be increased to a moderate extent without causing solution of the jelly, the
effect of a maximum swelling, diminishing as the concentration of the acid
increased, has been observed. Other salts also produce similar effects to
sodium chloride; thus the swelling caused by sulphuric acid is repressed by
sodium sulphate. Sodium chloride seems to diminish the swelling caused by all
acids, but in presence of large excess of sodium chloride, most of the acid in
combination with the gelatine will probably be hydrochloric, whatever the acid
used to originally produce the swelling. A curious fact observed by the author,
is that absolute alcohol, which so effectually dehydrates neutral gelatine, is
almost powerless to remove either water or acid from gelatine swollen by
hydrochloric acid. HCl is freely soluble even in absolute alcohol, but H- and
Cl-ions can only exist in it to a very small extent, so that we may conclude that
the acid which causes the swelling and retains the water of the jelly exists
either in actual combination with the gelatine, or in an ionised condition.
[57] The acid retained by the gelatine, as measured by deducting from the total contained in
the jelly, a quantity equivalent to the volume of solution absorbed, at first rises rapidly to
a maximum, then slightly diminishes and remains practically constant. On the theory
suggested, it is evident, however, that the absorbed solution must be more dilute than that
outside, and the actual combined acid greater than that shown by the above calculation.
The “combined” acid, as determined by indicators, shows slight but continuous increase.
It is acid to phenolphthalein, but neutral to methyl orange.
Solutions of caustic alkalies are in most respects analogous in their swelling
action to those of strong acids. A portion of the alkali is in some way fixed by
the gelatine, while another portion is simply absorbed as solution. A maximum
swelling effect is also noticed with dilute solutions, which is diminished as the
concentration increases. Swelling by alkalies is not diminished by chlorides so
far as has been observed, and especially it may be noted that the swelling
produced by caustic soda is not diminished by sodium chloride. On the theory
which has been suggested there is no reason why alkaline swelling should be
reduced by chlorides, since the swelling agent has no Cl-ion, but it is
somewhat singular that the sodium salt, having a common Na-ion should
produce no repression of the swelling by caustic soda. In the present state of
our knowledge no definite explanation can be given, but it is quite possible that
the swelling in this case is not produced by the sodium-ion but by some more
complex one, or even by the hydroxyl-ion, like most of the characteristic
reactions of alkalies. Apparently the gelatin-alkali compound is still strongly

alkaline, affecting phenolphthalein indicator like uncombined alkali—an effect
which is known to be due to the presence of free HO-ions.
The effect of acids and alkalies has been studied by Procter and others on
actual pelt as well as on gelatine, and has been found to be qualitatively, if not
quantitatively quite similar to that on gelatine, though from the acid retained
mechanically in the interfibrous spaces, exact quantitative determination is
more difficult. The amount of swelling produced is not proportional to the
strength of the acid, some weak and little ionised acids such as lactic producing
larger swelling than stronger acids such as hydrochloric and sulphuric, of
which the ionic pressure in the external solution is greater. Dilute solutions
generally produce greater swelling than more concentrated, so that where
swelling is required without destructive effect on the fibre, dilute solutions of
such weak acids are to be preferred, and the presence of neutral salts is to be
avoided. On the other hand, where it is desired to remove lime, or to bring the
pelt into an acid condition without swelling, the addition of neutral salts, and
especially of chlorides is advantageous. A very important application of this
principle is the “pickling” of sheep-skins, and especially of sheep-grains, in
order to preserve them for export. The principle of this operation is that the
skins are first swollen slightly with sulphuric acid, and the swelling is then
reduced by salt, either added, or used in a subsequent bath. In practice, salt is
now generally also added to the first bath to moderate the swelling. A suitable
strength for the “rising solution” is about 80 grm. common salt, and 7·5 grm.
sulphuric acid per litre. 100 c.c. of this solution will therefore require about 15
c.c. of N⁄1 alkali to neutralise it, and it should be tested after each lot of skins,
and maintained at the same strength by suitable additions of acid. The acid
absorbed by the skins is mainly hydrochloric, sodium sulphate accumulating in
the bath. The salt is not absorbed by the skins in the same way as the acid, but
will be continually diluted by the water they bring in, and occasional additions
of salt must therefore be made, the density being maintained at about 65° Bkr.
(1·065 sp. gr.) After paddling or being stirred in this bath for about 1⁄2 or 3⁄4
hour the skins are transferred to saturated brine, and stirred in it till fully fallen
in thickness, the density of the liquid being maintained by excess of salt. They
may be allowed to remain some hours in the saturated brine with advantage.
Within moderate limits, the strength of the rising liquor is not of great
importance, since the skins will only absorb a certain amount of acid
(increasing with the concentration of salt). In the second or falling liquor the
large excess of salt forces all the acid present into the skins, none diffusing into
the bath. Skins may be effectively pickled with very much smaller quantities of
acid than those prescribed above, or ordinarily used, and are much easier to tan

satisfactorily; but it is said that they are more liable to suffer from mildew.
Pickling may also be done by placing the skins in a concentrated brine-bath,
and adding a calculated quantity of acid, not exceeding 0·1 grm.-molecule of
sulphuric acid per kilo. of dry hide substance, but the method is not
economical in practice from the dilution of the bath produced by the water
brought in by the skins and the necessity of constant large additions of salt.
Pickled skins must not be brought in contact with water, which by diluting
the brine they contain, allows the excess of acid to act upon and destroy the
fibre. Even drops of water, accidentally sprinkled on the skins produce this
effect, and it is said that it spreads to parts which have not been wet. For
similar reasons, it is necessary in tanning pickled skins, at least to begin the
process in liquors to which salt has been added, the quantity required being
dependent on the amount of acid used in pickling the skins, and where this is
reduced to a minimum, it is even possible to tan without further addition of salt
than that contained in the skins.[58] The pickling process converts the skins into
a species of white leather, and skins tanned in salted liquors after pickling, or
by addition of both acid and salt to sumach liquors give good colour, and tough
leather with a much diminished consumption of sumach. The permanency of
such leather is somewhat doubtful, but the writer was unable to detect free
sulphuric acid in a sample which he examined, and it may be that when no acid
is added to the later liquors, that derived from the pickling is expelled by the
tannin; but this is very doubtful.
[58] Instead of using salted liquors, the skins maybe “depickled” by a bath of whitening and
water, borax, or some other mildly alkaline solution before tanning.
The facts which have been discussed in the preceding pages offer a
sufficient explanation of the causes which operate in those deliming processes
which depend on the simple neutralisation of the alkaline matters present in the
hide, and of the swelling by means of acid which forms a step in the
manufacture of many sorts of sole-leather, but they by no means fully elucidate
the causes of the much more complete depletion of the pelt brought about by
the bacterial products of bates and puers. It has been pointed out (p. 82) that
gelatine and hide-fibre in a neutral condition are swollen by water, but that the
equilibrium so reached is an unstable one, easily influenced by slight causes.
Among these, as has been pointed out by Koerner,[59] the surface-tension
between the water and the swollen fibre holds a place; and surface-tensions of
this sort are greatly influenced by many substances of the class to which
bacterial ferments belong. Many salts also alter the water-absorption of
gelatinous fibres, sometimes causing swelling, and sometimes contraction,

according to temperature, concentration, and the nature of the salt. Though
most salts do not seem to be absorbed by hide-fibre, it is possible, as suggested
by Koerner (loc. cit.), that in some instances the base may combine with the
acid-groups, and the acid of the salt with the basic groups of the gelatine-
molecule, while other cases are known in which salts are actually dissociated,
and their acid fixed by the affinities of the hide-fibre. An interesting case of
this sort was recently proved by Paessler and Appelius,[60] who showed that
sulphuric acid was absorbed from a solution of hydric sodic sulphate, and the
neutral sulphate left in the solution. Similar reactions undoubtedly occur with
some salts of strong acids and weak bases, but this point must be more fully
discussed in connection with the theory of mineral tannages.
[59] Beiträge zur wissenschaftlichen Grundlage der Gerberei, Jahresberichte der deutschen
Gerberschule zu Freiberg, 1898-9 and 1899-1900.
[60] Wissenschaftliche Beilage des Ledermarkt, 1901, ii. p. 106.

CHAPTER X.
WATER AS USED IN THE TANNERY.

Of all the materials employed in tanning, none is of more indispensable
importance than water, and its quality has undoubtedly great influence on
tanning, though it is constantly blamed for faults and troubles which are really
due to the mistakes of the tanner.
Water is chiefly used in tanneries for soaking and washing hides and skins,
for making the limes, the bates, and the tanning liquors, for steam boilers, and
in dyeing. For all these purposes it should be as free as possible from
impurities, but since water is the most universal solvent in Nature, it is never
found pure, but always contains mineral matter derived from the rocks and soil
through which it has flowed, as well as organic impurities from decaying
animal and vegetable matter. Associated with the latter are usually living
organisms of putrefaction (bacteria) which may affect the quality of the water
for tanning even more seriously than the mineral impurities. The purest natural
waters are those which have flowed only over hard sandstones and volcanic
rocks. Water sufficiently pure for laboratory use can only be obtained by
distillation. The steam-water from heating pipes usually contains large
quantities of dissolved iron, and often also volatile organic matters from the
oil, etc., which finds its way into the boiler. It may sometimes be made fit for
use by boiling (which precipitates the ferrous carbonate present), and
subsequent settling or filtration. The use of steam-water containing iron is a
frequent source of stains and discolorations in the tannery which more than
counterbalances the advantage of its softness.
The “hardness” of natural waters is mostly due to the salts of lime and
magnesia which they contain, which precipitate soap in the form of insoluble
stearates and oleates, which are useless for washing. It is commonly estimated
by determining the amount of a standard alcoholic soap solution which must be
added in order to produce a permanent froth on shaking. Theoretically about 12
parts of soap (sodium stearate or oleate) are destroyed by 1 part of calcium
carbonate or an equivalent quantity of other lime salts, with formation of
insoluble lime soaps (calcium stearate or oleate). Really, the reaction is much
more complicated, owing to the dissociation of the soap into free alkali and
acid-salts on solution in water. Teed[61] estimates that 1⁄3 to 1⁄2 more is required

than the theoretical quantity, and more in hot water than cold. This uncertainty
is partially overcome by testing the soap solution against a known solution of
calcium chloride. The presence of magnesia also complicates the test and leads
to discrepant results.
[61] Journ. Soc. Chem. Ind., 1889, p. 256. Cp. also Allen, ibid. 1888, p. 795.

The methods of determining hardness originated by Hehner (see L.I.L.B., p.
19) are simpler and more accurate than the soap-test, and are to be preferred,
except for direct determination of the suitability of a water for scouring with
soap. “Degrees” of hardness in England are calculated as parts of CaCO3 per
100,000, or sometimes grains per gallon (70,000 grains).
Hardness is of two kinds, “temporary” and “permanent”; the former being
removed by boiling, while the latter is not so removed.
Temporary hardness consists of the carbonates of alkaline earths held in
solution by an excess of carbonic acid. Lime combines with 1 molecule of
carbon dioxide to form the ordinary normal carbonate (chalk), which is
practically insoluble in water. When, however, excess of carbonic acid is
present, hydric calcic carbonate (bicarbonate) which is fairly soluble is
produced. This is easily demonstrated by passing carbon dioxide into
somewhat diluted lime-water, which at first becomes turbid from precipitated
chalk, but soon clears by formation of soluble hydric carbonate. If the solution
be now boiled, the hydric carbonate is decomposed, and the excess of carbonic
acid is driven off as CO2, and the chalk again precipitated. The reactions are
represented by the following equations:—
Ca(OH)2 + CO2 = CaCO3 + OH2. (1)

CaCO3
CaCO3 + CO2 + OH2 = - (2)
H2CO3

Magnesia forms soluble double carbonates in a similar manner, but on
continued boiling gradually loses the whole of its carbonic acid, and is
precipitated as magnesium hydrate, Mg(OH)2.
One of the most important reactions in connection with temporary hardness
is that caused by the addition of calcium hydrate (slaked lime), which forms
the basis of Clark’s softening process. When an equivalent amount of lime is
added to a solution of hydric calcic carbonate, it displaces the water of the
“half-bound” carbonic acid, forming a second molecule of calcium carbonate,
which is precipitated together with that originally present, as is represented in
the following equation:—

CaCO3
H2CO3 -+ Ca(OH)2 = 2CaCO3 + 2OH2. (3)

Hydric magnesium carbonate is also precipitated by lime, but the reaction is
somewhat different, the magnesia being removed as hydrate as follows:—
MgCO3
- H2CO3
+ 2Ca(OH)2 = 2CaCO3 + 2OH2 + Mg(OH)2. (4)

It will be noted that 2 equivalents of lime are required to precipitate 1 of
magnesia. Two molecules of sodium hydrate (NaOH) or potassium hydrate
(KOH) may be substituted for 1 of Ca(OH)2 with similar results, and in some
cases it is practically advantageous to use the former, as the sodium carbonate
formed in precipitating the temporary hardness reacts again on the permanent,
throwing down the lime and magnesia as carbonates. (See p. 101.)

Fig. 19.—Plan of Archbutt and Deeley’s Apparatus.
Larger plan (110 kB)

The use of lime for softening temporary hard waters was originally proposed
by Thomas Henry, F.R.S., of Manchester, but was first applied as a practical
process by Clark, who, after adding the requisite quantity of lime to the water

in a mixing vat, allowed it to stand in a large tank to clear by subsidence, the
precipitated carbonate of lime taking from 6 to 12 hours to settle. The process
in its original form is a perfectly satisfactory one, except for the capacious
settling tanks which are required, which in some cases are inconvenient and
expensive. Messrs. Archbutt and Deeley[62] have patented a modification of the
Clark process, by which the time of subsidence is much shortened, and
according to which the precipitated carbonate of lime of previous operations is
allowed to remain in the tank, and the fresh charge of water and lime is mixed
up with it by means of steam-injectors, which blow in a current of air through
perforated pipes at the bottom of the tank, and at the same time very slightly
warm the water. The action goes on much more rapidly at a slightly raised
temperature than in the cold; and rather curiously, the stirred up precipitate,
instead of increasing the time of clearing, settles rapidly and carries down with
it that formed in the new operation. It is particularly suitable for treating waters
containing magnesia, from which a compound of lime and magnesia is apt to
be precipitated in a colloid form which chokes filter-cloths and will not readily
settle. After softening, the water is usually “carbonated” by passing the gases
produced by burning coke into the floating exit-pipe through which it falls, in
order to retain any remaining traces of carbonates of lime and magnesia in a
soluble form, and prevent their subsequent precipitation in the pipes. The
apparatus is made by Messrs. Mather and Platt, of Manchester, and its
arrangement is shown in Figs. 19 and 20.
[62] Journ. Soc. Chem. Ind., 1891, p. 511.

Fig. 20.
Larger section (115 kB)

Several modifications of the Clark process have been introduced, in which
the precipitation is carried on continuously instead of intermittently. The most
important of these is the Porter-Clark, in which one portion of the water to be
softened flows through an agitator containing excess of lime, with which it
forms saturated lime-water, which is passed slowly up a cylinder where it
deposits the excess of suspended lime. The clear lime-water so produced is
mixed with a fresh portion of the water to be softened in a second cylinder also
provided with an agitator, the proportion of the two liquids being regulated by
cocks. The carbonate of lime is at once precipitated, and is removed by passage
through a filter press. This process is in successful operation on a considerable
scale at Messrs. Hodgsons’ tannery at Beverley.
Several other forms of filter have also been employed with success, and also
methods in which the treated water traverses tanks with sloping partitions on
which the carbonate of lime is deposited. The latter plan was originally

patented in France by Gaillet-Huet, and has been introduced into England by
Stanhope.
So far as is yet known, from the tanner’s point of view, it is hardly necessary
to make any distinction between lime and magnesia, either or both of which
may be considered simply as “hardness.” A hard water probably softens dried
hides more slowly than a purer water, though it is possible that the observed
difference in the time required may be due in many cases to the lower
temperature of wells from which hard water is generally derived. In the actual
“limes” the hardness of the water can have no appreciable influence, though if
sodium sulphide be used alone for unhairing, a certain waste occurs from
temporary hardness which may render it advisable to add a little lime. It is in
washing the hides free from lime that the influence of hard water is first
distinctly felt. If limy goods, after unhairing, are placed in a water with much
temporary hardness, the same action occurs as in Clark’s water-softening
process, and chalk is deposited in the surface of the hides, making them harsh
and apt to “frize” or roughen the grain in “scudding.” The common, but not
wholly satisfactory expedient is to add a little lime, or better, a few pailfuls of
lime liquor to the water before putting in the hides. The best plan is to use a
properly softened water. Permanent hardness is not injurious in this way.
Unfortunately it is not the grain alone which is injured by the use of hard
water for washing the hides, but on coming into the liquors the precipitated
bases combine with the acids and tannins, forming compounds which oxidise
and darken when exposed to the air, and which are the commonest causes of
stains and markings on all descriptions of leather. Even when goods are
drenched or bated before tanning the injury is not prevented, since the weak
organic acids which are capable of removing the lime (as such) from the hide
have little effect on the precipitated carbonate, which can only be dissolved by
the use of stronger acids. It must be noted that the same injurious effect on
limed goods is produced by free carbonic acid, which may be present even in
soft waters.
When temporarily hard waters are employed for leaching tanning materials,
the carbonic acid is displaced by the tannins, which form compounds similar to
those just mentioned, which are incapable of tanning, and darken and discolour
when exposed to the air. Though the amount of lime present in a liter of even
the hardest water is very small, yet in the aggregate of thousands of gallons
used weekly in a good-sized yard it amounts to something very considerable,
and as the molecular weight of tannins is very high, the quantity destroyed is
many times that of the lime present. This loss can be prevented (a) by the
addition of sufficient mineral acid to convert the temporary into permanent

hardness, (b) by the use of oxalic acid, which precipitates the whole of the lime
as oxalate, or, (c) best of all, by softening the water by suitable treatment
before use. Each part of temporary hardness reckoned as CaCO3 (L.I.L.B., p.
19), requires 1·26 parts of crystallised oxalic acid or 0·98 parts of H2SO4, or
say one part of ordinary oil of vitriol of sp. gr. 1·840 per 100,000 parts of
water.
As the lime and magnesia of temporarily-hard water is thrown down by
boiling, it is deposited in steam boilers as a soft precipitate, much of which can
be blown out by suitable sludging; but if oils or fats obtain access to the boiler,
a soft, bulky, adherent deposit is formed, keeping the water from the plates,
which may become red hot, and lead to collapse or explosion. This effect is not
produced by mineral oils, which, on the contrary, tend to prevent adherence of
scale to the plates, and as suitable mineral oils are not only cheaper, but much
less injurious to the working parts of steam engines than animal or vegetable
oils or tallow, they should always be used in preference for cylinder purposes.
Water which is temporarily hard owing to calcium and magnesium
carbonates, is unsuitable for dyeing, as the carbonates react with basic dyes,
precipitating the colour-base, and so rendering a part of the dye useless.
Further, as this precipitate is deposited on the skins it causes uneven dyeing
and gives rise to spots and streaks. In dyeing with basic dyes, therefore, it is
advisable to add sufficient acetic acid to the water before use to exactly
neutralise the carbonates present. Of course this treatment is quite unnecessary
when acid dyes are employed, as acid is usually added with the dye, and with
dyewoods the presence of a little calcium salt is advantageous.
As each “degree” of total hardness represents a soap-destroying power of at
least 2 oz. of soap per 100 gallons of water, allowance must be made in making
up “fat-liquors” with soap and oil for the loss of soap due to its precipitation by
the mineral matter in the water. The sticky lime-soaps are apt to adhere to the
leather and interfere with glazing; so that it is much better to employ a soft
water.
Permanent hardness of water is generally caused by sulphates of lime and
magnesia, and more rarely by chlorides and nitrates. As none of these can be
precipitated by lime, permanent hardness cannot be removed by Clark’s
process, nor can it produce the injurious effect on limed hides which have been
attributed to temporary hardness. Neither can the lime and magnesia present
combine with the tannins if used for leaching, since they are already fixed by
stronger acids, and at most can only act injuriously by slightly lessening the
solubility of the tannins. Even this effect cannot be regarded as proved, though
it deserves further investigation.[63] Permanent hardness is therefore of little

moment as regards the ordinary uses of the tannery, though it has considerable
influence in some of the processes of dyeing, and acts very injuriously where
soap is used for scouring, as in the washing of sheep-skins for wool mats, since
each part of lime reckoned as carbonate destroys at least twelve parts of pure
soap (sodium stearate or oleate), producing a sticky and insoluble lime-soap
which adheres to the fibre. In sole-leather tanning, permanent hardness is
sometimes advantageous, especially if it be due to calcium and magnesium
sulphates, and Vignon recommended that sulphuric acid should be added to the
water before use in quantity sufficient to exactly neutralise the carbonates
which cause temporary hardness, as magnesium and calcium sulphates are not
injurious, but tend to plump the hides. It must be remembered, however, that
the carbonic acid liberated may still have prejudicial effects on limed hides.

[63] Recent investigations by Nihoul (‘Influence de la nature de l’eau sur l’extraction des
matières tannantes,’ Bulletin de la Bourse aux Cuirs de Liège, Sept. 1901) on the
tanning waters of Belgium seem to show that permanent hardness is more injurious in
the extraction of tannin than has generally been supposed.
Permanent hardness is most objectionable in waters employed for boiler-
feeding, and calcium sulphate is especially so, as it becomes nearly
insoluble in water at 150° C. or 55 lb. steam-pressure, and is deposited on
the plates as a hard crystalline scale which has to be chipped off with a
hammer. Where many boilers have to be worked with a hard water, it is
much the most satisfactory to soften the water with caustic soda, or with
lime and soda together before it comes into the boiler, but in cases where
the plant required would be too costly, boiler-compositions are sometimes
used with good effect, though considerable caution is advisable, since some
of them affect the plates injuriously. The active constituent of many boiler-
compositions is soda-ash or sodium carbonate, which acts by double
decomposition with the calcium sulphate, forming sodium sulphate, and
precipitating calcium carbonate as a sediment which is easily washed out.
Most tanning materials, and even spent tan liquors, will prevent or lessen
incrustation if mixed with the feed water, but sometimes corrode the plates
if used too freely. This danger is lessened if they are used in conjunction
with soda. Heavy mineral oils, either introduced in small quantity with the
feed water, or painted on the sides of the boiler when cleaned, are useful in
preventing the formation of a coherent scale.
The removal of permanent hardness from water is easily effected in most
of the forms of apparatus employed for the softening of water by lime, by
using a calculated quantity of sodium carbonate in addition. The reaction is
represented in the case of calcium sulphate by the following equation—
CaSO4 + Na2CO3 = CaCO3 + Na2SO4.

The conversion of magnesium sulphate into carbonate may be similarly
effected, but as the latter is somewhat soluble, an additional equivalent of
lime must be used to precipitate it as hydrate. Magnesium salts, from their
solubility, do not cause scale on boilers (though the chloride is apt to
produce corrosion), but they are equally destructive of soap with the
calcium salts. Caustic soda will remove temporary hardness, and after
becoming converted into carbonate will further react on any permanent
hardness present; and its use is therefore sometimes convenient in small

softening plants, but it is not more effective, and considerably more costly
than a suitable mixture of lime and sodium carbonate. Even with these,
Archbutt states that the cost of softening permanent hardness is about ten
times as great as that of removing temporary hardness with lime only.[64]
[64] Proceedings of Inst. of Mech. Engineers, 1898, pp. 404-54, in which much valuable
information on water-softening is given.
As regards the influence of other impurities, our knowledge is far from
complete, but the following are the most important matters likely to be
present.
Mud under any circumstances is objectionable. It frequently contains
organic slime and organisms which encourage the putrefaction of hides
placed in it to wash or soften. It also almost invariably contains iron as one
of its constituents, and hence stains leather and gives dark coloured liquors.
It is not easily removed by filtration, as large filter-beds are expensive and
difficult to keep in order, and much space is required to clear water by
subsidence. Some mechanical filter which can be easily cleaned, and used
under pressure, offers the best chance of success. The Pulsometer Company
make one consisting of sponge tightly packed below a perforated piston. To
cleanse the filter a stream of water is passed the reverse way, and the piston
raised and worked up and down, either by hand or power, so as to loosen
and knead the sponge. Filter-presses, in which cloths, or in some cases sand,
are used as the filtering medium, are also well adapted for the purpose. If a
water be softened by Clark’s or other process the precipitated chalk carries
down the mud with it, together with most of the organisms.
Iron is always an objectionable impurity in the tannery, though it is less
injurious to the quality than the appearance of the leather produced, and
indeed German sole-leather tanners frequently put old iron in the handlers
to darken the colour of the leather, and apparently, if not really, to quicken
the tannage. It must not be present in waters used for dyeing. Iron oxide is
frequently present as a mud merely, and in this case can be removed by
filtration. It is rarely in solution in any other form than that of acid
carbonate, since sulphate or chloride could not exist in presence of
bicarbonate of lime. In this form, iron is precipitated at once by boiling or
on the addition of lime, like the temporary hardness due to other bases, in
the form of ferric hydrate, and more slowly by oxidation on exposure to the
air. The mud produced by softening waters which contain iron must be
completely removed by filtration, or subsidence, before the water is used

for leaching, or the iron will redissolve in the acids of the liquors. Iron is
not perceptibly injurious in the limes, but in the bates and wash-pits
sometimes causes stains, which are scarcely visible till blackened by the
tanning liquors. In presence of sulphur (from sulphide of sodium or the
decomposition of sulphates by the sulphur-bacteria nearly always present in
bates and soaks), the stains become bluish or greenish black, and a black
deposit is frequently produced on the sides of the pit, in which the threads
of sulphur-bacteria (Thiothrix) can often be recognised by the microscope.
As ferric salts not only combine with the tannins, but are themselves
tanning agents (see p. 198), they are rapidly absorbed by leather, and iron is
always present in leather ash. (For detection and estimation see L.I.L.B., p.
218.)
Alumina, except as clay, is rarely present in waters, and probably
harmless in any water likely to be used in tanning.
Soda is sometimes present in considerable amount, as sulphate, chloride,
or carbonate. The sulphate is probably inoperative. The chloride, if present
in material quantities, prevents plumping, and may be the cause of thin and
soft leather, and in large amounts will greatly impede the proper exhaustion
of many tanning materials. Sodium carbonate is sometimes present in
considerable quantities, as in some of the waters of the Leeds district. It
may coexist with temporary hardness, and produces similar injurious
effects. Waters in which it is present cannot have any real permanent
hardness. It may be neutralised by the very cautious addition of an acid; or
by admixture of a permanent-hard water. It tends to increased plumping in
the limes, but neutralises the free acids of the tan-liquors which are
necessary in sole-leather tanning.
Copper, lead, and other metallic bases are not likely to be present in any
waters used for tanning in quantities sufficient to be injurious.
Sulphuric acid rarely occurs free in water, and then only in such traces as
would be harmless for tanning, though possibly injurious to steam boilers.
As sulphates it is most common. Alkaline sulphates are not known to have
any deleterious action. The sulphates of lime and magnesia are the principal
cause of permanent hardness, q.v. Iron sulphate is sometimes found in
colliery waters.
Nitrates and nitrites in water are usually the result of “previous” sewage
contamination, and are only important as an indication of the possible
presence of the putrefactive ferments, and are of little moment in waters

only used for manufacturing purposes, while they seem to be even useful in
promoting the “working” of bran drenches, by supplying the nitrogen
required by the ferment.
Chlorine is seldom or never present in water in the free state, but only in
the form of chlorides, most frequently of sodium chloride (common salt),
the effect of which has been referred to above, and also at p. 88. The action
of other chlorides is probably similar as regards the swelling of hide.
Magnesium chloride is very objectionable as a constituent of boiler-waters,
as it liberates hydrochloric acid at high temperatures, and corrodes the
plates at the surface of the water. This injury can be prevented by addition
of soda.
Carbonic acid has been referred to under temporary hardness. Its
presence in the free state is a matter of some importance to the tanner (see
p. 99).
Silicic acid in a soluble form is present in some waters in considerable
quantity. Such waters are said to harden leather, but of this the writer has no
personal experience.
Few accurate researches have been made on the effect of the impurities
of water on tanning,[65] and though, from what has already been said, it will
be seen that they are not without effect it is probable that in many cases the
water is blamed for troubles which are simply the result of mismanagement,
and credited with virtues which are really due to careful and skilful
manufacture.
[65] See Nihoul, ‘Influence de l’eau sur l’extraction des matières tannantes,’ Bulletin de
la Bourse aux Cuirs de Liège, Sept. 1901.
The hardness of water, and the dissolved carbonic acid which it contains,
are, together with its temperature, the principal factors which determine
whether a hide will plump or fall in it. Almost the only accurate
investigation of this point has been made by W. Eitner.[66] He placed pieces
of hide, unhaired by sweating, and quite flat and fallen, in water for four
days at a temperature of 46° F. (8° C.), with the following results:—
1. In distilled water Scarcely at all plumped.
2. „ water saturated with CO2 Well plumped.
3. „ „ with lime bicarbonate, 20° German scale of hardness Tolerably plump.
4. „ „ „ magnesia bicarbonate, 20° do. „ „
5. „ „ „ lime sulphate 20° do. Well plumped.
6. „ „ „ magnesia sulphate, 20° do Best plumped.

7. „ „ „ magnesium chloride, 20° do. Not at all plumped.
8. „ „ „ common salt, 20° do. „ „
(1 German degree of hardness corresponds to 1 of CaO in 100,000.)

[66] Gerber, iii. (1877) p. 183.

The peculiarities which were shown by the hide pieces on removal from
the water were maintained throughout the tanning, which was conducted in
imitation of the German method, the hide being swollen and coloured
through in weak birch-bark liquors, made with distilled water and acidified
in each case with equal quantities of lactic acid, and finally laid away, till
tanned, in a mixture of oak bark and valonia. No. 6, from magnesium
sulphate, was the best; then No. 2; No. 3 was less good, but all the pieces
from 1 to 6 were firm, close and of good substance and texture, No. 1
having swelled well in the sour liquor. On the other hand, 7 and 8 scarcely
swelled in liquor, but remained flat throughout, and were looser, thinner and
of finer fibre. From this experiment it is clear that while sulphates and
carbonates exert a favourable influence on plumping, chlorides do the
reverse, as they themselves not only do not plump, but they place the hides
in an unfavourable condition for the plumping action of acids in the liquors.
These experiments are quite borne out by the writer’s experience in
practice. The water at the Lowlights Tannery, which in dry weather was
mostly obtained from beds of what was originally sea-sand, and which
consequently contained a very abnormal proportion of chlorides (up to 68
pts. NaCl per 100,000), required special and very careful management to
make thick leather, notwithstanding the fact that it contained a considerable
quantity of calcium and magnesium sulphates. These facts also indicate the
importance of the thorough removal of salt from hides intended for sole-
leather. Plumping is not a desirable thing in leather intended for dressing
purposes, and it is possible that the use of a small percentage of salt in the
liquors or wash waters might in some cases enable bating to be dispensed
with. Like a bate, salt would dissolve a small proportion of hide substance
(see p. 65). There is no practicable means of removing chlorides from
water, but Eitner suggests the addition of a small quantity of sulphuric acid
to water containing much temporary hardness (bicarbonates), in order to
convert it into permanent hardness (sulphates), which, as stated above,
plumps better. The amount required may be calculated from an acidimetric
determination of temporary hardness (see L.I.L.B., p. 19). A simple but not
very accurate guide, is to add enough acid to purple, but not to redden

litmus paper even after moving the latter about in the water for some
minutes. In practice the acid must of course be very thoroughly mixed with
the water by stirring and plunging. It must be borne in mind that Eitner’s
experiment was on sweated hides, and that with limed hide, which is kept
plump by the dissolved lime retained in the hide, different results as regards
carbonic acid and bicarbonates would be obtained. Both these would
convert the lime in the hide into chalk, which is insoluble and inert, and the
hide would fall, at any rate when the lime was completely carbonated, while
hides would remain plumpest in waters most free from substances capable
of neutralising lime. From this we may conclude, what may be a priori
expected, that the purer the water, the plumper limed hides remain in it. In
soft but peaty waters, hides fall rapidly, from the neutralisation of the lime
by the weak organic acids of the peat. Such waters are dangerous for
domestic use from their solvent action on lead, but this danger can be
entirely removed by storing the water in limestone reservoirs, or allowing it
to flow slowly through a limestone culvert before use. In some towns in the
north of England a small quantity of lime is added so as to neutralise the
water as it leaves the reservoir and before it enters the mains.
Wherever the conditions of putrefaction or decaying organic matter are
present, as in a bate, hides fall rapidly, and in extreme cases even the
presence of the stronger acids will not maintain plumpness. Eitner mentions
the case of a stream at Vissoko in Bosnia, which was in special repute
among the tanners from its power of pulling down hides rapidly, and which
took its rise in a common on which the pigs of the town were pastured. The
causes of this action are no doubt due to the products of putrefaction, but
are somewhat obscure. Bacteria present in water are a frequent source of
injury in the soaks, and probably in other stages of the tanning process.
Rain water and the water of streams in mountain districts of hard igneous
rock are generally nearly free from mineral constituents. This is the case
with the Glasgow water from Loch Katrine, and the Thirlmere water which
supplies Manchester. Such water, if cold enough, and free from mud and
organic impurity, is the best for almost every purpose in the tannery. Most
river-water contains material quantities of mineral matter, though it is
usually softer than that of springs or wells.
For further details as to the chemical examination of water, and the
methods of determining the amounts of its different constituents, see
L.I.L.B., pp. 18 et seq.

CHAPTER XI.
SOAKING AND SOFTENING OF HIDES AND SKINS.

As has been explained in the last chapter, hides and skins come into the
hands of the tanner either uncured (“green”), as they are taken off the
animal, preserved with salt or some other antiseptic, dried, or “drysalted” in
which both methods are combined. His object in each case is to remove
blood and dirt, and to restore the hide to its soft and natural condition; but
the treatment required varies much with the state of the hides.
Fresh hides merely require cleansing from blood and dirt. This is
necessary because the blood causes bad colour, and both blood, lymph and
adhering dung are sources of putrefaction, which ultimately attacks the
grain and fibrous structure of the hide. Hence washed hides keep better than
unwashed. Cold water is most desirable, as checking putrefaction. If the
water is much over 10° C., or if it is charged with organic matter and
ferment-germs; or if, as is too generally the case, the hides are in a partially
putrid state when received, the time of soaking must be reduced as much as
possible, and it may be necessary to sterilise the water with carbolic acid or
creolin (pp. 26, 28). In such cases the use of a wash-wheel, or tumbler, is
very desirable, rapidly cleansing the hides and removing adhering dung,
which interferes with the liming, and is a serious cause of damaged grain.
The American pattern of wash-wheel shown in Fig. 21 is very suitable for
the purpose. In no case is it desirable to allow green hides to lie for more
than a few hours in water; and unwise treatment at this time is the cause of
many troubles, which are only detected at later stages, and which are very
difficult to trace to their source. “Weak grain,” in which the hyaline layer
(p. 50) is destroyed, and which tans a whitish colour; “pricking,” or
perforation of the grain with small pinholes, which may go on to “pitting”
with larger holes, and a general weakening of fibre, with softening and
needless loss of weight, are among these results. An instructive instance
may be quoted. A large tanner found that his curried leather was affected
with small spots and rings of darker colour, which rendered it quite unfit for
staining, and which reappeared even when the leather was buffed. When

finished as black grain, these spots had a tendency to “spue,” or rise as little
pimples of resinous matter. Before the leather was stuffed no defect was
noticeable to the eye, but either then, or on stripping the grease by a solvent,
they could be seen under the microscope as lighter patches of open and
porous grain which absorbed more than their share of fat. During the
tanning process they could hardly be detected, but in the first colouring they
appeared for a few hours as blackish specks almost exactly like those
caused by particles of iron or iron-rust. By careful observation they were
traced back to the limes; specimens of the limed hide were submitted to
Director Eitner, who identified the defect as “Stippen,” caused by a species
of bacteria, which cannot subsist in limes, and which therefore must have
been in the soaks. These, which had been somewhat neglected from
pressure of work, were cleaned out and sterilised with creolin solution, and
the mischief ceased. It is worth noting that the tanner dated the beginning of
the trouble from the soaking of some “Spanish” horse-hides, which may
have introduced the infection. Several very similar cases have come under
the writer’s notice.
It is not absolutely necessary to soak fresh hides or skins at all before
liming, and where the water is scarce or unfavourable, or the skins tainted
or “slipping” hair, it is best to pass straight into a weak lime. In this case the
limes must be worked in shifts (see p. 131) and the whole of the oldest
liquor run away and the hides rapidly changed into a fresh lime, or the limes
will become so charged with organic matter and bacteria that the hides will
cease to plump, and may even putrefy.
Salted hides and skins require more soaking and more thorough washing
than fresh ones, as it is not only necessary to remove the salt, but to soften
and plump the fibre which has been dehydrated and contracted by salting. If
goods with salt in them are taken into limes, they will not plump properly,
[67] and creases and wrinkles (drawn grain) are formed which no after-
treatment will remove. This is especially important in sole leather. In
deciding on a method, we must bear in mind that salt is easily soluble, and
diffuses rapidly into water or weaker solutions, and that weak salt solutions
tend to prevent the plumping of the fibre, while those of about 10 per cent.
have considerable power of dissolving the cementing matter of the fibres (p.
65) and so lessening weight and firmness. It may also be noted that though
salt is not a true disinfectant (p. 22), salted hides are much less prone to

putrefaction than fresh ones, and therefore a longer soaking may be safely
given.
[67] Experiments mentioned on p. 89 throw some doubt on the power of salt to prevent
plumping in the limes, though the opinion in the text is generally held by tanners.
These conditions point to the desirability of free exposure to water,
attained by suspending, handling frequently, or tumbling, and repeated
changes to remove the salt. The degree of removal of salt is easily
determined by the estimation of Cl in the last wash-water (L.I.L.B., p. 18).
American tanners universally soak wet-salted hides three or four days with
as many changes of water, and frequently finish by a few minutes in a
wash-wheel. Any washing tumbler may be used; but the cheap and simple
construction of the American wash-wheel will be easily understood from
Fig. 21. The sides are open, so that hides can be put in or removed between
the spokes. The rim of the wheel is generally perforated, for the escape of
water which is supplied by a pipe passing through the axis; and the wheel is
often driven by a chain or rope round its circumference. No severe
mechanical treatment, such as “stocking,” is necessary or desirable for
green or salted hides.
Dry and dry-salted hides require much longer soaking than wet-salted,
the amount naturally depending on the thickness of the hide and the
character of drying. Even thin skins when strongly dried require
considerable time to soften and swell the fibres, although they soon become
wet-through and flexible. Many different methods of soaking have been
employed. Sometimes hides are suspended in running water; sometimes
laid in soaks which may be either renewed, or allowed to putrefy;
sometimes in water to which salt, borax or carbolic acid has been added, to
prevent putrefaction; and more recently weak solutions of caustic soda,
sulphide of sodium or sulphurous acid have been used with much success.

Fig. 21.—American Wash-wheel.

The first of these methods, were it desirable, is rarely possible in these
days of River Pollution Acts; of the others, it is difficult to say which is
better, since the treatment desirable varies with the hardness of the hide and
the temperature at which it has been dried. The great object is to thoroughly
soften the hide without allowing putrefaction to injure it. As dried hides are
often damaged already from this cause, either before drying, or from
becoming moist and heated on shipboard, it is frequently no easy matter to
accomplish this. The fresh hide, as has been seen, contains considerable
portions of albumin, and if the hide is dried at a high temperature, this may
become wholly or partially coagulated and insoluble. The gelatinous fibre
and the coriin (if indeed the latter exists ready formed in the fresh hide) do
not coagulate by heat, but also become less readily soluble. Gelatin dried at
130° C. can only be redissolved by acids, or water at 120° C. Eitner[68]
experimented with pieces of green calf-skin of equal thickness, which were
dried at different temperatures, with results given in the following table:—

Sam- Tempe- Remarks. Time of Remarks. Coriin
ple. rature Softening Dissolved
of in Water. by Salt
Drying. Solution.

I. 15° C. In vacuo 24 hours Without 1·68 per cent.
- mechanical -
II. 22° C. In sun 2 days work 1·62 „
In drying Twice
III. 35° C. 5 „ 0·15 „
closet worked
Refused to soften
IV. 60° C. „ - sufficiently - traces
for tanning

[68] Gerber, 1880, p. 112.

Hence it is evident that, for hides dried at low temperatures, short
soaking in fresh and cold water is sufficient, and, except in warm weather,
there would be little danger of putrefaction. With harder drying, longer time
is required, and more vigorous measures may be necessary. A well-known
tanner recommended a brine of 30°-35° barkometer (sp. gr. 1·035, or about
5 per cent. of NaCl). This has a double action, not only preserving from
putrefaction, but dissolving a portion of the hide-substance in the form of
coriin, which is undoubtedly a loss to the tanner, though it is questionable if
there is any process which will soften overdried hides without loss of
weight; since even prolonged soaking in cold water at a temperature which
is too low to allow of putrefaction taking place will dissolve a serious
amount of hide-substance. Chlorides, however, do not seem well adapted
for the purpose in view, from their weak antiseptic power and tendency to
prevent swelling. To prevent this Jackson Schulz advised the use of water at
80° F. for soaking during the winter months. Water containing a small
quantity (0·1 per cent.) of carbolic acid has been recommended for the
purpose, and will prevent putrefaction, while it has no solvent power on the
hide, but, on the contrary, tends to coagulate and render insoluble
albuminous matters. Borax has been proposed for the same purpose, and, in
1 per cent. solution, certainly prevents putrefaction, and has considerable
softening power, but is far too costly. Other methods of chemical softening
are described on p. 115.
For some descriptions of hides, and notably for India kips, putrid soaks
were formerly much employed, the putrefactive action softening and

rendering soluble the hardened tissue. In India the native tanners soften
their hides in very few hours by plunging them in putrid pools, into which
every description of tannery refuse is allowed to run. Putrefactive
processes, however, are always dangerous, as the action, through changes of
temperature, or variation in the previous state of the liquor, is apt to be
irregular, and either to attack one portion of the hide before another, or to
proceed faster than was expected. Hides are also frequently more or less
damaged by putrefaction and heating during the process of cure, and these
damages are accentuated in a putrid soak. Hence hides in the soaks require
constant and careful watching, and the goods must be withdrawn as soon as
they are thoroughly softened, for the putrefaction is constantly destroying as
well as softening the hides. It is possible that putrefactive softening is less
injurious to kips, and such goods as are intended for upper-leather, than to
those for sole purposes, as it is generally considered necessary in the former
case that a good deal of the albumen and interfibrillary matter be removed,
and that the fibre be well divided into its constituent fibrils for the sake of
softness and pliability; and thus the putrid soak, if acting rightly,
accomplishes part of the work which would afterwards have to be done by
the lime and the bate, as the actual fibre of the hide seems less readily
putrescible than the softer cementing substance.
Putrefaction is caused, as we have seen, by a great variety of living
organisms, each of which has its own special products and modes of action.
It is quite possible that, if we knew what precise form of putrefaction was
most advantageous, we might by appropriate conditions be able to
encourage it, to the exclusion of others, and obtain better results than at
present. Putrid soaks (in the old sense) are, however, disused in the present
day by all enlightened tanners, as it is recognised that the risks outbalance
the advantages, and when drysalted hides are worked, the soluble salts of
the cure accumulate to an injurious extent. The modern method, where no
chemicals are used, is to give one fresh water at least to each pack of hides
or skins. Even in this case considerable putrefaction takes place where the
soaking occupies 7 to 14 days, as is the case with kips and hides, and it is
probable that the use of chemical and antiseptic methods of soaking will
ultimately be generally adopted, both on technical and sanitary grounds.
The use of dilute acids for softening has much to recommend it, their
power of causing the fibre to swell and absorb water being quite equal to
that of the alkalies, while few, if any, putrefactive bacteria can thrive in an

acid liquid. Very dilute sulphuric acid has been used with success to
dissolve the alkaline “plaster” of East India kips (p. 39). It has considerable
disinfectant power (p. 23), but its action on the hide-fibre is undesirably
strong.
Sulphurous acid is much more suitable. Its use for this purpose was
patented by Maynard, along with a number of other possible uses, but the
patent has now lapsed, and he does not seem to have succeeded in
introducing it into practice. Experiments at the Yorkshire College, and also
at a tannery on a manufacturing scale, have shown that the method is
capable of excellent results. The hides are soaked for 24-48 hours in a
solution of sulphurous acid containing about 2 per cent. of SO2 (for
manufacture, compare p. 24; for testing, L.I.L.B., pp. 16, 37), and are then
transferred to water, where they swell freely to their full thickness. They
may be either limed at once, or first neutralised with dilute caustic soda,
ammonia, or sulphide of sodium, which, for dressing leather, is perhaps
desirable. No putrefaction takes place, even if they are retained for a
considerable time in water, and the acid has little or no solvent effect on the
hide-fibre, the strength of which is well preserved. The liming, however,
must either be conducted with the aid of sodium sulphide or in old limes,
since the sterile condition of the hides renders liming in fresh lime very
slow (cp. p. 137). For experimental purposes a 1⁄2 per cent. solution of
Boakes’ “metabisulphite of soda” may be used, to which 1⁄4 per cent. of
concentrated sulphuric acid previously diluted with water is gradually
added during the soaking, the hides being first withdrawn. For permanent
work it will be found much cheaper to manufacture the acid on the spot by
burning sulphur.
The use of solutions of caustic soda (1 part per 1000), or of sodium
sulphide (11⁄2-3 parts per 1000) as suggested by Eitner, seems at present
likely to supersede all other methods of softening from their simplicity and
safety. Twenty-four to forty-eight hours in either of these solutions, which
may if necessary be followed by a short soak in plain water, seem sufficient
to soften either kips or hides. Experiments at the Yorkshire College have
shown that solutions of this strength have little or no solvent action on the
hide-fibre, but promote its swelling in water so effectively that no
mechanical softening is needed (though a slight drumming is
advantageous), while putrefaction is almost entirely prevented, so that the
solution may be repeatedly used if kept up to its original strength, which is

easily determined with standard acid and phenolphthalein (see L.I.L.B., p.
17). Neither caustic soda nor sodium sulphide have any injurious effect on
liming, though it may prove somewhat slower than with the older methods,
where the epidermis was partially destroyed by the action of putrid
ferments. The dilute solutions used are not only less injurious to the hide
than those of greater strength, but they are also more effective in softening.
Eitner (Gerber, 1899, p. 584) states that when using a solution of caustic
soda of 1 part in 1000 strength, the time required to soften some hides was
only two days, as against three days for a sodium sulphide liquor, and four
days for pure water, and that with the soda solution only about 0·6 per cent
of the hide-substance of the skin was dissolved out, whilst when sodium
sulphide was used it was 0·7 per cent., and with pure water alone no less
than 1·9 per cent. was lost by solution.
The use of moderately warm water (40° C.) in a drum is quite successful
in rapidly softening sound hides after they have previously been soaked for
some days in cold water; but if they are tainted in the cure, it is very apt to
intensify the mischief. Hides which have partially putrefied internally, or
which have been exposed to a hot sun while the interior is still moist, are
very apt to appear sound while dry, but to blister or go to pieces from the
destruction of the fibres as soon as they are limed, and this in spite of even
the most careful treatment. For tainted hides, caustic soda is probably
preferable to sodium sulphide.
Many chemicals have been patented for softening hides. Sulphide of
arsenic is said to be in use, and if dissolved in caustic soda solution would
differ little in its effect from ordinary sulphide of sodium. Saltpetre has also
been employed, but its effect, if any, was probably merely antiseptic.
Ordinary sodium carbonate has been used, but is less effective than caustic
soda. Gas liquor and mixtures of this with tar and water were patented by
Barron, and probably the first would soften by virtue of its ammonia and
sulphides, while tar contains carbolic acid. Probably the most absurd
mixture of all was patented by Berry, which consisted of 1⁄2 bucket of slaked
lime, 1⁄2 bucket of wood-ashes, 12 lbs. of potash, 5 lbs. of oil of vitriol, and
4 lbs. of spirit of salt!

Fig. 22.—Faller Stocks.

Beside merely soaking the hides, it is sometimes necessary to work them
mechanically, to promote their softening; this was formerly accomplished
by “breaking over” the hides on the beam with a blunt knife. This process is
still in use for skins of many sorts, but for the heavier classes of leather is
now usually superseded or supplemented by the use of “stocks,” or drums.
The former consist of a wooden or metallic box, of peculiar shape, wherein
work two very heavy hammers, raised alternately by pins or cams on a
wheel, and let fall upon the hides, which they force up against the curved
end of the box with a sort of kneading action. The ordinary form of this
machine is shown in Fig. 22. A more modern form, which seems to possess
some advantages, is the American “double-shover,” or “hide-mill,” seen in
Fig. 23. “Crank stocks,” similar in form to the faller stocks, but driven by
cranks, are sometimes used for softening, but are better adapted to lighter
uses.

Fig. 23.—American Hide-Mill.

The number of hides which can be stocked at once naturally varies with
the size of both hides and stocks, but should be such that the hides work
regularly and steadily over and over. The whole number should not be put
in at once, but should be added one after another, as they get into regular
work. The duration of stocking is 10-30 min., according to the condition
and character of the hides. Hides should not be stocked until they are so far
softened that they can be doubled sharply, without breaking or straining the
fibre. After stocking, they must be soaked again for a short time, and then
be brought into an old lime. A small quantity of sodium sulphide added to
the soaks or in the stocks has been recommended as of great value in
softening obstinate hides, and probably with justice, from its well-known
softening action upon cellular and horny tissues.
Tumbler drums of various forms may also be used with good effect for
softening purposes, especially for skins, and are much less detrimental than
stocking, both as regards the weight and quality of the goods.

For sole leather, and even for kips, the use of stocks has in recent years
been entirely discarded by many of the more advanced tanners. If
mechanical work is required at all, the drum is preferred, and is sometimes
employed after a few days’ liming, the goods being first merely softened in
fresh water. The use of caustic soda, sodium sulphide, or sulphurous acid
renders mechanical softening almost unnecessary.

Fig. 24.—Drum for Washing or Tanning.

The drums employed are in principle like a barrel-churn, and are large
cylindrical wooden chambers 6 to 12 feet in diameter, and fitted inside
either with shelves like the floats of a water-wheel, or with rounded pegs on
which the hides fall. The American wash-wheel figured on p. 111 is a
machine of this kind, and one of a more elaborate description is shown in
Fig. 24. Drums are not only used for softening, but for tanning, dyeing, and
many other purposes in leather manufacture. It is advantageous to be able to
reverse the direction of their rotation to prevent the rolling up of the hides.

CHAPTER XII.
DEPILATION.

After the softening and cleansing of the hide or skin is completed, and
before proceeding to tan it, it is usually necessary to remove the hair or
wool. The earliest method of accomplishing this was by means of incipient
putrefaction, which attacks in the first instance the soft mucous matter of
the epidermis, and thus loosens the hair without materially injuring the true
skin. This loosening of the hair often takes place accidentally in hides
which have been kept too long without salting, and is known as “slipping,”
and is apt to be accompanied by some degree of injury to the grain. The old
method of loosening the hair by putrefaction, or, as it is generally called,
“sweating,” was to lay the hides in piles, usually in some warm and damp
place. Occasionally a slight preliminary salting was given to prevent too
much putrefaction of the hide. The action in this case, however, was very
irregular, and it has been quite abandoned in all civilised countries.

Fig. 25.—Sweat-Pit.

The method which is now used is to hang the hides in a closed chamber,
generally called a “sweat-pit,” Fig. 25, but usually constructed above the

ground-level and protected from sudden changes of temperature by double
walls, or by mounds of earth. The hides are hung in the sweat-pit, in small
chambers each capable of holding 50 or 100 hides. The temperature is kept
at about 15° to 20° C., the air being warmed, if necessary, by the admission
of steam below a perforated floor, or cooled by a shower of water from
sprinklers, so arranged as not to play directly on the skins, and is thus
always kept saturated with moisture. Little if any ventilation is allowed, and
a large quantity of ammonia is given off from the decomposition of the
organic matter, and no doubt contributes to the solution of the epidermis
and the loosening of the hair, as the writer has found that ammoniacal
vapours alone very speedily produce this effect.
After 4-6 days of this treatment, the hair is sufficiently loosened to be
removed by working the skin over the beam with a blunt knife, or by means
of the stocks or hide-mill (see p. 116). Great care and watchfulness are
required to avoid injury to the grain by putrefaction.
The hide is in a slimy and completely flaccid and “fallen” condition, and
some trouble is occasioned by the hair being worked into the flesh by the
hide-mill, to obviate which, a slight liming is frequently given after the
sweating. Hides which have been unhaired in this way require to be swollen
by acid in the liquors in order to produce a satisfactory sole-leather, as the
sweating process does not swell or split up the fibres.
In some European tanneries a similar process, but at a higher
temperature, is employed, and it is also largely used for sheep-skins under
the name of “staling,” but in this case is sometimes conducted in a very
rude and primitive manner, and frequently with the result of considerable
injury to the pelt.
The great objection to the sweating process, however carefully
conducted, is the liability of putrefaction to attack the skin itself, causing
“weak grain.” Its most advantageous use is for sole leather, as, although the
solution of the hide-substance may not be very much less than in the case of
liming, the dissolved matter remains in the hide instead of being washed
out, and being fixed by the tannin, contributes to the solidity of the leather.
In England, lime is the agent almost universally employed for unhairing,
though every tanner admits its deficiencies and disadvantages. It is hard,
however, to recommend a substitute which is free from the same or greater
evils, and lime has one or two valuable qualities which will make it very
difficult to supersede. One of these is that, though it inevitably causes loss

of substance and weight, it is also impossible, with any reasonable care,
totally to destroy a pack of hides by its use; which is by no means the case
with some of its rivals. Another advantage is that, owing to the very limited
solubility of lime in water, it is of comparatively small consequence
whether much or little is used; and even if the hides are left in a few days
longer than necessary, the mischief, though certain, is only to be detected by
careful and accurate observation. With all other methods, exact time and
quantity are of primary importance, and it is not easy to get ordinary
workmen to pay the necessary attention to such details. Again, the qualities
of lime, its virtues and failings, have been matter of experience for
hundreds of years, and so far as such experience can teach, we know
exactly how to deal with it. A new method, on the other hand, brings new
and unlooked-for difficulties, and often requires changes in other parts of
the process, as well as in the mere unhairing, to make it successful. As our
knowledge of the chemical and physical changes involved becomes greater,
we may look to overcoming these obstacles more readily.
The universal source of lime is chalk or limestone, which consists of
calcium carbonate, and from which the carbon dioxide is driven off by
burning in a kiln. Many limestones, however, are far from being pure
calcium carbonate, but contain large proportions of magnesia, iron and
alumina, the latter perhaps originally deposited in the form of clay with the
sediment from which the stone was formed. Such clay limestones when
burnt yield natural cements, like oolite and other “hydraulic” limes, which
are capable of setting even under water. The presence of magnesia and clay
is injurious not only by diminishing the amount of lime present, but by
making the lime much more difficult to slake; and iron oxide, though quite
insoluble, may become mechanically fixed in the grain of the hide, and may
be the cause of subsequent stains. The burning of lime in the kiln is
probably not quite so simple an operation as the equations of the text-books
would suggest. By mere heating, the carbonate can, it is true, be
decomposed, but to do this completely a good white heat is required, which
is rarely attained in practical burning, and it is probable that at least a part
of the carbon dioxide present is reduced to carbon monoxide by the
combustible fuel-gases, and so separated from the lime, for which it has no
affinity. Carbon monoxide is the cause of the intensely poisonous character
of limekiln gases, the pure dioxide being irrespirable, but not strictly
poisonous.

Quicklime, CaO, on coming in contact with water, combines with it with
the evolution of considerable heat, becoming slaked or converted into
hydrate, Ca(OH)2. This change takes place rapidly and easily when the lime
is light and porous, such as is obtained by the burning of chalk or good
limestone at a low temperature; but if it has been too intensely heated or
“over-burnt,” or contains silicates or other salts which fuse at the
temperature of the kiln, a compact lime is formed which slakes with
difficulty and extreme slowness, thus being lost to the tanner, or leading to
the still more serious result of burning holes in the hides by the heat
produced by slaking in contact with them. It is stated by Le Chatelier[69]
that for dense limes 24-48 hours is frequently required for complete slaking
in the cold, while magnesia is still more obstinate, months being sometimes
necessary for the complete hydration of hard-burnt samples; and mixtures
of lime and magnesia are intermediate in their character. Slaking is greatly
assisted by heat, even heavily burnt magnesia being hydrated in about six
hours at 100° C. Slaking is also much more rapid in a dilute solution (2 per
cent.) of calcium or magnesium chloride. From these facts it is easy to
deduce the reason why a suitable quantity of water, neither too much nor
too little, is desirable for the rapid and effectual slaking of lime. If too little
is used, the lime is only partially slaked, and it is not easy for further
portions of water to gain access to the interior of the powdery mass. On the
other hand, if it is “drowned” by excess, the temperature is lowered, the
process goes on slowly, and the mass does not readily fall into powder, and
so fails to be utilised in the liming process. Of all methods of slaking lime,
the ordinary one of tipping it direct into the lime-pits is perhaps the most
irrational, leading to the formation of unslaked lumps which may burn the
hides, and which, together with stones and dirt, rapidly choke the pits with
useless matter. The best process is that adopted by builders and in many
Continental yards, in which a large quantity of lime is slaked in a shallow
tank by throwing on it sufficient water to thoroughly wet it, and after
allowing it to heat and fall for 24 hours, adding enough water to convert it
into a stiff paste. In this form it may be kept for months without material
deterioration. When required for use, a suitable quantity of the paste is dug
out, and well stirred with water in a tub or tank before running into the pit
when the stones and sand remain in the tank. In this way all nuisance from
dust is also avoided. If lime is stored unslaked, it gradually absorbs
moisture from the air, falling, and soon becoming dusty and difficult to

slake completely, while the traces of carbon dioxide in the air gradually
convert it into useless carbonate.
[69] Bull. de la Soc. d’Encouragement, 1895, x. pp. 52-62; Journ. Soc. Chem. Ind., 1895,
p. 575.
The solubility of lime in water is very limited, and the figures determined
by different chemists do not agree very satisfactorily. The following table
gives the result of determinations made by Mr. A. Guthrie in the Author’s
laboratory, and is probably one of the most accurate[70]:—
100 c.c. of saturated lime water at 5° C. contain 0·1350 grm. of CaO.
„ „ 10° „ 0·1342 „
„ „ 15° „ 0·1320 „
„ „ 20° „ 0·1293 „
„ „ 25° „ 0·1254 „
„ „ 30° „ 0·1219 „
„ „ 35° „ 0·1161 „
„ „ 40° „ 0·1119 „
„ „ 50° „ 0·0981 „
„ „ 60° „ 0·0879 „
„ „ 70° „ 0·0781 „
„ „ 80° „ 0·0740 „
„ „ 90° „ 0·0696 „
„ „ 100° „ 0·0597 „
[70] Journ. Soc. Chem. Ind., 1901, p. 224.

It will be noticed that unlike that of most substances, the solubility of
lime in water diminishes as the temperature is raised. It is therefore
necessary in employing lime-water as a standard solution to take care that it
is saturated at a constant temperature. The results given in the above table
are those from pure marble lime. Where the ordinary impure limes from
limestone are employed, a somewhat stronger lime-water is often obtained.
This is difficult to explain, but possibly some double hydrate of lime and
magnesia is formed which is more soluble than either hydrate alone. The
results harmonise with the old belief of tanners that chalk-lime is milder in
its action on skin than that made from less pure limestones. The solubility
of any given lime is easily determined by adding it in excess to water in a
stoppered flask, and shaking frequently until a solution of constant strength
is obtained. A known volume of this solution (which must be clear and free
from undissolved lime) is then titrated with N⁄10 hydrochloric acid, using
phenolphthalein as the indicator.

Saturated lime-water may be conveniently used as an alkaline standard
solution for many purposes, and if kept on excess of lime is always caustic,
and varies very little in strength at ordinary laboratory temperatures. The
solution is nearly 1⁄20 normal, but for accurate work its strength should be
exactly determined with N⁄10 acid. 1 liter of pure lime-water at 15° C. should
require 471·4 c.c. of N⁄10 acid for neutralisation.
Lime is much more soluble in sugar solutions than in water. Such
solutions have been used as standard solutions, and sugar has been added to
limes to increase the action on the hides.
The following is the analysis of a lime used in a Leeds tannery, which
was made by Mr. G. W. Flower, B.Sc., in the Leather Industries Laboratory
of the Yorkshire College[71]:—
Per cent.
SiO2 and insoluble matter 17·70
Fe2O3 6·42
CaO 49·86
CaCO3 14·21
CaSO4 3·01
CaCl2 0·33
MgO 2·09
Organic matter 0·80
Moisture by difference 5·58
100·00
[71] Journ. Soc. Chem. Ind., 1901, p. 224.

The sample only contained 31·02 per cent. of available lime, the
remainder being probably combined with the silica. It also contained an
appreciable quantity of iron oxide, which might lodge mechanically in the
pores of the skin and become dissolved in later processes, darkening the
colour of the leather. The lime was also under-burnt, judging from the
amount of carbonate it contained.
For comparison with this, the analysis of a good specimen of
carboniferous-limestone lime from Buxton may be given:—
Per cent.
CaO 91·95
MgO 1·30
CO2 and moisture 6·75
100·00

Determination of “Available” Lime.—The practical value of lime for the
tanner is easily determined by drawing a sample by breaking off small
pieces from a number of lumps of the bulk, coarsely pulverising them in a
mortar, and then rapidly grinding a portion as fine as possible, and
transferring it at once to a stoppered bottle for weighing. A portion of this,
not exceeding 1 grm., is shaken into a stoppered liter flask, which is filled
up roughly to the mark with hot and well-boiled distilled water, and allowed
to stand for some hours with occasional shaking. When cold it is filled
exactly to the mark with cold distilled water, well shaken again and allowed
to settle, or rapidly filtered, and 25 or 50 c.c. of the clear liquid withdrawn
with a pipette and titrated with N⁄10 hydrochloric or sulphuric acid and
phenolphthalein. Each cubic centimeter of N⁄10 acid equals ·0028 grm. CaO.
It is generally a very mistaken economy to make use of an inferior lime for
tanning purposes, as any saving in cost is discounted by the larger quantity
required, the more frequent cleaning of the pits, and the danger of stains and
of burns from imperfect slaking.
The action of lime on the hide has already been spoken of to some extent.
It is throughout a solvent one. The hardened cells of the epidermis swell up
and soften, the mucous or growing layer and the hair-sheaths are loosened
and dissolved, so that, on scraping with a blunt knife, both come away more
or less completely with the hair (constituting “scud” or “scurf,” Ger. Gneist
or Grund). The hair itself is very slightly altered, except at its soft and
growing root-bulb, but the true skin is vigorously acted on. The fibres swell
and absorb water, so that the hides become plump and swollen, and, at the
same time, the “cement-substance” of the fibres is dissolved, and they
become split-up into finer fibrils: the fibrils themselves become first
swollen and transparent, and finally corroded, and even dissolved. A similar
swelling of the fibres is produced by both alkalies and acids, and is
probably due to weak combinations formed with the fibre-substance, which
have greater affinities for water than the unaltered hide.[72] This swelling is
useful to the tanner, since it renders the hide easier to “flesh” (i.e. to free
from the adhering flesh) on account of the greater firmness which it gives to
the true skin. It also assists the tanning, by splitting up the fibre into its
individual fibrils, and so exposing a greater surface to the action of the
liquors. This is advantageous in dressing-leather which is afterwards tanned
in sweet liquors, and which must have the cement-substance of the fibres
dissolved and removed for the sake of flexibility; and, in the case of sole-

leather, it is necessary for sake of weight and firmness that the hide be
plumped at some stage of the process; but it is probable that this effect is
produced with less loss of substance and solidity by suitable acidity of the
tanning-liquors. Another advantage of lime is that it acts on the fat of the
hide, converting it more or less completely into an insoluble soap,[73] and so
hindering its injurious effects on the after tanning process, and on the
finished leather. If strong acids whether mineral or organic are used later on,
this lime soap is decomposed, and the grease is again set free. In sweated or
very low-limed hides this grease is a formidable evil, causing darkening or
grease spots on the finished leather.
[72] Cp. p. 84.
[73] This has been questioned, but I have satisfied myself it is correct.

The customary method of liming is simply to lay the hides horizontally
one at once in milk of lime in large pits, taking care that each hide is
completely immersed before the next is put into the pit, so as to ensure a
sufficiency of liquor between them. Every day, or even twice a day, the
hides are drawn out (“hauled”), and the pit is well plunged up, to distribute
the undissolved lime through the liquor. The hides are then drawn in again
(“set”), care being taken that they are fully spread out. How much lime is
required is doubtful, but owing to its limited solubility, an excess, if well
slaked, is rather wasteful than injurious. Great differences exist in the
quantity of the lime used, the time given, and the method of working, not
only for various classes of leather, but for the same kinds in different yards.
Lime, as we have seen, is only soluble to the extent of about 1·25 grm. per
litre, or (as 1 cub. foot of water weighs about 1000 oz.) say 11⁄4 oz. per cub.
foot, or, in an ordinary lime-pit, not more than 1⁄4 lb. per hide. Only the lime
in solution acts on the hide, but it is necessary to provide a surplus of solid
lime which dissolves as that in the liquor is consumed or absorbed by the
hide; and this is especially the case where, as is generally customary, the
hides are laid flat in pits, so that no circulation of liquor is possible. Where
hides are suspended in lime-water, which is constantly circulated and kept
up to its full strength by agitation in another vessel with solid lime, they
unhair as quickly as with milk of lime, but the method seems, in the case of
lime, to present no special advantage over the ordinary one, if in the latter
the hides are hauled sufficiently often to keep the lime uniformly
distributed. The case is otherwise in dealing with more soluble depilatories.
Various patents have been taken for methods of liming by suspending in

liquors, but the idea is now public property, and is largely used on the
Continent. It is necessary that the lime which settles to the bottom of the pit
should be agitated and kept in suspension, which may be effected either by
moving the hides on a frame as in “suspenders” (p. 221), or by agitators
acting on the principle of pumps, and raising the liquor and sludge from the
bottom. Such agitators have been patented in Germany, but had been in use
much earlier in the Author’s tanyard. An agitator on the principle of the
screw-propeller of a steamship, placed near the bottom of the pit, and
protected by a lattice, may also be usefully employed (Fig. 26). Skins are
frequently limed in paddles, or stirred up by blowing air into the pit. The
latter method is neither effective nor economical in power.

Fig. 26.—Suspension Lime-Pit.

As has been noted, the solubility of lime, and consequently the strength
of the lime-liquor, is diminished by rise of temperature, but its solvent
action on hide-substance is much increased. As a consequence, the
loosening of the hair proceeds much more rapidly in warm limes, but the
hides do not plump well, and become loose, hollow and inclined to “pipe”
in the grain, and to weigh out badly, and for sole leather the method is
therefore in every way disastrous. In the few cases among the lighter
leathers where a decided softening and loosening of the texture of the skin
is required, it is possible that useful advantage may be taken of this effect;
but it would be exceedingly difficult to regulate the temperature of an
ordinary lime-pit with accuracy, and better results could probably be
obtained with suspenders in which the liquor could be constantly circulated.
When limes are very cold, in spite of the greater strength of solution, the
action is very much checked, and where goods are frozen into pits in severe
weather, there is but little danger of overliming, although the usual time
may be much exceeded. It is generally best to work limes at about the
ordinary summer temperature, and this is better done in winter by warming
the limeyard than by any direct heating of the limes. If lime which has
cooled after slaking is used, the water with which limes are made may
safely be warmed in midwinter to a temperature not exceeding 20° C.
The quantity of lime used by different tanners, and for different sorts of
hides and skins, is very variable, not only according to the effect which it is
desired to produce, and the way in which it is used, but from the arbitrary
fancy of the user, since its limited solubility renders an excess
comparatively innocuous. For sole-leather, the amount recommended varies
from under 1 per cent. to 10 or 12 per cent. on the green weight of the hide;
but probably 2-3 per cent. is all that can be really utilised, the remainder
being wasted. In order, however, to utilise the whole of the lime, very
frequent handling or agitation is required to ensure its uniform distribution.
It must also be borne in mind that the strength of commercial limes varies
from above 80 down to 30 per cent. of available calcium oxide.
Von Schroeder has found that a strength of 6 grams of calcium oxide
(CaO) per liter was sufficient, but, in practice, much more is generally
added. It is also noteworthy that a perfectly fresh milk of lime must be
made much stronger than one which has been used. This is partially due to

the fact that some bacterial action takes place in an old lime and that
ammonia is formed which assists unhairing, in addition to the effect of the
lime itself, and partially because the lime in old liquors remains in
suspension for a much longer time, and is thus more evenly distributed.
A method of liming, sometimes known as the “Buffalo method,” has
been largely adopted for sole-leather in America, and is now used in many
Continental yards. It consists in a very short liming and the subsequent use
of warm water. The limes are also often sharpened by the addition of a little
sodium sulphide or of some other sulphide. Thus, in one large yard in the
States, the hides for sole-leather (salted “packers”) are limed for 10 hours
only with 2 lb. lime and 21⁄2 oz. of sulphide of sodium per side, and after
lying overnight in water of a temperature of 35-45° C., are easily unhaired.
A Continental firm lime two days in weak fresh limes with a little tank-
waste, and then treat with water at 32° C. for 6-8 hours, when the hides are
unhaired and returned to the warm water for two hours before scudding. All
sorts of combinations between liming and hot water treatment can be
employed. The longer and stronger the liming, the lower temperature or
shorter time in the water will suffice. The method is much to be
recommended for firm sole-leather, but it does not saponify grease or swell
the fibres thoroughly, and usually vitriol is used for the latter purpose in a
later stage. The hide goes into the liquors practically free of lime, and the
loss of hide-substance is much less than in the ordinary method of liming.
A point of probably much greater importance than the quantity of lime
used is the length of time during which a lime is worked without change of
liquor. An old lime becomes charged with ammonia and other products of
the action of lime upon the skin, such as tyrosin, leucin (amidocaproic
acid), and some caproic acid, the disagreeable goaty odour of which is very
obvious on acidifying an old lime-liquor with sulphuric acid, by which
considerable quantities of partially altered gelatin are at the same time
precipitated (compare p. 64). Lime has considerable antiseptic power, and a
new lime is practically sterile, but very old limes, especially in hot weather,
often contain large numbers of active bacteria, which may be seen in the
microscope under a good 1⁄6-inch objective. Their presence is always an
indication that putrefaction is going forward, and if their number be very
excessive, the leather out of such limes will generally prove loose, hollow
and dull-grained, and in extreme cases hides may be totally destroyed.
Spherical concretions of calcium carbonate may also be seen under the

microscope, resembling on a smaller scale those found in Permian
limestone, and caused perhaps in both cases by crystallisation from a liquid
containing much organic matter. It is hardly probable that in many tanneries
the ammonia would pay for recovery from the lime-liquors, though it could
be easily done by steaming the old limes in suitable vessels, and condensing
the ammoniacal vapours in dilute sulphuric acid. Its quantity rarely exceeds
0·1 per cent. of NH3. For methods of estimation of ammonia, see L.I.L.B.,
p. 30.
Up to a certain point, it is found that old limes unhair much more readily,
and have a greater softening effect than new ones, which is often
advantageous for dressing goods; though for sole leather, where weight and
firmness are of primary importance, the use of stale limes must be kept
within the narrowest limits. In the finer leathers also, such as kid and
moroccos and coloured calf, where a sound and glossy grain is desired, the
effects mentioned are generally better obtained in other ways, such as by
the use of sulphides. On East India kips and other dried hides, which are
difficult to soften, and which have great power of resistance to the action of
lime, old limes are distinctly useful, but, even there, there are limits which
should not be passed. Probably no lime ought to be allowed to go for more
than three months at the outside limit without at least a partial change of
liquor, and the system of allowing all the limes in a yard to run for twelve
months, and then cleaning them all together, is almost the worst which can
be planned. A very much better way is to clean the limes in regular rotation,
using, if desired, a portion of the old liquor in making the new lime, so as to
avoid a too sudden transition. The old liquor is valuable, if at all, for the
ammonia and organic matter which it contains, as the amount of lime in
solution is not worth considering. The ammonia considerably increases the
solvent and unhairing power, while swelling the hide less than an equivalent
amount of lime. In some cases it may be desirable to add ammonia
artificially for this purpose. In this case it will be cheaper and more
convenient to add it in the form of ammonium sulphate than as liquid
ammonia. If it be desired to retain ammonia, the lime should be kept
covered. Very old limes containing excess of ammonia and lime, sometimes
in hot weather cause a transparent swelling of the goods, with destruction of
the fibrous texture.[74] The writer has observed a similar phenomenon in
very weak and old limes strengthened with sulphides, in which hide was left
experimentally for several weeks. The principal effect of the dissolved

animal matter is to enable bacteria to thrive in it, which they will not do in a
fresh lime, but putrid limes probably also contain liquefying ferments
produced by the bacteria present (p. 17), and which dissolve hide. Eitner
has published researches on the amount of hide-substance dissolved by
limes,[75] in which he shows that the loss of substance in liming sufficiently
to unhair is materially greater in old limes than in fresh ones, although
during the first two days of liming the new limes are decidedly the most
active. As he remarks, this justifies the wisdom of the method, now largely
adopted, of working limes in shifts, and beginning the operation in old
limes and completing it in fresh ones. (See also p. 131.)
[74] Gerber, 1884, pp. 150, 184.
[75] Gerber, 1895, pp. 157-9, 169-72.

For details of the analytical methods employed, Eitner’s original paper
must be consulted, but the annexed table (see next page) summarises his
results. The letters heading the columns have the following meanings.
A. Hide substance precipitated by neutralisation of the lime with carbonic
acid.
B. A further precipitate obtained by slight acidification with hydrochloric
acid.
C. Soluble peptones precipitated by hypochlorous acid or mercuric
nitrate.
It is obvious that none of these figures represent the total dissolved
organic matter, and it is to be regretted that this was not determined. It is,
however, fairly safe to assume that the table correctly represents the relative
solubility in the different liquors. In each case 2 liters of liquor were used
for each kilo of green hide. When old liquors were employed, the hide-
substance they originally contained was determined, and deducted from the
final result.
Hide Description of Days Hide-substances in Loss
Used. Lime Liquor. Liming. Grams per Liter. per
cent.
on
A. B. C. Total. Dry
Pelt.

1 Oxhide Fresh lime 30 grm. per liter - 6[76] 1·068 0·324 2·370 3·762 2·35

2 „ Ditto 9 2·764 0·540 3·624 6·928 4·14

3 „ Fresh lime 30 grm., 1⁄2 grm.
- 5[76] 0·852 0·172 1·816 2·840 1·75
sulphide of sodium per liter

4 „ Ditto 8 1·240 0·514 3·846 5·600 3·36

5 „ 5 weeks old lime, through which
- 2 0·180 0·212 0·988 1·380 0·87
four packs had passed

6 „ Ditto 5[76] 0·868 1·318 3·356 5·542 3·46

7 „ 5 months old lime, with sodium
- 2 0·196 0·188 0·864 1·248 0·77
sulphide

8 „ Ditto 5[76] 0·928 1·198 3·004 5·130 3·06

9 Cowhide Fresh lime as above - 5[76] 1·982 0·413 4·501 6·896 4·30

10 „ Ditto 8 3·132 0·672 5·741 9·545 5·94

11 „ Fresh lime as above, and 1⁄2 grm.
- 5[76] 1·012 0·403 2·315 4·730 2·96
sodium sulphide per liter

12 „ Ditto 8 2·521 0·653 5·026 8·200 4·87

13 „ Old disused lime - 5 0·344 0·291 2·341 2·976 1·84

14 „ Ditto 8[76] 2·119 1·697 6·952 10·768 6·45

15 „ Used sulphide of sodium lime 4
- 5[76] .. 1·600 1·047 2·527 1·58
weeks old

16 „ Ditto 8 0·791 0·519 4·592 5·892 3·43

[76] Hides unhaired.

Taking into account the liming necessary for unhairing only, as shown in
the table, it will be noted that the percentage of loss is invariably greater in
old limes than in new ones, and less in limes sharpened with sulphide of
sodium than where lime alone is used. The only exception to this rule is in
No. 15, where a sulphide lime 4 weeks old shows the least loss of any in the
time required for unhairing; and indeed sulphide limes if kept strengthened
with the requisite addition of sulphide, seem to deteriorate very slowly, No.

8, with a lime 5 months old, showing a result which may still be considered
good. Another point especially noted by Eitner is the slight action of old
limes during the first stages of liming, as compared with their rapid solvent
effect as the hair becomes loosened. The loss in any case does not appear to
be so great as the advocates of other unhairing processes have often
claimed. If we assume that all the dissolved hide-substance might have
made leather, the worst loss on oxhide only limed to the point of unhairing
amounts to less than 31⁄2 per cent. on the possible total; and it must be
remembered that at least a part of this consists of dissolved epidermis
matter, which could not by any possible method have been converted into
leather. It will be noted in Nos. 2, 4, 10, 12 and 16, what considerable losses
are produced by plumping limes after unhairing, but it must be borne in
mind that, in the case of dressing-leather, solution of at least a part of the
cementing matter is essential to produce the necessary softness and
flexibility. Eitner calculates the dry pelt-weight from that of the green hide
on the assumption, based on experiment, that 100 parts of the original skin
corresponds to 32 parts of dry pure pelt in green oxhide, 25 parts in green
calf-skins, and 56 parts in dried calf-skins. In some of the smaller skins,
such as kid worked for glove leather, where great softness and stretch is
required, the loss is necessarily much greater than in ordinary dressing-
leathers, amounting, in the case of kid, to from 20 to 27 per cent.
The parts taken by the purely chemical activity of the lime, and by the
action of bacteria and bacterial ferments in the unhairing process must still
be regarded as uncertain. The late Professor von Schroeder[77] carried out a
series of experiments on liming and sweating which were characterised by
his usual care and thoroughness, and which tend to prove that the chemical
action is far more important than the bacterial. He had fresh hides well
washed in a tannery immediately after slaughter, and fleshed. The butts
were then cut into pieces of about 10 cm. (4 inches) square, and salted in
brine repeatedly changed, and finally preserved for use in glass jars in
saturated salt solution. He found that when washed free from salt, and
placed in a moist chamber at a temperature of 16° C., the hair was
sufficiently loosened by bacterial action in four to five days. Pieces placed
in the moist chamber without previous removal of the salt only showed
signs of sweating after about ten weeks’ exposure. Liming experiments
were made with similar pieces of salted hide, both after three days’ washing
to free them from salt, and unwashed, and in both cases the pieces unhaired

freely in three to four days. These experiments were varied by using 6, 18
and 30 grms. of lime per liter of water in which about 200 grms. of hide
were placed, but neither in the washed, nor unwashed portions was there
any material difference in the time required to loosen the hair. Addition of 1
vol. of used lime-liquor to 3 vols. of water in making up the limes was
equally without perceptible influence, and careful bacteriological
examination of hide and liquors showed that the former was almost
sterilised by the intense salting, and that the lime-liquors were practically
free from bacteria.
[77] Gerberei-Chemie, Berlin, 1898. p. 646.

Von Schroeder’s conclusion that no gain arises from the use of excessive
quantities of lime, so long as the solution is kept saturated, is fully justified
both by experience and scientific reasoning, but his results with regard to
the effect of old liquors and bacteria contradict the conclusions both of
practical tanners and of other scientific experimenters.
The different effects of old and new limes are too well known to practical
tanners to be discounted by laboratory experiments, even if they were not
confirmed not only by Eitner’s results, but by a considerable amount of
work done in the Author’s laboratory and elsewhere; while the necessity of
bacterial action is at least rendered probable by the fact that soda solutions,
which are completely sterile to bacteria, fail to unhair hides which have not
previously undergone some putrefaction (see p. 137). In some experiments
undertaken at the suggestion of the Author it was found that a perfectly
fresh and sterilised calf-skin which was not unhaired after ten days’ liming
in sterilised lime-liquor unhaired rapidly on the addition of a bacterial
culture to the lime. It is extremely difficult to exclude bacteria, and even
where perfectly fresh skins treated with chloroform or carbon disulphide
were employed, bacteria were always to be recognised when the skin was
ready for unhairing. Von Schroeder’s work, is, however, so painstaking and
reliable, that these divergent results must be explained as other than
experimental errors. With regard to old liquors, it is known that ammonia is
a powerful aid to the unhairing process, and it is not certain to what extent
the liquors he used were charged with it. It is also certain that old limes
containing much organic matter, support bacterial life freely, while 25 per
cent. of a possibly not very old liquor would probably be sterilised by the
addition of lime and 75 per cent. water. In order to test the matter fairly
under exact tannery conditions, the lime should have been made up entirely

with old lime-liquor well charged with ammonia and organic matters,
instead of with water. It is also probable that the hides had undergone a
sufficient amount of bacterial change in the tannery before they came into
Von Schroeder’s salt solutions, and it is not at all unlikely that the salt
solution itself exercised some specific effect on the unhairing. It is also
possible that his bacterial cultures were made on gelatine media unsuitable
for the growth of alkaline bacteria, and therefore gave blank results. Under
these circumstances it is scarcely possible to arrive at any very definite
conclusions, and it is obvious that further experiments on these points are
extremely desirable.
Sodium and Potassium Hydrates.—From the earliest antiquity, wood-
ashes, consisting mainly of potassium carbonate, have been used for
unhairing, either alone or in conjunction with lime, and indeed the German
name of the process (Aeschern) is derived from the fact. In more recent
times, caustic soda, either ready formed, or causticised on the spot by the
addition of lime, has often been recommended as a substitute for lime. Its
action is very similar to lime, but, from its greater solubility, is far more
powerful, and probably this has hitherto formed one of the greatest
obstacles to its use, since a solution of the strength of lime-water is almost
immediately exhausted, while a much stronger one is too violent in its
action on the hides. Experiments made in the Author’s laboratory show that
caustic soda, in solutions of the same strength as lime-water, dissolve
considerably less hide substance than the latter, but it is more antiseptic than
lime, and does not unhair readily without the aid of bacterial action (cp. p.
137). It also swells more violently, and it is difficult to keep the grain
smooth and unwrinkled.
Caustic soda has the great advantage that from its solubility, and that of
its carbonates in water, it is much more easily and completely removed by
washing than is the case with lime. It has been successfully applied in some
instances to soften skins of which the texture is naturally too compact for
moroccos and the softer leathers; and is usefully employed in softening
dried goods (p. 115). Where caustic soda is required merely to “sharpen”
limes, it is best added in the form of sodium carbonate (soda-ash or
crystals), which are causticised by the lime in the pits. One-quarter or one-
half per cent. on the weight of hides added in this way decidedly increases
the plumping power of the lime. It may be noted that in the use of sodium
sulphide in conjunction with lime, caustic soda is one of the products of its

decomposition,[78] and is probably one great cause of the difference of
effect of this material for sharpening limes as compared with red arsenic.
[78] This has been denied, but is probably correct, though the actual reaction is not easy
to prove analytically; but the effect on the hide is practically what is stated.
An indirect method of liming has recently been patented by Messrs.
Payne and Pullman of Godalming,[79] which is of both scientific and
practical interest. From the difficult solubility of lime, and the consequently
weak solutions which must be employed, the ordinary process of liming is a
slow one. Caustic soda, however, can be used in much stronger solutions
without producing injury to the hide, or larger solution of hide substance,
and from its great diffusibility, it penetrates very rapidly. Used alone,
however, the hide becomes too much swollen for most purposes, and for
certain classes of leather at least (e.g. buff and chamois leather) the
presence of a portion of lime in the hide appears to be necessary for
successful work. If a hide which has been swollen with caustic soda be
afterwards treated with a solution of calcium chloride, double
decomposition takes place, and caustic lime is formed actually in the
interior of the fibre of the hide, while the sodium unites with the chlorine to
form common salt. Both solutions may be used in any convenient way, and
by the employment of drums, the whole liming process may be
accomplished in five or six hours. It is found, however, that perfectly fresh
hides treated in this way cannot be unhaired, and the explanation appears to
be that in the ordinary liming process, the epidermis is made soluble by the
joint action of bacterial ferments and of the alkaline solutions. If sodium
sulphide be added to the caustic soda used for unhairing, the goods will
unhair without the use of putrefactive means, but the process is difficult to
manage without destruction of the hair, and Messrs. Pullman now
recommend that all hides or skins for unhairing by their process should be
soaked for forty-eight hours in winter, and twenty-four hours in summer in
a really putrid stale soak. This necessity constitutes for very many purposes
a serious weakness in the method, as putrid soaking is always extremely
dangerous to the grain of the hide, and especially so in hot weather. For
certain purposes, however, advantage may be taken of the fact that the hide
or skin can be fully limed by Pullman’s process and the fibres swollen so as
to be prepared for tanning without any loosening of the hair, and the Author
has seen deerskins which have been treated in this way, on which the hair

was perfectly firm, while they possessed a softness and fulness which could
not be attained without liming.
[79] Eng. Pat. 2873, 1898.

Messrs. Pullman now recommend that the treatment with their solutions
should take place in pits, in preference to drums or paddles, and that the
caustic soda should not exceed a strength of one pound in ten gallons (1 per
cent.). The hides or calf-skins remain in this for about forty-eight hours,
during which they are once drawn and returned, by which time, if the putrid
soaking has been properly done, the hair should be fully loosened. The
hides are then drained for two hours, and passed into another pit containing
a solution of calcium chloride, which should be slightly stronger than the
caustic soda, say of about one and a half pounds per ten gallons. The goods
remain in this for about forty-eight hours, during which they are drawn
once, and are then well washed in soft water (free from temporary hardness)
in which they may be kept for some time without injury. As both the caustic
soda and the calcium chloride solutions are quite sterile to ordinary
putrefactive bacteria, both can be used for an almost unlimited time, and
they are conveniently kept up to strength by the addition of strong stock-
solutions. These may be made of a sp. gr. of 1·4 (80 deg. Tw.) which gives a
strength of about 51⁄2 lb. of caustic soda and 53⁄4 lb. of calcium chloride per
gallon.
In addition to the advantage of considerable saving of time, the effects
can be much more easily regulated than in ordinary liming, and the amount
of soda (and subsequently of lime) absorbed by the hide can be exactly
determined by titration of the liquors. Grease is better removed than by
ordinary liming, as soda-soaps are soluble in water, but if this result is to be
obtained, the soap must be worked out before passing into the calcium
chloride solution, which would otherwise convert it into an insoluble lime-
soap. A great gain in many districts is that the process yields practically no
effluents and no lime slab, both of which are frequently very difficult to
dispose of. The serious disadvantages of the stale soaking, however, have
already been mentioned.
In place of applying the caustic soda first, and the calcium chloride
subsequently, hides may be first treated with calcium chloride solution, and
then with caustic soda, or the caustic soda may be applied to the flesh side
of the hide by painting. These modifications are covered by Messrs.

Pullman’s patent, but they are willing to grant licences for experiments at a
nominal fee.
Alkaline carbonates are much milder in their action on hide than the
corresponding hydrates, and although they will unhair hides, in absence of
lime, their action is somewhat uncertain and slow. “Polysulphin”
(Polysulphin Co., Keynsham) owes its unhairing power principally to the
sodium carbonate, and not to the small traces of sulphur compounds which
it contains.
Sodium carbonate occurs in commerce in three forms: “soda ash,” a more
or less pure dry sodium carbonate; “soda crystals,” or washing soda,
Na2CO3·10Aq, containing 62·95 per cent. of water of crystallisation, and
efflorescing in the air; and Gaskell and Deacon’s “crystal soda,”
Na2CO3·1Aq, containing only 14·5 per cent. of water of crystallisation. It
must be remembered that where carbonate is used in conjunction with lime
it becomes causticised and converted into NaOH.
Sulphides.—The practice of using realgar, or red sulphide of arsenic (Ger.
Rusma) as an addition to limes for fine leathers is one of considerable
antiquity. It has the property of loosening the hair and epidermis structures
with less solution of cement-substance than lime alone, and hence produces
a leather of fuller and closer texture. It will, however, be convenient to defer
the consideration of this agent till after that of some of the more modern
and simpler substitutes, such as the sulphides of sodium and calcium.
Sulphides of the alkalies and alkaline earths, if used in strong solution, say
5 per cent. or upwards, have the effect of very rapidly reducing the harder
keratin-structures, such as hair and wool, to a pulp, attacking first the
interior cells, so that the hair crumples up like a string of sausages, and in a
few hours, or even, with very strong solution, in a few minutes, the whole
mass becomes so completely disintegrated that it can be swept off the hide
with a broom, or washed off in a tumbler. At the same time, the action on
the substance of the hide, and especially on the cementing substance, is
very slight, though the grain is swollen and temporarily rendered somewhat
tender. On the other hand, when used in weak solutions, say 1⁄4 per cent. and
under, in conjunction with lime, the hair is but little injured, while the hair-
roots and dirt are rapidly loosened, and results are obtained very similar to
those with arsenic.
Sodium Sulphide (Na2S·9OH2).[80]—For the methods of valuation and
determination of sodium sulphide, see L.I.L.B., p. 28.

[80] In the Laboratory Book the water of crystallisation is given as 10 Aq. Later
researches show that pure crystals of the commercial sulphide only contain 9 Aq., or
67·5 per cent. of water.

Hides suspended in solutions of sulphide of sodium of 2 to 3 per cent.
strength unhair rapidly.
For the commoner classes of sole-leather, hair is frequently removed by
painting on the hair side with a 15°-28° Tw. (30-40 per cent.) solution of
(crystallised) sulphide of sodium thickened with lime, applied with a fibre-
brush, and folding the hide in cushions in a damp place, or packing in a tub.
The hair is reduced to paste in a few hours. The same effect is produced by
drawing the hides through a similar solution without lime, of which
sufficient is retained by the hair to destroy it. The workmen must be
provided with indiarubber gloves to prevent the caustic effect of the
solution on the skin and nails. Skins and lighter hides are conveniently
unhaired by painting the mixture on the flesh side, when it will loosen the
hair or wool in a few hours without destroying it.
For dressing-leathers and the finer sorts of sole it is best employed as an
addition to ordinary limes to the extent of 1⁄4-1⁄2 per cent. on the weight of
the hides or skins, when the hair is loosened more rapidly than with lime
alone, and with less loss of hide substance.
Good samples of sulphide of sodium consist of pale-brown, almost
colourless crystals, containing 28 to 32 per cent. of dry sodium sulphide,
which readily deliquesce on exposure to air. Fused sodium sulphide can
now be obtained, which contains nearly twice as much actual sulphide as
the crystalline form. The dark green colour possessed by many samples of
sodium sulphide is due to the presence of iron sulphide. If carefully used no
serious harm can accrue from its presence. If allowed to stand a short time
in solution the iron sulphide will settle out.
Calcium sulphydrate, Ca(SH)2, sometimes called Böttger’s Grünkalk, is a
powerful depilatory, while it has probably less destructive action on the
hide-fibre than even the sulphide of sodium, and would no doubt be largely
used but for its unstable character. It is probably the principal active product
produced by the use of sulphide of arsenic in conjunction with lime, though
it is possible that a sulpharsenite may be formed. It may be produced by
passing hydrogen sulphide (SH2), into milk of lime. According to von
Schroeder, it is not formed by the reaction of sodium sulphide on lime
solutions (see note, p. 136). It may be obtained crystallised, and is soluble
in water, but is decomposed on boiling. The sulphide, CaS, is insoluble in

water, but by the action of steam under pressure it is said to be converted
into a mixture of equivalent parts of hydrate and sulphydrate. It may also be
dissolved in a solution of hydrogen sulphide, forming a solution of
sulphydrate. In this way it might be formed on a large scale from the “tank
waste” of the Leblanc soda process.
Gas-lime is principally active on account of the calcium sulphide which it
contains, but is very variable in its strength, as both sulphydrate and
sulphide are decomposed by the carbon dioxide always present in the gas,
forming carbonates. Lime has nearly gone out of use for purifying gas, its
place being now taken by iron oxide, but formerly gas-lime was a good deal
used for unwooling the small lambskins used for the commoner sort of
glove-kid, usually by painting a cream of it on the flesh side, but sometimes
by immersing in a strong solution, which of course destroyed the wool. Its
place is now taken by a solution of sodium sulphide of 15°-18° Tw.
(approximately 30-35 per cent. crystals), thickened with lime to a soupy
consistence, the use of which is much to be recommended for unwooling
sheep-skins.
The tank-waste from the Leblanc process, consisting principally of
calcium sulphide, is, when fresh, quite insoluble, and has no depilatory
powers; but when exposed to air and moisture, decompositions take place,
resulting in the formation of sulphydrates and polysulphides, which form a
solution which has been the subject of several patents for unhairing.[81]
Polysulphides alone have probably no unhairing effect, but in conjunction
with lime, sulphydrates are formed which rapidly loosen the hair. This fact
was the basis of an ingenious and effective unhairing process used very
many years ago by Mr. John Muir, of Beith, who, after liming for 24 hours
in the usual way, submitted the hides to a pretty strong solution of
weathered tank waste for 24 hours, and finally to water for 24 hours, to
remove the surplus lime and sulphides. The sulphydrates formed in the hide
attacked the hair-roots with little injury to the hair itself, and the hides
contained so little lime that they could be tanned for dressing without
bating, and made about 10 per cent. more weight than those treated in the
ordinary way. Some trouble was occasioned by stains caused by impurities
in the tank-waste.
[81] Squire, E. P., 756, 1855; Claus, E. P., 1906, 1855.

A somewhat similar unhairing mixture to that obtained from tank-waste,
which is now seldom to be got, was patented by Prof. Lufkin,[82] who

mixed equal parts of sulphur and soda-ash with a little water till combined,
and then added 8 to 10 parts of lime, slaked and still hot. Schultz[83] states
that such a mixture containing 10 lb. of sulphur, will unhair fifty hides in
the same way, and in about the same time as an ordinary lime, the pelt being
little plumped and easily reduced without bating by a few minutes’
wheeling in warm water. By boiling lime and sulphur with water a yellow
solution is obtained which can be used in the same way as that from the
tank-waste. A further quantity of water can be boiled on the same materials,
more lime and sulphur being added as required. Polysulphides appear to
have a marked effect in preventing plumping.
[82] Eng. Pat. 2053, 1860.
[83] ‘Leather Manufacture,’ p. 35.

Barium sulphydrate has been put on the market experimentally as an
unhairing agent, in the form of a strong solution containing yellow
polysulphides, and which deposits crystals of sulphydrate in cold weather. It
is more stable than calcium sulphydrate, but, on the whole, does not seem to
present any advantages over sodium sulphide.
Realgar or red sulphide of arsenic, As2S2, is made by fusing arsenious
acid and sulphur. (Orpiment is As2S3, but its action is different from that of
realgar.) Mixed with lime it produces calcium sulphydrate and possibly
hyposulpharsenite. To produce a rapid and complete reaction it must be
mixed with hot lime, and the hotter the mixture is made the more powerful
is its unhairing action. Milder forms may be made by mixing cold, or with
the aid of hot water only. It is used with great advantage in conjunction with
lime in varying proportions for unhairing lamb- and kid-skins for glove-kid
and other fine leathers, to which it gives the necessary stretch and softness
and cleanness of grain, without the loosening of texture and loss of hide-
substance which would be caused by an equivalent amount of ordinary
liming. For glove-kid about 0·1-0·3 per cent. of realgar and 5 per cent. of
lime is used, reckoned on the green weight of the skin.
For painting the flesh side of calf- and lamb-skins 1 part of realgar is
mixed with 10 parts of hot lime, made into a paste with water. Calf will
unhair in 8 or 10 hours.
“Inoffensive” unhairing solution contains a large quantity of arsenic
sulphide apparently dissolved in caustic soda, although Moret’s original
patent claimed the use of wool-sweat potash only!

W. R. Earp[84] has suggested the use of compounds of sulphur and
arsenic (thio-arsenates, thio-arsenites, etc.), in 5 per cent. alkaline solution.
He prefers to add the compounds to the ordinary lime-liquors, or to
manufacture them in situ by adding the proper quantities of arsenious or
arsenic acid mixed with one-third of its weight of sulphur to a solution of an
alkaline sulphide in lime-liquor. The pelt is not bated or drenched in the
ordinary way, but, after unhairing, is passed directly into the tanning liquor
to which sulphurous acid has been previously added.
[84] Eng Pat., No. 2052, Feb. 12, 1886.

There is more danger of injury to the hide from the very prolonged action
of weak solutions of sulphides, which tend ultimately to destroy the
structure and reduce the fibre to a gelatinous condition, than there is from
too concentrated solutions. No danger need, however, be apprehended in the
course of any ordinary liming. Arsenical limes are not suited for tainted
skins, and they should not be made so strong as to destroy the hair or wool.
For methods of analysis of both old and new lime-liquors, see L.I.L.B.,
pp. 27 to 34.
Whichever method of loosening the hair be adopted, the actual removal
must be effected by placing the hide on a sloping beam with a convex
surface, and then scraping it with a blunt two-handled knife (Fig. 27), the
workman pushing the hair downward and away from himself. The beam
may be either of cast iron or of wood, usually covered with zinc to increase
its wearing capacity. The hides after being removed from the lime-pits, are
allowed to drain for half an hour or so before the hair is removed, and
immediately this operation has been completed, they should be placed in
soft water. It is of great importance that the limed hides should not be
exposed to the air longer than is absolutely necessary for the removal of the
hair, as the carbonic acid present in the atmosphere quickly carbonates any
lime contained in the surface of the skin, forming chalk, and leading to
uneven tanning at a later stage.
When hide has been insufficiently limed it is often easy to remove the
longer hair but excessively difficult to get rid of the short under-growth of
the young hairs, which even in properly limed skins can often only be
removed by shaving them with a sharp handknife. This difficulty is caused
partly by the small resistance which the short hairs offer to the unhairing-
knife, and partly by their being more deeply rooted in the skin than the
older hairs (see p. 49).

Fig. 27.—Unhairing (Penketh Tannery).

Various machines have been devised to accomplish the removal of the
hair, but owing to the rapidity with which it may be worked off by hand,
and the fact that the work is not difficult, no machine has as yet come into
general use. Hand-work has the further advantage that in those portions of
skin where the hair is tighter than usual it may be removed by greater
pressure of the knife or by hand-shaving, whereas after goods have been
unhaired by machine they must always be examined and any patches of hair
removed by hand on the beam. The edges invariably require to be gone over
by hand.
Several machines with spiral knives have been introduced for the
purpose. That made by the Vaughn Company (Peabody, Mass.) for fleshing
is one of the most satisfactory for unhairing, though any other machine of a
similar type, and provided with spiral knife-blades, purposely kept blunt,
may be used. The Leidgen unhairing machine, shown in Figs. 28 and 29, is
one of the latest and most ingenious.[85]

[85] E. H. Munkwitz, Milwaukee.

Fig. 28.—Leidgen Unhairing Machine.

Occasionally goods are unhaired by fulling in the “stocks”; but it is very
doubtful whether the saving in labour is not more than counteracted by the
loss of weight caused by submitting the hide, while its gelatin is in a
partially dissolved condition, to such rough usage.
The use of the wash-wheel (see pp. 111, 118) for the same purpose is
much more satisfactory, and may be profitably employed for common
goods, especially when the hair has been loosened by painting with a
sulphide mixture.
After being unhaired, the hides are “fleshed” on the beam. This work,
which consists in removing any small pieces of flesh and fat left by the
butcher on the inner side of the skin, should be carefully and thoroughly
done; but the closeness of the fleshing required is dependent on the purpose
to which the hides or skins are to be applied.

Fig. 29.—Leidgen Unhairing Machine.

It is necessary not only to remove those portions of fat which are easily
visible, but also to force out that contained in the loose areolar tissue. The
form of knife used in England in fleshing is shown in Fig. 30. It differs
from the one used for unhairing in being somewhat broader and heavier,
and both its edges are sharp, so that where the flesh is too tight to remove
by mere friction of the knife, it may be actually cut away by holding the
knife almost flat on the beam, and using the convex sharp edge. The strokes
in cutting must not be too broad, or, from the convexity of the beam, the
substance of the hide will be cut into in the middle, or flesh will be left at
the edges of the stroke. This difficulty is avoided by the flexible knife
commonly used in Germany, but in other ways its work is less rapid and
effective.

Fig. 30.—Fleshing.

Machines have long been used for fleshing and scudding light goods,
such as lamb-, kid-, and goat-skins, and their use for fleshing dressing hides
has now become very general in the United States, and is gradually gaining
ground in England. The type of machine used for these heavier leathers,
varies considerably from that used for light skins, but the general principle
is the same. In most cases the working tool of the machine is a cylinder
with spiral blades, which are generally arranged right-handed on one half,
and left handed on the other, so as not only to scrape the hide in the
direction in which the cylinder works, but also to extend it sideways. Much
of the efficiency of these machines depends on the exact adjustment of the
pitch of the spiral, and in the Vaughn machine, which is probably most in
practical use, the blades are so arranged as to form two intersecting spirals,
one of steeper pitch than the other. The great difference in the machines for
skins and for heavy work, consists in the means adopted to support the skin,
and to carry it under the spiral blades.

Fig. 31.—Jones Fleshing Machine.

In the machine invented by the late J. Meredith Jones, the skins are
supported upon an india-rubber blanket stretched over two rollers, so that
the knife-cylinder works on that part of the blanket which is between them,
by which great elasticity is obtained, and this machine has proved most
successful in treating delicate skins. In some other forms of machine,
cylinders thickly covered with rubber have been substituted for this
arrangement. The Jones machine is shown in Fig. 31. For heavy hides the
Vaughn machine is most generally used, and may be taken as the type of the
rest, as the Vaughn Company certainly originated the semi-cylindrical
“beam,” which forms a very important feature. Its construction will be seen
from Fig. 32.

Fig. 32.—Vaughn Fleshing Machine, front view.

It will be easily noticed that if a hide be thrown over the half-cylinder so
that one half hangs outside it, and the other half falls in its hollow, and it be
then rotated, the hide is first caught firmly by a spring-clamp, which has
been supported above the edge of the half-cylinder by blocks attached to the
frame. As the edge rises, it lifts this clamp off the blocks, and thus carries
the hide under the spiral knife-cylinder. The blades of this spiral knife-
cylinder are ground to a sharp rectangular edge, and partly scrape and partly
cut the loose tissue of the flesh. When the half-cylinder has made a semi-
revolution, it returns to its original position, and the sizes of the driving
pulleys are so arranged that the cylinder travels downwards more rapidly
than it rises, in order to economise time, though in both cases the hide is
worked upon by the knife-spiral which is rotated at a still higher speed. The
hide is of course turned on the beam-cylinder and the other half is similarly
fleshed. The beam-cylinder reverses automatically, or may be reversed by
hand, and its nearness to the spiral knife is also under control. It is usually
covered with a thick sheet of rubber.
It is obvious that machines of this type can not only be used for fleshing,
but for unhairing and scudding, by the substitution of suitable knife-

cylinders, and in the case of light skins, cylinders fitted with slates are
frequently employed for the latter operation. The slate for the purpose must
be of a peculiarly fine and even grain, and is mostly obtained from a single
quarry in Wales. The Vaughn machine is frequently used in America for
fleshing hides after soaking but before they go into the limes, and much is
to be said in favour of this method, as the removal of the flesh permits even
and uniform action of the lime. It is, however, a distinct disadvantage to the
method that the flesh appears rough-looking after tanning, and the method
is most suitable in conjunction with the American system of splitting the
tanned leather.
In the production of sole-leather, fleshing machines have not as yet come
into very general use. This may be accounted for by the fact that if used
before liming a rough flesh is produced, which is unsightly on sole-leather,
and which cannot well be afterwards improved, while something of the
same objection attaches to fleshing after liming, with the added
disadvantage that the hide is too much pressed, and is not easy to plump
again, so as to make a satisfactory sole-leather.
In America, both sole- and dressing-leathers are usually tanned in sides,
the hide being cut down the centre of the back. In England, the hide is
usually “rounded” for sole-leather into “butts” or “bends” and “offal,” as
shown in Fig. 33. The rounding is done by hand with a sharp knife on a
table, and in some of the best tanneries frames made of wood or metal are
employed, to mark the sizes required. The chief advantage of rounding
before tanning is that the different parts of the hide can be differently
tanned, and appropriated to the purposes for which they are most suitable.
The offal is now frequently split and worked up for light leather, or in other
cases is tanned with a cheaper and more rapid tannage than the butts.

Fig. 33.—Diagram of Hide.

Dressing leather is more frequently rounded after tanning, according to
the purposes for which it may be required.

CHAPTER XIII.
DELIMING, BATING, PUERING AND DRENCHING.

Although lime is in many respects the most useful and satisfactory means
of loosening hair from hides and skins, it is of the greatest importance that it
should be completely removed when it has done its work, since its action on
tannins is most injurious, and it is often harmful in tawing. For soft leathers
it is also necessary that the skin should be brought from a swollen to a soft
and flaccid condition.
In practice this is mainly accomplished for dressing leathers by bating,
puering and drenching; while sole-leather and strap-butts are only too
frequently left to chance, and to the natural acidity of the tanning liquors.
Bating consists in handling, or steeping the goods in a weak, fermenting
infusion of pigeon- or hen-dung for a time usually extending over some
days, and is applied to the heavier classes of dressing leather, such as
“common” and shaved hides, kips and calf-skins.
Puering is a very similar process, applied to the finer and lighter skins,
such as glove- and glacé-kids and moroccos, in which dog-dung is
substituted for that of birds, and, as the mixture is used warm and the skins
are thin, the process is generally complete in a few hours at most. Neither
bating nor puering are very effective in removing lime, and seem to act
principally by some direct effect of the bacterial products on the swelling of
the pelt.
Drenching is occasionally used (e.g. on calf-kid) as a substitute for bating
or puering, but more frequently follows the latter, and serves to cleanse and
slightly plump the skins before tanning, and complete the removal of lime.
The drench-liquor is an infusion of bran made with hot water, and allowed
to ferment under the influence of special bacteria, which are always present
in vats used for the purpose, and which develop lactic and acetic acids.
It will be noted that all these methods are fermentative, and their effect is
not simply the chemical one of removing the lime, but the bacterial action
leads also to solution of the cementing substance of the hide-fibres, and
produces a marked softening effect on the leather, together with

considerable loss of hide-substance. In the manufacture of the softer
leathers this effect is generally desired, and no process would be
satisfactory which did not produce it; but in other cases, such as harness-
and strap-butts, firmer and heavier weighing leathers would be preferred, if
it were known how to make them. The putrefactive processes would be
gladly relinquished, if satisfactory substitutes could be found, not only on
account of their offensive character, but because of their uncertainty and
danger to the goods; and even if lime only were removed, the necessary
softness could often be obtained by appropriate liming and tanning.
It will be best, therefore, to deal first with the purely chemical methods
which aim only at removal of lime, before considering those involving
bacterial action. Unfortunately, the chemical problem is not so simple as it
might at first sight appear. The alkaline lime clings obstinately to the hide-
fibre, and can only be removed very slowly, if at all, by mere washing. On
the other hand, the use of any excess of strong acid is absolutely precluded,
because of its powerful swelling effect on the pelt, in the tanning of which it
would prove even more injurious than the lime, making dark-coloured and
brittle, or tender, leather. This effect is not to be avoided by the use of even
very dilute solutions of strong acids, since the affinity of hide-fibre for them
is so strong that it will abstract practically all the acid from even a
decinormal solution, leaving it quite neutral. What is required is an acid of
extremely weak affinities, forming soluble lime salts, and obtainable at a
low cost; or, on the other hand, a salt of some weak base which could be
displaced by lime, and which would not act injuriously on the pelt. With
certain precautions, and in special cases, however, the stronger acids may
be used successfully.
In the cases of sole- and belting-leather no softening is desired, and
formerly tanners usually contented themselves with a very perfunctory
washing in water, trusting to the acids present in the liquors to complete the
removal of the lime. Even pure distilled water effects this removal very
slowly and imperfectly, owing to the strong attraction of the lime for the
fibre; and if “temporary hard” water is used, the lime present in the hide
combines with that present in the water and is precipitated as chalk in the
surface of the hide. This may be prevented by previously adding a small
quantity of lime or lime-liquor to the water before use to soften it (see p.
95); but unless this is very carefully done, the free lime present in the water
prevents it from removing any from the hide. The safest way is not to add

lime direct to the water, but to change the latter gradually, so as to allow the
lime already present to soften the new portion of water.
A much more efficient method is to suspend the butts in water to which
small portions of diluted acid are successively added till the lime is nearly,
but not quite, neutralised. If carefully used, sulphuric acid[86] is perhaps as
good as any, but, of course, any excess will spoil the colour or “buff” of the
leather.

[86] The use of sulphuric acid for this purpose was patented by H. Belcher of Wantage
(No. 14,943), but was used some years previously in several tanneries known by the
author.
Acetic, formic, and lactic acids are safer than sulphuric, but are
somewhat costly, and must not be used in appreciable excess. Crude
pyroligneous acid may be used, and it has a considerable antiseptic effect
owing to the phenols, etc., which it contains. Hydrochloric acid is not
suitable for sole-leather, on account of the bad effect of chlorides on
plumping. Sulphurous acid[87] is perhaps the best, and its acid properties are
so weak that slight excess does little harm, but the neutral calcium sulphite
is insoluble, and to actually dissolve the lime the hydric sulphite must be
formed, which can only occur in presence of excess of the acid. Unless such
excess is used, the colour of the pelt in the early liquors is apt to be
somewhat greyish. Probably a very good method would be to suspend the
butts in a solution of sulphurous or some other acid of about N⁄20 strength,
sufficiently long to remove all lime from the surface and slightly to plump it
but not to penetrate to the centre of the hide, which should then be
suspended in water until any excess of acid had been taken up by the
unneutralised lime still present in the middle of the butt, which at the end of
the operation should be rather alkaline than acid. The course of this, or any
other bating operation can be followed by cutting the hide, and moistening
the cut surface with alcoholic solution of phenolphthalein, which is turned
red, or pink, by the least trace of free lime.
[87] Manufacture of sulphurous acid, see p. 24; testing, see L.I.L.B., p. 37.

In using mineral acids it is of great importance that they should be
perfectly free from iron, and that the vat employed should contain no iron
which could become dissolved, since, if present in the bating liquid, it is
sure to be fixed by the hide, especially if the quantity of acid used is
insufficient to neutralise the whole of the lime.
Besides the direct use of mineral acid which has been described,
sulphuric, or still better, oxalic acid may be very advantageously employed
in precipitating lime from used bating liquids containing weak organic
acids, or other lime solvents, so as to restore their original activity. Not only
is the bate economised by being used repeatedly, but some of the organic
products dissolved from the hide have themselves considerable power of
removing lime. Putrefaction should not be allowed to take place; but many
of the organic acids which have been proposed for bating belong to the

aromatic series, and have considerable antiseptic power. Where organic
acids are employed, the presence of their neutral lime-salts in the liquor,
resulting from previous operations, will reduce the swelling action of the
acid on the skin, without diminishing its power of removing lime (cp. p.
81).
In place of sulphuric acid, some tanners have employed a material
advertised under the name of “boral.” This substance consists simply of
sodium anhydrosulphate melted up with about one-seventh of its weight of
boric acid, the quantity of which is, however, too small to have appreciable
influence as an antiseptic, while it is said to form insoluble borates with the
lime present, which are sometimes a source of subsequent trouble.
There is no reason why ordinary sodium bisulphate should not be used
for the purpose, and its action is more mild than that of sulphuric acid itself,
but great care must be taken that no nitric acid is present, as is frequently
the case in the crude product obtained in the manufacture of nitric acid from
sodium nitrate, and known in commerce as “nitre-cake.” The presence of a
trace of sodium chloride would not be disadvantageous for dressing leather,
but would tend to prevent plumpness in sole. Paessler and Appelius[88] have
recently shown that raw hide absorbs sulphuric acid from sodium
bisulphate, leaving the neutral sulphate in solution.
[88] ‘Wissenschaftlich-Technische Beilage des Ledermarkt,’ 1901, p. 107.

Boric (boracic) acid, though used to a slight extent for a number of years
past, has recently come much into favour as a deliming agent, for which
purpose it is in many respects particularly suitable. Sole-leather may be
improved in colour by giving a short bath in 11⁄2-2 per cent. boric acid
solution to remove surface-lime. In this case the acid is best applied just
before the hide enters the suspenders. Boric acid may also be suitably
employed on hides which have been bated. It then acts as a drench and
removes traces of lime still left in the hides, so that the liquors have a more
even effect on them. Experience has shown that the skins should never be
allowed to lie for any length of time in the boric acid solution in a
motionless condition, as this tends to produce patches of partially delimed
skin, which cause irregular colour. It is best to keep the skins in fairly
constant motion in a paddle or by frequent handling. Boric acid has
considerable influence in preventing drawn grain in the early liquors, but if
it gets into the forward liquors it renders the leather loose and light (cp. p.
229, and L.I.L.B. p. 37).

Borax has also been suggested as a deliming agent, and as it is
chemically an acid salt, it has naturally some deliming effect, but it cannot
compare with boric acid in either price or efficiency.
Both boric acid and borax are antiseptics (see p. 25).
In the employment of either sulphuric, boric, or any other acid forming
calcium salts of limited solubility, it must be borne in mind that if the
solution is repeatedly re-strengthened, it will become saturated with the
lime-salt, and although the acid will still combine with the lime and render
it neutral, it will no longer remove it from the hide. Under these conditions,
sulphuric acid may cause the deposition of crystalline calcium sulphate in
minute nodules between the fibres. Calcium borate may be similarly
deposited, and has the further disadvantage of becoming decomposed by the
tanning liquors, which form dark compounds with the lime. In using
sulphuric acid alone it is therefore best to renew the water each time. When
it is used in conjunction with some other acid, forming very soluble lime
salts, this danger is not to be apprehended, while oxalic acid precipitates the
lime almost completely from the solution.
It is to be borne in mind that in all cases of using acids, any carbonate of
lime present on the pit sides or elsewhere will be decomposed, and the
carbonic acid will become dissolved in the liquor, and unless acid is used in
sufficient quantity to remove the whole of the lime, may tend to fix the
remainder as carbonate. In the case of dressing leather there is less danger
of this, as warm water is generally used, in which little carbonic acid
dissolves. It is probable that some of the coal-tar acids which have been
advertised for bating dressing leather might be advantageously employed
for sole. Hauff’s “anticalcium” (see pp. 29, 163), would appear to be very
suitable for this purpose, and if the liquor were regenerated by the addition
of sufficient sulphuric acid to neutralise the lime dissolved from the hide,
might be used repeatedly, and would not then prove expensive; while its
sterilising power would be very advantageous to the proper swelling of the
butts in the handlers, since nothing tends to check plumping so much as
putrefactive action.
Turning from sole to dressing leather, mineral acids are very successfully
employed for “pulling down,” the goods being thrown into a paddle
containing warm water of about 30°-35° C., and the calculated quantity of
sulphuric or hydrochloric acid, previously largely diluted with water, is then
added in two or three successive portions at intervals of perhaps ten

minutes. The acid must in no case be sufficient to neutralise quite the whole
of the lime. Goods treated in this way can be further bated, puered, or
drenched as required by the ordinary methods, if they are not sufficiently
soft. If too much acid has been used, and the skins show signs of swelling,
they may be brought down by the addition of a little ammonia, borax, or
even soda.
In many cases the addition of salt in small quantity to the acid liquor will
tend to deplete the hides, and at the same time prevent any injurious action
of the acid. Ammonium chloride may also be used with advantage (see p.
159). A solution containing about 15 per cent. of salt and 0·3 per cent. of
sulphuric acid, with some molasses, has been a good deal used in the States
as a bate, and seems to answer well on some classes of goods, but the acid
and salt are apt, ultimately, to find their way into the liquors and destroy
tannin. The process is well suited for chrome-leather, and may also be
usefully applied in cases where goods have become “wind-blasted” or
otherwise impregnated with carbonate of lime, since in presence of salt the
acid can be used in sufficient excess to dissolve the carbonate. Vegetable
acids may, of course, be used in conjunction with salt in the same way. The
salt does not neutralise the acid, but simply controls the swelling of the
skin, and if acid has been used in any material excess, the first part of the
tanning must be done in salted liquors, or the acid neutralised with
ammonia, sodium carbonate, or chalk, previous to tanning, as, otherwise,
the goods will plump up in the liquors, and be tender when tanned (cp. p.
91).
Lactic acid has recently come largely into use as a deliming agent. It is
best known as the acid which gives a characteristic taste to sour milk, and is
the chief product of the lactic ferment. It may be very successfully used for
neutralising the lime left in the skins after the depilation, but, if used in
excess, it tends to plump or swell the leather very strongly, being one of the
best plumping agents known. When used for deliming, a solution of 2 lbs.
in 100 gallons is very suitable. It may, in many cases, be substituted for the
bran-drench with advantage, and is much more rapid and less dangerous in
hot weather, but the effect is not in all respects identical.[89]
[89] On the manufacture of lactic acid by fermentation, see Claflin, Journ. Soc. Chem.
Ind., 1897, p. 516. Campbell states that practically pure cultures of the lactic bacteria
are obtained by continued culture in milk. These cultures employed as a ferment for
drenches have given good results in the Yorkshire College Experimental Tannery.

When lactic acid is used for bating, or drenching, the operation should
always be conducted in a paddle, and the liquid works more satisfactorily if
it is at a temperature of 30-35° C. As regards cost, it will be found that in
practice it is not appreciably more expensive than dung or bran. About an
hour’s paddling will generally suffice, if the right quantity of acid has been
used, but in some cases it is best to add the acid in several portions and take
more time.
The estimation of the amount of lactic acid in the commercial article may
be carried out by diluting exactly 9 grms. with about ten times its volume of
water, and then titrating it with normal caustic soda as described in L.I.L.B.,
p. 16, for acetic acid. As each c.c. of normal alkali is equivalent to ·090
grm. of lactic acid it will represent one per cent. of real lactic acid in the
sample. If other acids are present, they are of course included. Commercial
lactic acid is usually of about 50 per cent.
It is important that the lactic acid should be free from iron, a dilute
solution should give no blue coloration on addition of either potassium
ferrocyanide or ferricyanide. Acid perfectly free from iron is now easily
obtained.
Formic acid in 60 per cent. solution, formed synthetically by the
combination of carbon monoxide with caustic soda and the subsequent
decomposition of the sodium formate so produced, has recently been
brought into commerce at a cheap rate, and will probably form a
satisfactory substitute for acetic acid in the deliming of hides and many
other technical operations.
Instead of acids, many neutral salts may be used to neutralise lime, and in
sole-leather, it is not generally disadvantageous to leave the lime in the
hide, so long as it is in an insoluble and fixed condition, and combined with
an acid which cannot be displaced by tannin. Thus phosphates, or oxalates
of sodium or ammonium will convert the lime into insoluble phosphate, or
oxalate, setting free sodium- or ammonium-hydrate which form soluble
tannates and other salts which are easily washed out of the hide. Zinc
sulphate will form sulphate of lime and zinc oxide in the hide, and seems
worth further experiment for sole-leather, but must be free from iron. Alum,
or sulphate of alumina, would similarly form calcium sulphate and alumina,
but the tanning effect of alumina salts is too great to admit of their general
use for bating. Ammonium sulphate will form calcium sulphate with
liberation of ammonia.

For dressing leather, the use of ammonium chloride would be still more
advantageous, and it is a powerful bating material, converting the lime into
calcium chloride with the evolution of ammonia, which has but little
plumping power, and which is easily washed out. Ammonium chloride has
been very successfully used in calf-kid manufacture as a preparation for
drenching, instead of puering, which was formerly in vogue. As, however,
only about 3⁄4 oz. per dozen skins was employed, the cleansing must have
mainly depended on the warm water with which it was used, and the free
ammonia evolved.
The use of ammonium chloride as a bate was patented by Zollickoffer in
1838.
A bating liquor which was proposed by the writer, and which has been
used with some success on harness-leather, is made up with a 1⁄4 lb. of good
white ammonium chloride (sal ammoniac) and a 1⁄4 lb. of Boakes’
“metabisulphite of soda” per hide, and for successive packs sufficient
sulphuric acid to neutralise the ammonia formed, together with a small
quantity of metabisulphite and ammonium chloride to restore that carried
out by the hides is added. It is probable that this would also answer well for
deliming sole-leather as it entirely removes lime without pulling down the
hides much, and they would remain still plumper if ammonium sulphate
were substituted for ammonium chloride, while the sulphuric acid might be
safely increased till the liquor was but slightly alkaline when the bating was
finished. About 2-4 oz. of good white oil of vitriol is required per hide, but
the exact quantity will depend on the mode of liming, and the amount of
washing the hides receive before going into the bate, and can therefore be
only ascertained by experience. As no free sulphuric acid can exist in the
liquor so long as the quantity of metabisulphite is maintained, there is no
practical danger of spoiling the leather if the acid be in slight excess. The
quantities given may in most cases be advantageously diminished, since it
is not always advisable in practice to remove the whole of the lime, which
in small quantity renders tannage and penetration of the liquor much more
rapid, either by acting as a mordant to the tannin, or by temporarily
neutralising it and diminishing its astringent action on the hide-fibre.
Turning to dressing leather, we find that the use of cold water alone has
been practically abandoned in this country, though the finest French calf is
produced by repeated soakings in cold water with alternate workings over
the beam, sometimes extending to nine or more. In this case, from the

lengthened exposure to waters which are only gradually renewed it is
probable that putrefactive action takes place, and that a sort of bating is
effected by the decomposing products of the hide itself; in fact, in many
French yards, bran-drenches have been introduced to supplement the action
of the water alone. Waters differ greatly in their power of removing lime
from skin. Slightly acid and peaty waters, and those in general which
contain much organic matter, are much more powerful in reducing than
those which are purer (cp. p. 107).
Warm water has much more effect in removing lime than cold, since the
heat lessens the risk of dissolved carbonic acid, and seems to have a direct
depleting effect on the pelt. A good tumbling in warm soft water will
remove a great deal of lime, and is an excellent preparation for bating, but
heat must be used cautiously, and should never exceed 30°-35° C.; some
skins, such as seals, being very readily tendered by its action, while others,
especially sheep-skins, will stand a comparatively high temperature.
The use of a solution of carbonic acid for removing lime has been
patented by Nesbitt,[90] who takes advantage of the fact that calcium
carbonate is soluble in excess of carbonic acid (p. 94). The gas, which he
generates, as for soda water, by the action of acids on chalk, or limestone, is
received in a gasholder, and forced by a compressing pump into the vessel
containing the hides, which is preferably a rotating drum lined with copper,
and capable of bearing a pressure of about three atmospheres. The invention
excited considerable interest on its introduction, as the gas is, certainly,
quite uninjurious to the hides, and it was claimed that it enabled the grease
and dirt to be better removed than by the ordinary methods. Further
experience has shown, however, that the removal of the lime is far from
complete, since, for success, it is not only necessary to bring it into solution,
but to wash it out with carbonic acid solution under pressure, as on
exposure to the air, solutions of lime in excess of carbonic acid rapidly
deposit calcium carbonate. At the present time, the only tannery in which to
my knowledge the process is in use is that of Messrs. Mossop and Garland,
of Capetown, who state that it answers very well for harness-leather when a
pure lime made by calcining sea-shells is used for liming, but is not
satisfactory with ordinary stone lime. It is difficult to account for this on
chemical grounds. Gluestuff may be treated very satisfactorily by simply
blowing carbon dioxide, or washed and cooled lime-kiln- or furnace-gases,
into an open pit in which the material is kept agitated. In this case, however,

there is no need for the actual removal of the lime, so long as it is
carbonated and its caustic character destroyed. Carbonic acid does not
decompose lime-soap, and hence sets free no fatty acids, which, together
with grease, are the main cause of the turbidity of glue, and the process
therefore yields a more brilliant though darker coloured glue than does
treatment with sulphurous acid.
[90] Eng. Pats. 7744 and 12,681, 1886.

Several acids of the aromatic series have been from time to time
recommended as deliming agents, and generally possess the merit of acting
at the same time as powerful antiseptics. In this connection it may be well
to mention the solution of 1 per cent. of phenol and 2 per cent. of boric acid
used by Dr. Parker and the writer for preparing and preserving skins for
colour tests (L.I.L.B., p. 133). This answers very well as a bate even when
much diluted, and may be rendered cheap enough for use in practice by the
employment of a good commercial carbolic acid instead of pure phenol, and
the use of sulphuric acid to remove lime from the solution and render it
capable of repeated employment. The carbolic acid should not be too dark
in colour, and should be carefully dissolved, or “carbolic” stains will result.
“Cresotinic acid,” a mixture of impure acids obtained from cresols in the
same way as salicylic acid is manufactured from pure phenol, was
introduced as a bate and unhairing and deliming agent by J. Hauff, of
Feuerbach.[91] He also claims the use of hydrochloric acid to liberate the
acid after it has been combined with lime in the deliming process. It is only
soluble to the extent of about 1 in 800 of water, so that, even if used in
excess, no dangerously strong solution is formed, but it has a tendency to
slightly swell, and somewhat harden, the hides or skins, so that it is perhaps
more suitable for sole than dressing leather. It has also powerful disinfectant
properties (see p. 29).[92]
[91] Eng. Pat. 14,889, 1888.
[92] Compare also Journ. Soc. Chem. Ind., 1889, p. 954.

Hauff states that a solution of 18 lb. of cresotinic acid in 500 gallons of
water at 30° C. will bate one lot of 50 heavy hides, and that the same liquor
may be used continuously, by adding 4-5 lb. more cresotinic acid for each
successive 50 hides. For bating glove-leather, Hauff recommends the use of
5 kilos. cresotinic acid dissolved in 1000 liters of warm water for every 500
kilos. of wet skins, to which is added ammonia nearly sufficient to

neutralise the cresotinic acid, leaving the solution still slightly acid to litmus
paper; and he also advises the addition of 5 kilos. of ammonium chloride or
sulphate. The goods are paddled in this solution for about half an hour.
“Oxynaphthoic acid,” the corresponding mixed acids of the naphthols (p.
30), has also been patented by Hauff as a bate, since cresotinic acid
sometimes acts too powerfully on light skins.[93] He mentions that mixtures
of this and cresotinic acid, or salicylic acid, may also be used.
Oxynaphthoic acid requires for its solution 20,000 to 30,000 parts of water.
[93] Eng. Pats. 10,110 and 12,521. Journ. Soc. Chem. Ind., 1889, pp. 124, 809; 1890, p.
85.
A mixture of the α and β mono- and di-sulphonic acids of naphthalene
has also been patented for bating,[94] under the name of “Acrilene bating
and puering acid.” 150 calf-skins, weighing 880 lb., were pured in a 3 per
cent. solution of the α acid, and gave 266 lb. of leather as against 255 lb.
from a lot of similar weight treated with hen-dung, and this gain was more
than maintained on stuffing, while the shoulders were plumper and fuller.
This patent appears to anticipate a part of Hauff’s claim mentioned in the
next paragraph.
[94] Burns and Hull, Eng. Pat. 8096, 1891; Journ. Soc. Chem. Ind., 1892, p. 48.

More recently Hauff has patented, under the name of “anticalcium,” a
mixture of impure sulphonic acids of various cresols and hydrocarbons.
This is cheaper than cresotinic acid, and like it, possesses considerable
antiseptic powers. One-half to one-quarter per cent. solution will keep hides
uninjured for a considerable time, but at this strength it plumps
considerably, and seems more suitable as a deliming agent for sole-leather
than as a bate for dressing-leather, though it may replace drenching. No
doubt, by the use of warm water, and avoidance of excess of acid, skins
could be pulled down satisfactorily, or the plumping could be controlled by
addition of salt, but the disinfectant powers of the acid would render further
treatment with an ordinary bate or puer very difficult.[95]
[95] J. Hauff, Eng. Pat. 22,546, 1894; Journ. Soc. Chem. Ind., 1895, p. 170, Gerber,
1895, p. 133.
The “C. T. Bate,” manufactured by the Martin Dennis Chrome Company,
is of a very similar character; and is in the form of a greyish crystalline
paste, consisting mainly of sulphonic acids of naphthalene and probably
other hydrocarbons. It is very possibly made by sulphonating coal-creasote

oils, which contain much naphthalene and phenanthrene. The following
directions are given by the company for its use.
“1. After unhairing and fleshing from the lime, the skins should be
thoroughly washed with water (preferably warm) so as to remove as much
lime as possible.
2. If, in the liming process, the sulphide of sodium is used in combination
with the lime, it will render the lime more soluble and therefore more easily
removed with water.
3. The more completely the skins are cleansed with warm water the less
will be the quantity of bate required.
4. After washing, the skins should be thoroughly worked on the beam,
especially on the grain.
5. A solution of C. T. Bate is now prepared in the proportion of from one-
half pound to one pound of bate in 100 gallons of warm water (90° F.). In
making the solution do not have the water over 140° F. Under no
circumstances boil it.
6. If the hides or skins have been treated as above indicated, one pound
of bate should be sufficient for 400 pounds wet hide, washed from the
limes. The hides or skins are placed in the bating solution and worked for
an hour. They are then allowed to rest in the solution with occasional
stirring for some hours or over night.
7. The length of time that the bating should continue will depend upon
the degree of softness and pliability required in the leather. For instance, for
sole-leather fifteen minutes is sufficient; for satin leather thirty minutes; for
glove-leathers four to six hours or even longer.
8. On removing the skins from the bating solution it is sometimes
desirable, especially for the finer grades of leather, to wash them in warm
water and again work them over the beam. They are then ready to be placed
in the tanning liquors.
9. In preparing the bating solution for the second pack, draw down the
old solution one-third and replace with fresh water; then add in solution just
one-half the quantity of bate used at first, and so on with each succeeding
pack.
10. When fresh white limes are used toward the end of the liming
process, and a manure bate is deemed necessary to reduce the harshness of
grain caused by the fresh lime, it is very beneficial to give the skins from
the manure bate a drench of C. T. Bate, thereby arresting the bacterial action

of the manure bate, preserving the grain, besides cleansing, bleaching and
neutralising the skins preparatory to placing them in the tanning liquors.
11. Again, when it is considered desirable to use a manure bate, it is good
practice to treat the skins as above indicated (down to item No. 7), and then
place them in the manure bate. By this previous treatment the antiseptic
action of the C. T. Bate tends to arrest the destructive bacterial action of the
manure bate, thereby lessening the risk of damage to the grain. In all cases
where the value of the leather is dependent on the quality and perfection of
the grain, this is an important advantage to gain.”
All these coal tar “bates” are rather suitable to replace drenching than
bating or puering, as their effect is mainly that of removing lime. From their
antiseptic character they are very useful in stopping the effects of
putrefaction, and preventing ferments being carried into the tanning liquors,
and skins may safely be kept at least for some days in weak solutions, but
any necessary fermentive puering or bating should usually be done before
and not after their use.
A writer in the ‘Gerber,’ 1875, p. 279, recommends the use of dilute
solution of sulphide of sodium as a bating agent. Possibly it removes lime
as sulphydrate, and the writer named seems to have obtained good results
with glove lamb-skins. In experiments made at the Yorkshire College, a
solution of 4 grm. per litre used on 40 grm. of pelt was found to plump it
considerably, but probably a much weaker solution might be sufficient and
more satisfactory. Polysulphides, such as “liver of sulphur,” or the yellow
solution obtained by boiling dilute sodium sulphide or sodium hydrate
solution with excess of sulphur, have great power of “bringing down” the
pelt, and seem well worthy of experiment as bating agents.
In India, the pods of the babool (Acacia arabica) are much used as a
bate, the infusion being allowed to ferment. In their dry state they contain
about 12 per cent. of an easily changeable tannin, which does not
precipitate lime-water, and which by fermentation is very probably
converted into gallic acid. The use of gallic acid itself as a bate has been
patented by Albert Hull,[96] and would undoubtedly accomplish the removal
of the lime if used in sufficient quantity; but as he only uses a solution of 25
mgr. per litre (one part in 40,000) any effect must be mainly due to the
washing with water. Gallic acid forms dark oxidation products with lime.
[96] Eng. Pat. 14,595, 1889.

Of the fermentive methods of removing lime, “drenching” with
fermenting bran-infusions is the simplest in theory, and has been very
carefully investigated by Mr. J. T. Wood.[97] It will, therefore, be convenient
to consider this process first, although it is frequently employed as a means
of cleansing and slightly plumping the skin after the lime has been removed
by puering or bating. In calf-kid manufacture, however, it is now used
without previous puering, and in some other cases it is substituted for the
use of dung bates. The most important of the active ferments are two
species of bacteria, named by Wood Bacterium furfuris α and β, which are
very similar in their form and action (see L.I.L.B., p. 264), but produce a
somewhat better fermentation together than separately. They are shown in
Figs. 34 and 35.
[97] Journ. Soc. Chem. Ind., 1890, p. 27; 1893, p. 422; 1897, p. 510; Brit. Assoc. Rep.,
1893, p. 723.

Fig. 34.—Bacterium furfuris α. Fig. 35.—Bacterium furfuris β.

Neither species has any direct action on the hide substance, but ferments
the glucose produced by the action of the cerealin of the bran on the starch
which is present. A considerable quantity of hydrogen, with carbon dioxide,
nitrogen and small quantities of hydrogen sulphide, are produced during the
fermentation, together with lactic and acetic, and traces of formic and
butyric acids and amines. Active drenches contain 1-3 grm. of mixed acids

per liter, to which they owe their action, a perfectly satisfactory drenching
being produced by an artificial drench containing 0·5 grm. of glacial acetic
acid and 1 grm. of lactic acid (sp. gr. 1·210) per liter in which the skins
were worked for 11⁄2-2 hours, while 12-16 hours would have been required
in the ordinary drench. An experimental drench gave the following results
on analysis:—
Formic acid 0·0306 grm. per litre
Acetic acid 0·2402 „
Butyric acid 0·0134 „
Lactic acid 0·7907 „
Total 1·0749 „

It is probable that other organisms are capable of producing similar
fermentations, and it is not certain that in all tanneries the same ferments
are present. Mr. A. N. Palmer states that at the Cambrian Leather Works at
Wrexham, he has been unable to detect lactic acid in the drenches, all the
acids present being of the acetic series.
The drench-ferments investigated by Wood are incapable of attacking or
injuring the hide, and, in his opinion, when the skin is attacked, it is
generally due to putrefactive and gelatine-liquefying organisms introduced
from the bates, or from the air in hot sultry weather. Drenching takes place
most safely and satisfactorily at temperatures not exceeding 30°-35° C.,
when the process is usually complete in 12-24 hours. In hot sultry weather a
butyric fermentation of an active character sometimes suddenly takes the
place of the normal one (Ger. Umschlagen), the skins swell rapidly, become
translucent (glasig) and finally dissolve to a jelly. If tanned in the swollen
condition, tender and useless leather results, and the injury, once begun,
proceeds with alarming rapidity, skins being sometimes completely ruined
in a few hours. Prompt action is therefore necessary, and the first step to
take is to add salt, which checks the fermentation, and acts in the same way
as in the pickling process, controlling the action of the acid, and producing
a sort of tawing. Such skins will yield sound leather, though the grain is apt
to be somewhat drawn. If the skins can be immediately got out of the
drench, the acid may be neutralised by the cautious addition of ammonia,
soda, or whitening to the water in which they are placed, preferably in a
paddle, and if they are insufficiently drenched they may then be paddled in
tepid water, though this is hardly likely to be needed, as the effect of the
acid is to remove the lime very completely. The objection to the use of

whitening, which otherwise is the safest and best material to employ for
removing acid from pelt, is that it is apt to become mechanically fixed in
the grain, and, thus, to produce bad colour with vegetable tans. For white or
chrome leather it would do no harm. Precautions to prevent the recurrence
of the injury are to keep the temperature of the drench low, and to free the
bran from flour by washing in two or three cold waters, before adding to it
the hot water with which the actual drench-liquor is made, since the flour,
or at least its starch, is the source from which the butyric acid, as well as the
lactic, is formed. In cold weather, where drenching is proceeding in a
normal way, the flour is useful, since it is the natural nutriment of the
drench-ferment; and, in England, flour is frequently added purposely to the
bran to increase the activity of the drench. To retain the flour, the bran may
be washed first with boiling water, which gelatinises the starch and makes it
adhere to the bran, and, according to Eitner, removes a sticky fatlike matter
from it, and fits it better to remove the fat of the skin. After soaking in hot
water for two hours, it is washed in several cold waters and infused at about
40° C. for use.[98] Many tanners use the bran without previous washing, but
if much flour is present it rises to the top with the gas evolved by the
fermentation, and forms a pasty mass on the skins, which interferes with
even drenching.
[98] Gerber, 1882, p. 246.

The quantity of bran used in ordinary drenching is very variable, but
about 4 parts per 1000 of water used and from 5 to 10 per cent. on the
weight of pelt may be taken as an average quantity, more being frequently
employed. The temperature may vary from 10° up to about 30°-35° C., and
the time inversely from days or weeks down to two or three hours,
according to the temperature of the drench, the amount of ferment present,
and the thickness and character of the skins. The skins are usually thrown
into the freshly prepared drench, to which a few pailfuls of old drench-
liquor is frequently added as a ferment. Fermentation soon sets in, and the
gas evolved causes the skins to float to the surface; this is called the
“working” of the drench. Thin skins may be sufficiently drenched after once
rising, while thick ones require to be put down two or three times. A certain
sign of sufficient drenching is the appearance of small blisters on the grain,
caused by the evolution of gas in the substance of the skin. When these are
seen the drenching should be at once discontinued, as otherwise the blisters
will increase in number and burst through the grain, causing minute holes or

“pricks” (one of the many forms of the complaint called in German Pikiren
or Piquieren). When a bubble of air is enclosed in a fold of the sufficiently
drenched skin and pressed, it raises the grain without actually separating it
from the substance of the skin. The properly drenched skin also falls easily
in folds when held between the hands either lengthways or crossways, and
if thin, the skin tightly stretched over the hand shows grains of bran
underneath it as little lumps, round which the skin clings to the hand. The
drenched skin should not be transparent, but white and soft; and when
pressed should retain the mark of the finger. Some experience is required to
determine certainly the point of sufficient drenching, which, of course,
varies with the character of the skins, and the kind of leather which is to be
produced; and the feel of the skin to a practised hand is one of the most
important criteria.
A writer in the ‘Gerber’[99] divides drenching into three classes
—“sweet,” “alcoholic” and “sour.” Sweet drenching is done in a bath of
tepid bran-water, made by infusing in hot water and drawing the clear liquor
off the bran, which settles to the bottom. The skins are only allowed to
remain in 2-3 hours, or not long enough for fermentation to set in. The
process is only suited for very thin or soft skins, which will not stand any
further loosening. The use of bran-water has the advantage of saving the
labour of “branning,” or removing adhering bran with the knife on the
beam, but it is doubtful if unfermented bran has much actual effect. Bran-
water can, however, be used for drenching by fermentation, and for small
glove-lamb has largely superseded the older method. The mechanical action
of the bran in cleansing the pelt is however often useful. In sour drenching
the bran is allowed to steep and soften in cold water for many hours, and
boiling water is then added till the temperature is raised to 75° C., and it is
allowed to infuse with frequent stirring for some hours, and after cooling to
45° a considerable quantity of old drench-liquor is added as a ferment. If
the drench is used warm (30°-35°, or, in cold weather, even 40° C.), the
skins only remain in 1-3 hours, but if cold the drenching can be extended
over a period of 2-3 days, the skins being frequently handled. This
modification is suitable for glacé-kid and the harder sorts of skins, but
glove-lamb are always treated by the warm and rapid process. What the
writer in the ‘Gerber’ describes as the “alcoholic” bran-drench is probably
the method of fermentation investigated by Mr. Wood, in which ordinary
inflammable gases, but no alcohol, are produced.

[99] Gerber, 1888, p. 257.

A normal drench plumps the goods slightly, but if it contains much of the
putrid ferments carried in from the bate or puer the skins fall in it as they
would do in a bate. To increase this effect, putrid soak-liquor is sometimes
added to the drench, but with doubtful advantage.
In drench-liquors the total acidity may be determined by titration with
lime-water or N⁄10 caustic soda, with phenolphthalein as indicator; and the
volatile acids may be distilled off as described under the analysis of tanning
liquors (L.I.L.B., p. 126). For more complete methods of analysis the reader
is referred to Messrs. Wood and Willcox’s paper on the “Nature of Bran
Fermentation.”[100]
[100] Journ. Soc. Chem. Ind., 1893, p. 422.

Drenches are said to “work” somewhat better if made with water
containing nitrates, and this is quite probable; but the necessary nitrogen
can easily be supplied if required by the addition of a very small quantity of
saltpetre.
Wood is of the opinion that the ferments found in bran do not originate in
the drench itself, but come from the bated skins, as the drench-bacteria soon
die out without finishing the fermentation, and constant renewing of the
nutrient material is necessary (cp. p. 18).
Bating and puering, though differing practically in many ways, are
identical in theory, and most of what follows applies to both of them. The
action is much more complex than that of the drench, involving both
chemical reactions and those of organised and unorganised ferments, and it
is a matter of no little difficulty to say what proportion of the observed
effect should be ascribed to each of these agencies.
Formerly, the principal effect was attributed to organic salts of ammonia
and its homologues, and to amido-acids which combine with lime.
Phosphoric acid is also present, and if any exists in the form of soluble
salts, it will combine with lime, and render it insoluble and inactive. It is
probable, however, that most if not all the phosphoric acid is already in the
form of tricalcium phosphate, and therefore without effect.
It is now, however, recognised that the effects of these chemicals are of
no importance as compared with the products of bacterial action, and the
researches of J. T. Wood have cleared up much that was until recently quite
inexplicable.[101]

[101] Journ. Soc. Chem. Ind., 1894, p. 218; 1895, p. 449; 1898, pp. 856, 1010; 1899, pp.
117, 990.

Much effect has been ascribed to the digestive ferments, such as pepsin
and trypsin, which are present in fresh dung. It is known that the animal
organism secretes these in considerable excess of its requirements, but it is
doubtful whether any exist undecomposed, even in fresh dung; though they
are apparently more resistant to putrefaction and decomposition than would
a priori have been expected of such complex organic compounds, and there
is therefore a possibility of their existence in the dung, even as it comes to
be used in the tannery. Both pepsin and trypsin are enzymes (see p. 16), and
belong to the great class of albuminoids. They are soluble in water, but
insoluble in alcohol, and hence are precipitated by the addition of the latter
to their solution, but are not altered by it, and regain their activity on
solution in water. By heat they are coagulated and decomposed, and their
activity permanently destroyed.
Pepsin is the active principle of the secretion of the glands of the
stomach, and large quantities are prepared for medical use as an aid to
digestion from the stomachs of pigs. Pepsin only acts in slightly acid
solution, and, though fresh bate liquor is slightly acid to litmus, it speedily
becomes alkaline from the lime of the skins and the ammonia present, so
that the action of pepsin in a bate can only be a very limited one. Wood[102]
compared the action of a 1 per cent. solution of pepsin, acidified with 0·2
per cent. of hydrochloric acid, with that of a dogs’ dung puer liquor, both at
the temperature of 40° C. At the end of one hour the skin in the pepsin-
solution was considerably fallen, but that in the puer-solution was almost
dissolved. Since the solution here employed was much stronger than is
likely to occur in practice, and the conditions much more favourable to its
action, it may be assumed that the practical effect of traces of pepsin in the
bate may be neglected.
[102] Journ. Soc. Chem. Ind., 1894, p. 220.

Trypsin or pancreatin[103] if present, is more likely to have an effect,
since it is active in neutral and in alkaline solutions. It is the product of the
pancreas, and is largely concerned in intestinal digestion. Chemically it
much resembles pepsin, but is more resistant to heat, retaining its power of
digestion after heating to a temperature of 160° C. in a dry condition. Its
warmed solution dissolves fibrin almost instantly, and in large quantity, and
peptonises gelatin and hide-fibre, so as to render them soluble in water.

Wood found that a 1 per cent. solution of pancreatin acted far more rapidly
than a solution of pepsin of equal strength. At 40° C. in neutral solution, the
skin fell rapidly, and the action continued even in the cold. In 15 hours the
liquid was swarming with minute bacteria. At the suggestion of the Author,
the experiment was therefore repeated, with the addition of 15 per cent. of
chloroform, which prevented the development of bacteria, while it did not
stop the action of the pancreatin. The skin fell as before, but in neither case
had it the peculiar touch of puered skin, nor were the characteristics of the
leather produced from it the same. We may therefore conclude that, though
trypsin may contribute to the action of the bate or puer, it can only do so in
a minor degree, and that the principal effect of the bate or puer is due to
other causes. It is certain, however, that fresh bird-dung, and probably that
of all animals, contains ferments capable of liquefying gelatin. An instance
of this is found in the observation, common in glue manufacture, that if the
dropping of a sparrow falls on a cooler full of solidified gelatine size, it will
liquefy a track quite down to the bottom of the cooler. Trypsin, or at least
the secretion of the pancreas, as well as the gall from the liver, have great
power of wetting and emulsifying fats, and this has possibly something to
do with the action of the bate in enabling the skins to be cleansed of fat.
[103] Loc. cit. and Beilstein, iii. p. 1308, 2nd ed.

Bacterial fermentation and its products are however the main factor in the
action of puers and bates, and on this subject we owe most of our
knowledge to the work of J. T. Wood, since, though Popp and Becker have
worked over much of the same ground, they have not nearly so freely
published their results.
Wood showed that a fresh puer liquor, even when boiled for half an hour
and so freed from living organisms and albuminoid ferments, has still
considerable action on a limed skin, though much less than the unboiled
puer. He found that this action was principally due to amines and their
compounds with organic acids, which removed lime, but did not remove the
interfibrillary substance or give the proper feel of puered skin. A very
similar result was obtained with aniline (phenyl-amine) hydrochloride in 1
per cent. solution.
A considerable variety of bacteria from dung and other sources were
cultivated in various media and their puering power tested, but though
greater than that of the unorganised chemical compounds such as amine
salts and organic acids, it was in no case equal to that of an ordinary puer,

or sufficient for practical use. When, however, a small quantity of the amine
salts obtained from the puer were added to a mixed bacterial culture the
effect on the skin was almost as rapid and considerable as with an actual
puer.
In order to determine whether the puering effect was due to the direct
action of the bacteria or to their enzyme-products, the latter were separated
from a filtered puer solution by adding it to a large volume of 98 per cent.
alcohol in which the enzymes are insoluble. When redissolved in water,
they had a decided puering effect, and a solution of 0·5 grm. of the mixed
enzymes and 0·5 grm. of the mixed amine hydrochlorides in 100 c.c. of
water at 350° C. brought down a piece of limed sheep-skin in thirty minutes
exactly like a puer. The action is therefore dependent on the mutual action
of the enzymes and amine salts, but as the separation of these would be too
costly for practical use, and the puering proved more effectual when they
were formed in contact with the skin by active bacteria, Wood adopted the
method of preparing a suitable sterilised nutritive liquid, which was
inoculated before use with a mixed culture of suitable bacteria. For
laboratory purposes a suitable culture-medium was obtained by digesting 10
grm. of gelatine with 5 grm. of lactic acid (reckoned water-free) and 100
c.c. of water for three hours in a closed vessel on the water-bath. The
resultant solution was neutralised with sodium carbonate and diluted to 1
litre with addition of a small quantity of potassium phosphate.
The bacteria of fresh dog-dung were not found to possess a satisfactory
puering effect, but those from dung which had been fermented a month (as
in practice) gave a result nearly equal to actual puer. A still better result was
obtained by a mixed culture from the roots of wool loosened by sweating.
The bacteria were principally of two species, of which neither separately
was capable of satisfactory puering; but which together acted more rapidly
than an actual puer. These bacteria do not liquefy gelatine.
During the course of his experiments, Wood found that filtered puer
solutions were less active than turbid ones and that their activity was
increased even by the addition of inert substances, such as kaolin.
Wood attributes the differences in action between dog-dung and bird-
dung not only to different bacteria, but to the fact that in the latter case the
urinary products, and especially uric acid are contained in the dung.
From the results of these and similar researches, Wood in England, and
Popp and Becker in Germany succeeded in producing a practical artificial

puer, which they now manufacture in conjunction under the name of
“Erodin.”
“Erodin” consists of a solid nutrient medium and a liquid “pure culture”
of the bacteria necessary to effect the required bating or puering.
The following are the directions for working with erodin bate, as
supplied by the manufacturers:—
“For 100 lb. of wet skin washed ready for bating, about 1 lb. of erodin is
required. Or in the metric system, 1 kilo. wet skin requires about 10 grm.
erodin. The strength or concentration of the bate must not fall below 3 grm.
per litre of bate liquor, i.e. 1⁄2 oz. per gallon.
For preparing the bate a sufficiently large cask or tub carefully cleaned
and steamed out is placed near the bating paddle. The cask should be fitted
with a steam pipe easily screwed on and off, and also furnished with a clean
cover.
The requisite quantity of erodin is weighed out and put into the tub with
fifty times its weight of water, and the whole brought up to a temperature
reaching but not exceeding 40° C. (104° F.) by direct admission of steam,
thoroughly stirred, and the pure culture of Bacillus erodiens added to the
mixture. The temperature must not be allowed to fall below 25°C. (87° F.),
and a little steam should be admitted first thing in the morning, again at
noon, and in the evening, to bring the temperature up to 40° C. (104° F.).
A practical mode of procedure is as follows:—On Friday make up and
start fermenting twice as much erodin as will be required for a day’s work.
This is allowed to remain under the above-mentioned conditions until
Monday. On Monday half the amount will be used for bating; this is
replaced by an equivalent amount of fresh erodin powder, dissolved in fifty
times its weight of water, which is added to the already fermented erodin in
the tub. Proceed in this way each day until the following Friday, when there
will be left in the tub sufficient erodin for one day. This is put into a smaller
tub for use on Saturday, and the cycle of operation begun again.
One pure culture of Bacillus erodiens should be used for every 11 lb. (5
kilos.) erodin powder or less quantity.
Suppose the amount of erodin required for a day’s work to be 11 lb. (5
kilos.), then on Friday 22 lb. (10 kilos.) erodin must be mashed as above
described in 110 galls. (500 litres) water, 2 pure cultures added, and allowed
to ferment until Monday.

On Monday half of this is used, and to the remainder 11 lb. (5 kilos.)
erodin and 55 galls. (250 litres) water is added. This is repeated on Tuesday,
Wednesday and Thursday; and on Friday half is used and the remainder put
into a separate cask for use on Saturday, and in the mashing cask a fresh
quantity of 22 lb. (10 kilos.) erodin with 110 galls. (500 litres) water is
made up for use next week.[104]
[104] Mr. Wood has found that in many cases it is unnecessary to start afresh at the end
of each week, but that additional quantities of erodin solution with the accompanying
bacterial culture may be added continuously to the stock-tub as required. In puering,
the concentrated solution from the tub may be diluted with 4 to 6 times its volume of
warm water. The diluted liquor should usually only be used for one pack of skins.
On Saturday the remainder of the old mash is used up.
In case this mode of procedure is for any reason not suited to the
conditions of work, erodin may be used by making up every day a fresh
quantity with fifty times its weight of water, adding the pure culture, and
allowing it to ferment three days before use.” In some cases the solution
may be used for several consecutive packs, merely adding water and a small
quantity of erodin without a new culture.
Erodin is being used most successfully in several large works both in
England and abroad, and on calf-skins and sheep-skins has proved quite as
effective and much safer than dog-dung; the skins coming out clean and
free from stains. It has been a good deal used in the experimental tannery of
the Yorkshire College, and has proved a satisfactory substitute for puer, but
with the present bacterial cultures can only be employed warm, and does
not answer used cold like the ordinary pigeon-dung bate. No doubt a
suitable bacterial medium and culture can be found for cold bating, which
for thicker leathers is often preferable to puering, and experiments in this
direction are being undertaken.
From the multiplicity of germs present, and the adaptability of the dung
infusion as a nutrient medium for any putrefactive organisms which may
gain access to it, the bating and puering process is necessarily a dangerous
one for the goods, always leading to loss of weight, and, if the process is
carried on too long, to the more or less complete destruction of the skins.
Loss of weight, however, in greater or lesser degree is inevitable, and
indeed necessary where a soft leather is to be produced. If the skins are
allowed to lie in the bate or puer liquor, mud, containing organisms, and
zooglœa-forms of bacteria settle in the folds, and produce marbled
markings, streaks and lines by the destruction of the grain surface (hyaline

layer). Black or bluish stains are also often produced, known as bate-stains,
and either due to bacterial pigments, or in some cases, to the action of
evolved hydrogen sulphide on iron present from salting or other sources.
Frequent change of position is therefore necessary, especially when the
liquor is active from being used at a high temperature, but it does not seem
to be desirable to keep the skins in constant motion, and if puering is done
in a paddle, it should only be run at intervals.
T. Palmer[105] determined in experiments on pigeon-dung bates that there
is considerable loss of nitrogen during the process, and recommended
bating in pits from which the air was excluded as much as possible, both as
effecting a considerable economy in the dung, and in excluding false
ferments, which, he concludes, are mostly aerobic. It is not improbable that
the method is advantageous, since it has been shown by Roscoe and
Scudder that liquefaction of gelatin only takes place in presence of oxygen,
and its partial exclusion would therefore lessen the risk of overbating, and
consequent damage and loss of weight.
[105] Leather Trade Circular, 22nd Sept., 1891; 1887, p. 667; and Sanford, Journ. Soc.
Chem. Ind., 1893, p. 530.
Starting from the presumption that bating and puering are, in the main,
bacterial processes, more or less successful attempts had been made
previous to those of Wood, Popp and Becker, to substitute other fermenting
substances for dung; and probably these efforts failed in many cases, not so
much because they were wrong in principle, as from want of knowledge of
the necessary details, such as the use of proper ferments, and the provision
of suitable culture-media. Guano, prepared horse-flesh, urine, yeast, and
fermenting vegetables have all been tried. A solution of glucose or treacle of
about 10 per cent., to which 3 per cent. of pasty dog-puer is added about a
week before use, was tried many years since in a morocco-factory, at the
suggestion of the writer, as at least a partial substitute for puer, and is still in
use there. The mixture keeps for some time in an active state, and is added
to the puer liquors in the same way and in approximately the same
proportions as the dung paste. Similar in principle is the solid bate supplied
by an American firm, in which glucose is mixed with a small amount of
nitrogenous matter and phosphates, together with a lactic ferment, and
which only requires dissolving in warm water some little time before use.
Its results are good for some purposes, but rather resemble those of a drench
than a bate. In a similar way, puer may be added to bran-drench liquors, and

induces in them a fermentation which brings the skins down much lower
than the ordinary drench. It is probable that a weak glucose solution, with
traces of mineral constituents similar to Cohn’s solution (see L.I.L.B., p.
269) and “set” with sour milk, or fermenting drench-liquor, might in some
cases be used with advantage for drenching, with a saving of cost. A writer
in ‘Hide and Leather’ describes a bate in which two parts by weight of
glucose are dissolved in about 25 parts of water, and fermented, for about
three days, till a foam gathers on the top, with about one part of old bran
drench-liquor, or 0·1 part of pressed yeast, and then made up with water to
1000 parts. The goods are bated 24-36 hours at a temperature of about 35°
C, and the bate is strengthened for a second pack with about one-fifth of the
original glucose, a new bate being made at the end of a week, and set with
one part per thousand of the old one. A short bating of say 10 hours
produced very nice harness-leather, but the general tendency was to make
the goods looser and more spongy than a dung-bate. It is obviously not a
matter of indifference whether old drench, or yeast, is used to start the
fermentation, since in the latter case only alcohol could be produced
directly by the ferment introduced, though this might be fermented later, by
other accidental organisms, into acetic acid. These mixed bates, containing
glucose, are however probably wrong in principle, since the true puering
and bating bacteria will not thrive in presence of acids, and require
nitrogenous nutriment.
As regards the relative effect of dog- and hen- or pigeon-dung bates, the
chief of the published experiments are those made by W. J. Salomon at the
Vienna Versuchsanstalt für Lederindustrie,[106] in which he determined the
relative solvent power of equal quantities as being, for dog-dung 21⁄2, for
pigeon-dung 2, and for hen-dung 1. It is obvious that these figures, though
interesting, must be taken with some reserve, as the composition even of
pure dungs is by no means constant, depending on the feeding of the
animals, and adulteration is common. The writer has heard stories of a
certain dealer who used to fabricate his product from clay by the aid of a
popgun, though he does not vouch for the statement! It is generally held
that the action of bird-dung is more penetrating, but less softening and
loosening than that of dog-dung, which is thus generally used for
descriptions of leather where great softness and stretch are required. It is to
be remembered in this connection that bird-dung bates are generally used
cold, and hence are much slower in their action, which allows them time to

penetrate thicker hides more uniformly. Few analyses of the dungs used in
leather manufacture have been published, and these mostly with a view to
manurial value. Schulze[107] gives the result of forty analyses of pigeon-
dung as follows:—
Min. Max. Mean.
per cent. per cent. per cent.
Water 3·80 40·00 21·00
Nitrogen 1·47 5·04 2·53
Phosphoric acid 1·00 2·77 1·79
Potash 0·71 2·57 1·46

One sample contained 43·3 per cent. of sand!
[106] Tech. Quart., 1892, v. p. 81.
[107] Der Landwirt, 1895, li. p. 301.

Wood[108] quotes the following:—
Hen-Dung.
Per cent.
Water 60·88
Organic matter[109] 19·22
Phosphates 4·47
Calcium carbonate and sulphate 7·85
Alkaline salts 1·09
Silica and sand 6·69
Dog-Dung.
Water 31·0
Ca 43·0
Na, K, Mg 0·8
PO4 3·4
CO2 7·5
Organic matter 14·2
Traces Fe, Cl, Si, loss 0·1
[108] Journ. Soc. Chem. Ind., 1894, p 220.
[109] Containing nitrogen equal to 0·74 per cent. of ammonia.

This was apparently a sample from a dog fed on bones; that from the
kennels, which is more commonly used in leather manufacture, contains
much less lime; a sample analysed by Wood gave 4·7 per cent. mineral
matter, 9·7 per cent. organic, and 85·6 per cent. of water, part of which was
no doubt added.

Analysis.—Little or no attention has been paid to the analysis either of
dungs for bating purposes, or of the bating liquors, and although the total
cost of manure bates is a high one, it is evident that such low- priced and
irregular articles will not pay for elaborate analysis. Probably in some cases
it would be worth while to make a determination of moisture and organic
and mineral constituents by drying and ignition. Where a further
investigation is desired, the determination of the soluble matter by filtering
and evaporating a portion of the solution to dryness, and that of the nitrogen
by Kjeldahl’s method (see p. 70), would be advisable, and of course in the
future, when the subject is better understood, a bacteriological examination
may be useful. If it is desired to estimate the solution of hide-substance in
the use of bate or drench liquors, the determination of the nitrogen in a
measured quantity by Kjeldahl’s method will afford the best basis of
calculation, allowance being made for the nitrogen present in the original
bate liquor. Hide-substance contains about 17·8 per cent. of nitrogen. In
many cases, simple weighing of the solid residue, left on evaporating the
liquor to dryness and drying for several hours at 100° C., with subsequent
ignition to determine lime and other mineral matters, will suffice.
The quantity of hen- or pigeon-dung used in bating hides is very variable,
but may be stated at from 12 to 60 litres per 1000 kilos of raw hide, in at
least 2000 litres of water. The bate is generally used cold, the hides
remaining in it 4-8 days, with frequent handling; but some tanners,
especially in the United States, prefer bating in a paddle or drum at a
temperature of about 35° C., in which case the time must be diminished to a
few hours. The dung is best infused with warm water in a separate vessel,
[110] and allowed to ferment for at least a week without use, when it will be
found to swarm with micrococcus-chains. Only the clear liquor should be
run into the bate-pit, the sediment and dirt being thrown away, or used as
manure. In this way the danger of stains and flaking is much reduced. Bates
may be mended with fresh portions of dung-infusion for several successive
packs of hides, but should not be used too long, as they gain in solvent
power by the dissolved hide-substance and the increased fermentation, and
the method is not without risk.
[110] This seems to have been first suggested by T. Palmer, Eng. Pat. 13,636, 1886.

After bating, the hides are usually “worked” (“scudded,” “fine-haired”)
on the beam, to remove dirt and grease, but in America a wash in the wash-
wheel is often considered sufficient. Goods are occasionally “stocked” (p.

116) from the bates, but this is not to be recommended, as it is likely to
drive out much of the partially dissolved hide-substance and produce undue
looseness and loss of weight.
It is difficult to give any definite marks of sufficient bating other than the
soft and fallen feel of the hides, which is easily recognised by a practised
hand. One of the earliest signs of commencing overbating is the occurrence
of bluish patches, or a bluish tinge somewhat similar to an iron-stain,
which, if slight, generally disappears in a few days after the hides are taken
into the liquors. Hen- and pigeon-dung is probably best kept air-dried,
though, if very wet, or for convenience for immediate use, it may be kept in
paste like dog-dung.
Dog-dung should never be allowed to lie exposed to the air, or it putrefies
and turns black, the bating ingredients are destroyed, and it will not puer the
goods which turn black and putrid without softening. Dung should,
therefore, be mixed to a paste with water and kept in tanks, so as to be but
little exposed to the air, when it will retain its puering properties for a long
time unaffected. Fresh dung should be allowed to ferment for at least a
week before use. No accurate statement can be made as to the quantities
required. Eitner states that 1-11⁄2 pails of dung-paste (say 14-20 litres) is
sufficient for 200 medium to large lamb-skins for glove-kid. It should be
sufficient to make the water quite turbid, but not thick or soupy. For lamb-
skins a temperature of 18°-20° C. is suitable, which may be raised in very
cold weather to 25° C., to allow for cooling. The time required is from two
hours for the thinnest slink skins, to 12-14 hours for strong ones. It is well
to use wooden, and not iron, utensils for handling the dung, and it should be
strained through a coarse cloth after diluting with water. As has been
remarked, it is not desirable to keep the skins in constant motion in the
puer; they should be stirred or paddled for the first 20-30 minutes, and then
for 10 minutes every hour for five or six hours, after which they can be
allowed to lie for a longer period without injury. Puering is sufficient when
the skins feel quite soft and flaccid, hanging in folds in any direction and
allowing the flesh to be scraped off with the finger-nail.
Wood recommends that, for the puering of sheep-skins, dung should be
allowed to ferment one month before use, and states that it deteriorates if
kept over three months. The puering products are the result of the
successive action of many sorts of bacteria, and Wood is of opinion that
those actually concerned in puering originate from the air, or from the

vessels in which the dung is stored, and are not present in it when excreted.
Borgman[111] advises that the dung should be kept in a dry condition, and
only made into a paste between a fortnight and three weeks before use, by
covering in a clean cask with cold water, and on the following day mixing
to a smooth paste with a clean wooden “poss-stick,” made from wood free
from tannin. The cask should then be covered up, and allowed to rest
undisturbed till required. Clean extract-casks are very suitable for the
purpose, if carefully and repeatedly steamed out, and Borgman advises that
a regular series should be arranged, so as to supply the dung required, the
date of mixing being carefully marked on each cask. Throughout the
process the utmost cleanliness should be observed, and the casks should be
carefully steamed out as soon as emptied. Immediately before use the dung-
paste should be heated by steaming nearly but not quite to boiling point,
care being taken to avoid the introduction of condensed water containing
iron, and the dung thoroughly mixed with a large quantity (say 100 gallons)
of water at 45°-50° C., allowed to settle, and drawn off through a basket,
and strained into the puering paddle through a second basket lined with
coarse open canvas (such as is used by plasterers to cover windows while
the plaster is drying). A further quantity of warm water should be poured on
the residue in the mixing tub, and used for diluting that in the paddle to the
proper volume. The temperature of the liquor may reach 42° C. before the
skins are introduced. The liquor should be of a light colour, greenish to
brownish yellow; if darker, it indicates decomposition of the dung by
improper storing, or too long fermentation, and will be liable to cause
staining and injury to the skins. About 33 liters of dry dung is required per
100 kilos. of wet skin prepared for puering (33 gallons per 1000 lb.). Dry
dung should be of yellow to brown colour, dark brown or black dung is
spoiled and unsuitable for use. Wet dung is more difficult to judge, but very
dark brown or black should be rejected, as well as that with a very strong
smell, indicating that it has already fermented. Borgman’s directions bear
the stamp of experience and common sense, and the book as a whole repays
study.
[111] ‘Die Feinleder-Fabrikation,’ Berlin, 1901, p. 69.

Borgman recommends that the skins should be warmed by paddling for
some time in water of about 40° C. to which a couple of pails of puer-paste
have been added, before bringing them into the puer, the temperature of
which they should reduce to perhaps 38° C. The puered skins should feel

silky on the grain, and even somewhat slippery, and when pressed between
the finger and thumb a dark impress should be left, and the flesh should be
tender and easily scraped off. The requisite condition will, however, vary
somewhat with the kind of skins, and the purpose for which they are
intended. After puering, the skins may be paddled for half an hour in water
of about the same temperature as the puer.

CHAPTER XIV.
ALUM TANNAGE OR TAWING.

We have now followed the raw material up to the final stage of
preparation for its actual conversion into leather, and it remains to consider
the means by which that important change is produced. Though as yet the
vegetable tanning process is most largely used, and possesses the greatest
commercial importance, the use of mineral salts has long been known, and,
through the advent of chrome tanning, has placed the permanent supremacy
of the vegetable tannins in considerable doubt. Not only the importance of
mineral tanning processes, but their greater simplicity from the scientific
side, justify their consideration before those of vegetable origin.
In the previous chapters it has been shown that to produce a permanent
leather, it is not only necessary to dry the fibres in a separate and non-
adherent condition, but so to coat them or alter their chemical character that
they are no longer capable of being swelled and rendered sticky by water.
All salts which produce a contraction or dehydration of the fibre similar to
that caused by alcohol are capable of the first effect in a greater or less
degree. Many sulphates, and particularly those of sodium and magnesium,
though they will not alone produce leather, will so far contract the fibres as
to greatly hasten tanning by vegetable tanning materials, and they are
therefore capable of useful application in quick tanning processes,
especially where tough and light-weighing leathers are aimed at, which may
be subsequently weighted and solidified by further treatment. Strong
solutions of ammonium sulphate are almost as strongly dehydrating as
alcohol, and will produce white leathers very similar to those formed by
pickling, a fact which is certainly of considerable commercial importance.
None of these salts, however, can form a complete leather in themselves,
but require the assistance of metallic salts which will permanently fix
themselves in the fibre, and diminish or destroy its attraction for water.
Many substances have this power in a greater or less degree, but all those of
commercial importance belong to the group of which aluminium, iron and
chromium are representative, and which are capable of producing salt-

forming oxides of the formula M2O3 (e.g. alumina, Al2O3). Manganese, of
which the salts of this type are very unstable, has very slight tanning power,
while titanium, which in many ways is allied to the group, though it does
not strictly belong to it, has recently been patented as a tanning agent. For
the present, however, we may limit our attention to the three metals first
named.
Alumina and its salts demand the first attention, not only as having been
used for leather manufacture in very early times, but as being still important
commercially. The metal aluminium is now well known, and its oxide,
alumina, Al2O3 is abundant in nature, combined with silica in the form of
clay and bauxite, as fluoride in combination with sodium fluoride in
cryolite, and in some cases as a native sulphate. Alum-shale, which was
formerly the principal source of alum, is a bituminous clay containing much
iron sulphide, and which when calcined yields aluminium sulphate. As
aluminium sulphate does not crystallise readily, and was difficult to free
from iron, potassium sulphate was added to the liquor obtained by leaching
the calcined shale, from which, after concentration by boiling, potash-alum,
a double sulphate of potassium and aluminium, Al2(SO4)3,K2SO4,24Aq, was
easily crystallised out. Alum is now usually made by decomposing clay or
bauxite with sulphuric acid, and ammonium sulphate is generally
substituted for the potassium salt, yielding ammonia-alum, a double
sulphate of aluminium and ammonium of similar constitution to potash-
alum. Ammonium alum is easily distinguished from the potassium salt, by
the strong smell of ammonia which it evolves on the addition of caustic
soda or lime. So far as is known, there is no practical difference in tanning
effect between the two salts, and ammonium alum is cheaper, and slightly
stronger, its molecular weight being 906, as against 948 for the potassium
salt. Either alum dissolves readily in cold water to the extent of about nine
parts in 100 of water, and more easily, and to a much larger extent in hot
water, from which the excess crystallises on cooling. It is said that for
purposes of leather manufacture, alum solutions should not be boiled, and,
though it is improbable that this produces any change, it must be
remembered that chrome alum on boiling really does undergo
decomposition to free acid and a more basic salt, indicated by change of
colour from violet to green, from which it slowly returns to the violet form
on cooling.

Alums are only valuable in leather manufacture in proportion to the
aluminium sulphate which they contain, the potassium or ammonium
sulphate taking no part in the reaction, and since improved methods have
rendered possible the production of aluminium sulphate practically free
from iron, it has largely taken the place of alum, than which it is both
cheaper and stronger. Crystallised aluminium sulphate, Al2(SO4)3, 18Aq,
has a molecular weight of 666, which is of equal value to 906 of ammonia-
alum, and 948 of potash-alum. Iron is the most objectionable impurity in
both alums and aluminium sulphate, and may be detected by the addition of
potassium thiocyanate, which will produce a red colour, or potassium
ferrocyanide (yellow prussiate of potash), which will produce a blue. As the
iron may be present in the ferrous condition, it is safer first to boil the alum
solution with a few drops of nitric acid or bromine water. For more accurate
determination of iron see L.I.L.B., pp. 20, 136.
No satisfactory leather can be produced with a solution of alum or
aluminium sulphate alone, the skin drying horny, and incapable of softening
by stretching. In practice, salt is always used in addition, the proportion
being very variable, but averaging about half the weight of alum, or two-
thirds the weight of sulphate of alumina employed. The mode of action of
the salt has long puzzled chemists, and it has been supposed that its use was
to convert the aluminium sulphate into chloride, a reaction which takes
place to some extent, but which fails to explain the production of a soft
leather, since aluminium chloride, though freely taken up by the skin,
produces no more satisfactory leather than aluminium sulphate. The real
explanation is found in Chapter IX. Alumina is a weak base, which readily
gives up its acid to the pelt, becoming converted into a basic salt (see p.
187). The acid not only swells the pelt, and renders it incapable of
producing a soft leather, but the swollen pelt is less ready to absorb the
alumina salt, and so remains undertanned. The addition of salt prevents the
swelling effect of the acid, and produces a partial pickling of the skin (p.
89), which, in conjunction with the tanning effect of the basic alumina salt
formed, yields a satisfactory leather, though one which is readily affected
by washing. If instead of adding common salt to the alum solution, an alkali
such as soda is added, it combines with a portion of the acid, forming
sodium sulphate, while the alumina remains in solution as a “basic salt.” As
the term “basic salt” must be frequently employed in connection with
mineral tannage, it may here be explained. Basic salts are compounds

intermediate between the normal salt, in which the whole of the base is
combined with acid, and the hydrated oxide in which the whole is combined
with OH groups. Thus aluminium chloride, Al2Cl6, is a normal salt, in
which the whole of the combining powers of the aluminium are saturated
with chlorine: aluminium hydrate, Al2(OH)6, is the hydrated oxide, and
Al2Cl5OH, Al2Cl4(OH)2, and so on are basic salts in which successively
more of the Cl is substituted by OH. Generally, as a salt becomes more
basic, its solution in water becomes more unstable, and very basic salts are
either insoluble, or are precipitated from their solutions by very trifling
causes, such as boiling, dilution, or the attraction of animal or vegetable
fibres; separating into free acid and either hydrate or a still more basic and
insoluble salt. On this property depends their importance in tanning and
dyeing, many of the metallic mordants being solutions of basic salts. Basic
salt solutions are formed in various ways, the most common being the direct
solution of a hydrated oxide in a solution of the normal salt, or the
neutralisation of a part of the acid of the normal salt by the addition of a
stronger base. This is what takes place on the addition of soda to an alum
solution. If the soda is added in excess, the whole of the alumina is
precipitated as hydrate, or as an insoluble basic salt, but if a proportion not
exceeding about four parts of crystallised sodium carbonate be dissolved
separately, and added slowly with constant stirring to the ten parts of alum
dissolved in water, no precipitation will take place. In this solution leather
can be tanned, either with or without addition of salt, the alumina is taken
up more freely than from the normal alum, and the leather is more easily
softened, and more resistant to water. In fact such leather bears a strong
resemblance to the chrome tannages, standing a great deal of washing, and
considerable temperatures without returning to a pelty condition. The more
basic the solution that is used, the fuller and softer is the leather produced.
The alumina-salt taken up by the skin from such basic solutions is always
basic, while that absorbed from alum or alumina sulphate is apparently the
normal aluminium sulphate. It is probable however that the actual tanning
salt is in both cases basic, and that the acid is fixed as free acid, as in the
pickling process, as the proportions of acid and base found in the residual
liquor are somewhat variable.
Basic alumina solutions have hardly taken the place in practice which
they deserve, though they were described by Knapp in 1858[112] and have
since been patented by Hunt, but the patent (probably invalid) has been

allowed to lapse. A good stock solution for practical use is made by
dissolving 10 lb. of sulphate of alumina in 10 gallons of water, and 4 lb. of
washing soda in 4 gallons, and gradually mixing the latter with the former.
Salt can be used in addition if desired, and flour and egg-yolk may also be
added.
[112] ‘Natur und Wesen der Gerberei,’ Braunschweig, 1858.

In curing small skins, where it is not desirable for the fur to come in
contact with the liquid, or in the tawing of wool rugs, it is often convenient,
after freeing the skin as much as possible from blood and dirt, and adhering
flesh, to stretch it on a frame, or nail it out on a board, and apply a strong
alum-and-salt solution, as hot as the hand will bear, with a sponge,
repeating the operation till the skin is struck through. About 1 lb. of alum
and 1⁄2 lb. of salt per gallon is a suitable strength. In place of applying the
solution, powdered alum and salt is sometimes rubbed into the wet skin.
Alumed goods should generally be dried out rapidly, and finally at a good
temperature, as this tends to fix the tannage, which is also made more
permanent and resistant to water by keeping the skins for a month or more
in the alumed condition, an operation known as “ageing.” When first dried,
alumed goods are invariably stiff and horny, and, to give them softness,
must first be damped back to a half-dry condition, and then gradually
softened by mechanical means. “Staking,” and “perching” are the usual
methods, the first consisting in drawing the goods vigorously over a
bluntish blade fixed on the top of a post, and the second in fixing the skins
on a horizontal pole (the “perch”), and working them with the “crutch
stake,” a tool formed somewhat like a small shovel with a semicircular
blade, in place of which a “moon-knife” (a round blade somewhat like a
broad thin quoit) is often fixed in a wooden crutch. The tools, and mode of
using them are shown in Figs. 36 and 37.[113] Machines, described on p.
192, are now generally used for these operations. After the first staking or
softening, the skins are allowed to become nearly dry, and are then staked a
second time. Some judgment is required as to the precise degree of moisture
in each case: in the first instance the skins must be sufficiently damp to
yield without injury to the mechanical stretching, but in this state they retain
sufficient moisture to enable the fibres again to adhere on drying; and at the
second staking or perching, they must be damp enough to allow these fibres
to be again loosened without violence, and dry enough to prevent their
again adhering.

[113] The process shown in Fig. 37 is not actually “perching,” but “grounding,” in which
a moon-knife with a sharp turned edge is used to reduce the thickness of the skin on
the perch, at the same time as it stretches and softens it.

Fig. 36.—Staking White Leather.

Fig. 37.—Grounding with the Moon-knife.

The following slight sketch of the manufacture of calf-kid will serve to
illustrate the practical manufacture of the finer alumed or “white” leathers.
The raw material is in England mostly large market-calf, though salted and
dried skins are sometimes employed. After sufficient soaking or washing in
water, they are limed without arsenic or other sulphides, in limes which
must not be allowed to grow stale or putrid, until the hair can be easily
removed. After unhairing and fleshing in the usual way, they receive a few
days in a pretty fresh lime, in order to plump them, and are then freed from
lime gradually but as completely as possible, by successive steepings and
washings in water softened by a mixture of that already used on other goods

and by working on the beam. This acts as a partial substitute for puering
with dung, which is now no longer used on calf-kid. The goods are next
drenched in the ordinary way, 3-4 % of bran being used, and the goods
allowed to rise two or three times in the drench, which should be conducted
with the usual precautions (p. 167) to avoid the danger of butyric
fermentation in hot weather. The goods should come out of the drench free
from lime, and unswollen by acid, but full, white, and soft. The tanning (or
“tawing” as it is usually called in the case of alumed goods) is done in a
rotating drum with a mixture of alum or sulphate of alumina, salt, flour,
egg-yolk, and olive oil. About 5 per cent. of flour, 2·5 per cent. of alum, 1
per cent. of salt, the yolks of 25 eggs, or 11⁄2 lb. of preserved egg-yolk, 2 oz.
of olive oil, and 11⁄4-11⁄2 gallon (12-15 lb.) of water are required per 100 lb.
of wet pelt. The flour is first made into a smooth paste with a little water,
the egg-yolk, somewhat diluted with warm water and strained, is mixed in
together with the oil, and finally the alum and salt solution is added at such
a temperature as to bring the whole mixture to blood-heat (38° C.). The
length of drumming depends on the thickness of the skins, several hours
being required for very thick ones, but care must be taken to stop and
ventilate the drum at frequent intervals, so as to prevent the skins becoming
hot by friction. This part of the process was formerly accomplished by
treading with bare feet in a tub. After tawing, the goods are allowed to lie in
piles over-night, or are sometimes laid in tanks for a day or so with any that
remains of the tawing paste, to complete the absorption of the salt and alum,
and are then frequently split with the band-knife machine, though it would
be better, as is often done on the Continent, to split them before tawing, the
materials of which are not only costly, but unfit the splits for many purposes
for which they might be employed. The drying should be rapid, but is best
done first at a moderate temperature, or in the open air, and then in a rather
hot stove. They may now be allowed to “age” from one to three months, but
it is usually better before ageing to do the first part of the finishing process,
consisting of damping back, staking, and if necessary, shaving. Machines
are now almost invariably used for the staking, the principle of which may
be described as that of a pair of tongs, carrying one or generally two staking
blades on one limb, and a roller on the other which closes on the skin, and
presses it against and between the blades, while the tongs are drawn
backwards, allowing it to slip through. Fig. 38 illustrates the Slocomb, one
of the most popular machines of this type. After staking and ageing, the skin

is soaked in water till thoroughly wet in all parts. This not only softens the
skin, and prepares it for dyeing, but takes out the superfluous alum and salt,
and at the same time a good deal of flour and egg. To replace these, “re-
egging” is necessary, and while some manufacturers give egg-yolk, or egg-
yolk and flour only, many add a proportion of salt, and sometimes also of
alum. This is done before dyeing, if the skins are to be blacked on the table,
but as tray-dyeing (see p. 406) would again wash out the egg, the re-egging
is deferred till after dyeing if this process is resorted to. Before dyeing, the
skins receive an alkaline mordant to overcome greasiness, and enable them
better to take the colour. In former times this was usually stale urine, but
this has mostly been superseded by solutions of “hydroleine” (a washing
powder), or of soap rendered more or less alkaline with ammonia. Eitner
gives the following recipe, viz. 1⁄2 lb. Marseilles soap dissolved in boiling
water, 5 or 6 egg-yolks added, and the whole made up to 4 gallons with
water and 1⁄4 lb. potash bichromate. The colour used is infusion of logwood
or its extract, or two-thirds logwood and one-third fustic, which is best
extracted without alkali, a small quantity of soda or ammonia being
afterwards added. It is fixed and darkened by a wash of iron-liquor or a
solution of 1 of ferrous sulphate in 75 of cold water. After being again
dried, the skins are sometimes grounded with the moon-knife, softened
again by staking or perching, for which a machine with inclined or spiral
blades attached to a drum and working on a sort of leather apron is often
preferred to machines of the Slocomb type, and rubbed over on the grain
with a composition containing oil, wax, etc., and are finally ironed with a
heavy flat-iron, to give them a fine and smooth surface. Eitner gives a
recipe for the gloss:—1 kilo gum arabic, 1⁄2 kilo yellow wax, 1⁄2 kilo beef-
tallow, 3⁄4 kilo Marseilles soap, 1 liter strong logwood infusion, and 5 liters
water. The water is brought to a boil in an earthen pot, and then the soap,
wax, gum, and tallow are added successively, each being stirred till
dissolved before adding the next, and lastly the logwood. After boiling for
an hour, it is allowed to completely cool, being incessantly stirred during
the whole process. After ironing the goods are rubbed over with a final
gloss, for which Eitner gives the following recipe:—8 liters olive oil, 500
grm. tallow, 500 grm. yellow wax, 500 grm. rosin, 500 grm. gum arabic.
(No water is given in the recipe, but the gum arabic is presumably softened
in water.) The mixture is cooked for two hours in an earthen pot till the
water is evaporated, and allowed to cool with constant stirring. The skins

are then rubbed with a flannel with a very small sprinkling of French chalk,
and are ready for sale.

Fig. 38.—The Slocomb Staking Machine.

The manufacture of glove-kid is quite similar in principle to that just
described, but varied in detail to suit the softer and more delicate skins
employed, to give greater softness, and especially the quality of stretching
in any direction without springing back, which is so characteristic of the
leather. Lamb-skins are the principal raw material, though genuine kid is
also employed for the best qualities. The manufacture varies much with the
quality and character of the goods. The skins, which are mostly dried, are
soaked in clean and cool water for three to four days, according to age and
thickness. Common qualities (small imported slink lambs) are often
unhaired by dipping in or painting with a paste of gas-lime, lime and
sulphide of sodium, or lime and red arsenic, so as to destroy the wool.
Better skins are sometimes unhaired by painting on the flesh with lime
alone or in mixture, and in other cases ordinary lime-pits are used, with
limes, which are most usually strengthened with red arsenic, which is added
to the lime while hot from slaking (cp. p. 142).
The calcic sulphydrate (and perhaps sulpharsenite) thus formed hastens
the unhairing, and preserves the gloss of the grain. Well conducted glove-
kid establishments avoid as much as possible the use of old limes, which
produce a loose, porous leather, with a rough, dull grain. The liming lasts on

the average ten days, and is of the greatest importance. It is essential that
the inter-fibrillary substance should be dissolved, that the leather may have
the quality known in Germany as Stand, that is to say, may be strongly
stretched in either length or breadth without springing back. It also depends
upon the liming (and this is of special importance in the case of lamb-
skins), whether the tissue of the fat-glands is well loosened, so that the fat,
either as such, or as lime- or ammonia-soap, may be readily and completely
worked out. Skins in which this is neglected can never be properly dyed.
When the hair (or wool) is well loosened, the skins are rinsed in water,
and then unhaired on the beam with a blunt knife. The water employed in
washing should not be much colder than the limes, or it will prevent the hair
from coming away readily. The wool or hair is washed and dried for sale.
The skins are thrown into water, to which a little lime-liquor has been
added, to prevent precipitation of the lime in the skins by the free carbonic
acid of the water, which would have the effect of making them rough-
grained.
Next comes the first fleshing (Vergleichen) or “levelling.” By this, the
loose cellular tissue on the flesh-side is removed, together with the head,
ears, and shanks; and the flanks are trimmed. The skins are then again
thrown into water softened with lime-liquor as above described, and then
into a puer of dogs’-dung. This is prepared by stirring up white and
fermented dogs’-dung with boiling water, and straining it through a sieve or
wicker basket. The puer must be used tepid, and not too strong. The skins
“fall” (lose their plumpness) in it rapidly, and become extremely soft and
fine to the touch; and the fat-glands, remaining hairs, and other dirt, can
now be very readily scudded out.
Too strong puers, or too long continuance in them, produce evident
putrefactive effects on the skins. (See also p. 181.)
When the skins come out of the puer, they are stretched and worked on
the flesh with a sharp knife, and any remaining subcutaneous tissue is
removed. This constitutes the second fleshing. They are then rinsed in warm
water, and beaten with clubs in a tub, or worked in a tumbler-drum, in either
case with a very little water only; and finally brought into a tank of water,
not too cold, and kept in constant motion with a paddle-wheel.
The skins are next cleansed on the grain-side by working on the beam
with plates of vulcanite set in wooden handles, so as to remove fat, lime-
and ammonia-soaps, and other lime compounds, together with all remaining

hair or wool. The skins are now a second time washed in the “paddle-
tumbler,” first in cold, and then in tepid water; and after allowing the water
to drain from them, they are transferred to the bran-drench.
This is prepared by soaking wheaten bran in water at about 50° C., and
diluting with warm water. Sometimes the mixture is strained, and the bran-
water only used, to save the trouble and cost of removing adhering particles
of bran from the delicate skins. Sufficient of the liquid must be employed to
well cover the skins, and the temperature may range from 50° F. (10° C.) to
68° F. (20°C.). These conditions are favourable to bacterial activity, which
comes into play, and, on the one hand, evolves acetic and lactic acids,
which dissolve any remaining traces of lime, and on the other, loosens and
differentiates the hide tissue, so as to fit it to absorb the tawing solution.
Much care is required in the management of the bran-drench, especially in
summer, since the lactic readily passes into the butyric fermentation (see
also p. 167). The tawing mixture is composed (like that employed in the
fabrication of calf-kid, q.v.) of alum, salt, flour and egg-yolks, in a quite
thin paste. A small quantity of olive oil is also generally used. The skins are
either trodden in it with the feet, or more generally put into a tumbler-drum
with it. Kathreiner pointed out, some years ago,[114] that a mixture of olive-
oil and glycerine might be partially substituted for the egg-yolks, in both the
tanning and dyeing of glove-kid leather.
[114] Gerber, i. (1875) p. 170; ii. (1876) p. 664.

The tawed skins are now dried by hanging on poles, grain inwards. Rapid
drying in well-ventilated, but only moderately heated rooms is essential to
the manufacture of a satisfactory product.
The dry leather is rapidly passed through tepid water, and after being
hung for a very short time, to allow the water to drain off, is trodden tightly
into chests, and allowed to remain in them for about 12 hours, so that the
moisture may be uniformly distributed. It is then trodden on hurdles
(German Horden) composed of square bars of wood, joined corner to
corner, so as to make a floor of sharply angular ridges. The next operation is
stretching with the “moon-knife”; after which the leather is dried nearly
completely, and staked again.
This completes the tawing process. The goods are now “aged” as in calf-
kid manufacture. Before dyeing they are washed with tepid water to remove
part of the tawing mixture, and especially, superfluous alum and salt, and
are re-egged much like calf-kid, before dyeing if the latter is done by

brushing, and after if in the dye-tray or paddle. Aniline colours are more
used than formerly, especially for topping and brightening the natural
colours, but the dyewoods and other mordant colours are still largely
employed. The leather is first prepared with an alkaline mordant (stale
urine, ammonia, etc.) (cp. p. 413), then repeatedly brushed with or dipped
in the dyewood liquor, and a wash (“striker,” German Ueberstrich)
containing some metallic salt is generally applied, with the object either of
bringing out the special tone required, or of making the colour more lively
and permanent. The striker is usually a solution of one of the so-called
“vitriols”: “white vitriol” (zinc sulphate), “blue vitriol” (copper sulphate),
“green vitriol” (iron sulphate), or occasionally other salts.
After the dyeing, the skins, if dipped, are wrung out and re-egged; if
brush-dyed, sleeked out with a brass or ebonite sleeker to get rid of
superfluous water. They are then dried in an airy room. Before staking
(stretching), the skins are laid or hung in a damp cellar, or in moist saw-
dust. They are staked twice: once damp, and once nearly dry; and are
finished by glassing or ironing.
Skins which are much damaged on the grain, or otherwise faulty, are
smoothed with lump pumice on the flesh side, or fluffed with fine emery on
the fluffing wheel. They are then dyed on the flesh side, mostly by dipping,
but occasionally with the brush, in which case, the method described is
slightly modified.
Tawing with alum and salt is frequently employed for commoner and
stronger leathers, such as aprons (of sheep-skin), leather for whip-lashes,
laces for belts, and “skivers” for capping chemists’ bottles. The process is
practically the same as for calf-kid, except that no egg, and little flour is
used. Often flour is entirely omitted, and the goods may then be alumed in
tubs, in which they are merely handled, as the alum solution penetrates
quickly. Goods which are required white are frequently handled or tumbled
with a milk of “whitening,” both to improve the colour, and to neutralise
any acid present, and fix the alum by rendering it more basic. Alumed
goods can be stuffed with greases, either by hand or in the drum, after
thorough softening by staking.
Alum, and other salts of alumina are frequently used in combination-
tanning with vegetable materials (see Chap. XVII.). “Green” leather for
laces, “dongola,” and “dog-skin” glove-leathers are made in this way.
Glazed kid for ladies’ shoes must be slightly vegetable-tanned on the

surface, or it will not glaze, but this is frequently accomplished by the use
of materials in the dye-liquor containing tannins.

CHAPTER XV.
IRON AND CHROME TANNAGES.

Iron tannages may be very shortly dismissed, as their practical interest is
at present either historical or prospective, but iron salts enter in so many
ways into the chemistry of leather manufacture, that their properties must
be briefly considered. Iron exists in salts in two states, the ferrous, and the
ferric, in the first of which it is divalent, and in the second trivalent. Thus
ferrous chloride is FeCl2; ferrous oxide, FeO; ferrous sulphate, FeSO4;
ferrous hydrate, Fe(OH)2. The compounds of ferrous iron are mostly green,
like ferrous sulphate (“green vitriol,” “copperas”): exposed to air and
moisture, they easily absorb oxygen, and pass into the ferric form. Ferric
chloride is FeCl3 (or, as it is sometimes written without much reason,
Fe2Cl6), ferric hydrate Fe(OH)3, ferric oxide Fe2O3, ferric sulphate Fe2
(SO4)3, and so on. The atomic weight of iron is 56. Ferric salts are mostly
yellow or orange, ferric hydrate is yellow-brown, and on ignition is
converted into deep red ferric oxide, which is very difficultly soluble in
acids. Ferric salts in contact with more easily oxidisable matters, readily
give up oxygen, and pass into the ferrous state; and especially does this
happen in the presence of organic matter, under the influence of sunlight.
Thus iron-salts often act as carriers of oxygen, and oxidisers of organic
matter, absorbing oxygen from the air, and giving it up again under the
influence of light or heat. There are several other oxides of iron which do
not form salts, and there is a ferric acid, apparently corresponding to
chromic acid, which is so unstable that it has been very imperfectly
investigated.
Ferric salts correspond in structure to those of alumina, and like these are
powerful tanning agents, and readily form basic salts, while the ferrous salts
have no tanning effect until they become oxidised, when they form basic
ferric salts. Ferric salts are characterised by giving blue-black or green-
black compounds with tannins, and with many other allied bodies, while the
corresponding ferrous compounds are mostly colourless, though they
rapidly oxidise and darken.

Ferric iron, like alumina, forms an “alum,” a double sulphate of iron and
potassium, Fe2(SO4)3K2SO4, 24Aq, forming fine pale-violet crystals, but
dissolving to a yellow-brown solution. (It must be distinctly understood that
iron-alum and chrome-alum contain no alumina, but are simply called
alums because of their similarity of constitution, iron or chrome taking the
place of the aluminium. Iron-alum, in conjunction with salt, can be used for
tanning, giving a pale buff-coloured leather very similar to an ordinary alum
leather. Thus the presence of a small quantity of iron in an alum used for
tawing is of no consequence, except as affecting the colour of the leather. In
impure sulphate of alumina such as “alumino-ferric,” it, however, generally
exists in the green ferrous state, and only acquires tanning properties on
oxidation. Without common salt iron-salts are still less satisfactory tanning
agents than those of alumina under the same conditions, as the acid is yet
more loosely held, and though basic ferric salts are taken up in considerable
quantities by hide, the leather produced is thin, and usually brittle. Professor
Knapp devoted much study to the production of a commercial sole-leather
by basic iron-salts; and took several patents, which did not prove practically
successful, though the brittleness was to some extent overcome by the
incorporation of compounds of iron with organic materials such as blood
and urine, of iron-soaps, and of rosin and paraffin in the leather. Like most
mineral tannages, the process was far more rapid than that with vegetable
materials. Knapp’s basic tanning liquor was made by the oxidation of
ferrous sulphate with a small quantity of nitric acid. Patents have also been
taken for the oxidation of ferrous sulphate by peroxide of manganese in
presence of sulphuric acid, which produces basic ferric sulphate in mixture
with manganese sulphate, which has also some tanning properties. Attempts
have also been made to tan by treatment of the hide with solutions of
ferrous sulphate, and subsequent exposure to the air, in order to oxidise the
iron on the fibre and convert it into a basic ferric salt, but have not proved
of any commercial value.
The principal use of iron at present in leather manufacture is in dyeing
blacks (see p. 413), but in this case, its feeble hold upon acids in the ferric
state, and its tendency to act as an oxidising agent, or oxygen carrier,
renders the blacks somewhat unstable, and is frequently injurious to the
leather. There is also little doubt that the presence of ferric salts in leather
blacks has a great tendency to cause the resinification of the oil, known as
“spueing,” by promoting its oxidation.

Chrome tannages, from a practical point of view, stand on a very
different footing to those which have just been mentioned; having
established their position in the manufacture of almost all sorts of light
leathers, in competition with all the older methods, and making a serious
claim to a share in the production of belting and even of sole leathers.
Chromium is a grey, and very infusible metal, which chemically much
resembles iron in its compounds, and has an atomic weight of 52, or a little
over. Like iron, it possesses a divalent and a trivalent form, but the divalent
has so strong an affinity for oxygen, and passes so readily into the trivalent
form, that until easier means are found for its preparation, it is of little
practical interest. Its salts are blue. On the other hand, salts of the trivalent
form, corresponding to the ferric salts of iron, are very stable, and powerful
tanning agents. They are mostly green, but violet modifications are known,
corresponding to the violet crystals of iron-alum, but of a much deeper tint.
There is also a hexavalent form, probably corresponding to that of iron in
the unstable ferrates, but in the case of chromium, of considerable stability.
Its oxide is chromic anhydride, CrO3, commonly called chromic acid, which
combines with bases, and especially with the alkalies to form yellow or
orange-red salts, and the anhydride itself is almost crimson in the solid
form, though dissolving to orange or yellow solutions. Chromic acid though
it hardens and preserves animal tissues, has no tanning properties till it
becomes reduced to chromic oxide. There is also a higher, but very unstable
oxide, perchromic acid, possibly corresponding to persulphuric acid, which
is soluble in ether to an intensely blue solution. The name chromium is
derived from the intense colour of many of its compounds.
Our supplies of chromium are derived from chrome-iron-ore, a mineral
which contains oxides both of chrome and iron. This is furnaced with a
mixture of lime, and soda or potash, when it absorbs oxygen from the air,
the chromium becoming converted into chromic acid which combines with
the alkali present, while the iron remains undissolved as ferric oxide.
Lixiviating the mass, and evaporating the solution, lime and potassium or
sodium chromates are obtained, according to the alkali used, and on adding
sufficient sulphuric acid to combine with half the base, potassium or sodium
dichromate (or as it is commonly called “bichromate”) can be crystallised
out. Potassium dichromate is most commonly made, because it crystallises
well, and is not deliquescent, but sodium dichromate is somewhat cheaper,
though less convenient. Dichromates, at least in the crystallised state, are

not hydric salts like bisulphates, but anhydrochromates corresponding to the
potassium anhydrosulphate obtained by fusing ordinary bisulphate, and to
fuming sulphuric acid. Thus the formula of potassium dichromate is

CrO2OK
- O , or Cr2K2O7
CrO2OK

and its molecular weight is 294, while that of sodium dichromate, which is
similar in constitution, but crystallises with 2Aq, is 298. The molecular
weight of CrO3 is 100. Chromic acid, and acidified potassium dichromate
are powerful oxidising agents, and are used as such in many processes, and
especially in the manufacture of alizarine. If sulphuric acid be used in
molecular proportions, the product of the reaction is chrome-alum: 4H2SO4
+ Cr2K2O7 = 3O + 4OH2 + K2Cr2(SO4)4. This, like ordinary alum,
crystallises with 24Aq, and hence has a molecular weight of 998. It forms
dark purple, almost black crystals, which are a fine garnet-red by
transmitted light. In cold water it dissolves to a violet solution, which
becomes green on boiling, but very slowly resumes the violet condition
when cold. This change, which is not uncommon in chrome solutions, is
probably due to a partial decomposition into free acid and a basic salt, the
basic salts of chromium being generally green. It has been noticed that raw
pelt swells much more in the green, than in the violet solution. Being
derived from waste products, chrome-alum is often a cheap and valuable
source of chromium for chrome tanning.
For the analysis of chrome compounds see L.I.L.B., p. 141 et seq.
Chrome oxide, and basic chrome salts, when strongly ignited, become
insoluble even in concentrated acids, and their analysis is therefore attended
with some difficulty. If, however, the ignited residue (for instance a leather-
ash) be finely powdered, and intimately mixed with a fusion-mixture
consisting of equal parts of pure calcined magnesia and pure dry sodium
carbonate, and ignited (preferably over a Teclu burner), in a platinum
crucible, in which it is occasionally stirred with a platinum wire, it will be
quantitatively converted into chromate, which may be dissolved in acid, and
estimated with potassium iodide and thiosulphate in the usual way. If it is
desired at the same time to estimate sulphuric acid, it is sometimes

preferable to substitute lime or calcium carbonate for the magnesia, which
is apt to be contaminated with sulphates.
Chrome is not only of importance in tanning, but in dyeing; on account
of its power of forming insoluble colour-lakes with many mordant
colouring matters. For this purpose normal or basic chromic salts are
sometimes used, sometimes chromic acid or dichromates, the latter acting
not only by yielding chrome-oxide on reduction, but as oxidising agents to
the colouring matters. Most of the colours produced with chrome mordants
are of dark shades, that with logwood being deep violet or black. The
mordanting power of chromium is important in the dyeing of chrome
leather. Bichromate of potash is often used in dilute solution for darkening
the shade of leather dyed with other materials, but is not to be
recommended on account of its destructive action on the leather.
Numerous patents have been taken for processes of chrome tannage. The
first practical method was described by Professor Knapp in 1858 (see p.
210), though he did not recognise its value. Some of the patents have a
historical interest, though of no importance. Among these may be
mentioned that of Cavallin, a Swedish apothecary, whose object was dyeing
rather than tanning, but who treated raw hide with a solution of bichromate,
which was afterwards reduced on the fibre by one of ferrous sulphate. The
leather produced is dark reddish brown, and tender from the amount of
basic ferric salt formed at the same time. Mr. J. W. Swan, well known in
connection with photographic processes, and electric lighting, also patented
a process of chrome tannage (as an addendum to a patent on carbon
printing), in which the chromic acid first fixed in the pelt was reduced by
“oxalic, or other suitable acid.” Although it is possible to produce leather
within the lines of the patent, the strongly acid reaction of the reducing
agent renders it unsuitable for practical use. The first chrome tanning
process which made any show of practical success, was that patented in
1879 by Heinzerling, which was acquired in this country by the Eglinton
Tanning Company, and also worked under their license for a short time by
the Yorkshire Tanning Company at Leeds. Though the process was not
commercially successful on any considerable scale, it possesses points of
interest which make a brief description desirable. The hides or skins, after
preparation in the usual way, were treated in a mixed solution of salt, alum
(or aluminium sulphate), and potassium bichromate, but no systematic
attempt was made to reduce the chromic acid to a tanning form, the product

being, at first at least, merely an alum tannage, coloured, and perhaps
somewhat hardened with chromic acid, though on keeping for a length of
time, reduction gradually took place at the expense of the hide-fibre, and of
the fats employed in currying, so that the leather internally became greyish-
green, and really chrome-tanned. Specimens of the early products of the
process, preserved in the museum of the Leather Industries Department at
Leeds, have now all undergone this change, but are still tough and flexible,
showing that the rapid tendering of the Heinzerling leather, which was one
of the causes of its failure, must have been due to some error in
manufacture, and was not inherent in the process. Interesting, historically, is
the fact, that at an early stage in the life of the patent, a specimen of the
leather was submitted to the late Professor Hummel, in order that he should
suggest some means of overcoming the disagreeable yellow colour of the
product. He reduced it with a bisulphite, and coloured it with an aniline dye,
and a piece is still in the possession of the Yorkshire College, and in
perfectly sound condition. If legal publication of this experiment could have
been proved, it would have invalidated the important Schultz patents under
which most of the chrome-kid of the United States has been manufactured.
As bearing on modern chrome-tanning, the most important reaction in the
process is that of the alum with the bichromate. It has been shown by Heal
and Procter[115] that pelt absorbs practically no chromic acid from
bichromate, unless it has been previously set free by acidification. When
however alum, or sulphate of alumina is added, its sulphuric acid liberates
the chromic acid, leaving a basic alumina salt in solution, and this fact has
been utilised in some modern tanning processes.
[115] Journ. Soc. Chem. Ind., p. 251, 1895.

The first really important advance in practical chrome tanning was made
by Augustus Schultz, in 1884. Schultz was not a tanner, but a chemist,
employed by a New York firm of aniline colour merchants, and his attention
was accidentally drawn to leather by a friend who asked him if it were
possible to produce a leather for covering corset steels, which would not
rust them as ordinary alumed leathers do. The process which he adopted
was probably suggested by a method then recently patented for the
mordanting of wool by chrome oxide, and depended on the power of the
pelt to absorb free chromic acid (as it does all other free acids), and the
subsequent reduction of the latter on the fibre to a basic chrome salt, which
produced the tannage. The reducing substance employed was the free

sulphurous or thiosulphuric acid of an acidified solution of sodium
thiosulphate (hyposulphite), and as it was not certain which of the two acids
was the really active agent, Schultz duplicated his patent, so as to cover
both. Though he made no claim in his patent to having discovered the best
proportions of his ingredients, those which he specified have proved
practically useful after allowing for the modifications required by different
skins, and slightly different methods of working. His first bath consisted of
a solution of 5 per cent. of bichromate of potash, and 21⁄2 per cent. of
concentrated hydrochloric acid (or 1·25 per cent. of concentrated sulphuric
acid), reckoned on the wet weight of the prepared pelt, and dissolved in
sufficient water for convenient use in the paddle or drum which was to be
used in the process. In this bath the skins were worked till they took a
uniform yellow colour throughout, but without any tanning effect being
produced. They were now freed from superfluous chrome liquor by
draining or “putting out,” and transferred to the second bath, which
consisted of 10 per cent. of “hypo” and 5 per cent. of hydrochloric acid
similarly dissolved. In this, they rapidly took a duck-egg green colour from
the reduction of the chromic acid; and when this was uniform throughout
the skin, the tannage was complete. The exact quantity of water is not of
great importance, and good results can be obtained with anything varying
from 20 to 50 gallons per 100 lb. of pelt (200 to 500 per cent.) if time be
allowed for the weaker solution to act. The quantities of “hypo” and
hydrochloric acid given for the second bath are often somewhat insufficient,
and have to be slightly increased to complete the reduction. The reactions
which take place are represented by the following formulæ, in which the
weights of the materials taking part in the reaction are also given below the
symbols. In the first bath—
Hydro- Potas-
Potassium chloric sium Chromic
dichromate. acid. chloride. acid. Water.
K2Cr2O7 + 2HCl = 2KCl + 2CrO3 + OH2
294 + 73 = 149 + 200 + 18

As ordinary concentrated hydrochloric acid does not contain more than
about 30 per cent. of actual HCl,[116] about 2·5 parts would be required to
completely decompose 2·94 parts of dichromate, while in Schultz’s formula
2·5 parts of hydrochloric acid are used to 5 parts of dichromate. This excess

has been found useful in the production of a good leather, both to prevent
accidents from an overdose of hydrochloric acid, and because of the
modifying effect of an excess of neutral salt on the action of the chromic
acid (cp. p. 82).
[116] Acid of S.G. 1·16 (32° Tw.) contains 31·5 per cent. of HCl by weight or 36·6 grm.
per liter, and therefore is practically 10 × normal strength. Acid of S.G. 1·2 (40° Tw.)
contains 39·1 per cent. or 469 grm. per liter.
The reactions which take place in the second bath are somewhat
complicated. Eitner, in a valuable series of articles on chrome tannage,
which have been appearing in the ‘Gerber’ since January 1900, states that
even better results are obtained by using the hydrochloric acid in slight
excess, as the action of chromic acid (in the presence of the potassium
chloride of the chrome-bath) is not swelling but hardening to the skin, and
the slight swelling action of the hydrochloric acid tends to counteract this,
and also to facilitate the subsequent reduction. The two views are not
contradictory, as the excess of bichromate behaves to the hide as an alkaline
salt, which also produces a slight swelling effect, and it is quite probable
that better results are attained when the solution is either alkaline or acid,
than when the potassium chromate is exactly decomposed. Eitner
recommends the use of four parts by weight of bichromate, and four parts
of the strongest hydrochloric acid, dissolved in 400 parts of water, for each
100 parts of wet pelt, which should yield about 40 parts of dry leather. He
states that if such a bath be used, it may be safely and economically
exhausted by a second pack of skins, which is impossible in a bath
containing excess of unacidified bichromate. He gives[117] the following
explanation of the successive changes which take place when acid is
gradually added during the reduction, but points out that in practice the
reactions always to some extent go on simultaneously.
[117] Gerber, p. 297, 1900.

In the first stage, very slight acidification is required, and if the skins
have been chromed with excess of hydrochloric acid, may be altogether
dispensed with. The skins become brownish from the conversion of the
chromic acid into so-called “chromium dioxide” (probably really a basic
chromic chromate, Cr2CrO4(OH)4, which on ignition leaves Cr3O6); no
sulphurous acid is liberated, or sulphur deposited, but sodium tetrathionate
is formed in the bath, and the reaction may be represented as follows:

(1) 3CrO3 + 6HCl + 6Na2S2O3 = 3Na2S4O6 + 6NaCl + 3OH2 + Cr3O6.

Further addition of hydrochloric acid brightens the colour of the skins,
while the liquid still remains clear, and chromium chloride is formed
instead of chromic chromate, the main reaction being:
(2) 2CrO3 + 12HCl + 6Na2S2O3 = 3Na2S4O6 + 2CrCl3 + 6NaCl + 6OH2.

On still further addition of hydrochloric acid, sulphur is separated
according to the following equation, and is deposited partly in the skins,
and partly in the bath:
(3) 2CrO3 + 6HCl + 3Na2S2O3 = 3Na2SO4 + 3S + 2CrCl3 + 3OH2.

After complete reduction, and consumption of the free hydrochloric acid,
further reactions take place at the expense of the excess of thiosulphate
which should be present, resulting in the production of basic chromic salts,
and the further deposition of sulphur, mostly within the skin, as shown in
the following equations:
(4) Cr2(SO4)3 + Na2S2O3 + OH2 = 2CrOH.SO4 + SO2 + S + Na2SO4.

(5) 2CrCl3 + Na2S2O3 + OH2 = 2CrOH.Cl2 + SO2 + S + 2NaCl.

The thiosulphate bath therefore not only reduces, but precipitates sulphur
in the skin, and reduces the chromic salt to a basic state. In boiling solution,
thiosulphate precipitates the whole of the chromium as chromic oxide, but
in the cold, and in presence of free sulphurous acid, it only reduces to a
basic salt. Eitner does not consider the possibility, which certainly requires
investigation, that instead of basic salts, sulphite-sulphates are formed at
least in the first instance. Such salts of one base and two acids are quite
possible, and it is very probable that in the use of chroming baths
containing organic acids, they have considerable influence on the tannage.
The free sulphur which is liberated is partially deposited on and among
the fibres of the leather, and adds to its softness, and also acts chemically on
the oils used in “fatliquoring,” so that it is probably one of the main causes
of difference between the products of the Schultz or “two-bath” method,
and the “one-bath” processes subsequently to be described.

It does not fall within the scope of this book to describe in detail the
working methods for the production of the different kinds of chrome
leather, but a few precautions common to all forms of the process may be
named. It is not absolutely important in all cases that goods should be
completely freed from lime before chrome-tannage, but in this case a
sufficiency of acid must be allowed in the first bath to neutralise the lime
introduced. Pretty thorough liming is generally advisable, to plump and
separate the fibres, but as a rule the bating or puering of goods for chroming
should not be excessive,[118] but should be planned not to remove more than
is absolutely necessary of the hide-substance, as the chrome tannage is in its
nature soft and light, and does not lend itself to artificial fillings, such as the
flour and egg-yolk of the calf-kid process. Skins are sometimes freed from
lime by “pickling” (p. 89), and pickled skins may be chromed without
depickling, which will be done by the dichromate, but in this case the acid
contained in the skins must be considered in the composition of the
chroming bath. Skins, indeed, which are pickled with a sufficiency of acid
may be chromed in a neutral dichromate bath, and this is sometimes a
convenient mode of procedure. To prevent drawing of the grain during
tanning, skins not unfrequently receive a preliminary tannage with alum, or
sulphate of alumina, and these materials, together with salt, may be
introduced into the chroming bath, in which case they will liberate a portion
of the chromic acid, as has been mentioned in connection with the
Heinzerling process. Alum, chrome-alum, and acid salts, such as sodium
bisulphate, may be substituted for the acid in the chrome bath, but organic
acids must not be used, as they would reduce the chromic acid. The quantity
of free chromic acid in the chrome bath is of the most vital importance to
success, as it, and not the dichromate (which may be present in considerable
excess), regulates the amount of chrome taken up by the skin, and the
subsequent degree of tannage. It is very possible to injure leather by
overchroming, rendering it rough, harsh and even tender. If a bath
containing excess of bichromate is to be re-strengthened, it may be assumed
as a rule that all the free chromic acid has been absorbed by the skins, and
while it is merely necessary to restore the strength of the dichromate to its
original amount, the full quantity of acid must be used which would be
required in preparing a new bath. Where, as in Eitner’s acid chrome bath,
the whole of the chromic acid is liberated, the bath may be exhausted by a
second pack of skins. Many tanners, in order to avoid the complications of

remaking a bath, run away their chrome liquors after once using, containing
all the excess of dichromate which has been used. With proper chemical
control, this should not be necessary, and is objectionable, not only from its
wastefulness, but on account of the very poisonous character of the
unreduced bichromate. Even weak dichromate solutions, especially if
warm, are liable to cause painful and obstinate eruptions on the hands, but
this rarely occurs to tanners, as the poisonous action of the solution is
removed on reduction. It is well, however, to arrange that men who handle
skins in the chrome bath, should subsequently also work in the reducing
bath. Methods of analysis of used chrome liquors are given, L.I.L.B., pp.
142 et seq. Those for the determination of acidity are not however easily
applicable in the presence of alum and salts of chromic oxide.

[118] Goat-skins for glacé kid need thorough puering to produce a smooth grain.

The skins, on coming from the chroming bath may be allowed to lie for
some time without serious injury, but should be carefully protected from the
action of light, which reduces the chrome at the expense of the skin, and
renders the subsequent tannage irregular. It is found that skins, if brought
into a weak or neutral reducing bath, are apt to “bleed” or lose chromic
acid, which is reduced wastefully in the bath. On the other hand a strong
“hypo” bath is apt to draw the grain and contract the skins, owing to the
tannage taking place too suddenly. A somewhat strong “hypo” bath is
therefore often employed as a preparatory “dip,” the skins being simply
drawn through it to fix the chrome on the surface, piled on a “horse” and
subsequently reduced in a bath of ordinary strength. The tendency to bleed
is lessened, but at the expense of the pelt, by the reduction which takes
place if the skins are allowed to lie overnight in the chromed state. Eitner
states that skins chromed in an acid bath (i.e. where the whole of the
chromic acid is in a free state) show little tendency to bleed. After
reduction, the skins are well washed with warm water, and their subsequent
treatment is the same as that of skins tanned by the one-bath process, which
is subsequently described (see p. 211).
Naturally in practical work, the reduction cannot be made to proceed
rigidly in the definite steps described by Eitner on p. 206, but all go on in
different proportions together, though by supplying the acid in proper
quantities, and at proper intervals, they may be made in the main to follow
in the given order. Both on this account, and because neither the exact
amount of chromic acid in the skins, nor the sulphurous acid lost by escape
into the air can be exactly determined, the reduction cannot be conducted on
theoretical principles, but the best conditions must be empirically
determined. Eitner states that 12 parts of thiosulphate dissolved in 400 parts
of water, and 6 parts of (40 per cent.) hydrochloric acid are sufficient for 4
parts of bichromate per 100 of wet pelt employed in the chrome bath, of
which not more than one-half to two-thirds is absorbed; and that if equal
parts of bichromate and acid are employed in chroming, the acid used in
reducing may be lessened to 5 parts. In this case it must not be forgotten,
that if the partially exhausted chrome-bath is used for a second pack of
skins, which are afterwards finished in a bath of full strength, nearly the
whole quantity of bichromate used in making up one bath will be absorbed
by the skins. The amount of acid consumed in reduction will be greater, the

more rapidly it is added, owing to increased escape of sulphurous acid. It is
better to add the acid, previously diluted with water, in 8 or 10 successive
portions, more rapidly at first, and more slowly during the latter half of the
operation, each portion of acid being added as soon as no further change of
colour appears to be caused by that already given. These changes are the
more rapid the lighter the goods. The colour darkens at first to olive-brown,
then gradually becomes green, and finally blue, and when this colour is
uniform throughout the thickness of the goods, no further acid need be
added. For goods which have been chromed in an acid bath, Eitner states
that no acid will be needed for the first twenty to thirty minutes. It is
important to have a sufficient excess of thiosulphate in the bath when
reduction is complete, in which case the goods may be left for some hours
or overnight in the bath, to complete “neutralisation,” but Eitner prefers to
use a fresh bath of 11⁄2 parts of thiosulphate in 400 parts of water for this
purpose, the bath being used, after settling, for making up the reduction
bath for the next lot of goods, for which 11⁄2 parts less thiosulphate is used.
The goods must be kept in motion during reduction, either in a drum or a
covered paddle.
In a paper on “Die Natur und Wesen der Gerberei” published by
Professor Knapp, in 1858, he describes clearly a chrome tanning process
with basic chromic chloride, formed by the addition of sodium carbonate to
a solution of the normal salt, but he expressly states that the product was not
more resistant to water than the ordinary alum tannages. How he fell into
this error is hard to explain, for leathers produced according to his
directions, resist not merely washing in cold but boiling water. As soon as
the Schultz process proved successful, many attempts were made to evade
the patent by the use of other reducing agents than the “hypo,” and other
salts of sulphurous acid which it covered. Among these, the use of hydrogen
sulphide, and acidified solutions of alkaline sulphides, and especially of
polysulphides,[119] proved capable of practical use, though less convenient
than thiosulphate, but were soon acquired by a combination, the Patent
Tanning Company, together with Schultz’s original patents.
[119] “Liver of sulphur” or solutions, made by boiling sodium sulphide or soda with
excess of sulphur.
Under these circumstances, Martin Dennis, either by fresh discovery, or
otherwise, revived the original process of Knapp, which he patented[120]
almost word for word, and offered a basic chrome tanning liquor for sale,

without further restrictions on its use. This liquor was made by dissolving
precipitated and washed chromic hydrate (easily prepared by precipitating
chrome-alum solution with excess of alkali) in hydrochloric acid to
saturation, and adding washing soda until the solution was rendered
sufficiently basic. Such a solution may be used on skins prepared in the
ordinary way, by diluting with water, and strengthening as the tannage
proceeds, like a vegetable tan-liquor. It is doubtful if the patent is a valid
one, as it was known that the use of such a solution was not new, and it was
only granted in America on the representation, which has since been found
to be mistaken, that chlorides alone were applicable for tanning, while
Knapp had not restricted his statement to these salts. In reality chlorides and
sulphates seem equally suitable, but to produce similar results the former
must be made more basic than the latter. In any case the patent cannot cover
the general principle of basic tanning, but only the particular liquor and
mode of preparation specified. It was soon afterwards shown by the writer,
[121] that a good chrome tanning liquor might be prepared by direct
reduction of dichromate with sugar in presence of such a limited quantity of
hydrochloric acid as to produce a basic salt. Suitable proportions are 5 mol.
HCl to 1 mol. potassium dichromate, which produces a salt approximately
Cr2Cl3(OH)3. The solution is easily made by dissolving three parts of
dichromate in a convenient quantity of water, adding six parts by weight of
concentrated hydrochloric acid, and then cane-sugar gradually till a green
solution is obtained, when the whole may be made up to one hundred parts,
and will be approximately of the same strength as a 10 per cent. solution of
chrome-alum. A little heat may be needed to start the reaction, but too much
should be avoided, as considerable heat is evolved by the oxidation; and as
much carbonic anhydride is produced, which causes the solution to
effervesce briskly, the vessel used should be of ample size. In place of cane-
sugar, a good quality of glucose may be used, but some samples contain
some impurity which produces a violet solution which will not tan
satisfactorily. This liquor is in regular use in many tanneries, producing a
good quality of chrome calf, but is somewhat variable in its effects
according to the temperature employed in its preparation, and it appears to
have no real advantage over a simple solution of chrome-alum, rendered
basic by soda and with some addition of salt. A somewhat similar
preparation is Eberle’s “chromalin,”[122] in which some organic substance,
probably crude glycerine, is used to reduce the bichromate. The organic

matters, and especially the organic acids which result from the oxidation of
the sugar or glycerine, are not without influence on the tanning properties of
the liquor. Of course these solutions may be rendered still more basic by the
addition of sodium carbonate. A good stock-liquor, of approximately the
same strength as that above described, is made by dissolving 10 parts of
chrome-alum in 80 parts of tepid, but not hot water,[123] and adding with
constant stirring a solution of 21⁄2 to 31⁄2 parts of washing soda in 10 parts of
water. The chrome alum dissolves somewhat slowly without the aid of heat,
and the solution is best made either in a small drum driven by power, or by
suspending the crystals in a basket near the surface of the liquor, so that the
saturated solution can descend.
[120] Martin Dennis, U.S.A. Pat. 495028, 1893; and 511411, 1893, 7732, 1893. E. Pat.
Gallagher.
[121] Leather Trades Review, Jan. 12, 1897.
[122] Compare Eberle’s German patents, 119042, 1898, and 130678, 1899. The last of
these appears to be anticipated, at least as regards the use of glucose, sugar and starch,
by the writer’s publication in 1897 above cited.
[123] Leather Trades Review. Later investigations have shown that the temperature of the
water is unimportant if alkali be added, but chrome-alum dissociates to some extent in
hot water, and comparative experiments have shown that solutions of the normal salt
made with the aid of heat act on skin as if more acid than those made in the cold.
Eitner[124] has pointed out the important effect that differences of basicity
have on the tanning properties of chrome solutions. Normal chrome
sulphate or chrome-alum colours the leather quickly and equally
throughout, and swells the pelt on account of its practically acid character,
but gives a thin and lightly tanned leather, from which much of the chrome
washes out, unless it is at once “neutralised” in alkaline solutions. As the
chrome solution is made more basic, the tannage penetrates more slowly,
but is heavier and more thorough, the colour is darker and bluer, and much
less of the chromic salt is removed by washing with water. When the
basicity becomes excessive, the solution becomes unstable, and
decomposes on dilution with water into a very basic salt which is
precipitated, and a more acid solution than that given by a moderately basic
salt. The effect of such solutions on the leather is very unsatisfactory,
producing the bad effects both of too acid and too basic salts. The pelt is apt
to be swollen and lightly coloured by the more acid salt, but at the same
time the actual tannage proceeds very slowly, and in extreme cases it is
difficult to tan through, while the surface becomes over-tanned, and the
grain often tender and even brittle from the incrustation of precipitated

basic salt. Eitner likens the effect of the more acid liquors to the quickly
penetrating and lightly tanning vegetable tans, such as gambier, and that of
the more basic to the heavier tannages, such as valonia; and within limits,
advantage may be taken of these facts in adjusting the liquors to the
character of the leather it is desired to produce. In sulphate liquors, he
considers the salt CrOH.SO4 as most suited to general use, and in the case
of chrome-alum, this is produced by the use of 286 parts of soda-crystals, or
106 parts of dry sodium carbonate (1 molecule) to 998 (or practically 1000)
parts by weight (1 mol.) of chrome-alum. (In using washing soda, care must
be taken to employ clear crystals of the salt, and not those which have
become white by loss of water.) In place of soda, Eitner makes a similar
basic liquor by boiling 1000 parts of chrome-alum with 248 parts (1 mol.)
of sodium hyposulphite until the whole of the liberated sulphurous acid is
driven off, and the sulphur deposited. In comparative experiments by the
Author, no difference could be detected between the tanning effects of the
two solutions, and that with soda is both cheaper and more easily made. If
the solution with hyposulphite is not boiled, a more acid liquor results, in
which part of the chromium is combined with sulphurous acid, forming an
unstable compound which may prove useful in certain cases.
[124] Gerber, 1901, pp. 3 et seq.

Eitner states that he has made chrome-solutions of various types,
containing organic compounds in combination with the chrome-salt, which
combine with the leather, producing a fuller and softer tannage, but he gives
no details as to their preparation, as they are made commercially by the
“Erste Oesterreichische Soda-Fabrik” at Hruschau. The writer has found
that in some cases by the addition of say 3 parts of sugar, or still better of
glucose, to 10 parts of the chrome-alum in making up the basic liquor, a
much fuller and plumper leather is produced, which dries perfectly soft,
even without staking or fat-liquoring; and it is probable that many other
organic compounds may be found which produce similar effects. The
addition of very small quantities of even neutral tartrates or lactates, and
probably of many other organic salts or acids, have a remarkable effect in
lowering the apparent basicity of the solution, and it is possible that these
may also be usefully employed in combination with very basic liquors. It is
highly probable that the unsatisfactory tanning liquors produced by direct
reduction with some samples of glucose are due to the presence of small
quantities of some organic acid produced during the oxidation. It has been

found that these solutions may be made to tan by the liberal addition of
soda. It is probable that more satisfactory results in chrome-tanning will be
attained by the direct addition of known organic substances to basic liquors
of definite constitution, than by the somewhat uncertain products of organic
oxidations.
The quantity of salt to be added depends on the qualities desired in the
leather, and upon whether chloride or sulphate liquors are employed; salt in
chloride-liquors increasing the softness of the leather, but in excess tending
to flatness, while in sulphate-liquors it practically diminishes their basicity
by converting the chromium sulphate into the equivalent chloride, which, as
Eitner points out, behaves as a less basic salt, and hence but little advantage
is to be gained from its use. It is best to begin with a very weak liquor, to
avoid drawn grain, and for the same purpose a preparatory tannage with
alumina salts, or an addition of alum or sulphate of alumina and salt may be
made to the first liquor, as the attraction of the chrome salt for the fibre is
sufficient to produce a chrome tannage, even in presence of excess of
alumina salts. 10 lb. of chrome alum will tan about 100 lb. of wet pelt, but
more must be used for the first parcel; as to avoid loss of time, the skins
may be tanned out in a pretty strong liquor. The bath has a tendency to
become acid by use, and before strengthening, it may be necessary to add
some more soda solution. Very little additional salt is required, as it is only
absorbed by the skins to a small extent, probably as chromic chloride. As
the liquors gradually become charged with sulphates, it is best to work them
out like bark liquors, and not to go on strengthening the same liquor
indefinitely. If old liquors are used for green goods, it is not necessary to
neutralise them with soda before use, as Eitner has shown that less basic
liquors colour more evenly and with less tendency to produce drawn grain.
Basic chrome liquors, such as have been described, may also be used in
chrome combination tannage. It is generally best to let the light vegetable
tannage precede the chrome, and lightly tanned skins, such as “Persians”
and East India kips, acquire many of the qualities of chrome-tanned leather
by the treatment. The effect is still further increased by a previous
detannisation of the leather with alkaline solutions (see p. 241). Several
firms beside Dennis now supply basic chrome liquors ready prepared for
use.
The time of tannage will of course vary with the thickness of the goods,
and for calf-skins will usually extend over some days, though it can be

much quickened by drumming. The tannage is generally best accomplished
in the paddle, but can be carried out by frequent handling in pits or tubs, or,
where very smooth grain is important, by suspension. When the goods
come out of the final liquor, they may be allowed to lie in pile for twenty-
four hours, or even for some days, with advantage, as the surplus chrome
liquor is pressed out, and the tannage becomes more complete. They are
then washed with plenty of warm water, till it ceases to be coloured with
chrome. They may be kept for an almost unlimited time in a wet condition,
as they do not bleed, and have little tendency to heat even in pile. They
have now reached the stage at which we left the “two-bath” leather, and the
subsequent treatment may be the same in both cases.
Although by both processes, the chrome-salt fixed in the fibre is of a
decidedly basic character, it still contains enough acid to act injuriously on
the leather in course of time, and to lead to serious inconveniences in its
subsequent treatment. Before proceeding further, this access of acid must be
removed or neutralised, and it is not too much to say that most of the
troubles experienced in the fat-liquoring arise from neglect or mistake in the
washing and neutralisation. The difficulty in the process arises from the fact
that while the acid should be reduced to a mere trace, it must not be entirely
removed,[125] as chromic oxide itself does not seem capable of tanning, and
at any rate the effect of excess of strong alkalies is at once to render the
leather hard and pelty. Borax is one of the safest neutralising materials,
about 3 per cent. on the wet weight of the pelt being required, in not more
than 1⁄2 per cent. solution. Eitner recommends the use of silicate of soda,
which, sold as a solution of S.G. 1·5, is somewhat stronger and much
cheaper than borax. Hyposulphite of soda and whitening together neutralise
more rapidly and completely than either alone. Other salts of weak acids
may also be used, the acids exercising a regulating influence which
prevents neutralisation going too far. Sodium carbonate or bicarbonate, or
ammonia may also be used, but with these it is difficult to get even
“neutralisation,” or to avoid the risk of carrying the process too far. Even a
thorough drumming with a milk of “whitening” (calcium carbonate) is
effective. With the latter there is no danger of overdoing the process, but in
some cases the adhering whitening and precipitated calcium sulphate are
troublesome in later operations. In any case the neutralising should only be
carried so far that the skins show no acid reaction to litmus paper.
[125] Procter and Griffith, Journ. Soc. Chem. Ind., 1900, p. 223.

It is probable that one of the great causes of difference between “one-
bath” and “two-bath” leathers is the presence of free sulphur in the latter.
This may also be introduced into “one-bath” leather, by treating it in the wet
chromed state, without washing out the chrome liquor, with excess of a
solution of hyposulphite, or of an alkaline polysulphide, which at the same
time will neutralise the skin. The more acid the chrome liquor, the greater
the quantity of sulphur which will be introduced. The simplest means of
distinguishing “two-bath” from “one-bath” tannages is to test for the
presence of sulphur, by wrapping up a silver coin, with a piece of the
leather in paper, and leaving the parcel for an hour in the water-oven, or
some other warm place, when the presence of sulphur will be shown by the
blackening of the coin. Of course a sulphurised “one-bath” leather will give
the same reaction.
The leather must now be dyed and fat-liquored. Which of these two
operations should be first undertaken will depend on circumstances. Most
leathers dye more easily before fat-liquoring, but as many dyes are soluble
in the alkaline fat-liquor, a good deal of colour is often lost. This may be
compensated by dissolving a suitable aniline (acid) colour in the fat-liquor.
“Bluebacking” is generally done before fat-liquoring by drumming with
methyl-violet, or some other aniline colour (with or without logwood,
which gives alone a very dark violet). Any shaving or splitting required
must of course be done before bluebacking.
The fat-liquor is an emulsion of soap and oil, which for chrome leather
should be as neutral as possible, if the neutralising has been thorough; but if
any acid be left on the skins, a neutral fat-liquor will be precipitated as a
greasy mass. This can sometimes be remedied by the addition of a little
ammonia or borax, or by re-fat-liquoring with soap solution only, but if the
washing of the skins has been incomplete, and soluble chrome-salts remain,
the mischief is almost irretrievable, as sticky chrome-soaps are formed,
often coloured with the aniline violet, which adhere to the skins, and which
can scarcely be removed by any solvent which does not injure the leather.
As regards the soaps and oils used, there is considerable latitude: 11⁄2 per
cent. of castor-oil soap, and 3⁄4 per cent. of castor or olive oil on the wet
weight of the pelt has done good service in my hands, but many
manufacturers employ soft soaps, curd soaps, etc., with castor, olive, cod or
neatsfoot oil, and sometimes sod-oil or degras. Eitner considers olive-oil
and olive-oil potash soap the most suitable, and particularly warns against

the use either of drying oils or of oils containing tallow (such as neatsfoot),
which are not only apt to cause a white efflorescence, but to give the leather
a disagreeable rancid smell. Fish-oils are unsuitable, but mineral oils are
often useful constituents of fat-liquors. Wool-fat also makes a good fat-
liquor, but is unsuitable for goods which are to be glazed. “Turkey-red oil”
(which is sulphated castor) may be used as a fat-liquor, simply mixed with
warm water, without soap, and has been recommended where delicate
colours are to be dyed after fat-liquoring; but it is said to have an
unsatisfactory after-effect, hardening and tendering the leather. Some soaps
made from the saponifiable part of wool grease, such as “Lanosoap,” also
act well in conjunction with olive, castor, or other oils. Where leather is to
be glazed, the amount of fat-liquoring must be kept very moderate. Fat-
liquors should be thoroughly emulsified, and are generally used warm. They
penetrate better if the leather is partially dried by sleeking out, or pressing,
or cautious “samming,” but the leather must not be completely dried out
before fat-liquoring and dyeing, unless it has been previously treated with
glycerine, glucose, treacle or some deliquescent salt, which will enable it to
be wet back. Chrome leathers are not “waterproof,” as has often been
stated, unless rendered so by treatment with soaps and greases, and are
apparently easily wetted, but the fibre will no longer absorb water after
thorough drying, and consequently will neither dye nor stuff satisfactorily.
In order to enable chrome leather to be kept in an undyed condition,
glycerine or syrup is sometimes mixed with the fat-liquor, but as the watery
portion of this is not generally completely absorbed, the process is
somewhat wasteful. Mr. M. C. Lamb avoids this difficulty by applying a
solution of glycerine to the grain-side with a sponge after fat-liquoring. In
this case the leather may be dried sufficiently for staking or shaving without
risk.
Chrome leather can be dyed by many of the acid aniline colours without
a mordant. Basic colours are only fixed when the leather has been first
prepared with a vegetable tannin, gambier, or a mixture of gambier and
sumach being the most suitable. Considerable care must be employed in the
application of tannins to chrome leather, as they have a tendency to harden
it and diminish its stretch, or even to render it tender, but traces of tannin in
the dye probably facilitate glazing. Before dyeing, it is advantageous to fix
the tannin with tartar emetic, or for browns and yellows, with titanium
potassium oxalate solution, which itself gives a good yellow-brown with

tannin. In place of employing the tannin and titanium salt in two separate
baths, they may be combined, using a weight of the gambier or tanning
extract (oakwood, chestnut, etc.) about equal to that of the titanium salt, or
titanium tanno-oxalate solution may be used. Chrome leather may be dyed
with the various dye-woods, which are mordanted by the chromium present,
but the colours are mostly dull, that of logwood being nearly black. A good
black of a very permanent character is obtained by dyeing with logwood,
and saddening with a hot solution of titanium oxalate in the drum. A little
iron-alum added to the chrome liquor in tanning will facilitate dyeing the
skins black with logwood and help it to penetrate through the leather, which
is sometimes desired. Several aniline blacks, and notably the “corvolines”
of the Badische Anilin und Soda Fabrik, Casella’s “leather black C,” and
Claus & Rée’s chrome-black give very satisfactory blacks by brushing or
dyeing.
Chrome skins may be glazed in the ordinary way with blood or albumen
mixtures under glass or agate, but require good pressure and repeated
seasonings and glazings, and much care is required in fat-liquoring. The
glazing is often assisted by the previous application of barberry juice (épine
vinette) or of lactic or tartaric acid solution with a trace of sugar. Much of
the difficulty which has been experienced in glazing chrome leathers is due
either to the natural fat of the skin, or to oils used in fat-liquoring in
excessive quantity or of unsuitable character.

CHAPTER XVI.
PRINCIPLES OF THE VEGETABLE TANNING PROCESSES.

The processes employed in the production of leather with the vegetable
tanning materials vary extremely according to the class of leather which is
being produced, both in the materials selected and in the time required. In
sole-leather tanning, where thick hides are used, and where diffusion is the
only force acting to carry the tannin into the hide, many months are
frequently needed, while with thin skins, and with the aid of mechanical
motion, which circulates the tanning liquid between the fibres, the process
is often complete within a few hours. Differences in the strength of the
liquors according to whether hard or soft leathers are to be produced, and
the mutual action of the acids naturally present in the liquors, and of the tan,
have also a determining effect upon the quality of the product.
The simplest form of tanning in principle is probably the old-fashioned
method of sole-leather manufacture. For this purpose, the hides are usually
“rounded” or trimmed after liming, unhairing and fleshing, so that the most
valuable part, the “butt,” can be tanned separately from the “offal.” The
butts are usually washed in water to remove a portion of the lime,
considerable care being required at this stage to avoid carbonation and
fixation of chalk by means of free carbonic acid, or hydric calcium
carbonate (temporary hardness) in the water employed, or by the free
carbonic acid of the air. This somewhat primitive process can at best only
remove a small portion of the lime, since so long as the lime remains in the
caustic condition, it is very obstinately held by the hide-fibre. Advanced
tanners now frequently employ weakly acid baths, in addition to washing,
in order to produce more complete deliming, and this effects a very
considerable improvement of colour in the early liquors. The use of lactic
acid (free from iron) or boric (boracic) acid in solutions of about 4 lb. per
100 gallons, in which the butts are kept in motion, are among the safest and
most satisfactory ways of removing surface-lime and improving the colour,
but even the stronger mineral acids may be used successfully with caution
(see Chap. XIII.).

Whether acid be used or not, the butts are now usually suspended in deep
pits containing old and nearly exhausted tan liquors. These liquors contain a
certain amount of lactic and acetic acids, derived by fermentation from the
sugary matters of the tanning materials, and also in some cases, weak acids
originally present in the materials themselves. These acids are most
important to successful tannage, and their effect is twofold; in the first
place, they neutralise and remove any lime which still remains in the butts;
and, secondly, they bring the butt into a slightly acid condition, in which it
remains plump and swollen in the liquors, while the tannin gradually
penetrates and tans the fibre. If, as frequently happens, especially in modern
yards where extracts are very largely used, the natural acid of the liquors is
not sufficient for this purpose, the lime combines with the tanning matters,
and the butts either become discoloured at once, or darken by exposure and
oxidation, when they come to be dried, while the pelt remains flat and
insufficiently swollen. To avoid this trouble, resort is sometimes had to
artificial acidification of the liquors. As a general rule, it may be stated that
it does not answer to mix the stronger mineral acids directly with the
liquors, but lactic and acetic acids may be used, or even oxalic acid may be
added to the suspenders in such quantities as to precipitate and remove the
lime which they contain, setting free the organic acids with which it had
been combined. The use of oxalic acid should never be pushed further than
this, as it has a most powerful swelling action on the hide; and goods which
are too much swollen by acids tan dark and brittle.
After the hides have remained from ten days to a fortnight in the
suspenders, they are usually laid in pits called “handlers” which are worked
in series of 6, 8, or 10 pits, containing the same number of packs of goods.
The weakest liquor from the youngest pack is run to the suspenders daily, a
new and stronger liquor is run to the pit, which now becomes the head of
the series, into which the oldest and most tanned pack of butts is moved;
and the next takes its place and liquor, and so on down the series, the
youngest pack finally occupying the place which had previously been taken
by the last but one. In this way each pack receives a change of liquor of
regularly graduated strength; and during the time which it remains in the
handlers, passes from a strength of perhaps 20° Bkr. (sp. gr. 1·020) to one
of about 40° Bkr. (sp. gr. 1·040). During this part of the process the butt is
completely or nearly coloured through, and is then ready for the “layers.”

In the forward handlers, dustings of ground bark or other tanning
material are very frequently given, and the layers only differ from these in
having much heavier dusting, stronger liquors, and being allowed to remain
undisturbed for greater lengths of time, ranging from a week up to a month
or even six weeks, as the tannage progresses. The handler-liquors are
principally from the old layers, though they are frequently made up with
weak liquors from the leaches, and strengthened with extracts or gambier.
Very varied materials are used in the manufacture of sole-leather. Oak-
bark is one of the oldest, and as regards quality one of the most satisfactory,
but it is costly, not only on account of its weakness in tannin but from the
light weight of leather which it gives. Valonia is one of the favourite
materials, giving heavy weight and a solid leather, in which it deposits a
great deal of bloom. Oakwood, chestnut-wood, and hemlock-bark extracts
are now very largely consumed, principally in strengthening the layer-
liquors; the great object being not only to lessen the cost in material, but to
save time, and produce greater weight and firmness. The layer-liquors in
some yards where extract is used, reach strengths of even 120° to 150° Bkr.
(sp. gr. 1·12 to 1·15), while in pure oak-bark yards it is difficult to get
above 30° or 35° Bkr.; and even these figures are only reached by
repeatedly strengthening the same liquor, in which large quantities of non-
tanning substances accumulate. The opinion of the most intelligent tanners
is, however, that better results are attained by a regular change of liquor,
even if the apparent strength is less.
When the leather has remained a sufficient time in the layers to have
attained all the weight and solidity of which it is capable, it is washed up in
a clear and somewhat weaker liquor or even in warm water, and taken into
the shed to be dried and finished. As this finishing is almost purely
mechanical, and scarcely comes within the scope of the present volume, a
very brief sketch must suffice.

Fig. 39.—Wilson’s Striking Machine.

The mode of finishing which was formerly, at least, in vogue in
Lancashire and Cheshire may be taken as a type of the best work. (In the
present day, the various methods are so widely known that they have ceased
to be local, and are varied according to the quality and tannage of the
goods.) The butts, which in earlier times were largely bark-tanned, are taken
wet from the pits, and scoured on a rounded beam or “horse” with stone and
brush, till the bloom is completely removed, and are then lightly oiled on
the grain, half dried (“sammed”), laid in pile to temper, and “struck out”
with the “pin,” a two-handled tool of triangular section shown in Fig. 29.
The use of this tool has now been largely superseded by Wilson’s striking
machine Fig. 39, in which knives or sleekers (or stones and brushes), held
in jointed arms, are made to work on the butt, which is extended over a
slowly rotating cylinder. The object of the pinning is not so much to remove
bloom or dirt, which has been previously effected by the scouring, as to
smooth and flatten the grain. After further drying, a second pinning is
generally given, and the goods are then twice rolled, first with a light
weight, and somewhat moist grain, and then more heavily with the grain
nearly dry. This was formerly accomplished by a sort of box or car, heavily
loaded with weights, supported on a smooth brass roller of about 5 inches
diameter and 9 inches long, and manipulated with a long wooden handle on

a floor of hard wood, or zinc plates. One type of the machines which have
now almost entirely replaced this primitive contrivance is shown in Fig. 40,
but is principally used for offal and common classes of goods. For better
work, traversing rollers, such as Wilson’s ingenious double bed roller
shown in Fig. 41, are to be preferred. After rolling, the goods are dried
pretty rapidly by the aid of moderate heat, and, after polishing with a brush
(hand, or machine, Fig. 42), are ready for sale. It may be pointed out that
although the tools are different, the process is almost the same as that used
for “vache lissée” in France and Belgium, and closely resembles that of
currying harness leather except that the “stuffing” with fats and oil is
omitted.

Fig. 40.—Offal Roller.

In contrast with the rather elaborate method just described, we may place
the American finish of red hemlock sides, which are tanned throughout with
a material which yields no bloom. On these, the scouring and “striking” is

altogether omitted: the goods are completely dried out from the pits, which
is found to fix the dark-coloured liquor, and result in better colour; they are
then damped back, and tempered, and heavily rolled under a rapidly moving
pendulum roller, which polishes at the same time that it smooths the leather.
The saving of cost by so simple a process is not inconsiderable.

Fig. 41.—Wilson’s Double-bed Butt Roller.

In the West of England, much heavy leather is still manufactured from
South American hides, which are tanned with a large proportion of valonia;
and which consequently are heavily bloomed. No attempt is made to
remove this bloom, which would too much lessen the weight and firmness,
but the goods, after a light oiling to preserve the colour, are hung up and
partially dried, and are then laid in pile to temper. The grain side is now wet
with soap and water, with which a little oil is often mixed, and the bloom is
“struck in” with the pin or machine; a somewhat blunt pin being used, or a
blunt tool in the striking machine; which is held at such an angle as to
smooth and compress the grain without taking too much hold on it. After a
little further drying, the striking is generally repeated, the goods are washed
over with water, and rolled “on.” They are now coloured with a mixture of
pigment colour, generally containing a large proportion of whitening, or

sometimes of French chalk coloured with ochres, chrome-yellow and
orange, or whatever may suit the tint preferred by the tanner, or best imitate
the colour of a clean-scoured tannage, and usually mixed with size and oil,
or sometimes with oil and tan liquor. This mixture is well rubbed in, and
smoothed over with a cloth, and then polished by brushing, when the goods
are “rolled off,” rapidly dried, and again brushed. If the work has been well
done, it is not easy to distinguish from clean scouring, and is much cheaper.

Fig. 42.—Brushing Machine.

A method intermediate between this and the first described, and which
was formerly much used in London, was to proceed as above, but using
more water and holding the pin in the first striking so as to scour out as
much bloom as possible, and assisting this by the free use of water and the
brush. Instead of using an opaque pigment-colour, the goods were generally
coloured either between striking and the first rolling, or between the two
rollings, with a transparent colour, such as dissolved annatto, or a mixture
of aniline dyes, so as to conceal the traces of bloom, and to render slight
damages to the grain less conspicuous.
The principles of the manufacture have been fully explained in previous
sections, up to the time when the goods are taken into the tanning liquors.
At this stage complicated reactions take place between the lime in the butts,

the free vegetable acids in the liquors, and the tannins; and on the right
adjustment of these three factors much of the success of the operation, and
indeed of the whole manufacture depends. If the lime is in excess of the
acids present, it forms insoluble compounds with the tannins in the surface
of the hide. If these are protected from the air, they are generally
redissolved as they advance into more acid liquors, but they readily become
oxidised into dark-coloured matters, which can no longer be removed. Their
presence in the finished leather is one of the great causes of darkening in
drying. If the hide in the limy condition has been exposed either to the
carbonic acid of the air, or to free carbonic acid, or acid calcium carbonate
dissolved in water (“temporary” hardness, p. 94), a precipitate of calcium
carbonate will be formed in the surface, which is much more difficult to
remove than free lime, and which is perhaps the most common cause of the
stains and discolorations which are so serious a source of loss to the sole-
leather tanner. These stains may, if not too much oxidised, be removed by
treatment of the tanned leather with weak warm sulphuric acid, but this
remedy brings other evils in its train, and should not be required. The great
remedy is to keep the goods from the time of unhairing till they go into the
liquors, under water in which there is always a trace of caustic lime, or
which at any rate are free from carbonic acid. In deliming sole-leather with
acids, it is best to give the full dose of acid required, at once, and not
gradually, so that it may act most powerfully on the exterior, and remove
any carbonates present, before it penetrates to and becomes neutralised by
the excess of lime in the interior. This is exactly the reverse of what is
advisable with dressing leather, where the object of the tanner is to remove
lime as uniformly and completely as possible, without excessive acidity of
any part. Of course hides should not, even in the case of sole-leather, be
allowed to go into the liquors while any acid swelling of the surface
remains, but this will soon disappear if the goods are suspended for a time
in cold water after deliming, unless excess of acid has been used (cp. p. 153
et seq.).
If the proportion of free acid in the suspender liquors is as it ought to be,
it is probably rather advantageous than otherwise for a little lime to remain
in the interior of the hide, as it keeps the pelt in a plump condition during
the first stages of colouring, quickens the penetration of the tannin, and
lessens the tendency to “drawn” or wrinkled grain, which arises when the
goods go into the liquors in a flat or fallen condition. The causes of drawn

grain are often a little obscure. Of course that case needs no elucidation in
which the hides are submitted to the tanning liquor in a creased or wrinkled
condition, which is simply fixed and made permanent. This may arise,
either from carelessness in handling the goods before taking into the
suspenders, or from the way in which they are slung to the sticks, which
often draws them into long wrinkles, afterwards difficult to remove. Drawn
grain in general, however, arises from the grain-surface becoming tanned
and fixed in area, while the substance of the hide is in a more extended
condition than that which it assumes as tannage proceeds. Hides in a flat
and unswollen condition are thinner, the fibres are slenderer and looser than
when swollen, and consequently the hide has a larger area. If, after the grain
is tanned, the substance of the hide becomes contracted in the liquor, either
by swelling with acids, or by the direct action of the tannin on the interior
fibres, the grain is certain to be shrivelled, like the skin of a dried apple. A
similar effect, produced in a mechanical way, may always be noted where a
hide has been coloured hanging grain-side out over a pole, so that the
surface is extended at the bend, on which long wrinkles are formed as soon
as it is straightened.
A hide in a slightly alkaline condition colours, and even tans more
quickly than one which is acid. In presence of a trace of lime, and
deficiency of free acid in the suspender liquor, tannages of valonia and bark
give the butt a sort of lemon-yellow colour, which is not in itself injurious,
and which disappears as the hides advance into more acid liquors, but
which is a sign of danger, as showing that no excess of acid exists in the
suspender-liquors. Gambier gives pelt perfectly free from lime a pale buff
colour, but where lime is present, the colour is always reddish, and much
darker, and this coloration does not disappear so readily as that with
valonia, so that if gambier is to be used in the first liquors, care should be
taken to remove all lime from the surface. The only known tannin which
gives no insoluble compound with lime is that of the babool pod
(sometimes called “gambia-pod”), which is frequently used in India as a
bate, and which would probably prove very useful in colouring liquors (pp.
165, 288).
When sole-leather first goes into liquors, it is generally swollen with lime
to some extent. If the liquors contain, as they usually do, sufficient free acid
(acetic, lactic) in addition to the tannins, these combine with and neutralise
the lime, and the pelt, without absolutely becoming flat and thin, loses its

firmness, and becomes soft and spongy. This is a favourable condition for
the absorption of tannin, but care should be taken not to allow the pelt to be
squeezed or pressed, or water will be squeezed out, and the pelt will not
easily resume its plumpness. As the tannage proceeds, both the tannin and
the acid of the liquors penetrate deeper into the pelt, the former tending to
contract and the latter to swell the fibres. Thus a given quantity of acid will
cause the greater swelling, the less tannin is present; and therefore in strong
tanning liquors more acid is required. The presence of certain products of
bacterial putrefaction has a great but unexplained effect in preventing hide
from swelling with acids; and in hot weather, much better swelling is
obtained by sterilising and deliming the hides with one of the coal-tar
products mentioned on pp. 30, 162. Boric acid may also be satisfactorily
used for this purpose, but should not be allowed to get into sole-leather
liquors, as it tends to produce a soft and loose tannage, and from its
inorganic and indestructible character, is apt to accumulate in a yard in
which it is used. The same reasons render unadvisable its introduction into
any liquors which are to be returned to the leaches even in the tannage of
dressing leather, though its presence in the colouring liquors is otherwise
very useful in lessening the astringency of the tannins (“mellowing the
liquors”), and making a fine grain. Its mode of action is by no means clearly
explained, but is in some way connected with its tendency to produce
“conjugated acids” (L.I.L.B., pp. 37, 46).
The so-called “mellowness” of old liquors requires a word of comment.
It is well known to practical tanners, that old liquors are much less liable to
produce drawn grain, and a harsh surface, when used to colour green goods,
than liquors, even equally weak, which have been made from fresh
materials. This is probably due, in part at least, to more than one cause.
Most natural tanning materials contain tanning matters of varied degrees of
astringency and power of attaching themselves to the leather-fibre. It is
obvious that if a tanning liquor is used, the most astringent and energetic
tannins will be first removed from it, leaving those of a milder character. It
is also known that the presence of neutral alkaline salts of weak acids has
considerable influence in producing mellowness; the addition, for instance,
of sodium acetate has a marked effect. This effect is probably due in the
first place to the action of neutral salts in diminishing the energy of weak
acids (see p. 81), and secondly to the fact, that their bases combine to some
extent with the tannins; and that, as was perhaps first pointed out by the

writer, such tannins are, as it were, partially paralysed in their action on
hide (p. 339). Sodium sulphite acts powerfully in this way, and may perhaps
prove of technical value in temporarily diminishing the astringency of
liquors in quick tannage. Borax has a similar effect, but is too alkaline, and,
unless used with extreme caution, spoils the colour of the liquors by causing
oxidation. It is probable that similar causes explain the mellowness of
palmetto extract, which contains large quantities of alkaline salts, and of
some extracts which have been treated with sulphites, when used undiluted
in drum tannage. The addition of free acid will generally restore these
tannins to an active condition.
As the tannage proceeds and penetrates further into the hide, the liquors
are used stronger, as the outside, once tanned, is to a large extent protected
from their action, and it is only by continuously increasing the strength of
the liquors that more tannin can diffuse into the interior, since diffusion
only takes place from a stronger into a weaker liquor. The liquor in the
interior of the butts is always exhausted of tannin so long as any part of the
hide-fibre remains untanned, but as the layer of tanned fibre between this
and the outside gets thicker, a greater difference is required to maintain a
reasonable rate of exchange, just as a greater head of liquor is required to
maintain a flow of liquor through an increased number of percolation-
leaches. If the strength of the liquor outside be allowed to fall off, this
graduation of strength from the outside to the inside of the butt is disturbed,
and takes some time to re-establish. As the liquors become stronger in
tannin, they may also become somewhat stronger in acid, since, as has been
stated, the two act to some extent in opposition to each other. The acid-
swollen fibre absorbs the tannin more slowly than if it were in more neutral
condition, but it absorbs it apparently in larger quantity, and at any rate,
makes a firmer, solider, and less flexible leather.
It has been mentioned that in the latter stages of the process, solid
tanning materials are generally strewed between the butts in the tanning
liquor. It may be pointed out that many materials vary in their tanning
effect, according to whether they are used in solid form or merely in
liquors. It has been shown by Youl and Griffith[126] that such materials as
valonia, oakwood and chestnut extracts, and myrobalans, which contain
both gallotannic and ellagitannic acids, lose strength rapidly when kept in
the form of liquor, the ellagitannic acid becoming decomposed with
separation of insoluble ellagic acid. Now it is just this ellagic acid, which