The manufacture of mineral and lake pigments _ Containing directions for the manufacture of all artificial artists_ and painters_ colours_ enamel colours_ soot and metallic pigment

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Title: The manufacture of mineral and lake pigments
Containing directions for the manufacture of all artificial
artists' and painters' colours, enamel colours, soot and
metallic pigments

Author: Josef Bersch

Translator: A. C. Wright

Release date: November 16, 2023 [eBook #72143]

Language: English

Original publication: London: Scott, Greenwood & Co., 1901

Other information and formats: www.gutenberg.org/ebooks/72143

Credits: Charlene Taylor and the Online Distributed Proofreading
Team at https://www.pgdp.net (This file was produced from
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*** START OF THE PROJECT GUTENBERG EBOOK THE
MANUFACTURE OF MINERAL AND LAKE PIGMENTS ***

THE MANUFACTURE OF
MINERAL
AND LAKE PIGMENTS
CONTAINING DIRECTIONS FOR THE MANUFACTURE
OF ALL ARTIFICIAL ARTISTS’ AND PAINTERS’ COLOURS,
ENAMEL COLOURS, SOOT AND METALLIC PIGMENTS

A Text-Book For
Manufacturers, Merchants, Artists and Painters

BY
Dr. JOSEF BERSCH

TRANSLATED FROM THE SECOND, REVISED EDITION

BY
ARTHUR C. WRIGHT, M.A. (Oxon.), B.Sc. (Lond.)

FORMERLY ASSISTANT LECTURER AND DEMONSTRATOR
IN CHEMISTRY AT THE YORKSHIRE COLLEGE, LEEDS

WITH FORTY-THREE ILLUSTRATIONS IN THE TEXT

LONDON
SCOTT, GREENWOOD & CO.
Publishers of the “Oil and Colourman’s Journal”

19 LUDGATE HILL, E.C.
1901
[The sole right of publishing this work in English rests with the above firm.]

PREFACE.
We know hardly another branch of chemical technology which has
made such remarkable advances of late as the manufacture of colours; a
large number of pigments have been recently discovered, distinguished by
beauty of shade and permanence. Chemists are continually endeavouring to
replace handsome and poisonous colours by others equally handsome but
non-poisonous.
In writing this work I have endeavoured to give it such a character that
it may be a text-book for the practical man, only those methods have been
given which certainly lead to a good result; in the case of new pigments I
have only described methods of preparation which I have myself found to
give good results.
Since it cannot be imagined that any one quite ignorant of chemistry
could successfully manufacture colours (colours being always made by
chemical processes which occasionally are rather complicated), I have,
therefore, presupposed a knowledge of the principles of chemistry. In the
short sketch of the chemical properties of the raw materials used in making
ordinary pigments, the principal properties of the materials used by the
colour maker are given.
In order to make this treatise useful to dealers in and consumers of
colours, the chapters dealing with the examination of pigments have been
so arranged that the nature or adulteration of a pigment can be determined
quickly and with certainty by any one.
Recipes, which originated at a time when empiricism ruled in chemistry,
have been omitted, since they would only detract from the clearness of the
matter.
As far as it is possible I have avoided the “recipe fetish,” and have
endeavoured to make clear to the reader the chemical processes to which
regard must be had in the manufacture of the different pigments. Since the
appearance of the first edition there have been many valuable innovations in

the mineral colour industry, to which regard has been given in preparing
this second edition in so far as they possess a really practical value.
A critical examination of proposals and formulæ, which are found in
large numbers in the journals, has been avoided, since I wished to keep for
my book that character of a reliable text-book and book of reference which
was ascribed to it in the form of its first edition.

Dr. JOSEF BERSCH.

TRANSLATOR’S PREFACE.
If excuse be needed for presenting a translation of Dr. Bersch’s book at
so long an interval after the publication of the original (1893), it must be
sought in the paucity of the English literature on the subject. It is hoped that
the practical nature of the work will make it acceptable to the English
reader.
The subject-matter of the original has been preserved in the translation
without alteration or addition, with the exception of an unimportant change
in the order of arrangement.
The metric system of weights and measures has been used throughout;
for the convenience of those who are not familiar with this system,
directions are given in an appendix for converting into English weights and
measures.
The section on paint grinding (Chapter LXIX.) is perhaps somewhat
incomplete; for a more detailed and modern account of this branch of the
subject the reader is referred to Practical Paint Grinding, by Mr. J.
Cruickshank Smith, B.Sc., shortly to be issued by the same publishers.

A. C. WRIGHT.
Hull, January, 1901.

TABLE OF CONTENTS.
page
Chapter I. Introduction 1
The Physico-chemical Behaviour of
Chapter II. 8
Pigments
Raw Materials Employed in the
Chapter III. 14
Manufacture of Pigments
Chapter IV. Assistant Materials 16
Water—Chlorine—Ammonia—The Hydrometer —Sal Ammoniac
or Ammonium Chloride—Ammonium Sulphide.
Acids.—Hydrochloric Acid—Sulphuretted Hydrogen
—Sulphuric Acid: Oil of Vitriol, Nordhausen Sulphuric
Acid—Nitric Acid—Aqua Regia—Carbon—Carbonic Acid Gas.
Organic Acids.—Acetic Acid—Oxalic Acid—Tartaric Acid.

Chapter V. Metallic Compounds 32
Alkalis.—Potassium Compounds—Potassium Carbonate
—Potassium Hydroxide—Potassium Nitrate—Potassium
Bitartrate—Potassium Bichromate—Potassium Sodium
Chromate—Chrome Alum—Potassium Ferrocyanide
—Potassium Ferricyanide—Sodium Salts—Sodium Carbonate
—Sodium Hydroxide—Sodium Thiosulphate—Sodium
Chloride—Salts of the Alkaline Earth Metals.
Calcium Compounds.—Calcium Oxide—Calcium Hydroxide
—Calcium Carbonate—Calcium Sulphate—Calcium Phosphate
—Magnesium Carbonate—Barium Compounds—Barium
Chloride.
Aluminium Compounds—Aluminium Sulphate.
The Alums.—Potassium Aluminium Alum—Roman Alum—Soda
Alum—Ammonia Alum—Alumina and Hydrate of Alumina.
Compounds of the Heavy Metals.—Zinc Compounds—Cadmium
Compounds—Iron Compounds—Ferrous Sulphate—Ferrous
Chloride—Manganese Compounds—Nickel Compounds—Cobalt
Compounds—Chromium Compounds—Molybdenum, Tungsten
and
Vanadium Compounds—Antimony Compounds—Bismuth
Compounds—Tin Compounds—Arsenic Compounds—Lead
Compounds—Lead Sulphate—Lead Nitrate—Lead Acetate
—Basic Lead Acetate—Lead Chloride—Copper Compounds
—Copper Sulphate—Copper Nitrate—Copper Acetate
—Mercury Compounds—Mercurous Nitrate—Mercuric Nitrate

—Mercurous Chloride—Mercuric Chloride—Silver Compounds
—Gold Compounds.
Chapter VI. The Manufacture of Mineral Pigments 71
White Mineral Pigments—White Lead.

Chapter VII. The Manufacture of White Lead 77
Manufacture of White Lead from Metallic Lead.—Dutch
Process: Casting the Lead into Sheets—Building
up the Stacks—Removal and Grinding of the White
Lead—White Lead Mills—Hard White Lead—Soft White
Lead.—German Process.—French Process: Preparation
of the Solution of Basic Lead Acetate—Preparation
of the Carbonic Acid and Precipitation of the White
Lead—Manufacture of White Lead by Means of Natural
Carbonic Acid.—English Process.—Other Methods.
—Oxychloride White Lead: Lead Sulphite—Lewis
& Bartlett’s White Lead Pigment.—White Lead-Antimony
Pigments: Lead Antimonite—Lead Antimonate.
Chapter VIII. Enamel White 116
Lithopone.

Chapter IX. Washing Apparatus 120
Filter Presses.

Chapter X. Zinc White 126
Griffith’s Zinc White—Tungsten White—White Antimony
Pigments: Antimony Trioxide—Antimony Oxychloride
—Bismuth White—Tin White—Manganese White
—Magnesia White or Mineral White—Annaline.
Chapter XI. Yellow Mineral Pigments 133
Chapter XII. Chrome Yellows 134
Lead Chrome Yellow—Preparation of the Lead Solution
—Precipitation of the Chrome Yellow—The Pale
Chrome Yellows.
Chapter XIII. Lead Oxide Pigments 143
Red Lead, Minium.

Chapter XIV. Other Yellow Pigments 148
Cassel Yellow—Montpellier Yellow—Turner’s Yellow
or English Yellow—Naples Yellow—Antimony Yellow
—Calcium Chrome Yellow—Barium Yellow, Yellow
Ultramarine or Permanent Yellow—Zinc Chrome Yellow
—Cadmium Chrome Yellow—Cadmium Yellow—Lead
Iodide—Mars Yellow—Siderin Yellow—Aureolin
—Tungsten Yellow—Nickel Yellow—Mercury Yellow

or Turpeth Mineral—Yellow Arsenic Pigments—Lead
Arsenite—Thallium Pigments.
Chapter XV. Mosaic Gold 160
Chrysean.

Chapter XVI. Red Mineral Pigments 163
Vermilion—Black Mercuric Sulphide—Red Mercuric Sulphide.

Chapter XVII. The Manufacture of Vermilion 166
Dry Method: Chinese Vermilion.—Wet Method: Firmenich’s
Method—Liebig’s Process—Infusible White Precipitate
—Electrolytic Process—Mercuric Iodide.
Chapter XVIII. Antimony Vermilion 178
Appendix—Antimony Blue.

Chapter XIX. Ferric Oxide Pigments 180
Vogel’s Iron Red—Macay’s English Red—Indian Red.

Chapter XX. Other Red Mineral Pigments 186
Chrome Red or Chrome Vermilion—Cobalt Red—Cobalt
Magnesia Red—Cobalt Arsenate—Chromium Stannate
—Silver Chromate.
Chapter XXI. Purple of Cassius 190
Magnesia Gold Purple—Alumina Gold Purple.

Blue Mineral Pigments—Iron-Cyanogen
Chapter XXII. 194
Pigments
Chinese Blue—Prussian Blue—Mineral Blue—Soluble
Prussian Blue—Special Processes for the Manufacture
of Chinese Blue—Turnbull’s Blue—Antwerp Blue.
Chapter XXIII. Ultramarine 204
Chapter XXIV. The Manufacture of Ultramarine 211
Preparation of Mixtures for Ultramarine—Ultramarine
Violet—Chlorine and Steam Process—Hydrochloric
Acid and Air Process—Ammonium Chloride Process
—Pale Blue Ultramarine—Ultramarine Red.
Chapter XXV. Blue Copper Pigments 226
Bremen Blue and Green—Neuberg Blue—Lime Blue
—Payen’s Mountain Blue—Oil Blue—Copper Hydroxide.
Chapter XXVI. Blue Cobalt Pigments 230
Cobalt Blue, Thénard’s Blue, Cobalt Ultramarine,
King’s Blue, Leyden Blue—Cæruleum—Cobalt
Zinc Phosphate.

Chapter Smalts
233
XXVII.
Preparation of the Charge—Fusion of the Charge
—Grinding the Fused Mass—Tungsten Blue
—Tessié du Motay’s Blue—Molybdenum Blue.
Chapter Green Mineral Pigments—Green Copper
240
XXVIII. Pigments
Green Copper Pigments: Copper Carbonate—Copper Arsenite
—Scheele’s Green—Swedish Green—Brunswick Green
—Green Verditer—Neuwied Green—Copper Oxychloride.
Chapter XXIX. Emerald Green 244
Manufacture of Emerald Green from Verdigris—Manufacture
of Emerald Green from Copper Sulphate—Mitis
Green or Vienna Green—Copper Stannate—Kuhlmann’s
Green—Elsner’s Green—Casselmann’s Green—Lime
Green—Patent Green—Copper Borate—Copper Silicate
(Egyptian Blue).
Chapter XXX. Verdigris 252
Blue Verdigris—Distilled or Crystallised Verdigris
—German Verdigris.
Chapter XXXI. Chromium Oxide 260
Chapter Other Green Chromium Pigments
264
XXXII.
Guignet’s Green—Emerald Green—Chrome Green Lake
—Turkish Green—Leaf Green.—Chromium Phosphate
Pigments: Arnaudan’s Green—Plessy’s Green—Schnitzer’s
Green—Chromaventurine—Chrome Blue (Garnier).
Chapter Green Cobalt Pigments
268
XXXIII.
Cobalt Green.

Chapter Green Manganese Pigments
270
XXXIV.
Manganese Green—Rosenstiehl’s Green—Böttger’s Barium
Green—Manganous Oxide—Manganese Blue.
Chapter Compounded Green Pigments
273
XXXV.
Chrome Green—Elsner’s Chrome Green—Silk Green
—Natural Green—Non-arsenical Green.

Chapter Violet Mineral Pigments
276
XXXVI.
Chromic Chloride—Manganese Violet—Tin Violet,
Mineral Lake—Copper Violet, Guyard’s Violet.
Chapter Brown Mineral Pigments
279
XXXVII.
Lead Brown—Manganese Brown—Pyrolusite Brown
—Prussian Brown—Iron Brown—Copper Brown
—Chrome Brown—Cobalt Brown.
Chapter Brown Decomposition Products
283
XXXVIII.
Humins—Bistre.

Chapter Black Pigments
285
XXXIX.
Charcoal Blacks: True Charcoal Black—Vine Black
—Vine Black from Wine Lees—Vine Black from
Pressed Grapes—Bone Black or Ivory Black.
Chapter XL. Manufacture of Soot Pigments 294
Chapter XLI. Manufacture of Lamp Black 307
Calcination of the Soot—Pine Black.

The Manufacture of Soot Black without
Chapter XLII. 313
Chambers
Chapter XLIII. Indian Ink 316
Neutral Tint Black—Appendix: Black Mineral Pigments
—Chrome Copper Black—Chrome Black.
Chapter XLIV. Enamel Colours 319
White Enamels—Coloured Enamels: Yellow Enamels—Red
Enamel—Blue Enamels—Green Enamel—Violet Enamel
—Black Enamel.
Chapter XLV. Metallic Pigments 326
Shell-Gold—Shell-Silver—Imitation Silver.

Chapter XLVI. Bronze Pigments 329
Electrolytic Copper Bronze—Tungsten Bronze Pigments.

Chapter XLVII. Vegetable Bronze Pigments 339
Appendix: The Brocade Pigments.

PIGMENTS OF ORGANIC ORIGIN.

Chapter Lakes
343
XLVIII.
Chapter XLIX. Yellow Lakes 348
Dutch Pink—Weld Lake—Gamboge Lake—Prepared
Gamboge—Fustic Lake—Quercitron Lake—Purree
or Indian Yellow—The Colouring Matter of Saffron
—Colouring Matter of Gardinia Grandiflora.
Chapter L. Red Lakes 354
Cochineal and Carmine

Chapter LI. The Manufacture of Carmine 357
Cenette’s Method—Munich, Vienna, Paris or Florentine
Lake—Ammonia-Cochineal.
Chapter LII. The Colouring Matter of Lac 363
Lac Dye.

Chapter LIII. Safflower or Carthamine Red 366
Safflower Carmine—Alkanet.

Chapter LIV. Madder and its Colouring Matters 370
Garancin—Garanceux—Madder Extract—The Constituents
of Madder.
Chapter LV. Madder Lakes 375
Madder Carmine.

Chapter LVI. Manjit (Indian Madder) 378
Chica Red, Curucuru, Carajuru—Bigonia Chica.

Chapter LVII. Lichen Colouring Matters 380
Archil—French Purple—Cudbear—Litmus.

Chapter LVIII. Red Wood Lakes 384
Chapter LIX. The Colouring Matters of Sandalwood 388
and Other Dye-woods
Chapter LX. Blue Lakes 390
Indigo—The Constituents of Indigo.

Chapter LXI. Indigo Carmine 394
Indigo Mills—Blue Lake.

Chapter LXII. The Colouring Matter of Logwood 398
Logwood Extract—Kohlrausch’s Process for Obtaining
Concentrated Extracts of Colouring Matters and
Tannins.
Chapter LXIII. Green Lakes 409
Chlorophyll—Sap Green—Chinese Green, Lokao
—Charvin’s Green.
Chapter LXIV. Brown Organic Pigments 414
Asphaltum—Sepia.

Chapter LXV. Sap Colours 416
Chapter LXVI. Water Colours 419
Moist Water Colours.

Chapter Crayons
423
LXVII.
Crayons for Earthenware.

Chapter Confectionery Colours
427
LXVIII.
Chapter LXIX. The Preparation of Pigments for Painting 429
Paint Mills.

Chapter LXX. The Examination op Pigments 434
Mineral Pigments—Examination with the Blowpipe
—Reactions of the White Pigments—Reactions
of the Yellow Pigments—Reactions of the Red
Pigments—Reactions of the Blue Pigments
—Reactions of the Green Pigments—Reactions
of the Brown Pigments—Reactions of the Black Pigments.
Chapter LXXI. Examination of Lakes 445
Reactions of the Organic Colouring Matters.

Chapter The Testing of Dye-Woods
449
LXXII.
The Colorimeter.

Chapter The Design of a Colour Works
457
LXXIII.
Chapter Commercial Names of Pigments
460
LXXIV.

APPENDIX.

The Conversion of Metric into English 469
Weights and Measures—Centigrade
and Fahrenheit Thermometer Scales

INDEX.

CHAPTER I.
INTRODUCTION.

It is doubtful whether another branch of applied chemistry is recorded
of so great an age as the colour industry; at the present time there is hardly a
race on the face of the earth which does not make use of colours in some
form, either for the decoration of their persons or surroundings. The art of
preparing colours is as ancient as their use. It is true that we find from the
most remote historical records that the so-called earth colours were almost
solely employed, and principally those which exist ready formed in nature.
But these natural colours also require their particular process of preparation
before they fulfil their object, even though this be merely a mechanical
operation, such as powdering or levigating. That the oldest nations of whom
we possess lasting records, either written or otherwise, really understood
the preparation of colours by chemical processes is shown by the common
occurrence in the Egyptian mural pictures of figures clad in brightly
coloured garments, a proof that the Egyptians not only understood the
science of colour manufacturing, but also the more advanced art of
fastening colours upon fabrics—dyeing.
The writings of the ancient Greeks, and in part also the scanty remains
of their buildings, prove to us completely that they understood the use of
colours to such an advanced degree that they already employed them for
pictures as works of art. That the Greeks were also acquainted with the
preparation of colours and dyeing follows from various passages from the
classical writers, in which magnificently decorated rooms and beautifully
coloured garments are often described.
Among the Romans, who were the pupils of the Greeks in the arts and
manufactures, the prodigal luxury which existed in Rome, especially under
the emperors, caused a great demand for colours, which were used in the
most profuse manner for the decoration of house and attire. The Roman
colour makers had advanced so far in their art that they could colour the
human hair rose-red.

A glance at East Indian fabrics and pictures, or at the ancient Chinese
buildings, whose colouring is a matter of marvel to-day, shows that the
Oriental were not behind the Western nations in the discovery of colours
and the art of manufacturing them.
In so old an industry it is not remarkable that great changes have taken
place in the course of time. The thousands and thousands of experiments
made by the alchemists in the attempt to prepare gold failed in their main
object, but the tremendous expenditure of time and trouble in this work was
not fruitless; upon the great mass of chemical facts discovered by the
alchemists were laid the foundations of scientific chemistry. We find on
reading the writings of the alchemists that the colour industry is indebted to
them for an immense number of its products; the reason being that the
alchemists worked by preference on metals, earths and mineral compounds,
and from these substances a large number of colours are obtainable, of
which many are still in use to-day, and, on account of their cheapness, will
continue in use.
The period in which the painters were also the colour makers lies not far
behind us. The preparation of many a colour of particular beauty was
treated by the fortunate owner of the recipe as a great secret. It was sold by
him at a great price. What a difference between that time and the present!
There is now no painter among civilised races to whom it would occur to
prepare his own colours; the chemical works provide them for him at a low
price and in such a condition that they can be immediately used for
painting. The Italian painters prepared the highly prized blue pigment,
ultramarine, by laborious toil from the costly lapis lazuli; to-day, this same
colour, more beautiful and deeper in hue, is made by several works, and
sold at a price which bears no comparison with that of the colour obtained
from the mineral. The latter was worth many times its weight in gold: a
pound of the finest ultramarine can now be bought for a shilling or two.
We find a similar comparison in the case of the fine scarlet pigment
known as vermilion: formerly the natural vermilion, cinnabar, was sold at a
very high price; at the present time the finest vermilion, prepared
artificially, can be bought at a low rate. It is no longer necessary for any one
to use natural Chinese vermilion as an artists’ colour.

Whilst formerly mineral colours were used in great preponderance, we
now know a great number of vegetable and animal colouring matters. The
discovery of the sea route to India and the discovery of America had an
important influence in this development. From these countries, as from
other tropical lands, come the majority of the plants which contain
colouring matters. The attempt to change these colouring matters into
insoluble compounds led to the discovery of the lake pigments.
With the advance of chemical knowledge the number of colours grew
apace; e.g., the discovery of chromium was of great importance to the
colour industry: it presented us with a large number of new colours. To a
more limited extent, the discovery and study of uranium, molybdenum and
other metals were the occasions for the invention of new colours.
In more recent times, efforts in the colour industry have been especially
directed to making colours more permanent and, at the same time, harmless.
In the first respect, the position at present leaves much to be desired; but, as
regards the second property, great advances have been made. The colours in
use in former days were almost all very poisonous compounds; the greater
number were derived from lead, copper, mercury or arsenic. More recently
these poisonous substances have been in many cases replaced by innocuous
materials, so that among the colours now in use, though the list is much
more comprehensive than of old, there are but few poisonous to a high
degree.
In all civilised states the use of poisonous colours has been much
restricted by law, and in those cases in which an article is to be
manufactured for use as food the employment of such colouring matters has
been absolutely forbidden. For example, in Germany by the law of 5th July,
1887, concerning the use of dangerous colours in the preparation of foods
and condiments, the application of the permissible colours has been exactly
defined.
During the last decades the colour industry and, still more, dyeing have
undergone a complete change. The momentous discoveries which have
been made in these departments leave far behind the advances which have
been made in other branches of chemical technology, the manufacture of
explosives, perhaps, excepted. We allude here to the beautiful colours which
have been made from coal-tar, colours which far surpass in beauty all

hitherto known, and which we can already prepare in every shade and hue.
Unfortunately, we can only employ the coal-tar colours, as such, in a
restricted measure among the pigments; they are of more importance in
dyeing. We use the term pigments here in the narrow sense of such
substances which, when spread out on certain materials, provoke a certain
sensation of colour. Dyeing is, on the contrary, that branch of colour
chemistry which generally has for its object the simultaneous production of
the colour and its fixation upon a fabric. This definition was at least
applicable to the majority of the colours which were in use before the
discovery of the coal-tar colours and their introduction into the industry.
Since, however, the latter have acquired so great a preponderance in dyeing,
it is no longer applicable, for the dyers use at present a large number of
substances which are included in the narrow definition of pigments. The
greater part of the coal-tar colours are substances which, in solution, when
brought in contact with a fabric, adhere to it and colour it permanently.
According to their use and preparation, pigments are divided into a
number of classes, and one speaks of painters’, artists’, enamel, porcelain
and glass colours, also of oil, honey, water and cake colours. Although this
division is important for trade purposes, it is of little moment for the colour
maker, for he can prepare the same colour for both purposes, either for oil
or water colour. What is of the greatest interest for the colour maker is the
preparation of the pigment itself. The conversion of the prepared pigment
into (oil or water) paint is unaccompanied by difficulties.
When we look for a practical classification for pigments, we find that
there are colours which exist ready formed in nature, and others which can
only be obtained by certain chemical processes, at times very complicated.
As regards the first group of pigments—those which exist ready formed
in nature—the processes which they undergo at the hands of the colour
maker are almost entirely mechanical treatments—grinding, sieving,
levigating and similar operations—in order to convert them into such a
condition that they can be used for painting. Since a large number of these
pigments belong to that class of minerals which mineralogists call earths,
these pigments have also been designated earth pigments, a term which we
shall retain on account of its general use, although it is incorrect, since
many of the so-called earth pigments are not obtained from “earths” in the
mineralogical sense.

Among the pigments which are prepared by human skill many divisions
can be drawn. A large number of pigments are prepared from mineral
sources; an equally important number are derived from the animal and
vegetable kingdom, the latter consisting of combinations of organic
materials with certain inorganic substances. Some few pigments (putting
aside the coal-tar colours) are simply organic products, as, for example, the
majority of the black pigments, which consist of carbon.
The following classification is drawn up on the lines indicated above:—
1. Natural Colours or Earth Pigments.—Found ready formed in nature
and requiring only mechanical preparation to be usable. A large number of
handsome and also cheap colours belong to this class.
2. Artificially Prepared Mineral Pigments.—Obtained by certain
chemical processes, and, according to their composition, either compounds
of metals with sulphur, oxygen, iodine, cyanogen, etc., or of oxides with
acids, i.e., salts.
3. Lakes.—Compounds of colouring matters from the animal or
vegetable kingdom with a mineral substance, such as lead oxide or alumina.
As a fourth group we might take those colours which do not fall into the
previous classes, as, for example, the black pigments composed of carbon;
but since this division is not made in practice we shall not regard this
species of pigment as a particular group, but shall discuss them in the
proper place.
As an entirely new group of colours are to be classed those which are
generally called coal-tar colours. These colours, which, at present, are the
most important in dyeing and calico printing, are prepared from so-called
organic compounds (more properly, carbon compounds). The manufacture
of these colours is a separate branch of chemical industry.

CHAPTER II.
THE PHYSICO-CHEMICAL BEHAVIOUR
OF PIGMENTS.

In a work which, as its title indicates, is devoted to a description of the
manufacture of pigments, the properties of those substances which are
necessary for the preparation of colours cannot be exhaustively considered;
we must, therefore, presuppose a knowledge of the elements of chemistry.
We have to consider in this book the chemistry of colours; the reader will,
therefore, not expect an exposition of general chemical laws; we shall only
state certain facts which are of value to the manufacturer. With the
description of the manufacture of each pigment and of the materials
required for that manufacture, we shall still discuss the chemical processes
which must be conducted in the preparation of the colours, so far as it is
necessary in order to understand them. In this chapter we shall say a few
words about the physical and chemical behaviour of pigments in general.
The great majority of pigments are prepared by the process of
precipitation, generally by mixing the solutions of two substances, upon
which an interchange of the constituents occurs and the less soluble
compound separates in pulverulent form from the solution as a precipitate.
Most of these colours are obtained by the admixture of the solutions of two
salts; the preparation of the so-called chrome yellow may be taken as an
example. In the preparation of this pigment, a solution of a lead salt, sugar
of lead (lead acetate), is mixed with a solution of bichromate of potash,
whereupon a precipitate of lead chromate (chrome yellow) is formed, whilst
potassium acetate remains dissolved. The lead chromate is formed because
the acetic acid has a greater affinity for potash than for lead oxide,
wherefore an interchange of acid and base takes place, but the lead
chromate being insoluble in water consequently separates in the form of a
precipitate.
Many mineral pigments are produced in the form of precipitates by
passing sulphuretted hydrogen or carbonic acid gas into certain metal

solutions. In these cases a similar exchange takes place between the
reacting substances to that given in the case of chrome yellow; the metals
have a greater affinity for the sulphur or for the carbonic acid than for the
substances with which they are already united, they unite with the former,
and the new compound separates as an insoluble substance. We have
examples of such compounds in cadmium sulphide, which is obtained by
passing sulphuretted hydrogen into the solution of cadmium in an acid, and
in white lead, which is formed by the saturation of a solution of lead acetate
by carbonic acid.
Many organic colouring matters, soluble in water, have the property of
forming compounds with metallic oxides, soluble with great difficulty,
when their solutions are mixed with a salt of lead, tin or aluminium, and the
oxide is separated from the solution by an alkali. The precipitates obtained
in this way are insoluble compounds of the colouring matter and the oxide
of the metal; they are called lake pigments, or, briefly, lakes. A large
number of pigments, often of great beauty, is obtained in this manner. The
lakes are widely used in all branches of painting and dyeing.
Of great importance for the quality of the pigment is the physical
condition of the precipitate; this is either crystalline or amorphous, that is,
non-crystalline. When a crystalline precipitate is examined under the
microscope, it is seen to consist of very small, coloured, transparent
crystals. The amorphous precipitates are, however, in such a fine state of
division that even with the highest magnification they transmit little or no
light, and consequently appear opaque. These different characters of
precipitates have the greatest influence on that property of pigments which
we call covering power. In consequence of its transparency, a crystalline
precipitate will allow the colour of the surface upon which it is spread to
appear through, hence it must be laid on much more thickly than is
necessary with an opaque pigment, of which a thin coating is sufficient to
make the colour of the surface beneath invisible.
How extremely important is the crystalline or non-crystalline nature of a
precipitate in practice is seen by a consideration of white lead. This
pigment, lead carbonate, can be made by mixing solutions of a lead salt and
a soluble carbonate (soda); but in this case a lead carbonate of crystalline
nature is formed, which, being transparent, is of so small covering power
that this process has no application in the manufacture of white lead; but a

far more troublesome method is used by which a non-crystalline product,
amorphous lead carbonate, is obtained.
Many pigments are formed by burning (oxidising) metals, as, for
example, zinc white; others are prepared by melting salts together, as
Naples yellow; others again are formed by very complicated processes still
partially unexplained, as is the case with ultramarine. In the manufacture of
colours we find all chemical processes in use.
It may be here remarked that it is quite possible to manufacture some
colours, indeed a large number, according to fixed directions, without any
particular chemical knowledge being necessary to carry on the processes.
Indeed, in works we find most processes being carried out by ordinary
labourers who are quite destitute of any knowledge of chemistry. We must,
however, add that we are convinced that any colour maker who works
simply in a purely empirical manner, according to a stereotyped recipe, will
never be in the position to raise himself above the position of a workman;
he will not be able, when a slight mishap occasions a change in the ordinary
course of the process, to devise a means of overcoming the defect, but will
be compelled to dispose of the faulty product in the condition in which it
exists. Such a manufacturer is in a condition of blind dependence on the
chemical works and dealers from whom he receives the raw materials
requisite for the preparation of his colours. If he should receive materials
which contain impurities not to be detected by empirical methods, the
inevitable result will be that the colours produced from them will not be
equal to the standard. If, in making a colour which is the outcome of several
processes, a workman once makes a mistake, the product will not be of the
required quality.
On the contrary, if the manufacturer possesses a certain amount of
chemical knowledge, it will not be difficult for him to ascertain the causes
of a failure in a process, and, at the same time, to devise means by which
the defects may be removed. The manufacturer is more and more in the
habit of buying the chemicals which he requires for his manufactures rather
than of making them himself. He should, therefore, be in a position to form
an opinion as to the usability and purity of these substances, which will
only be possible when he has the knowledge requisite for subjecting them
to a chemical examination.

Although we shall presuppose, as we have said, that those who intend to
concern themselves with colour manufacturing possess an acquaintance
with the principles of chemistry, yet this book has been so planned that it
may be of use (we hope) to the practical man who is innocent of chemical
knowledge. On this account, we have devoted care to the description of
those raw materials which are bought in large quantity, and to the simple
investigation of their purity.
When the manufacturer has the advantage of a chemical education,
apart from his endeavours to produce colours lacking nothing in beauty or
depth of shade, he will direct his endeavours in two directions, in respect of
which great advances are yet to be made—the permanence and
harmlessness of his colours.
Many pigments possess the undesirable property of losing their
brightness under atmospheric influences; many, indeed, fade away
completely in the course of time. We have only to examine a picture some
centuries old; in spite of the care bestowed on its preservation, we can say
with certainty that, in the course of time, it will be so completely altered
that nothing will remain of the original colours. It is the endeavour of the
sensible manufacturer of colours to make only such as remain unaltered by
atmospheric action, and also undergo no change when they are mixed with
other pigments. Although it may be highly desirable that the painter should
possess a knowledge of the chemical properties of the colours he uses, still
it should be the first object of the maker to take care that he places on the
market only colours which will remain as much as possible unaltered when
used alone, and will remain undecomposed when mixed. This is,
unfortunately, not the case with many colours now in use. We shall return
later to this point, of such extraordinary importance to the artist.
The second point to be observed, is to produce only harmless colours.
The advances of chemistry have made known to us a series of colours
which have the advantage over others known for a longer period that they
are non-poisonous. Unluckily, these harmless colours frequently fall behind
the poisonous colours in brilliance, and generally they are more expensive.
Here, too, is opened to the manufacturer a wide field of activity. The more
completely poisonous substances disappear from the colours in use, the
more widespread will be the use of colours. We should remark that the
expression “poisonous colours” is to be used with a certain reserve. Many

pigments which contain lead, copper, antimony, mercury, etc., are
poisonous, because they contain poisonous metals; but poisoning with them
will not readily take place on account of their insolubility. It is different
with the very poisonous arsenic compounds, which should be removed from
the list of colours in common use; many a misfortune caused by them
would then be avoided.
Endeavours to produce innocuous colours have been more successful
than the efforts after permanence. There are now very few commonly used
colours which can be accounted very poisonous compounds, and which
cannot be replaced by other colours of equal beauty. On the whole, we are
now in the position to prepare harmless colours suited to most purposes.
Special endeavours should be made to sell these, so that such cases of
poisoning should not occur as, for example, caused by gingerbread which
had been wrapped in paper coloured by emerald green.

CHAPTER III.
RAW MATERIALS EMPLOYED IN THE
MANUFACTURE OF PIGMENTS.

As we have mentioned before, the manufacturer of colours now
generally uses materials supplied to him by chemical works. The purer
these are, the easier it will be to work with them, and the finer will the
colours turn out. We have indicated that it is important for the manufacturer
to know accurately the properties of his materials in order to be able to
estimate their value. Many substances required in certain cases must be
made by the colour manufacturer, since, on account of their condition, they
cannot form articles of commerce—chlorine and sulphuretted hydrogen, for
example.
In addition to the substances which are not to be bought, there are others
which do occur in commerce, but are sold at so high a price that the
manufacturer is compelled to make them himself. This is the case with the
cobalt compounds, from which many beautiful colours are made. The
producers of these demand such prices that it is to the interest of the colour
maker to prepare them for his own use.
In the following chapters, we shall deal with the more important raw
materials which are employed in colour manufacturing, and shall restrict
our remarks to what is of particular importance thereto. For more detailed
accounts of these raw materials the reader is referred to the text books of
chemistry, in which he will find them minutely described, in so far as they
are chemical products.
The materials employed may be divided into assistants in the processes
and components of the manufactured pigment. The assisting substances are
those which are used in the manufacture of a colour without entering into its
composition; from the component materials the colours are directly derived.
For example, in the manufacture of Prussian blue, yellow prussiate of
potash, an iron salt, water (in which the salts are dissolved) and nitric acid
are used. In the blue obtained are contained portions of the iron salt and of

the yellow prussiate, these are, therefore, component materials, whilst water
and nitric acid are simply assistants, since they do not enter into the
composition of the pigment.
In colour making a large number of assisting materials are employed,
which comprise a considerable number of elements and compounds. Since
these are of great importance for our purpose, we shall describe their
properties, and, when necessary, briefly the method of preparation.
Among the component materials are to be reckoned a large number of
salts of the alkaline earth and earth metals and of all the heavy metals. In
addition, there are also the substances of animal or vegetable origin used in
lake making.
In the description of the raw materials, if we were to overstep the line
drawn here, we could include a great variety of compounds, those, for
example, used in the manufacture of the so-called aniline dyes. These
substances form, however, as we have stated, the object of a particular
branch of manufacture, which forms a separate division of colour
chemistry, but with which is not to be confounded what has been hitherto
designated the manufacture of colours.

CHAPTER IV.
ASSISTANT MATERIALS.

Water, H₂O = 18.[1]—This substance plays a tremendous part in colour
making; almost all the substances which are used in solution are dissolved
in water; the removal from precipitates of admixed foreign bodies, the so-
called washing, is always accomplished with water. The chemist does not
understand by water quite that liquid which in general speech is so
designated. We must consider the water which is at the disposal of the
colour maker.
[1] We append the chemical formula and the molecular weight to the description
of each compound.

Water, in the chemical meaning of the word, is a liquid composed only
of hydrogen and oxygen, and leaving no residue when evaporated. Such
water is not found in nature; it can only be obtained by distillation of well
or river water. The water which falls in long continued rain, or is obtained
by melting snow, is most nearly like distilled water; it contains only small
quantities of dissolved substances, and generally such as would be without
influence in colour making. Water of this description is available for but a
limited use; the large quantities of water required in a colour works must be
taken from springs or streams. These waters contain, however, more or less
large amounts of dissolved salts, which act in a marked manner upon the
substances dissolved in them.
In almost all spring and well waters is found carbonate of lime; such
waters are called “hard”. River water contains generally little carbonate of
lime; it is then called “soft water”. The influence of the carbonate of lime is
especially evident when salts of lead, copper, iron and other heavy metals
are dissolved in water; the carbonate of the particular metal gradually
separates from solution, and the liquid becomes very turbid.
When only hard waters containing much lime are at the service of the
manufacturer, turbid solutions are often obtained, which must be filtered
before use. In many cases this can be avoided by adding milk of lime to the

water in a large vessel; the free carbonic acid unites with the lime, and thus
the carbonate of lime, which is only soluble in water containing free
carbonic acid, separates as a fine precipitate. Water which has been treated
in this way becomes clear after some time, through the deposition of the
carbonate of lime; it is then soft water. In order to separate the carbonate of
lime in this way, no more than the requisite quantity of milk of lime should
be added, so that no lime remains in excess, since this would cause
precipitates when salts of lead, copper, iron, etc., were dissolved. In many
cases—for example, when lead or barium salts are dissolved—the lime
contained in the water can be made harmless by slightly acidifying with
acetic or nitric acid. Water which contains sulphate of lime (gypsum) is
equally useless for many purposes, as, for example, the solution of lead and
barium salts. These metals form insoluble compounds with the sulphuric
acid, which render the solution turbid, and can be removed only with
difficulty by filtering, on account of their great fineness. They are more
easily removed by allowing to settle.
Water containing gypsum often contains in addition small quantities of
sulphuretted hydrogen. However small the quantities of this gas may be,
they still make the water absolutely useless for certain purposes in the
manufacture of colours; for example, for the preparation of all pigments
containing lead which are obtained by precipitation. The sulphuretted
hydrogen forms black compounds with lead, copper, bismuth, mercury and
other metals, which impair the brilliance of the colour. A colour made under
these conditions is never clean, its hue is injured by the admixture of the
black substance.
Water which contains much common salt (sodium chloride) is
unsuitable for the solution of lead, mercury and silver salts. In consequence
of the great affinity of these metals for chlorine, turbid solutions are
obtained when their salts are dissolved in water containing common salt.
Some waters contain a considerable quantity of iron. Such waters
deposit on evaporation, and often on standing exposed to the air, a brown
powder of ferric hydrate, which would have considerable influence on the
shade of a pigment. White pigments, in the preparation of which such a
water is used, have always a brownish tinge; yellow and red pigments are
also unfavourably affected.

Carbonate of lime and common salt occur in small quantities in every
well water. The colour maker must do the best he can with such a water; its
use will not particularly harm the shade of the colours prepared with it if the
amount of the impurities is not very large. Water containing much iron is
practically useless; the oxide of iron would injure the colours so much that
it would not be possible to obtain brilliant shades. Water from wells in the
neighbourhood of deposits of turf or cemeteries often contains considerable
quantities of organic substances which act injuriously on the shade of
pigments; such water should not be used in colour making.
The impurities in a water are more or less harmful according to the
purpose for which it is to be used. Sulphate of lime is generally more
injurious than carbonate of lime, since the precipitates which the latter
causes in solutions of the salts of certain metals can be prevented by the
addition of acids. This is not the case with sulphate of lime; when lead or
barium salts are dissolved in water containing this substance, a precipitate
of lead or barium sulphate is obtained, which is insoluble.
In dealing with the salts of costly metals, such as mercury or silver, it is
better to dissolve them in distilled water, or, at least, very pure rain water.
The rain water which runs from zinc or well tiled roofs is generally very
pure; for practical purposes it may be regarded as free from carbonate and
sulphate of lime, sulphuretted hydrogen and common salt. The colour
maker should take care to obtain as much of this pure water as possible by
erecting large rain-water tanks.
The less impurity a water contains the more useful it is for our purpose.
After rain water soft river water is the best, and after this the softer well
waters. All mineral waters distinguished by a high content of salts or gases
are quite useless for colour making; for this reason sea water is disqualified.
An accurate analysis of a water is much too complicated for the
manufacturer; it is sufficient for him to convince himself of the absence of
certain substances. Water which, some time after the addition of a little
tannic acid solution, acquires a clear green or a bluish to black shade
contains much iron, and is useless. Water which coagulates a large quantity
of a solution of soap in alcohol is very rich in carbonate or sulphate of lime.
In order to decide approximately in what relative proportion these salts are
present, a solution of barium chloride is added to the water so long as a

precipitate forms. If this disappears completely on the addition of nitric
acid, the water contains only carbonate of lime; if it only partly dissolves,
sulphate of lime is also present. The presence of chlorine is shown by a
considerable turbidity on acidifying the water with nitric acid, boiling and
adding silver nitrate. If the precipitate obtained on the addition of a lead salt
is not pure white, but discoloured, the water contains sulphuretted
hydrogen, which has formed black lead sulphide. In order to test the water
for organic substances, about a litre is evaporated to dryness in a porcelain
dish, and the residue heated to redness; if it turns brown and black, and
possibly gives off a smell of burnt feathers, the water contains much
organic matter.
Pure water is coloured permanently red by a solution of potassium
permanganate; but if it contains organic matter, the solution is decolourised
after some time and a brown precipitate is deposited at the bottom. From
the amount of this precipitate an idea of the quantity of organic matter
present may be obtained.
It is only necessary to be very scrupulous concerning the quality of the
water when it is to be used for the solution of salts or the extraction of dye-
woods. For washing precipitates, which requires a large volume of water,
there can generally be used, without detriment, water containing much lime,
but it must be free from iron and sulphuretted hydrogen. The latter is
particularly harmful to most of the lead colours, which would lose in beauty
by washing with water containing this substance.
It is hardly necessary to say that the water used in colour making must
be quite clear. Muddy river water must in every case be completely freed
from the solid particles contained in it, either by settling or by filtering.
Filters filled with well washed sand give good results for this purpose.
Chlorine, Cl = 35·5.—For some operations in colour making it is
necessary to employ chlorine. This is a greenish yellow gas at ordinary
temperatures, which is characterised by a suffocating smell and the energy
with which it unites with most elements. On account of its injurious effects
on man certain precautions have to be observed in preparing chlorine, and it
is advisable to erect the apparatus necessary for its production in a separate
room, so that the workmen are not injured by the gas.

Fig. 1.

Formerly chlorine was exclusively made in lead apparatus, because this
metal is one of the least readily attacked. When such an apparatus is used
for the first time a layer of lead chloride is formed, which, like a varnish,
protects the metal beneath from further attack. Fig. 1 represents an
apparatus formerly employed for the preparation of chlorine in chemical
works. In the upper part of the pear-shaped vessel, K, there are four
openings, two of which, D and C, are provided with water lutes. This means
that the opening is surrounded by a moat containing water, into which the
rim of the cover dips, thus making a joint. Through the middle opening goes
the axle of the stirring apparatus, R; in the fourth is a lead safety funnel, J.
Solid materials are introduced through D, liquid through J; the tube C
carries away the chlorine formed; the tube A, furnished with a stop cock,
can draw off the fluid contents of the apparatus.
Since lead melts at low temperature, the apparatus cannot be heated
over the fire without danger, therefore it is surrounded by an iron jacket, W,
which is filled with water, or else the apparatus is heated by steam

introduced into W. Larger quantities of chlorine are more conveniently
prepared in an apparatus, of similar structure, made of stone or earthenware,
which have the advantage over lead that they are not at all attacked by
chlorine.

Fig. 2.

Fig. 2 exhibits the construction of such an apparatus of medium size. It
is constructed of sandstone or earthenware; the lid and some of the smaller
parts can be made of either earthenware or lead. The pyrolusite is
introduced at G in large pieces; H is the funnel for pouring in the acid; E K
D, the steam pipe; C, the perforated false bottom upon which the pyrolusite
lies; F, the delivery tube for the chlorine; J, the opening for running off the
manganese chloride; a, the leaden cover.

To prepare chlorine, 1 part (by weight) of common salt, 1 part of
powdered pyrolusite, 2½ parts of vitriol and 1¼ part of water are used. The
salt and the pyrolusite are introduced through D into the apparatus (Fig. 1);
the acid, diluted with the water, is poured in through the funnel, the
materials are mixed by the stirrer and gently warmed until chlorine appears,
when the application of heat must be considerably diminished or the
chlorine will be violently evolved.
Pyrolusite and hydrochloric acid are now generally used for the
preparation of chlorine, because the solution of manganese chloride, left at
the end of the operation, is valuable.
If all the chlorine made in one operation is not at once required for the
manufacture of a colour, it can be utilised by sending it into a box filled
with slaked lime, which is converted into chloride of lime or bleaching
powder. The liquid run away from the apparatus at the conclusion of the
operation contains manganese and sodium sulphates, or manganese chloride
as the case may be, and can be used for the preparation of manganese
pigments.
Ammonia, NH₃ = 17.—Ammonia is obtained from chemical works in
the form of a strong solution of ammonia gas in water, which is generally
very pure. The density of an aqueous solution of ammonia is the smaller the
more ammonia it contains, and thus the strength of a solution of ammonia
can easily be formed by means of the hydrometer. The following table
shows the percentage of ammonia, NH₃, in a liquid of known specific
gravity at the temperature of 14° C.:—

Specific Ammonia Specific Ammonia
Gravity. per cent. Gravity. per cent.
0·885 36·00 0·953 11·50
0·886 35·00 0·955 11·00
0·889 34·00 0·957 10·50
0·891 33·00 0·959 10·00
0·893 32·50 0·961 9·50
0·895 32·00 0·963 9·00
0·897 31·30 0·965 8·50

Specific Ammonia Specific Ammonia
Gravity. per cent. Gravity. per cent.
0·905 25·39 0·970 7·50
0·925 19·54 0·972 7·00
0·932 17·52 0·974 6·50
0·947 13·46 0·976 6·00
0·951 12·00 0·978 5·50
The Hydrometer.—In the above table the percentage content of the
ammonia solution is given according to its specific gravity, that is,
according to the ratio between the weight of any volume of the liquid and
the weight of an equal volume of water. According to scientific principles,
only those hydrometers should be used which are graduated in specific
gravities. In spite of all exertions in this direction, manufacturers have not
yet been induced to use such instruments in every case. Hydrometers, with
quite arbitrary scales, such as those of Baumé and Twaddell, are frequently
found in works. These hydrometers generally only show that a liquid is of
so many degrees on the particular scale, and the manufacturer in using them
is restricted to the following out of a certain recipe which requires the use
of a liquid of a certain strength which is expressed in degrees Baumé, etc.
He does not learn by this how many per cent. of the particular substance are
dissolved in the water when the liquid has a certain hydrometric strength.
For the sake of uniformity, it is urgently to be desired that all
manufacturers who use the hydrometer to estimate the content of a liquid in
ammonia, potash, soda, hydrochloric, sulphuric, nitric acids, etc., should
employ simple specific gravities. This is desirable, because the percentage
strength of a solution, corresponding to the specific gravity, can be at once
accurately found from tables. On these grounds, in the present work, we
have restricted ourselves to tables showing simply the specific gravities of
solutions and the corresponding composition.
Sal Ammoniac or Ammonium Chloride, NH₄Cl = 53·5.—This
substance comes into commerce in the form of a white crystalline meal,
more rarely in the form of sugar loaves (crystallised sal ammoniac) or of
flat cakes (sublimed sal ammoniac). It is usually very pure, since impure
forms, generally containing much iron, are difficult of sale. At a particular

temperature sal ammoniac is volatile; it is used in certain mixtures in order
to prevent the temperature, on heating, from rising beyond a certain point.
Like ammonia, it is more used in dyeing.
Ammonium Sulphide, NH₄HS.—This compound is obtained by
leading sulphuretted hydrogen into ammonia solution so long as it is
dissolved, and a test portion of the liquid still gives a white precipitate with
a solution of magnesium sulphate. Ammonium sulphide decomposes by
long standing in the air, sulphur being separated. It gives precipitates with
the salts of certain metals, for example, iron, cobalt, manganese, zinc,
nickel. These precipitates, which consist of the sulphides of the metals, are
not formed by sulphuretted hydrogen in acid solutions.

Acids.

In colour making many acids are used for the solution of metals, the
production of precipitates, for oxidations and so forth. Commercial acids,
especially inorganic acids, generally contain not inconsiderable quantities
of impurities which are injurious in the manufacture of many colours.
Hydrochloric Acid, HCl = 36·5.—The commercial acid (muriatic acid,
spirits of salt) generally contains large quantities of iron, which colour it
yellow—fortunately, in many cases, this is not a disadvantage, and also at
times the iron can be removed from solutions made in the acid. Another
impurity is sulphuric acid. This can be detected by diluting and adding
barium chloride; if sulphuric acid be present, a white precipitate, or, at least,
a cloudiness, appears.
Ordinary hydrochloric acid is a solution of hydrochloric acid gas in
water. The strongest acid contains 42·85 per cent. of the gas, and has the
specific gravity 1·21. The following table gives the strengths of acids of
various specific gravities:—

Hydrochloric Hydrochloric
Specific Specific
Acid Acid
Gravity. Gravity.
per cent. per cent.
1·21 42·85 1·10 20·20
1·20 40·80 1·09 18·75

Hydrochloric Hydrochloric
Specific Specific
Acid Acid
Gravity. Gravity.
per cent. per cent.
1·19 38·88 1·08 16·71
1·18 36·36 1·07 15·49
1·17 34·34 1·06 13·86
1·16 32·32 1·05 11·49
1·15 30·30 1·04 8·97
1·14 28·28 1·03 6·93
1·13 26·26 1·02 4·89
1·12 24·24 1·01 2·03
1·11 22·22 —— ——

Sulphuretted Hydrogen, H₂S = 34.—This is a gas of acid properties
smelling like rotten eggs; it precipitates the sulphur compounds of many
metals when led into the acid solution of the corresponding salt. This
substance is seldom required in colour works, so that it is convenient to
have an apparatus which permits of the preparation of any required quantity.
Fig. 3 represents an apparatus devised by the author, which is well adapted
for the preparation of sulphuretted hydrogen. It consists of a small, wooden
tub, on whose upper edge lies a thick paper ring, so that the lid may be
pressed down air-tight by the screws B. Through the lid pass a tap-funnel, T,
a movable screw, S, and a tube, R, to carry away the gas. On the screw S
hangs a basket, K, by a handle; this is filled with pieces of iron sulphide as
large as nuts. The tub is filled to about one-third of its height with a mixture
of 9 parts of water and 1 part of sulphuric acid.

Fig. 3.

When sulphuretted hydrogen is required the basket is lowered by the
screw, S, until it dips in the liquid; according as the basket dips more or less
into the liquid a fast or slow current of the gas is obtained. When the gas is
no longer required the basket is raised out of the liquid, and the evolution of
gas at once ceases. The funnel, T, serves for the introduction of the liquid,
the tap, H, for drawing off the iron sulphate solution, which can be used
with advantage for the preparation of fine iron colours. The apparatus
should not be opened so long as sulphide of iron remains in the basket.
Sulphuric Acid comes into the market in two different forms: oil acid;
both are used in colour making. Oil of Vitriol, H₂SO₄ = 98, is a colourless,
oily liquid of high specific gravity; it is generally tolerably pure, and
contains, as a rule, only a small quantity of lead, the presence of which is
indicated by a turbidity on largely diluting the acid. The amount of pure
sulphuric acid in the liquid is practically determined by taking the specific

gravity. The table indicates the relation between the specific gravity and the
content of sulphuric acid.

Sulphuric Sulphuric
Specific Specific
Acid Acid
Gravity. Gravity.
per cent. per cent.
1·8485 100 1·8043 89
1·8475 99 1·7962 88
1·8460 98 1·7870 87
1·8439 97 1·7774 86
1·8410 96 1·7673 85
1·8376 95 1·7570 84
1·8336 94 1·7465 83
1·8290 93 1·7360 82
1·8233 92 1·7245 81
1·8179 91 1·7120 80
1·8115 90 —— ——

Nordhausen Sulphuric Acid, H₂S₂O₇ = H₂SO₄ + SO₃, is generally a
yellowish brown liquid, which gives off white fumes in the air. It contains
varying quantities of sulphur trioxide dissolved in sulphuric acid. It often
contains selenium, which separates as a red powder when the acid is
diluted. The presence of this impurity does not interfere with the use of the
acid for dissolving indigo, the only purpose for which it is required in the
colour factory.
Nitric Acid, HNO₃ = 63.—This acid, which is used in the preparation
of many colours, is distinguished by the readiness with which it gives up
part of its oxygen, and thus converts metals like antimony and bismuth into
oxides, and transforms other compounds into a higher state of oxidation.
There are two kinds of nitric acid: ordinary nitric acid, a colourless liquid
which is more or less pure; and fuming nitric acid, a yellow or orange-
coloured liquid, fuming strongly in the air, which consists of a solution of
nitrogen peroxide, NO₂, and nitric oxide, NO, in nitric acid.

Specific Gravity Nitric Acid Specific Gravity Nitric Acid
at 15° C. per cent. at 15° C. per cent.
1·530 100·00 1·323 55·00
1·520 97·00 1·284 50·49
1·509 94·00 1·251 45·00
1·503 92·00 1·211 40·00
1·499 91·00 1·185 33·86
1·495 90·00 1·157 30·00
1·478 85·00 1·138 25·71
1·460 80·00 1·120 23·00
1·442 75·00 1·089 20·00
1·423 69·96 1·067 15·00
1·400 65·07 1·022 11·41
1·346 60·00 1·010 4·00

Since the action of nitric acid chiefly depends on its oxidising
properties, which are possessed by both kinds, it generally does not matter
which is used. The usual impurities are chlorine and sulphuric acid; the
presence of the first is shown by silver nitrate solution, of the latter by
barium chloride, in each case added after diluting. When the acid is used for
oxidations these impurities do not interfere, but nitric acid containing
chlorine cannot be used to dissolve silver, because the chlorine would form
insoluble silver chloride.
The strength of nitric acid is gauged by its specific gravity as given in
the table.
Aqua Regia.—A mixture of 2 parts of hydrochloric acid and 1 part of
nitric acid gradually turns orange or yellow and evolves chlorine. This
liquid, which can dissolve gold in consequence of the free chlorine it
contains (hence its alchemistic name, from gold, the “king of metals”), is
used as a very powerful oxidising agent in colour making.
Carbon, C = 12, is the only one of the non-metallic elements to be
mentioned here; by itself it forms a group of very important pigments,
which we shall describe in detail at a later stage.

Carbonic Acid Gas, CO₂ = 44, is used in the manufacture of white
lead, which it precipitates from lead acetate. This is, however, a particular
branch of colour making carried on in special works. In describing this
manufacture we shall return to the preparation of carbonic acid on a large
scale.

Organic Acids.

The organic acids which are important in colour making are acetic,
oxalic and tartaric acids.
Acetic Acid, C₂H₄O₂ = 58.—The very dilute form of this substance is
known commonly as vinegar, the stronger as pyroligneous acid, and the
purest as glacial acetic acid; the latter is, however, scarcely used. Formerly
in colour making ordinary vinegar was used, but now pyroligneous acid is
almost exclusively employed. This is distinguished by its strong
empyreumatic smell, which, however, is without importance in colour
making.
The strength of a solution of acetic acid cannot be found by a simple
estimation of specific gravity, since the density does not increase with the
percentage of acetic acid. If an accurate estimation of the strength of acetic
acid is required, it must be obtained by neutralising the acid with an alkali
by a process of volumetric analysis.
For practical purposes, where it is generally known whether a very
strong or a more dilute acetic acid is under consideration, the following
table, showing the connection between specific gravity and percentage
strength, is sufficient.

Specific Acetic Acid Specific Acetic Acid
Gravity per cent. Gravity per cent.
1·0635 100 1·058 48
1·0670 98 1·055 46
1·0690 96 1·054 44
1·0706 94 1·052 42
1·0716 92 1·051 40

Specific Acetic Acid Specific Acetic Acid
Gravity per cent. Gravity per cent.
1·0730 90 1·049 38
1·0730 88 1·047 36
1·0730 86 1·045 34
1·0730 84 1·042 32
1·0730 82 1·040 30
1·0735 80 1·038 28
1·0732 78 1·035 26
1·0720 74 1·031 22
1·0710 72 1·027 20
1·0700 70 1·025 18
1·0700 68 1·023 16
1·0690 66 1·020 14
1·0680 64 1·017 12
1·0670 62 1·015 10
1·0670 60 1·012 8
1·0660 58 1·008 6
1·0640 56 1·005 4
1·0630 54 1·002 2
1·0620 52 1·001 1
1·0600 50 —— ——

Oxalic Acid, C₂H₂O₄.2H₂O = 126, has but a limited use in colour
making. It comes into commerce in the form of more or less pure white
crystals which readily dissolve in water, and are almost pure oxalic acid,
containing only small quantities of oxalate of lime, the presence of which is
without importance for the purposes to which the acid is put in colour
making. Frequently, instead of oxalic acid, the acid potassium oxalate (salt
of sorrel) is used.
Tartaric Acid, C₄H₆O₆ = 150, occurs as white or yellowish crystals,
with a slightly burnt smell, which dissolve readily in water, and have a
strong acid taste. The pure acid, which is white and without smell, is

considerably dearer than the yellow variety. The impurities of the latter,
which are small in quantity, are without influence on the colours prepared
by its help, so that this form is generally used.

CHAPTER V.
METALLIC COMPOUNDS.

Alkalis.

The compounds of the alkali metals, potassium and sodium, play a
considerable part in colour making. Formerly the potassium compounds
were in general use, but the sodium compounds are at present obtainable at
a much lower price, and in most cases they can be used equally well. Thus,
in colour making, sodium compounds are chiefly employed. The cyanogen
compounds are an exception; their potassium compounds are used
exclusively.
Potassium Compounds.—The potassium compounds which are chiefly
used in colour making are potassium carbonate (potashes, pearl-ash),
potassium hydroxide (caustic potash), potassium nitrate (saltpetre),
potassium tartrate (tartar), and potassium ferrocyanide and ferricyanide
(yellow and red prussiate of potash). The cyanogen compounds have
peculiar properties. We shall describe them separately after the potassium
and sodium compounds.
Potassium Carbonate (carbonate of potash), K₂CO₃ = 138, is known
commercially as potashes, a name derived from its former method of
preparation by heating the ashes of plants in pots. At present potashes are
prepared in large quantities from other sources.
Pure potash forms crumbling lumps with a slight yellow or bluish grey
tinge, rapidly absorbing moisture from the air, and in time completely
liquefying. The yellowish tinge is caused by oxide of iron, the bluish by
manganese compounds. The so-called calcined potash has been strongly
heated, and thus all organic substances contained in it have been destroyed.
Potashes are in no way pure potassium carbonate; they contain a
mixture of all those salts which are found in plants—potassium sulphate
and chloride, small quantities of silicic acid, etc. These impurities are rarely

harmful, still it is generally necessary to know the percentage of pure
potassium carbonate contained in potashes.
Although at present in commerce the strength of potashes is frequently
guaranteed, it is still desirable to estimate the strength. It is sufficient for
practice to allow a small quantity, say 100 grammes, to stand with an equal
quantity of very cold water for some hours, then to filter and pour a similar
quantity of water over the residue on the filter. The weight of undissolved
substance subtracted from 100 gives with sufficient accuracy the weight of
pure potassium carbonate contained in 100 parts of potashes. This method is
founded on the fact that potassium carbonate dissolves readily even in cold
water, but the other salts with difficulty. This procedure can also be used to
obtain pure potassium carbonate from crude potashes; it is only necessary to
filter and evaporate to dryness the solution obtained by pouring very cold
water on crude potashes.
Potassium Hydroxide (Potassium Hydrate, Caustic Potash), KOH =
56.—The commercial variety consists of very deliquescent white lumps,
generally containing a large quantity of impurities. On this account caustic
potash, or rather a solution of it, is prepared in the colour works.
With this object 11 parts of potash, contained in a tub with an opening at
the bottom, are mixed with 100 parts of cold water. Two hours later, the
clear solution is run off into a clean iron pan, in which it is heated to
boiling. To the boiling solution is added milk of lime prepared from water
and 3·5 parts of quicklime. After the liquid has boiled a few minutes, a
small portion is filtered and hydrochloric acid added to the clear filtrate; if
no effervescence occurs, then all the potassium carbonate is converted into
caustic potash. Should effervescence occur, milk of lime is added until a
new portion no longer effervesces on the addition of hydrochloric acid.
Then the pan is covered with a well-fitting lid, and the cooled liquid, if not
required for immediate use, preserved in well-corked glass bottles.
The strength of a caustic potash solution can be found by means of a
hydrometer. The following table shows the relation between the specific
gravity of a solution and the percentage of caustic potash it contains:—

Specific Caustic Potash
Gravity per cent.
1·06 4·7
1·11 9·5
1·15 13·0
1·19 16·2
1·23 19·5
1·28 23·4
1·39 32·4
1·52 42·9
1·60 46·7
1·68 51·2
Potassium Nitrate (Saltpetre), KNO₃ = 101, consists of large crystals,
which quickly dissolve in water. On heating it readily gives up oxygen, and
thus finds use as an oxidising agent. In former times, when the colour
manufacturer was compelled to make his own materials, saltpetre was of
great importance in colour making; at present, when such materials are to
be bought at low prices and no colour maker prepares his own, saltpetre is
little used.
Potassium Bitartrate, C₄H₅KO₆ = 188.—This salt, known as tartar in
large crystals, and as cream of tartar in the form of meal, is occasionally
used in colour making. It is little soluble in cold water, but more easily in
hot. The hot solution is generally used.
Potassium Bichromate (Bichromate of Potash), K₂Cr₂O⁷ = 295.—This
salt is made in special works, by melting chrome iron ore with saltpetre and
extracting the mass with water, when a yellow solution of potassium
chromate is obtained; to this sulphuric acid is added, which unites with half
the potassium, thus leaving potassium bichromate, which is obtained by
evaporation of the solution in fine red crystals. These are purified by
recrystallisation. At present, in place of the above method, calcium
chromate is formed by roasting chrome iron ore with lime; the calcium
chromate is then decomposed by a soluble potassium salt, thus forming
potassium chromate.

Potassium bichromate is unaltered in air; it dissolves easily in water,
and is of great importance in the preparation of many colours, in particular
chromium oxide and the lead pigments. The commercial salt generally
contains potassium sulphate, with which at times it is intentionally
adulterated. The adulteration is detected by dissolving in water, adding half
the volume of pure hydrochloric acid, and cautiously and carefully dropping
in spirits of wine. A rapid action takes place, which is only assisted by
warming when necessary. The red liquid changes to emerald green. If
barium chloride be now added, a white precipitate is obtained in the
presence of potassium sulphate.
Potassium Sodium Chromate, KNaCrO₄ = 279, is also used in colour
making. Its solution is made by adding soda to a solution of potassium
bichromate so long as an effervescence of carbonic acid occurs, and until
the liquid turns red litmus paper blue; the solution of the double salt is
yellow.
Chrome Alum, KCr(SO₄)₂.12H₂O = 499.—This salt occurs in
commerce as beautiful violet crystals. It is obtained as a by-product in the
manufacture of aniline and anthracene dyes, and may often be bought at
lower prices than other chromium salts; 100 parts of water dissolve
approximately 20 parts of chrome alum.
Potassium Ferrocyanide, K₄Fe(CN)₆.3H₂O = 422.—The potassium
iron cyanogen compounds are made in special works, particularly in the
neighbourhood of large towns, by melting potashes with nitrogenous
organic substances and iron, washing out the mass and purifying the salt so
obtained by recrystallisation. Potassium ferrocyanide (yellow prussiate of
potash) forms large transparent crystals of a peculiar soft nature, which
dissolve readily in water. It often contains considerable quantities of
potassium sulphate, up to 5 per cent., and it is to be noted that the impurity
is much the cheaper of the two salts. When barium chloride is added to a
solution of the salt, a white precipitate forms if sulphate be present.
The behaviour of yellow prussiate towards iron salts is noteworthy.
With ferrous salts, for example green vitriol (copperas), it gives a white
precipitate which gradually turns blue in the air; with ferric salts, for
example ferric chloride (“nitrate of iron”), it at once gives a blue
precipitate.

Potassium Ferricyanide (Red Prussiate of Potash), K₃Fe(CN)₆ = 329,
is obtained by passing chlorine through a solution of yellow prussiate until
the liquid smells strongly of chlorine and no longer gives a precipitate with
a solution of a ferric salt. The solution then contains potassium ferricyanide
and chloride. The former is obtained by evaporating and allowing to
crystallise.
Pure potassium ferricyanide forms beautiful dark red crystals, which
readily dissolve in water. The solution gives a blue precipitate with ferrous
salts, but only a brown colouration and no precipitate with ferric salts. Both
yellow and red prussiate are used in the preparation of several much used
colours, for Prussian and Chinese blues, and several others. All cyanogen
compounds, with the exception of yellow prussiate, are extremely
poisonous. The following table gives the solubility of potassium
ferricyanide at different temperatures:—

100 Parts of Water Temperature. Specific Gravity
dissolve Parts of Salt. °C. of Solution.
33 4.44 1.151
36 10.00 1.164
40.8 15.50 1.178
58.8 37.80 1.225
77.5 100.00 1.250
82.6 104.40 1.265

Sodium Salts.—In chemical properties the sodium salts are very similar
to the potassium salts, and, being cheaper, they are generally used in place
of the latter.
Sodium Carbonate (Soda Crystals), Na₂CO₃.10H₂O = 286, is made in
enormous quantities in great works and in a very pure state. It forms large
transparent crystals, which effloresce in the air, losing a large quantity of
water, and so falling to a white powder. Although this property does not
interfere with the use of soda, since it is generally used in solution, yet
efflorescence should be as far as possible avoided by keeping the salt in
well-closed packages, because effloresced soda dissolves more slowly than

crystallised, since it has to combine with water before it can enter into
solution.
In the retail trade a form of soda is found which is adulterated with very
large quantities of Glauber’s salt. This is recognised by the different form of
the crystals. Manufacturers sell soda stating its strength. The colour maker
should only buy with this guarantee.
Sodium Hydroxide (Sodium Hydrate, Caustic Soda), NaOH = 40,
comes into commerce in the form of hard masses, the use of which would
be very convenient for the colour maker if it were not often very impure.
Thus it is better to prepare a solution oneself, which is accomplished in the
manner given above for caustic potash.
Caustic soda and caustic potash have similar properties; they have a
corrosive action on the skin, readily unite with carbonic acid from the air,
and separate the heavy metals from their solutions in the form of hydrated
oxides.
The strength of caustic soda solutions is given in the following table:—

Specific Caustic Soda Specific Caustic Soda
Gravity per cent. Gravity per cent.
2·00 77·8 1·40 29·0
1·85 63·6 1·36 26·0
1·72 53·8 1·32 23·0
1·63 46·6 1·29 19·0
1·56 41·2 1·23 16·0
1·50 36·8 1·18 13·0
1·47 34·0 1·12 9·0
1·44 31·0 1·06 4·7

Besides soda and caustic soda, few soda salts are used in colour making.
However, sodium nitrate (Chili saltpetre), NaNO₃, is frequently used
instead of ordinary saltpetre, from which it differs in being deliquescent.
Sodium Thiosulphate (Hyposulphite), Na₂S₂O₃.5H₂O = 248, is a
common article of commerce, being much used by photographers; it is used

in a few cases in colour making. It forms large crystals with a somewhat
bitter taste, permanent in air and readily soluble in water.
Sodium Chloride (Common Salt), NaCl = 58·5, which has a little
application in colour making, is sufficiently pure in the form in which it is
generally used for household purposes.

Salts of the Alkaline Earth Metals.

The metals which are known as the alkali earth metals have much
similarity with the alkali metals in their compounds, with the difference that
their alkalinity is much less, and that their salts are much less soluble in
water. For colour making the compounds of three of these metals—calcium,
barium and magnesium—are used.

Calcium Compounds.

The most important calcium compounds are lime and carbonate and
phosphate of lime. Carbonate of lime is used as a pigment, and will be dealt
with in detail among the mineral colours; it will be but briefly described
here.
Calcium Oxide (Quicklime), CaO = 56.—When chalk and limestone,
which consist of calcium carbonate, are heated, carbonic acid is evolved,
and calcium oxide, commonly called quicklime, is left. For our purposes
only very pure quicklime is to be used. Its ordinary impurities are iron
oxide and magnesia: the former is found in lime made from red or brown
limestone; the latter in lime made from dolomitic limestone. The presence
of oxide of iron is recognised by the reddish tinge of the quicklime; if
magnesia be present, a small quantity of the quicklime, when mixed with a
very large quantity of water, leaves an insoluble residue which consists of
magnesia.
When quicklime and water are brought together they unite very
energetically and form calcium hydroxide or slaked lime. According to the
quantity of water used for slaking, either dry slaked lime, lime paste, or
milk of lime is produced, all of which find a use in colour making.

Calcium Hydroxide (Slaked Lime), Ca(OH)₂ = 74.—In order to
prepare slaked lime, which contains lime united with just the necessary
quantity of water, the pieces of quicklime are sprinkled with water from a
watering can. The water is rapidly taken up and the sprinkling is repeated
until the lumps begin to fall to a fine powder; in the process the lime
becomes very hot. The slaked lime is then passed through a sieve in order to
separate the larger pieces of quicklime which have not been slaked; the
powder must be kept in well closed packages, since it energetically absorbs
carbonic acid out of the air.
If so much water is added to the lime that a homogeneous wet mass is
formed which can be readily moved with a shovel, one has then lime paste,
which can be conveniently kept in pits as the masons do; it may be stored in
this way for many months without appreciable alteration, still it is better to
keep it covered. To prepare milk of lime, so much water is used in slaking
that a milky liquid is formed, or the lime paste is mixed up in the proper
quantity of water. Slaked lime dissolves in 700 to 800 parts of water; on
standing, the undissolved slaked lime settles to the bottom of the milk of
lime: thus it is better to prepare milk of lime immediately before use, and to
stir it well to prevent the settling of the solid particles.
Slaked lime in one of its forms is often used instead of the more costly
caustic soda in order to precipitate metallic oxides from their salts.
At times one finds a too strongly burnt lime, so-called “dead-burnt”
lime, which is very slowly slaked by water. Such quicklime is slaked by
allowing it to lie in water for days, or by means of hot water, which
accomplishes the slaking more quickly.
Calcium Carbonate, CaCO₃ = 100, is found naturally in large
quantities as chalk, which consists of the skeletons of extremely small
animals. By powdering and levigating, it is converted into a soft powder,
which is used to lighten the shade of lakes and other colours.
Calcium Sulphate (Gypsum), CaSO₄.2H₂O = 172.—This mineral,
when finely powdered, is added to some colours.
Calcium Phosphate (Bone Ash), Ca₃(PO₄)₂ = 310, is sometimes used
to lighten the shade of certain colours which might be injured by calcium
carbonate. It comes in large quantities as a fine, white powder from South

America. Naturally, only quite white bone ash can be used; if not
completely burnt it is grey, and will then impair the shade of colours with
which it is mixed.
Magnesium Carbonate (Magnesia), MgCO₃ = 84, is also used as an
addition to colours in order to obtain pale shades. It is most cheaply
obtained by dissolving magnesium sulphate (Epsom salts) in water and
adding soda solution so long as a precipitate forms, which is then washed
and dried. The magnesium carbonate prepared in this way is a very fine,
light powder, insoluble in water, which can be mixed with the most delicate
colours without harming them. White magnesia is an extremely light
powder, which may be used when it can be bought at as low a price as the
above preparation.
Barium Compounds.—The raw material used for the preparation of
barium pigments is either barium sulphate (barytes, heavy spar) or barium
carbonate (witherite). The latter is much more rare than barytes, which is
almost exclusively employed in the preparation of barium compounds. The
barium compounds of particular importance for our purpose are barium
chloride and nitrate.
Barium Chloride, BaCl₂.2H₂O = 244.—This salt is now a common
article of trade, and can be bought at a low price. When pure it forms
colourless crystals readily soluble in water. If the colour maker is able to get
cheap barytes and fuel, it may be advantageous for him to prepare barium
chloride himself.
To prepare barium chloride from barytes, the latter is very finely ground
and levigated, intimately mixed with coal, and the mixture subjected to a
very high temperature, when barium sulphide is formed, which is dissolved
by washing out the mass with water and converted into barium chloride by
adding hydrochloric acid, sulphuretted hydrogen being evolved.
The best method is to mix 4 parts of barytes with 1 part of bituminous
coal and so much coal-tar that a plastic mass is formed, which is well
kneaded and made into small cylinders 3 centimetres in diameter and 10
centimetres long. These cylinders are placed in layers in a cylindrical
furnace with a good draught, which contains at the bottom a layer of coal 15
to 20 centimetres thick, then a layer of the cylinders, then again coal, and so
on until the furnace is full. The lowest layer of coal is lighted, and the

whole burnt at a bright red heat, when the barium sulphate is changed into
sulphide. Hydrochloric acid is poured over the residue and the insoluble
part, consisting chiefly of unaltered barytes, is used for the next operation.
Witherite (barium carbonate) can be converted into barium chloride in a
very simple manner. Hydrochloric acid is added, in which it dissolves with
the evolution of carbonic acid. The solution is allowed to stand twenty-four
hours with excess of witherite; the whole of the dissolved iron is thus
precipitated. The solution is then filtered, evaporated down and left, when
pure barium chloride crystallises out.
Barium chloride and all soluble barium salts should only be dissolved in
pure water (rain or distilled). Water which contains carbonates or sulphates
always gives a turbid solution by precipitating barium carbonate or
sulphate.

Aluminium Compounds.

The compounds of the earth metal aluminium play a very important part
in colour making, since they form beautifully coloured compounds with
many organic colouring matters. Formerly alum was the only material used
in colour factories for the preparation of the alumina compounds; at present
aluminium sulphate is used, and when it is sufficiently pure it is the most
valuable material, because it contains the greatest quantity of alumina.
Under the designation of alum only one compound, the so-called potash
alum, was at one time found in commerce, but now there are other alums,
which contain soda or ammonia in place of potash. These salts are of equal
use in colour making to potash alum. The preference is to be given to the
compound which contains the largest proportion of alumina. The chief point
to be observed in connection with alumina compounds for use in colour
making is that they shall be free from iron, because iron oxide, which would
be precipitated out of the solution along with the colours, in consequence of
its red colour would spoil the shade of the pigment.
Aluminium Sulphate (Sulphate of Alumina), Al₂(SO₄)₃.18H₂O = 664.
—Any manufacturer who can obtain cheap china clay (kaolin) and
sulphuric acid can himself prepare this compound with advantage. The
apparatus used for this purpose is an iron pan containing sand, in which is

placed a large earthenware dish. In this dish are put very finely ground
kaolin and strong sulphuric acid, and the mixture is heated so strongly that
the acid boils, evolving heavy, white vapours. It is absolutely necessary to
heat in this manner in order to avoid dangerous accidents. Sulphuric acid
bumps so violently on boiling that it may even break a thick earthenware
dish. The use of a sand bath makes the bumping harmless.
China clay, which consists of silicate of alumina, is decomposed by
heating with sulphuric acid into silicic acid and sulphate of alumina. The
original milky liquid becomes more transparent during boiling, and has at
last the appearance of starch paste. Kaolin contains varying quantities of
silica. The quantity of sulphuric acid necessary for its decomposition can
only be found by trial. The quantities are chosen so that a small amount of
kaolin remains undecomposed in order that the aluminium sulphate shall
contain no free sulphuric acid.
When the decomposition is finished the pan is allowed to cool and the
solid mass is brought into a vat filled with water, in which it is stirred until
dissolved; then the liquid is left until the jelly-like mass of silicic acid has
sunk to the bottom, when the clear solution of aluminium sulphate is drawn
off and can at once be used.
If solid aluminium sulphate is required—and this is to be recommended
when large quantities are to be prepared—the solution is evaporated in
earthenware dishes until a portion solidifies when dropped on a cold plate.
The molten aluminium sulphate is then cast in prismatic blocks, which are
preserved in boxes. These blocks are of a pure white colour and very
crystalline; they dissolve with difficulty in cold, but readily in hot water
without residue. The solution has an acid taste, even when it contains no
excess of sulphuric acid. When the blocks have a yellowish tinge, this
denotes the presence of iron, and the solution must be freed from this
impurity. Nowadays sulphate of alumina can be obtained so cheaply that it
is hardly of advantage to make it.

The Alums.

These are double salts of aluminium sulphate and potassium, sodium or
ammonium sulphate. There are also other double sulphates known as alums

which, in place of aluminium, contain chromium, iron, etc., but they are not
of interest here. It may still be observed that all alums, whatever their
composition, possess the property of crystallising together from mixed
solutions, so that crystals can be obtained in which every existing alum is
contained.
The potassium, sodium and ammonium aluminium alums are used in
colour making.
Potassium Aluminium Alum, KAl(SO₄)₂.12H₂O = 474.—This is the
substance commonly called alum. Like all alums it crystallises in fine
octahedral crystals, which at first are quite transparent, but slowly effloresce
in the air and become covered by a white powder. It dissolves with
difficulty in cold, but readily in hot water. The solution has at first a sweet
taste, with an astringent after-taste.
Alum comes into the market in different forms, of which the following
are the most important: as crystallised alum, in the form of large crystals
united together; as alum meal, a coarse crystalline powder obtained by
rapidly cooling and stirring a hot alum solution. On account of the larger
surface this form dissolves more quickly than the large crystals. Roman
alum is the name of a variety chiefly imported from Italy; it owes its
reputation to its great purity—it contains a very small quantity of iron.
In order to prepare alum quite free from iron from the ordinary alum
containing iron, it is recrystallised, that is, as much as possible is dissolved
in boiling water and the solution quickly cooled with continual stirring; the
small crystals so obtained are then washed with cold water. The residual
saturated solution of alum, which contains the greater part of the iron, can
be used for the preparation of those colours which are not injured by the
presence of iron.
The solubility of alum in water varies greatly at different temperatures.
The table gives the weight of alum dissolved by 100 parts of water at
different temperatures.

Temperature. Crystallised Alum. Anhydrous Alum.
0° C. 3·90 2·10
10° ” 9·52 4·99

Temperature. Crystallised Alum. Anhydrous Alum.
20° ” 15·13 7·74
30° ” 22·01 10·94
40° ” 30·92 14·88
50° ” 44·11 20·09
60° ” 66·65 26·76
70° ” 90·67 35·11
80° ” 134·47 45·66
90° ” 209·31 58·64
100° ” 357·48 74·53

When potash alum is heated it loses water, 75 per cent. of the total at
61° C.; at 92° C. it melts completely, and all the water is lost by continued
heating at 100° C. The residue is known as burnt alum.
In alum the whole acidity of the sulphuric acid is not neutralised; the
solution has always an acid reaction; if soda solution is added, the escaping
carbonic acid causes the liquid to effervesce. If soda solution is added until
a further addition would cause a precipitate, a solution of so-called neutral
alum is formed which has no longer an acid reaction. Neutral alum is
occasionally required in colour making. In preparing it the soda solution
must be added with great care when the liquid is near its point of
neutralisation. Any addition of soda solution after this point is reached will
cause a separation of alumina. This is not desirable, since it is generally
only wished to precipitate the alumina in combination with colouring
matters.
Roman Alum.—Under this name, or that of “cubic alum,” a variety of
alum is sold, generally at a rather higher price than ordinary alum, from
which it is distinguished by its crystalline form. Ordinary alum forms
octahedral crystals often the size of a child’s head, but cubic alum well
formed cubes.
The property of crystallising in cubes may be imparted to any alum
solution by the addition of a little potash. Much so-called Roman alum is
made in German works in this way. When this alum contains very little iron
it is quite equal in quality to the best Roman alum, for the higher value of

the latter is entirely due to its small content of iron. The alum prepared in
the province of Naples is still better than Roman alum; it contains less iron.
When alum is required for the preparation of lakes of bright and delicate
shades, it is indispensable to use a preparation free from iron, because the
brownish yellow oxide of iron would appreciably injure the shade. Alum,
free from iron, is most simply prepared by dissolving alum in boiling water,
running the boiling solution quickly through a cloth, and quickly cooling
with constant stirring. The alum meal prepared in this way contains much
less iron than the original alum, the iron salts remaining dissolved in the
mother liquor. When this alum meal is collected and cold water poured over
it to free it from mother liquor, it is generally sufficiently pure to be used
for any purpose, but if not, it is again recrystallised. The alum liquors
containing the iron are used for the preparation of colours which are not
injured by the presence of iron.
To test alum for iron yellow prussiate of potash is used, which gives a
blue precipitate with ferric salts. The test is carried out by dissolving 10
grammes of alum in 1 litre of water, placing the solution in a tall, narrow
cylinder standing on white paper, adding 10 to 20 drops of a saturated
solution of yellow prussiate, and well stirring. On looking down through the
liquid, if a distinct colouration is at once evident, the alum contains much
iron, and must be recrystallised; indeed, the crystals would generally be
coloured yellow. On the contrary, if the solution does not show a blue tint
until after standing several days, the alum contains but a small quantity of
iron, and can be used for most purposes without further purification. Alum
quite free from iron is a rare commercial article; the test will generally show
a feeble blue colouration. If this is not intense and no blue precipitate is
deposited at the bottom, the alum is tolerably pure, and can be used in
colour works. The longer the time before the blue colouration appears the
poorer is the alum in iron.
Soda Alum, NaAl(SO₄)₂.12H₂O = 458, is made in some alum works. It
has the greatest similarity in properties with the potash salt, but is
distinguished by a much greater solubility in water and more rapid
efflorescence in air. Soda alum can be bought at varying prices; that
containing iron is much cheaper than that free from iron. When the latter is
to be bought at a fair price it is to be preferred to potash alum, since, as we
shall show later, it contains a larger proportion of alumina.

Ammonia Alum, (NH₄)Al(SO₄)₂.12H₂O = 453.—This compound of
aluminium sulphate and ammonium sulphate is now often met with; the
more expensive potassium sulphate in ordinary alum is replaced by
ammonium sulphate, which is cheaply obtained from the ammoniacal liquor
of the gas works.
Ammonia alum is better for our purpose than potash alum since it
contains more alumina, is generally cheaper and dissolves more easily in
water. Unfortunately most commercial ammonia alum contains so much
iron that it has to be recrystallised before it can be used in colour works.
One hundred parts of water dissolve at different temperatures the
quantities of ammonia alum given in the table:—

Crystallised Anhydrous
Temperature.
Ammonia Alum. Ammonia Alum.
0° C. 5·22 2·62
10° ” 9·16 4·50
20° ” 13·66 6·57
30° ” 19·29 9·05
40° ” 27·27 12·35
50° ” 36·51 15·90
60° ” 51·29 21·95
70° ” 71·97 26·09
80° ” 103·08 35·19
90° ” 187·82 50·30
100° ” 421·90 70·83

Of the different alums, ammonia alum contains the largest and potash
alum the smallest proportion of alumina. The composition of the three
commonly occurring alums is given in the following table:—
Potash Soda Ammonia
Alum. Alum. Alum.
Potash, K₂O 9·95 —— ——
Soda, Na₂O —— 6·80 ——

Potash Soda Ammonia
Alum. Alum. Alum.
Ammonia, NH₃ —— —— 3·89
Alumina, Al₂O₃ 10·83 11·20 11·90
Sulphuric acid,
33·71 34·90 36·10
SO₃
Water, H₂O₄ 45·51 47·10 48·11

Thus ammonia alum is to be preferred to soda alum and soda to potash
alum, whilst the latter is used on account of its greater purity. In a colour
works, in which large quantities of alum are used, it is advantageous to
work with ammonia alum which is recrystallised on the works.
The alums and aluminium sulphate are the alumina compounds in
ordinary use in colour works; aluminium acetate could also be used if it
were to be had at a reasonable price.
Alumina, Al₂O₃ = 102, and Hydrate of Alumina.—Pure alumina, or
rather hydrate of alumina, is required in the preparation of many colours.
When the solution of an aluminium salt is precipitated by soda, the carbonic
acid escapes with effervescence, and a gelatinous precipitate is formed
which it is extremely difficult to wash clean. The precipitate, which consists
of hydrate of alumina, shrinks very greatly in drying, and turns to a horny
mass; when strongly heated it loses water, and becomes a white, insoluble
powder of anhydrous alumina.
A variety of hydrate of alumina, heavy, and therefore easily washed, is
obtained by boiling a solution of alum with a plate of zinc lying on a copper
plate until all the alumina has separated. By collecting this on a filter and
pouring hot water over it a number of times it is obtained quite pure.
Alumina plays a particularly important part in the manufacture of cobalt
colours. When treated with a cobalt salt and heated it takes a fine blue
shade.
In the foregoing those metals and their compounds have been treated
which are extensively used in preparing colours without themselves
forming coloured compounds. The chromates and prussiates are an
exception to this. The ammonia compounds, the alkalis and alkaline earths,

also the acids, are used in making many colours, although they do not
contain colouring principles. The alumina compounds are in similar case.
Themselves colourless, they form at the same time a carrier for the coloured
compound and bring it into a suitable form for use as a pigment.
An example will explain what we mean by “carrier” of the colouring
matter. Logwood contains a very handsome colouring matter which can be
extracted by water. In order to be able to employ this colouring matter as a
pigment it is combined with alumina, a compound insoluble in water being
formed, which is called a lake. In this compound the alumina is to be
regarded as the carrier of the colouring matter, which it has fixed in an
insoluble form.
In dyeing, which in many respects is closely allied to colour making, the
property of certain metallic compounds, themselves colourless, of fixing
dyes is commonly utilised, the metallic compound being called the
mordant. The fabric is first prepared with the metallic compound or
mordant, and the colour then formed by bringing the mordanted material in
contact with the colouring matter.
The “heavy metals” form, among their numerous compounds, a great
number of coloured substances, and several of them are distinguished by a
great wealth of coloured derivatives; for example, copper, chromium and
cobalt form coloured compounds only. Although the use of pigments
derived from the heavy metals has been considerably restricted in recent
years by the discovery of a series of colouring matters which replace them,
yet they are now, and always will be, of very great importance in the
manufacture of colours. It is, therefore, necessary briefly to describe the
various metals which are used in colour making, so that the manufacturer
may know what metals produce harmless colours, permanent and unaltered
by the atmosphere, and what do not.
The metals are divided into two great groups, designated, according to
their specific gravities, the group of the light and of the heavy metals. The
light metals comprise the alkali, alkaline earth and earth metals, whose
important compounds we have just described. The specific gravity of each
of these metals is less than five times that of water.
The heavy metals have a specific gravity exceeding 5; they are
generally divided into groups, which are known by the name of the

commonest metal in the group. These are as follows:—
Zinc group Zinc, Zn = 65.
Iron group Iron, Fe = 56.
Tungsten group Tungsten, W = 184.
Antimony group Antimony, Sb = 120.
Tin group Tin, Sn = 119.
Lead group Lead, Pb = 207.
Silver group Silver, Ag = 108.
Gold group Gold, Au = 197.
Platinum group Platinum, Pt = 194.
To the zinc group belong the metals zinc, cadmium and indium, of
which only the two first are of importance here. The iron group comprises
iron, manganese, cobalt, nickel, chromium and uranium, all of which are
used in the manufacture of colours. The antimony group contains antimony
and bismuth, the latter of which is of little importance. To the tin group
belong tin and the rare metals titanium, zirconium, thorium, niobium and
tantalum; of these tin alone is important in colour making. In the lead group
are lead and thallium; lead produces many important pigments. The silver
group contains silver, copper and mercury, the two latter of which are
important. Of the gold group gold alone is of interest, and its importance
has been diminished by the discovery of far cheaper substances which
replace it. In the platinum group, which contains platinum, iridium,
rhodium, ruthenium, palladium and osmium, only platinum itself is used as
a colour; it is employed in porcelain painting to produce the peculiar
metallic shimmer known technically as “lustre.”
The behaviour of the compounds, of the metals mentioned above
towards sulphuretted hydrogen is of the greatest importance to the colour
maker, since on it depends the alterability of the pigments when exposed to
the atmosphere. Many metallic compounds are unaltered by the
sulphuretted hydrogen present in the air, whilst others are in a high degree
affected by it and become gradually darker, so that their colour may in the
end approximate to black.
The pigments containing lead, copper, mercury and bismuth are
extremely susceptible to the action of sulphuretted hydrogen, by the action

of which they form black compounds. Since colours which contain these
metals are not permanent, but darken considerably, endeavours have been
made for a long time to replace them by others not susceptible to the action
of sulphuretted hydrogen. Thus it is desirable to manufacture only colours
free from metals forming black compounds with sulphuretted hydrogen. For
the same reason pigments which contain sulphur should not be mixed with
those containing metals which form black sulphur compounds.
The rules laid down in the preceding paragraphs are of the greatest
importance for the artist, for by following them he will succeed in
composing a “permanent palette,” that is, containing only such colours as
will not, by their composition, bring about the speedy ruin of the painting.
Although the majority of the compounds of the heavy metals are
poisonous, some possess this property in an eminent degree. These are
chiefly colours which contain arsenic, antimony, copper and lead. So far as
it is possible, these colours should be dispensed with and harmless pigments
sold in their place, though this is not always possible, since several
poisonous colours cannot be replaced by innocuous ones.

Compounds of the Heavy Metals.

Zinc Compounds.—Zinc oxide, ZnO, and zinc sulphate,
ZnSO₄.7H₂O, are the compounds of this metal used in the colour industry.
Zinc oxide, which is used as a white pigment, is a powder which turns
yellow when heated, and is not acted upon by sulphuretted hydrogen. Zinc
sulphate (white vitriol) occurs as colourless crystals, or more frequently as
greyish white crystalline masses. The freedom of zinc sulphate from iron is
of particular importance; the commercial article is seldom satisfactory in
this respect. In order to free commercial zinc sulphate from iron, the
property of zinc hydroxide of precipitating iron oxide from neutral solutions
may be employed. The zinc sulphate is dissolved in water, and ammonia
added in small quantities until the precipitate of zinc hydroxide remains on
stirring. When the liquid is left in contact with the precipitate and stirred up
once or twice a day, if iron is present the precipitate will turn yellowish
brown, owing to the separation of ferric hydroxide, and in the course of a
few days all the iron will be removed from solution. The liquid should then
give no blue colouration with yellow prussiate of potash.

Zinc oxide is used as a white pigment, zinc chromate as a yellow, and
zinc cobalt compounds as green colours.
Cadmium Compounds.—Cadmium is a metal which possesses great
similarity to zinc, with which it occurs in nature. In the preparation of
cadmium compounds the metal is generally used. This is dissolved in dilute
sulphuric acid, hydrogen is evolved, and a solution of cadmium sulphate
obtained.
Cadmium is used in colour making only for the preparation of the
beautiful cadmium yellows.
Iron Compounds.—These are of the greatest importance to the colour
maker. Several, in which iron alone is the colour principle, are very
valuable: ochre, rouge, Venetian red, sienna and umber, for example. Iron
compounds are also used in the preparation of many colours. The most
important is:—
Ferrous Sulphate (Green Vitriol, Copperas), FeSO₄.7H₂0.—This
substance, which occurs commercially in a form of great purity at a very
low price, is generally the starting point in the preparation of iron pigments.
When pure, it forms fine sea-green crystals, with an astringent metallic
taste, which are not poisonous and are easily soluble in water. After long
exposure to the air, ferrous sulphate becomes covered with an ochre-
coloured crust, consisting of basic ferric sulphate. The ferrous oxide
contained in the green vitriol has united with oxygen and been converted
into ferric oxide. The latter requires a larger quantity of acid than ferrous
oxide for the formation of soluble salts, so that an insoluble basic salt is
separated. The same thing occurs when a solution of ferrous sulphate is
exposed to the air.
When green vitriol, or any other ferrous salt, is exposed to the action of
oxidising agents, such as chlorine or nitric acid, the iron is rapidly changed
into the ferric state. This transformation is of particular importance in the
manufacture of certain blue pigments.
We give in a table the relation between the percentage of crystallised
ferrous sulphate (FeSO₄.7H₂O) contained in a solution at 15° C. and its
specific gravity:—

Percentage of Percentage of
Specific Specific
Ferrous Ferrous
Gravity Gravity
Sulphate. Sulphate.
1·000 0 1·118 21
1·005 1 1·125 22
1·011 2 1·131 23
1·016 3 1·137 24
1·021 4 1·143 25
1·027 5 1·149 26
1·032 6 1·155 27
1·037 7 1·161 28
1·043 8 1·168 29
1·048 9 1·174 30
1·054 10 1·180 31
1·059 11 1·187 32
1·065 12 1·193 33
1·071 13 1·200 34
1·077 14 1·206 35
1·082 15 1·213 36
1·088 16 1·219 37
1·094 17 1·226 38
1·100 18 1·232 39
1·106 19 1·239 40
1·112 20 —— ——

Yellow and red prussiate, which have been already mentioned, also
belong to the iron compounds. They have been separately mentioned
because the iron is contained in them in a peculiar form as a portion of an
organic radical.
Ferrous Chloride, FeCl₂, may sometimes be used instead of green
vitriol. When iron is dissolved in hydrochloric acid hydrogen is given off
and a solution of ferrous chloride is obtained; but when rouge is dissolved

in the same acid, ferric chloride is formed. When iron is dissolved in nitric
acid, in consequence of the oxidising properties of this acid a ferric salt is
obtained. Iron forms two series of salts: in the ferrous compounds the iron
is in the same form as in green vitriol and the corresponding salts; in the
ferric compounds the iron is contained in a higher state of oxidation. By
powerful oxidising agents, as nitric acid or chlorine, ferrous compounds are
converted into ferric.
Manganese Compounds.—Manganese (Mn) is a metal whose
compounds show great similarity with those of iron, like which it forms two
oxides (also others), manganous oxide (MnO) and manganic oxide
(Mn₂O₃). The salts of manganous oxide are not oxidised in the air like those
of ferrous oxide.
The raw material used in the preparation of manganese compounds is
the mineral pyrolusite, which is manganese dioxide (MnO₂).
Manganese sulphate (MnSO₄) forms rose-red crystals containing
varying quantities of water. The residues from the preparation of chlorine
can be used as the material for the preparation of colours. According as
pyrolusite and hydrochloric acid or pyrolusite, salt and sulphuric acid are
used for this purpose a solution of manganous chloride or sulphate is
obtained.
Manganese compounds have but a restricted use in colour making.
Nickel Compounds are generally coloured green, but they are not used
as pigments.
Cobalt Compounds.—Among these are found many important
pigments. All cobalt compounds are coloured; in beauty and variety of
shade they can only be compared with those of chromium. In properties
cobalt is very similar to iron and nickel.
The form in which cobalt is used in preparing colours is either cobalt
nitrate, Co(NO₃)₂.6H₂O, or cobalt chloride, CoCl₂.6H₂O. Both salts are
articles of commerce, but generally they are so dear that it is more
profitable for the colour maker to prepare them direct from the cobalt
minerals. A simple method for preparing cobalt compounds from the ores is
therefore given. The most important cobalt ores are speiss cobalt, a
compound of cobalt and arsenic, and cobalt glance, a compound of cobalt,

arsenic and sulphur. The former mineral often contains only small quantities
of cobalt, and it is advisable for our purposes to use cobalt glance, which
contains from thirty to forty per cent. of cobalt. This mineral is first roasted,
that is, is heated with a plentiful air supply, by which means the arsenic is
driven off. On account of the poisonous nature of the arsenic vapours the
roasting must be conducted in a furnace with a very good draught.
Under the name of zaffre, roasted cobalt ores come into commerce.
These may be used in the preparation of cobalt compounds, by which
means the operation of roasting is avoided. According to the quality of the
ore which has been used to obtain zaffre, it contains a very varying
proportion of cobalt. The varieties richer in cobalt must be used; they are
technically known by the mark FS, or FFS (the best).
The roasted cobalt ores or zaffre are treated with fused acid potassium
sulphate, when the salts of iron and manganese are decomposed, whilst
cobalt and nickel sulphates remain unchanged. In a Hessian crucible are
melted 300 parts of acid potassium sulphate, and 100 parts of the powdered
zaffre are gradually added, mixed with one part of green vitriol and one part
of saltpetre; the mixture is heated so long as sulphuric acid escapes. The
mass is then boiled with water and the red solution treated with sulphuretted
hydrogen so long as a precipitate is formed; this may contain copper,
manganese, and bismuth. After filtering, soda is added to the boiling liquid;
cobalt carbonate is precipitated, which can be converted into nitrate or
chloride by solution in the corresponding acid. If cobalt sulphate is
required, the solution, after treatment with sulphuretted hydrogen, need
only be evaporated to crystallisation, when the sulphate separates in fine red
crystals. Cobalt nitrate and chloride are very soluble in water; to obtain
them their solutions must be strongly evaporated and quickly cooled whilst
stirring. The crystals of cobalt nitrate and chloride absorb moisture from the
air and deliquesce; they must be kept in glass vessels with well-ground
stoppers.
The cobalt compounds which are to be used in colour making must be
free from iron, nickel and arsenic, which would detract from the cleanness
of the colours. If the precipitate produced by soda contains iron it is mixed
with excess of solution of oxalic acid, and after a few hours the cobalt
oxalate is filtered from the liquid, in which all the iron is dissolved. The

cobalt oxalate can then be converted into nitrate or chloride by treatment
with nitric or hydrochloric acids.
These salts form the material for the preparation of the cobalt
compounds, a large number of which are used as extremely durable red,
blue and green pigments; several of them, such as cobalt blue, cannot be
exactly replaced by other pigments. On account of the industrial importance
of the cobalt colours, these directions for the preparation of the soluble
cobalt salts from the ores have been given with some detail. The preparation
of the cobalt colours will be given in extenso later on.
Chromium Compounds.—As the name indicates, this metal yields
numerous coloured compounds (χρῶμα, colour); in fact, only coloured
chromium compounds are known, and the colours are most varied—yellow,
green, red and violet. On this account the chromium compounds are among
the most important used in colour making; a great number of colours are
prepared by their aid. Chrome ironstone, as we have already stated, is the
raw material for the preparation of chromium compounds. From it
potassium bichromate is made on a large scale in special works, so that no
colour maker is compelled to prepare chromium salts himself.
When the chromium pigments contain no metal blackened by
sulphuretted hydrogen, they have the desirable property of being unaltered
by the atmosphere. Like the cobalt compounds, they are distinguished by
their great stability when heated; on this account, they have a large use in
porcelain painting.
Molybdenum, Tungsten and Vanadium Compounds on account of
their cost have a very limited use as pigments. Molybdenum compounds are
obtained from molybdic acid; compounds of tungsten from the metal; and
those of vanadium from ammonium vanadate.
Antimony Compounds can be used in the preparation of several
pigments, but, on account of their behaviour towards sulphuretted
hydrogen, the pigments cannot be regarded as really permanent, and their
use is generally diminishing. The so-called antimony vermilion is the only
antimony compound at all extensively employed.
Bismuth Compounds possess properties very similar to those of
antimony. Only one bismuth preparation is used as a pigment, and this is

very sensitive to the action of sulphuretted hydrogen, being changed into
black bismuth sulphide.
Tin Compounds are employed in two ways: some are themselves
colours, such as stannic sulphide (mosaic gold); others, themselves
colourless, are used in making pigments, as stannous and stannic chlorides.
Stannous Chloride, SnCl₂.2 H₂O, is obtained when tin is dissolved in
hydrochloric acid, hydrogen being evolved. Stannic Chloride, SnCl₄, is
formed when tin is dissolved in a mixture of hydrochloric and nitric acids
(aqua regia).
Tin compounds have, similarly to aluminium salts, the property of
forming coloured insoluble compounds (lakes) with many organic colouring
matters. Their use for this purpose is extensive.
Arsenic Compounds formerly played an important part in colour
making. They were used in the manufacture of a large number of pigments,
very beautiful but extremely poisonous. At the present time, we can,
fortunately, entirely dispense with arsenic in colour making; the arsenic
colours can be replaced by others equally handsome and less, or not at all
poisonous. The most important of the arsenic compounds is the trioxide
As₂O₃, or Arsenious Acid, commercially known as white arsenic. This
substance is obtained in large quantities as a by-product in the extraction of
several metals. It forms masses which are either glassy or have the
appearance of porcelain. Freshly sublimed arsenic trioxide is glassy; this
form gradually changes into the porcellaneous variety, it dissolves with
difficulty in water. A strong solution can only be obtained by boiling for
many hours.
The compounds of arsenic with sulphur, formerly extensively used as
pigments, have now almost fallen into disuse.
Lead Compounds belong to the substances most largely used in
making colours. Unfortunately all lead colours have two very important
drawbacks. They are all very poisonous and at the same time extremely
sensitive to sulphuretted hydrogen, so that they are very considerably
altered by the action of the small quantities of that gas contained in the
atmosphere of an ordinary dwelling. A striking example of this is seen in
the lead paints used on the doors of water-closets. The paint, at first pure

white, becomes gradually darker, and at last almost black, the lead
compound having been changed into black lead sulphide.
On account of this great sensitiveness of lead compounds, it would be
better if they could be excluded from the list of colours. Great care must be
taken not to mix lead compounds with others which contain sulphur; a
discolouration of the mixture would be the inevitable result in a very short
time.
The oxides of lead and a number of its salts are themselves pigments,
for example, litharge, PbO, red lead, Pb₃O₄, and white lead (basic lead
carbonate). These pigments are prepared on a large scale in particular
works. At this point only those lead compounds will be mentioned which
are generally used for the preparation of other lead pigments; they are: lead
sulphate, nitrate, acetate and chloride.
Lead Sulphate, PbSO₄, is formed when sulphuric acid or the solution
of a sulphate is added to the solution of a lead salt; so obtained it is a white
crystalline powder insoluble in water. This substance is generally not made
in colour works, but is purchased from chemical works or dye houses, of
which it is a by-product. In this form (lead bottoms) it is generally not
sufficiently pure, but contains admixtures of sulphuric acid or aluminium
salts, from which it is freed by washing. The lead sulphate is stirred up in
water, the heavy precipitate allowed to settle, the wash water drawn off, and
after repeating this process until the wash water no longer shows an acid
reaction, the purified precipitate is dried. In this condition it is a heavy,
white powder, and can alone be ground into paint. But on account of its
crystalline nature, which reduces the covering power, such use is
inadvisable.
Lead Nitrate, Pb(NO₃)₂.—This very important compound may be
bought, but it is advisable to prepare it in the works. Water is placed in a
wooden tub, then half the volume of nitric acid is added, and finely
powdered litharge gradually stirred in, the liquid being kept in constant
movement. When it is seen that the litharge is only slowly dissolved, the
liquid is well stirred after each addition of litharge and then tested by litmus
paper. When this is no longer reddened, the nitric acid is completely
saturated, and the liquid contains only lead nitrate in solution. It is allowed
to stand until the insoluble portions have settled, and then drawn off into

another vessel where crystals of lead nitrate separate in a few days. If the
salt be required in solid form, the solution may be evaporated in
earthenware dishes; generally the solution is used as it is obtained.
Pure lead nitrate forms white crystals which are not particularly soluble
in water, 1 part requiring 2 parts of water at the ordinary temperature. Lead
nitrate is decomposed on heating, like all nitrates, and litharge remains. The
solution of this salt is used in the preparation of those lead pigments which
are obtained by precipitation, for example, chrome yellow.
Lead Acetate, Pb(C₂H₃O₂)₂.3H₂O.—The compounds of lead with
acetic acid are of great importance. Two of these are to be considered:
neutral lead acetate, commonly known as sugar of lead, and basic lead
acetate. It may be advisable to manufacture both these compounds, the
latter always. Neutral lead acetate comes into commerce in the form of
colourless heavy crystals, which are often covered by a white powder of the
basic acetate; they dissolve readily in water, and the solution has a sweetish
taste, hence the name “sugar of lead”. Frequently the solution is very turbid;
this is caused by the carbonates contained in the water. The turbidity may be
removed by the addition of a little acetic acid. It is only economical for the
colour maker to prepare sugar of lead when he can obtain cheap raw
materials, lead or litharge and vinegar. Pyroligneous acid may also be used
if it is colourless, its odour being without importance for this purpose.
The best method for preparing lead acetate from litharge is to place the
vinegar in a tub and hang in it a strong linen bag filled with finely ground
litharge. The tub is kept covered for a few days and its contents then tested
with red litmus paper. When this is turned blue, the liquid is drawn off and
vinegar gradually added whilst stirring, until blue litmus paper is just turned
red. In this process, after neutral lead acetate has been formed, more lead
oxide is dissolved, and the liquid thus acquires an alkaline reaction. The
further addition of acetic acid reconverts the basic salt into neutral lead
acetate.
Lead acetate solution may, with advantage, be prepared directly from
metallic lead. For this purpose, lead is granulated by melting and pouring in
a thin stream into cold water, where it solidifies in irregular pieces. This is
done in order to give the lead as large a surface as possible. Three high
narrow tubs placed one above the other so that liquid may flow from the

highest to the middle, and from this into the lowest, are filled with
granulated lead. Vinegar is placed in the uppermost vessel to cover the lead;
after twenty-four hours it is allowed to flow into the middle, and after a
further twenty-four hours into the lowest tub. In this way, a solution of basic
lead acetate is formed, to which the necessary quantity of acetic acid is
added to bring it into the neutral condition. If crystalline lead acetate is
required, the liquid is evaporated down and quickly cooled with stirring, so
that small crystals are formed. Generally, however, evaporation is
unnecessary, since lead acetate is always used in solution in preparing
colours.
If the lead acetate solution be not colourless, which is generally the case
when coloured acetic acid is used, the defect may be removed by stirring a
little bone black into the liquid and filtering after twenty-four hours, when a
completely colourless solution is obtained.
It is always necessary to know exactly how much lead acetate is
contained in the solutions prepared by these processes. The lead or the
litharge is therefore weighed and the volume of the lead acetate solution
measured. One hundred parts by weight of crystallised lead acetate are
obtained from 62·54 parts of lead.
Lead acetate solutions must be kept in well-covered vessels. The
carbonic acid of the air will turn the liquid turbid. The turbidity may be
removed by the addition of acetic acid.
In the following table is given the percentage of crystallised lead acetate
contained in solutions of different specific gravities:—

Crystallised Crystallised
Specific Specific
Lead Acetate Lead Acetate
Gravity Gravity
per cent. per cent.
1·0000 0 1·1869 26
1·0064 1 1·1955 27
1·0127 2 1·2040 28
1·0191 3 1·2126 29
1·0255 4 1·2211 30
1·0319 5 1·2303 31

Crystallised Crystallised
Specific Specific
Lead Acetate Lead Acetate
Gravity Gravity
per cent. per cent.
1·0386 6 1·2395 32
1·0453 7 1·2486 33
1·0520 8 1·2578 34
1·0587 9 1·2669 35
1·0654 10 1·2768 36
1·0725 11 1·2867 37
1·0796 12 1·2966 38
1·0867 13 1·3064 39
1·0939 14 1·3163 40
1·1010 15 1·3269 41
1·1084 16 1·3376 42
1·1159 17 1·3482 43
1·1234 18 1·3588 44
1·1309 19 1·3695 45
1·1384 20 1·3810 46
1·1464 21 1·3925 47
1·1544 22 1·4041 48
1·1624 23 1·4156 49
1·1704 24 1·4271 50
1·1784 25 —— ——

Basic Lead Acetate, Pb(C₂H₃O₂)₂.2PbO.—This salt may be regarded
as a compound of neutral lead acetate with lead oxide. It is obtained by
digesting vinegar with excess of litharge or with metallic lead, and also by
treating lead acetate solution with litharge so long as the latter is dissolved.
In the method last given, 100 parts of sugar of lead require about 118 parts
of litharge to produce a saturated solution of basic acetate. The solution of
this compound is alkaline; it turns red litmus paper blue. When exposed to
the air, a turbidity is quickly produced owing to the separation of lead

carbonate. In one white lead process basic lead acetate is the starting point
of the manufacture.
Lead Chloride, PbCl₂, is seldom used in making colours. It may be
prepared by stirring powdered litharge in common salt solution until the
powder appears white. This, when washed, constitutes basic lead chloride.
On adding hydrochloric acid to the washed mass until the liquid remains
acid, lead chloride is obtained in the form of crystalline needles, which are
very little soluble in cold, but more easily in hot, water.
Like any other soluble lead salt, lead chloride may be used in the
precipitation of colours, but is seldom employed on account of its small
solubility. Basic lead chloride was, at one time, used as a white pigment,
and after melting, by which it is turned yellow, as a yellow pigment; it is no
longer in use for these purposes.
Copper Compounds.—These are generally green or blue, and have an
extended use in the production of colours. The metallic copper which is
used in the preparation of colours is of the ordinary commercial quality. The
impurities which it contains are generally so small in quantity that they are
without importance for our purpose.
Copper Sulphate (Bluestone, Blue Vitriol), CuSO₄.5H₂O.—This is the
commonest of the commercial copper salts, and on that account deserves
our especial attention. It forms large sky-blue crystals, which effloresce
slightly in the air, possess an unpleasant metallic taste, and are poisonous,
like all soluble copper compounds.
Copper sulphate comes into commerce in a very pure form, but some
qualities contain zinc sulphate or ferrous sulphate. The presence of zinc
may be detected most easily by boiling the solution with excess of caustic
soda, when copper oxide separates as a black powder, whilst zinc oxide
remains dissolved. When sulphuretted hydrogen is passed through the
liquid, a white precipitate of zinc sulphide is formed.
Iron is detected by passing sulphuretted hydrogen through the solution
so long as a precipitate is formed, allowing the liquid to stand in a covered
vessel, pouring it off from the precipitate, adding nitric acid, boiling and
adding a solution of potassium ferrocyanide; a blue precipitate denotes the
presence of iron.

Copper sulphate is rarely found which is quite free from iron and zinc.
If these impurities are present in but small quantity, the zinc not exceeding
1 per cent. and the iron at most 0·5 per cent., the copper sulphate may be
regarded as sufficiently pure for our purposes.
It may be here remarked that copper sulphate obtained from mints is
generally of great purity, and hence particularly adapted for colour making.
When copper sulphate and other copper salts are dissolved in water,
pale blue flocks of copper carbonate generally separate. This is due to the
carbonate of lime contained in the water. An addition of a few drops of
sulphuric, nitric or hydrochloric acid suffices to prevent this separation.
Copper Nitrate, Cu(NO₃)₂.6H₂O.—This salt may occasionally be
obtained in colour works as a by-product. When nitric acid is poured over
copper, there follows a copious evolution of nitric oxide, which produces
brown fumes of nitrogen peroxide in air. Nitric oxide may be used to
convert ferrous into ferric salts, a transformation required in making
Prussian blue. In working in this way, nitric acid is poured over copper
contained in a vessel provided with a delivery tube for the gas. The blue
solution is at once used. Pure copper nitrate forms fine blue crystals, which
very readily deliquesce in the air. The solution is therefore generally used as
it is prepared.
Copper Acetate, Cu(C₂H₃O₂)₂.H₂O.—Copper is readily attacked by
acetic acid. A number of salts are formed, of which some are used as
pigments. For our purpose it will be sufficient to describe the manufacture
of verdigris; few colour makers prepare any other copper acetate. A solution
of this salt is most simply prepared in the following manner: Slaked lime is
stirred with strong vinegar, and the solution left in contact with the excess
of lime so long as it has a weak acid reaction. The solution, which contains
acetate of lime, is poured into a solution of copper sulphate so long as a
precipitate of sulphate of lime is formed. When this has been separated
from the liquid, the latter is ready for further treatment. It contains only a
very small quantity of dissolved sulphate of lime, which is not harmful in
the preparation of colours.
In addition to the copper compounds mentioned here, several others
were formerly used as pigments, or in the preparation of pigments which

are no longer employed, because copper compounds of good colour can be
obtained in a cheaper manner.
The same precautions should be taken in the use of copper colours
which were mentioned for lead pigments; copper compounds are equally
sensitive towards sulphuretted hydrogen, by which they are gradually
discoloured.
Mercury Compounds.—Mercury forms compounds which, vermilion
in particular, are used as pigments, and others which are used in the
preparation of pigments. In many cases metallic mercury is the starting
point in the preparation of the mercury compounds. The compounds
commonly known as calomel and corrosive sublimate are also used.
Mercurous Nitrate, HgNO₃.—Nitric acid acts upon mercury in a
manner differing according to its strength, and according to whether the
mercury or the nitric acid is used in excess. In order to prepare mercurous
nitrate, acid free from chlorine must be diluted at least with four times its
volume of water, and the mercury must be in excess. On warming, the
mercury is gradually dissolved, and, on cooling, the solution deposits
colourless crystalline needles of the salt. A further crop of crystals is
obtained after evaporating the solution.
When the action of nitric acid is over, the solution must be at once
separated from the excess of mercury to prevent the formation of basic
salts. If the salt has been properly made, it is completely soluble in water,
but if a lemon yellow precipitate is formed on dissolving, the nitrate
contains a basic salt, which can only be dissolved by warming and adding
more nitric acid.
Mercuric Nitrate, Hg(NO₃)₂, is most simply obtained by warming
mercury with very strong nitric acid. The heating must be continued until a
test portion of the solution no longer gives a precipitate with hydrochloric
acid. When this solution is evaporated, nitric acid is given off, and a salt
crystallising in white needles is obtained, which dissolves in water with the
separation of a yellow basic salt. It is therefore better to use the hot
solution, which contains a little free acid, without evaporating.
Instead of mercurous and mercuric nitrates, the corresponding sulphates
may be used, but the chlorides are more frequently employed since they can

be readily obtained from the makers.
Mercurous Chloride (Calomel), HgCl, is obtained pure by adding
common salt solution to a solution of mercurous nitrate and washing the
precipitate, which is insoluble in water.
Mercuric Chloride (Corrosive Sublimate), HgCl₂ a common article of
commerce, is prepared by heating a carefully made mixture of mercuric
sulphate and common salt, when mercuric chloride sublimes. It is a white
crystalline mass, soluble in 13·5 parts of water at 20° C., and soluble in 3
parts of alcohol. Although all mercury compounds are very poisonous,
corrosive sublimate requires particular care in handling, since its easy
solubility makes it surpass all other mercury compounds in poisonousness.
The mercuric sulphate required in the above preparation is obtained by
heating mercury with sulphuric acid. Corrosive sublimate can also be
prepared by adding hydrochloric acid to mercurous nitrate, and heating,
with gradual addition of hydrochloric acid until a clear solution is formed,
from which mercuric chloride crystallises on cooling.
Silver Compounds.—Silver nitrate, AgNO₃, is the only one of
importance here. It is obtained by dissolving silver in nitric acid, when a
blue solution is obtained because commercial silver contains copper,
evaporating the solution to dryness, melting the residue, and keeping it
molten until all the copper nitrate is decomposed. This point is recognised
when a small portion of the melt dissolved in water does not give a blue
colouration with excess of ammonia. Fused silver nitrate forms a white
crystalline mass readily soluble in water and turning black when exposed to
light, like many other silver compounds.
Gold Compounds.—Gold is now very little used in preparing colours.
The compound used for this purpose is gold chloride, AuCl₃, which is
obtained by heating gold with hydrochloric acid, and adding nitric acid in
small quantities until all the metal is dissolved. By careful evaporation of
the yellow solution gold chloride is obtained in brownish yellow crystals,
which easily dissolve in water.
The compounds of molybdenum, vanadium and uranium are less used
than those of gold, yet these metals find a special use in the preparation of
colours for porcelain painting and for colouring glass, for which they are of

great importance. The preparation of their compounds from the raw
materials is complicated and not remunerative to the colour maker; they
should be obtained from chemical works.

In the foregoing, the most important compounds of inorganic origin
used in making colours have been briefly described, less in order to teach
the methods for their preparation than to give the manufacturer the means
of learning their properties. Since the great development of chemical
industries during the last decades it is more advantageous for the colour
maker in most cases to draw his supply of these substances from works of
good reputation than to make them himself; only in the case of a few
substances, which are sold at unreasonably high prices, will it be profitable
for him to prepare them himself.

CHAPTER VI.
THE MANUFACTURE OF MINERAL PIGMENTS.

By mineral pigments we understand those which consist of compounds
of metals with elements such as sulphur, chlorine and iodine, or with
compound radicals, such as cyanogen, or of metallic salts.
Looking at the classification of mineral pigments from the chemical
standpoint, a grouping according to the constituents would appear
preferable, and we should have groups of colours consisting of metallic
oxides, sulphides, salts, etc. In such a division of pigments, according to
their constituents, no regard would be taken of the colour of the pigment.
The white zinc oxide would be placed in the same group with yellow lead
oxide and red lead oxide; and red mercury sulphide and yellow cadmium
sulphide would fall into the same group of the sulphur compounds.
The chemical composition of a pigment is of less importance to the
colour manufacturer than its shade, and it therefore appears to us more
reasonable to prefer to classify mineral pigments according to similarity of
colour. We shall therefore place all those mineral pigments together which
possess the same colour.
Common usage differs from the scientific in the description of colours.
In the physical sense, yellow, red and blue are the so-called “simple
colours,” between which lie orange, green and violet as “mixed colours”.
Physics knows no white colour and no black colour, but describes white as
a mixture of all the simple colours and black as the absence of colour. A
grey or brown shade, produced by different mixtures of simple colours, is
just as little known in the scale of colours as white or black.
The colour maker follows, as we have said above, the common manner
of speaking; to him white and black are equally as much colours as red and
green. Besides the pure principal colours (yellow, red and blue), and the
mixed colours obtained from them (orange, green and violet), colour
makers distinguish many shades of each colour—lemon yellow, sulphur
yellow, cherry red, blood red, violet blue, etc. For the present purpose it is

of great importance to accurately distinguish the several shades, for the
value of many colours is in proportion to their beauty of shade. The colour
maker is often required to produce a colour of some particular shade, which
he accomplishes in many cases by a suitable alteration in the process by
which the colour is made, in other cases by mixing different colours, in
which event chemistry is of no help to him; he must depend on the
sensitiveness of his eyes to colour.
White Mineral Pigments.—We are acquainted with a great number of
white or, more properly, colourless mineral compounds; they possess the
property of reflecting, undecomposed, all the rays of light which fall upon
them, in consequence of which they produce that impression upon the eye
which we call white. According as a white substance reflects every ray of
light or absorbs a portion, we see it as a brilliant pure white or, in the latter
case, as a white with a grey tinge. If a white body reflects the majority of
the rays of light falling on it, but decomposes a small number, we perceive a
white which has a yellow, blue or red tinge.
The most valuable white for the colour maker is evidently that which
reflects, unaltered, all the rays of light; it is the most brilliant, and free from
every tinge of colour. The physical condition of the substance is most
important. Solid substances are either crystalline, that is, possess definite
shapes formed according to a regular law, or they are amorphous, that is, are
composed of irregularly formed particles. Snow and white lead may serve
as representatives of these two classes. Snow is composed of small
colourless crystals of ice, the flat surfaces of which reflect, undecomposed,
the light falling on them. The smaller are the crystals, the more pure appears
to us the whiteness of the snow, and the thinner is the layer of snow
required to produce the sensation of whiteness. But if the snow crystals are
larger, the white appears to have a bluish tinge, and only a thick layer of
snow is opaque. White lead, being an amorphous substance in a condition
of very fine division, reflects the light very regularly, so that a thin layer of
white lead appears quite opaque.
Among the artificial pigments, crystalline or amorphous, exactly the
same conditions hold good as between snow and white lead. Of amorphous
pigments a very thin layer is in most cases sufficient to make the surface
upon which they are spread invisible, or, as the technical expression runs,
“to cover,” whilst crystalline substances possess a smaller covering power.

A striking example of this is seen by a comparison of two white pigments,
white lead and “patent white” (lead oxychloride). The former is amorphous,
the latter crystalline. Both are completely colourless and reflect white light,
but in consequence of its amorphous condition and finer particles, white
lead possesses far greater covering power than “patent white”.
Among all other colours the same rule holds. Amorphous pigments have
always a greater covering power than crystalline. The smaller the crystals of
the latter the greater is their covering power, so that in preparing pigments
of a crystalline character care must be taken to make the crystals as small as
possible.
From the above definition of white pigments it follows that an immense
number must exist, since every colourless substance in a state of fine
division appears white. Generally only those bodies are used which are
insoluble in water, or almost insoluble, and which possess great covering
power. The following may be mentioned as white pigments, only a few of
which are in use: white lead, white zinc, permanent white, lead oxychloride,
lead sulphate and sulphite, zinc oxychloride, lead antimoniate, antimony
white, tin white, tungsten white, and in addition certain earths, pipe clay,
china clay, etc. Several of these pigments are far too expensive for ordinary
use, and have no advantage over much cheaper pigments except for very
special purposes, such, for example, as bismuth white for cosmetics.
In general use we find very few artificial white pigments; these are lead,
zinc and barium compounds. Circumstances may arise which make it
expedient for the colour maker to manufacture other white pigments, for
example, a demand for them, or favourable opportunities for obtaining the
requisite raw materials.
The white lead pigments, of which there is a large number, as we have
indicated, all have the great disadvantage that they are not permanent, that
is, are changed by atmospheric influences. It is well known that lead is a
very delicate reagent for sulphuretted hydrogen, with the sulphur of which
it forms a black compound. Now the air, especially in towns, contains
sulphur in the form of sulphuretted hydrogen or ammonium sulphide;
though the quantity is very small, the fate of every white or coloured lead
pigment is decided by it; after a longer or shorter time it will be
discoloured, will gradually darken, and finally be turned black. In spite of

this great changeableness of lead pigments they are used by artists and
painters, although the majority could be replaced by more permanent
pigments entirely unaltered by the atmosphere.

White Lead.

This pigment, which in addition to other good properties has remarkable
covering power, was amongst the earliest known artificial pigments.
Already in the fourth century before Christ, Dioscorides described the
preparation of white lead, which was obtained by exposing lead to the
action of the vapours of vinegar, removing the white layer and treating it
with water. The Roman writers describe a similar method; they use the
name cerussa, under which white lead is known to-day in commerce.
Although white lead has been so long known, it was left to Bergmann,
in 1774, to show that it contained carbonic acid; before that it was believed
to be lead acetate. The development of analytical chemistry was followed
by a knowledge of its constitution and the use of more rational methods of
manufacture. Whilst in the middle ages the manufacture of white lead was
almost exclusively in the hands of the Dutch and Venetians, in later years it
gradually spread, and now many works are concerned in the manufacture of
this pigment. That adulterations of white lead were not rare in former times
appears from the writings of Basil Valentine, an alchemist of the fifteenth
century.
Commercially white lead is known under most varied titles, of which
the following are the principal: White lead, Venetian white, Dutch white,
Krems white, Kremnitz white, flake white, etc.
According to its chemical composition, white lead is a compound of
lead carbonate and lead hydroxide, that is, a basic lead carbonate.
Commercial white lead, apart from intentional admixtures of other white
substances, contains lead carbonate and lead hydroxide in varying
proportions, as is shown by the following analyses by Mulder, who found
that all the samples examined by him were composed according to one or
other of the following formulæ:—

2 PbCO₃.Pb(OH)₂, containing 86·27 per cent. of PbO.

5 PbCO₃.2Pb(OH)₂ ” 85·86 ” ”
3 PbCO₃.Pb(OH)₂ ” 85·45 ” ”
4 PbCO₃.Pb(OH)₂ ” 85·00 ” ”
According to Hochstetter, the manufacture of white lead must be
directed to obtaining the compound 2 PbCO₃.(OH)₂, which possesses the
following percentage composition:—
PbO 86·32
CO₂ 11·36
H₂O 2·32
The compound of this composition is distinguished by being completely
amorphous, and so possesses the greatest covering power. Commercial
white leads, as is seen from the formulæ of Mulder, may differ appreciably
from this composition, when they will have a smaller covering power, since
they will contain some quantity of neutral lead carbonate, PbCO₃
(containing 83·46 per cent. of PbO), which is crystalline.
White lead is prepared according to very different methods, the
principle of which consists in subjecting a solution of tribasic lead acetate
to the action of carbonic acid, by which the basic carbonate is produced and
the neutral acetate formed, the latter being then reconverted into the basic
acetate, which again serves to produce white lead and so on.

CHAPTER VII.
THE MANUFACTURE OF WHITE LEAD.

The processes by which white lead is or was manufactured may be
divided, according to the principal operations, in the following manner:—
1. Processes in which metallic lead is subjected to the action of acetic
acid vapour, whilst the vessel in which this operation is conducted is
exposed to a higher temperature. In the oldest so-called Dutch method this
increase of temperature is effected by the decomposition of manure, by
which the vessels containing the lead and acetic acid are surrounded. In
consequence of the heat produced by this fermentation, acetic acid and
water are volatilised, and oxygen also being present, lead acetate is formed.
Also, as a consequence of the oxidation of the lead, heat is produced,
which accelerates the process, and lead oxide is formed in large quantity,
which unites with the neutral acetate already formed to produce a basic
compound. The vessels in which this process is taking place are in an
atmosphere containing much carbonic acid produced by the fermentation of
the surrounding organic matter; this carbonic acid converts the basic lead
acetate into white lead.
The German or Austrian method is to be regarded as an improvement on
this rough process. The heat necessary for the normal course of the
chemical reactions is produced from fuel; the carbonic acid formed by the
combustion of the fuel is used to convert the basic lead acetate into white
lead.
2. In the above methods the manufacture of white lead commences with
the production of white lead from metallic lead and acetic acid. In the so-
called French method a solution of basic lead acetate is decomposed by
carbonic acid into white lead and neutral lead acetate, which latter is again
converted into basic acetate.
3. The English method. The principle of this process consists in
moistening litharge with a solution of lead acetate and exposing it to the
action of carbonic acid, whereby white lead is formed.

Methods for manufacturing white lead, which are often advanced as
entirely new processes, may be always traced to one of the above, from
which they deviate but little in principle, and the deviations cannot always
be regarded as improvements. In the following detailed account of the
manufacture of white lead we shall adhere to the classification just given,
according to which there are three principal methods: (1) Manufacture of
white lead from metallic lead, acetic acid and carbonic acid (Dutch and
German processes); (2) manufacture of white lead from basic lead acetate
(French process); and (3) from litharge moistened with lead acetate solution
(English process).
We should remark that every year “new” processes for the manufacture
of white lead are patented. The majority of these will not be mentioned; to
those acquainted with the principles of chemistry they at once appear
impracticable.

Manufacture of White Lead from Metallic Lead.

(a) Dutch Process.

This essentially primitive process, when properly conducted, produces
white lead of good colour and covering power, which are the properties for
which this pigment is valued. It is now seldom used,[2] because other
methods give a product of equal colour in a shorter time. It is, however, of
economic interest, as showing how a branch of industry may rise from
crude beginnings to a high state of perfection. The operations comprised in
the Dutch process are as follows:—
[2] In England the Dutch process is in general use.—Translator.

(1) Casting the lead into sheets; (2) placing these sheets in pots and
arranging the pots in the stacks (placing the pots containing lead and acetic
acid in the bed of manure); (3) removing the pots from the stack; (4)
separating the white lead formed in the pots; (5) further purification of the
impure white lead by grinding, washing and drying.
1. Casting the Lead into Sheets.—At first sight it would appear
unsuitable to cast lead in sheets, since this metal can be readily rolled into
sheets of any thickness. Experience has, however, shown that rolled lead is

only slowly attacked by acetic acid vapours, whilst cast sheets are rapidly
attacked.
An iron pan about one metre in diameter, with an iron cover furnished
with a pipe opening into a flue, is used for melting the lead. This
arrangement is designed to protect the workman from the dangerous
vapours evolved from the molten metal. At the workman’s side of this cover
is a counterpoised slide, which only remains open when the counterpoise is
held in check by a lever. In front of the kettle there is an iron plate movable
about a horizontal axis. The lead being heated to just above its melting
point, the workman takes 7 to 8 kilogrammes of metal in a ladle, and pours
it on the plate, which is horizontal. The lead solidifies in a very short time,
but before it is completely solid the plate is inclined towards the pan so that
the still liquid lead runs back into it, leaving a very thin sheet on the plate.
The hard sheet is removed from the plate, and the latter cooled by cold
water to be ready for a new casting. The sheets made in this way are not
more than 1 to 2 millimetres thick. They are then cut into strips of a width
to suit the size of the pots in which they are converted into white lead; the
width of the sheets is generally 5 to 6 centimetres. Since the rate at which
the white lead is formed depends on the surface of the metallic lead, instead
of continuous sheets the lead is generally cast into gratings. For this purpose
an iron plate, upon which are intersecting strips, is used instead of the flat
plate. Plates for the casting are also used containing grooves intersecting at
right angles. In the first case, plates are obtained in which are openings
meeting at right angles, and in the second case, according to the distance of
the grooves apart, a more or less wide-meshed lattice work.

Fig. 4.

2. Building up the Stacks.—The rolled-up lead plates are placed in the
pots. These (Fig. 4) are somewhat conical in shape; they have at some
distance from the bottom a projecting ring, or sometimes three projections
only, upon which the lead spiral rests. Before the spirals are put in position,
a quantity of ordinary vinegar, about a quarter of a litre, is poured in. There
must be sufficient room below the spiral so that it shall not be in contact
with the vinegar. The insides of the pots are glazed at least half-way up, so
that the liquid does not penetrate the porous earthenware.
The pots have a capacity of about 1 litre, and a diameter at the top of 10
centimetres. If lead plates are used, the pots are about 20 centimetres high;
if gratings are used the pots may be lower, by which there is economy in
room and a larger number can be placed in one stack.
The stacks, built up of pots and manure, are of different sizes; it is not
advisable to make them too small, or the loss of heat would be considerable.
A stack 4 to 5 metres long, 3½ metres wide and 6 to 7 metres high will
contain 6,000 to 8,000 pots and 9,000 to 11,000 kilogrammes of lead.
The stack consists of a rectangular pit walled on three sides; the fourth
side is open, with the earth dug out in the form of an inclined plane, in order
to permit the introduction of the pots and the manure. The construction of

the stack is commenced by placing the pots at the bottom in rows, avoiding
interspaces as much as possible. In a stack of the size mentioned a layer
contains 1,000 to 1,200 pots. Between the pots containing lead and acetic
acid are arranged a number of larger ones containing acid only; the object
of these is to furnish acetic acid vapour. When the pots are in place, 3 or 4
lead plates are placed on each spiral, the top plate forming the cover;
immediately over the pots strong wooden planks are laid, and upon them a
layer of boards, which must fit so tightly that nothing can fall through. On
the boards is spread out a layer of fresh stable manure, with which the space
between the outside row of pots and the wall is also filled. The layer of
manure is 30 to 40 centimetres thick.
Upon the lowest layer of pots, a second, third and so on are built up
exactly in the same manner, so that the whole stack is filled with alternating
layers of pots and manure. In order to prevent the cooling of the uppermost
layer of pots, it is covered with a thicker layer of manure 60 to 70
centimetres thick. When lead plates and the taller pots are used, a stack will
generally contain 15 layers; but when gratings and the smaller pots are
employed, 18 layers can be packed into the same space. In arranging the
layers of pots, care should be taken to leave spaces at tolerably equal
distances, so that the air necessary for the oxidation of the lead may enter.
To prevent the cooling of the stack at the front, where it is not protected by
masonry, when full it is walled up with boards; a board roof also protects
the erection from rain.
In place of manure, spent tanners’ bark can be used, which in the same
manner ferments, producing heat and carbonic acid. In places in the
neighbourhood of large tanneries this spent bark is generally obtainable at
lower prices than stable manure, which is more valuable for agricultural
purposes; the former has also a very considerable advantage, white lead
made by means of spent tanners’ bark being generally of a purer white than
that made with manure. The reason for this is that in the decomposition of
animal excrement small quantities of sulphuretted hydrogen are produced, a
gas which produces black lead sulphide when it comes into contact with
lead compounds.
According to the results of practical experience, pigs’ dung cannot be
used in the manufacture of white lead; so much sulphuretted hydrogen is
evolved from it that the white lead is not white, but has a greyish tinge.

When bark is used in place of manure, a discolouration of white lead by
sulphuretted hydrogen is not to be feared, but there is the drawback that a
longer time is necessary for the corrosion of the lead, because the bark
decomposes more slowly than the manure, and accordingly gives out less
heat and carbonic acid.
According to the climate of the country, the stacks may be differently
erected. In colder countries it is necessary to sink them in the earth and
surround them with masonry, as directed above; but in warmer climates
such effectual protection against cooling is unnecessary, yet in all cases it is
better to sink the stack in the earth on account of the regularity of
temperature so obtained.
Instead of sinking the stacks in the earth, they may be built in the open
when there is a plentiful supply of manure or bark; but they must then be
surrounded by a very thick layer of manure to prevent cooling. It is a
desirable alteration in the construction of the stacks to provide the pots with
lids, and so avoid the use of the layer of boards separating each two layers
of pots. The object of the cover is simply to prevent dirt from falling into
the pot. It should not fit tightly on the edge, or the entry of carbonic acid
into the interior of the pot would be made difficult. The lids are therefore
rounded and fit loosely on the pot. When pots with lids are used, the lowest
layer is covered with manure in the ordinary way; upon this again comes a
layer of pots, and so on.
The transformation of the lead may be regarded as complete in four to
six weeks when manure is used, but with bark the time extends to ten
weeks. The quantity of white lead obtained varies in different cases; for
example, from a stack 5 metres long, 4 metres wide, and 6 metres high, into
which 12,000 kilogrammes of lead were put, 10,000 kilogrammes of white
lead were obtained, and 4,000 kilogrammes of lead remained unaltered. In
another case, for a stack of 8 layers 280 kilogrammes of vinegar and 9,600
to 12,000 kilogrammes of lead were used, and there was a residue of 10 to
15 per cent. of lead.
3. Removal and Grinding of the White Lead.—The stack is pulled
down after the lapse of the necessary time; the lead plates and rolls are
collected in wooden boxes and brought into the room where the white lead
is separated from the metallic lead. Formerly the white lead was removed

from the sheets exclusively by manual labour, an operation extremely
dangerous to the workman. It is quite impossible to prevent the formation of
white lead dust, so that the men were continuously in an atmosphere
charged with the poisonous material, and, as a consequence, suffered from
the various forms of lead poisoning.
In order to diminish as much as possible the injurious effects of white
lead on the health of the workmen, manual labour has been, as far as
possible, replaced by machinery, yet the greatest care is necessary in the
different manipulations of so poisonous a substance as white lead.
When the white lead is removed by manual labour, an operation which
ought to be forbidden, the lead sheets are unrolled and struck together,
whereupon the greater part of the white lead falls off. To remove the
remainder of the white lead the plates are laid one upon the other and struck
with a hammer until the white lead is loosened; or the plates are cleaned
with metal brushes.
The masses of white lead obtained in this way are contaminated by
larger or smaller quantities of metallic lead, from which they must be freed
by a further mechanical operation, namely grinding. The larger pieces of the
white lead, which have a thickness of several millimetres, were picked out
and sold separately under the name of flake white. This was formerly a
highly prized quality of white lead, its appearance being a guarantee of its
purity. The flake white generally found in the market nowadays is not
obtained in this manner, but by mixing white lead with a solution of dextrin,
forming plates from the paste and drying them slowly in the air.
In order to separate the white lead mechanically from the remains of the
lead sheets, grooved rollers are used, between which the sheets are passed.
In order to guard against dust, the rollers are surrounded by a closed casing,
in which there is also a sieve which serves to separate the larger pieces of
white lead (flake white) from the fine dust. The arrangement is represented
in Fig. 5. The unrolled plates pass through the opening, B, on to an endless
leather band, by means of which they are carried between the grooved
rollers, D and E; after they have passed through these they go between a
second pair of rollers, F and G, which are placed nearer together; they then
fall into the drum-shaped sieve, H, out of which they leave the apparatus in

the direction of K. The white lead falls through the sieve, is caught in trucks
placed at J, and carried away to the mills.

Fig. 5.

A mechanical arrangement for the separation of white lead from the
unaltered metallic lead, due to Horn, is represented in Figs. 6 and 7. It
consists of a drum, in which is the spindle, b, provided with the arms, c. The
teeth, e, on the arms reach nearly to the lower curve of the drum, but pass at
a somewhat greater distance from the upper portion so that the pieces of
lead can fall down again after they are carried up. The material is fed in
through the hopper, t, the rotation of the arms loosens the white lead from
the pieces of metallic lead, and it is carried on by the water which passes
through the sieve, 3, and the stop cock, h, into the settling tank. The lead
pieces which are carried forward have their progress checked by the bridge,
o, they are collected by the perforated scoop, n, are raised above the hopper,
m, and fall out of the apparatus at r. If it is required to continue the
treatment of the material in the drum for a longer period it is only necessary
to close the hopper, m, by the slide, n.

White Lead Mills.—Before the white lead is subjected to the real
process of grinding, it is generally first ground dry, or, more properly,
pressed or crushed. This crushing is accomplished by means of vertical or
edge-runner mills, which consist of mill-stones running round upon a stone
bed about a vertical axis. The mill is surrounded by a wooden casing to
prevent the escape of dust.
Wet grinding, which is done between mill-stones, may be carried out in
two ways, with the production of hard or soft white lead. The former is
obtained when the lead acetate is not removed, the latter when all the lead
acetate is washed out.

Fig. 8.

Hard White Lead presents a shining mass, broken with difficulty. This
appearance is a guarantee against adulteration with barytes; white lead,
which contains this adulterant, does not give a smooth, but an uneven
earthy fracture. Hard white lead is rather difficult to grind, and requires very
careful treatment to be brought into that state of fine division in which it is
usable.
The dry white lead, after powdering under the edge-runners, must pass
through a sieve, which retains the particles of metallic lead, before it is
subjected to wet grinding. The mills used for wet grinding differ little, or
not at all, from the ordinary pattern.

Fig. 9.

A mill designed by Richter of Königsee, in Thuringia, for wet grinding,
is constructed as follows (Figs. 8 and 9). The shaft, F E, is driven by a
water-wheel and the necessary gearing at 60 revolutions per minute; the
bevelled cog-wheel G, by means of the cog-wheels H and J, drives the
shafts K and L, which, by means of the bevelled cog-wheels a and b,
communicate to the runner stones of the mills, M, N and O, a speed of 140
revolutions per minute. C D is the framework for the support of the storied
arrangement of mills, f and g are bearings with cast-iron cups to receive the
escaping oil, i is a clamp fastening the axle d to k. A steel cup in k rests
upon the hardened top of the spindle, l; the bed stone rests upon the beam n
(80 millimetres thick) and is prevented from lateral movement by the
surrounding oo; p is a wooden support in the opening in the bed stone,
carrying the stuffing box, q, which prevents the white lead from running
out, and connects the spindle, l, with the bed stone. The runner stone, r, is
surrounded by the box, s, cemented with white clay inside and out and
fastened to the bed stone. The lever, t, turning about w, moved by the screw,
u, and handle, v, regulates the position of the runner. The diameter of the
stones is 95 centimetres.
Thirty kilogrammes of white lead, mixed with water, are fed into the top
mill of the series shown on the right; the mixture flows by the spouts, e, into
the lower mills, and is received in vessels, from which it is emptied into the

holder, P. From P it goes through the 3 mills on the left. After passing
through 6 mills, white lead without barytes is ready for use. If the white
lead contains an admixture of barytes, it is put through the 3 mills on the
left a second time. In 24 hours 900 to 1,200 kilogrammes (18 to 24 cwt.) of
white lead are ground.
In dry grinding, mills are used arranged so that the formation of dust is
avoided, a matter which is of particular importance. The usual size of these
mills has a diameter of 90 to 95 centimetres for both runner and bed stone.
The grinding surfaces of both stones have radial grooves.

Fig. 10.

Fig. 10 represents the construction of Lefèbre’s white lead mill, which is
designed to give the greatest protection possible to the workmen. At A, the
white lead masses are enclosed in a hopper lined with bronze, with internal
angular projections; by means of M, which is grooved in a similar manner,
the larger lumps are broken up and enter the mill itself by means of the
hopper under A. The mill consists of bed stone, H, and runner, K. The
grinding surfaces are grooved to facilitate the delivery of the ground white
lead. As the illustration shows, the stones are completely covered by M, so

that the escape of dust is almost completely prevented. The ground white
lead is conducted by the tubes, O O, to the drums, O′ O′, from which it is
packed.
In order to obtain hard white lead, the material which has been ground
dry under the edge-runners is mixed with water to a soft paste, or if water
has been added whilst the lead was under the edge stones, more is now
added to make the paste thin enough to be ready for the mills, where it is
ground, being fed in regularly by means of a copper spoon. The paste
issuing from the mill is collected in earthenware pans or plaster of Paris
moulds, in which it is dried.
These pans have usually the shape of a truncated cone, which was the
form in which white lead was formerly brought into trade from Holland. In
drying, water is lost, and the mass of white lead shrinks, so that the lump
can be removed from the mould after a few days on turning it upside down.
The drying is accomplished either in the air or in artificially heated stoves.
The heating must at first be gradual, or the mass of white lead would shrink
so rapidly that the cone would be full of cracks, and then easily fall to
pieces. When once the drying has reached a certain stage, the temperature
of the stove may be raised to 50° C. (122° F.) without danger of breaking
the lumps. When quite dry, the surface of the white lead, which is now
rough, must be smoothed by scraping, when it is ready for the market.
Soft White Lead.—The hard white lead prepared as just described
consists of very heavy and very hard lumps of the purest white. When soft
white lead is required, the admixed lead acetate must be removed by
washing. This is accomplished by adding a larger quantity of water, either
when grinding under the edge-runners or in the mills, so that a thin pulp is
formed; this is run into a receiver, in which is a stirrer. The white lead is not
completely prevented from sinking by the stirrer; a soft mud is deposited at
the bottom of the vessel, which can only be stirred up with difficulty on
account of its high density. When this vessel is full, the stirrer is stopped
and the milky liquid allowed to settle, which happens in a short time on
account of the high specific gravity of white lead. The clear liquid above the
white lead is drawn off into a tank lined with cement. It is advisable to
arrange the stirrer so that it may be placed at any height in the vessel; if this
is the case, by gradually lowering the stirrer whilst in motion, the white lead

lying at the bottom can be mixed up with fresh water. These operations are
repeated until all the lead acetate is removed.
Soda solution is added to the wash waters in order to recover the lead
dissolved in them as lead acetate; lead carbonate is precipitated and settles
at the bottom of the tank. This precipitation may also be effected by putting
lumps of limestone in the tank. The paste remaining in the washing tubs,
which now contains only pure white lead and water, is filled into bags of
closely woven material and the water pressed out by a gradually increasing
pressure, until a stiff pulp remains behind. This is then completely dried,
either in the air or in drying stoves.
Soft white lead forms either irregular lumps or a soft, heavy powder.
The lower qualities of white lead contain a smaller or larger quantity of
finely ground barytes; the higher the proportion of barytes the smaller is the
covering power of the mixture. A very simple method for detecting barytes
in white lead will be given later.

(b) German Process.

The German or Austrian method of making white lead is also known as
the chamber process, since the formation takes place in closed chambers,
constructed of wood or masonry. In the older processes, cast lead sheets
were bent double and hung on cross bars in a wooden box with a water-tight
bottom, with the precaution that the plates did not touch. A number of these
boxes, generally 90, were arranged in a hot room, each box being about 1·6
metre long, 0·4 metre wide and 0·3 metre high. On the bottom of each box
was poured a mixture of vinegar or dilute acetic acid and wine refuse, and
the box was covered with a well-fitting lid. The temperature of the room
was gradually raised week by week; during the first week remaining at 25°
C., during the second at 38° C., the third at 45° C. At the commencement of
the fourth week the temperature was raised to 50° C., at which it was kept
for a fortnight. At this high temperature a considerable quantity of acetic
acid is evaporated, causing the formation of lead acetate, which is
converted, by the carbonic acid evolved from the wine refuse, into basic
lead carbonate. When the proper temperatures have been maintained, on
opening the boxes almost all the lead is found changed into white lead,
which is knocked off, and the residual lead used in casting new plates. It is

easy to conceive that the boxes may be well replaced by brick chambers, in
which a large number of lead plates are brought and into which acetic acid
vapours and carbonic acid are introduced after the room has been closed.
Chambers are used which can contain 12,000 to 12,500 kilogrammes of
lead.
These chambers, in which the lead plates are hung upon wooden
supports, have an opening immediately above the bottom, which is
connected with a retort in which vinegar, containing 4 to 5 per cent. of
acetic acid, is boiled. After about 12 hours, by the simultaneous action of
the vapours of acetic acid and the oxygen of the air, lead acetate is formed.
Carbonic acid is now led into the chambers. The carbonic acid is obtained
by burning charcoal in a cylindrical furnace; it is cooled by being passed
through a long iron tube before it enters the chamber. For 12,500
kilogrammes of lead there are required every day about 482 litres of dilute
acetic acid, obtained by mixing strong acetic acid with water until the
mixture contains 4·5 per cent. of the acid, and 18 kilogrammes of charcoal,
from which the carbonic acid is obtained. The time required is 5 or 6 weeks,
and the residue of unaltered lead varies from 10 to 35 per cent.
The white lead obtained by this process is quite usable, but the process
has the considerable disadvantage that there is no control over the quantities
of materials used. In order to produce a certain quantity of white lead of a
certain composition, definite quantities of lead, oxygen, acetic acid and
carbonic acid are necessary. If, then, the apparatus can be so arranged that
the quantities of material employed can be accurately measured, a great
advance will have been made, for the operation will be no longer conducted
at random, but under definite unchangeable conditions. The quantities of
carbonic acid and acetic acid can be calculated beforehand. The volumes of
the gases required may be measured without difficulty by meters of the type
used for measuring coal gas. Upon this principle are based a number of
more modern methods.

Fig. 11.

According to the process of Major, the vapours of water and acetic acid
are introduced at the same time into the chambers filled with lead plates, in
order to produce basic lead acetate. This part of the process requires about
12 hours. Then carbonic acid is introduced; it is produced by burning
charcoal in an iron cylinder, through which air is forced. By means of this
carbonic acid, which is at a comparatively high temperature, about 60° C.,
the lead acetate is quickly changed into white lead. A portion of the lead
acetate remains undecomposed; in order to remove this, at the close of the
operation ammonia is injected, which decomposes the lead salt. The
ammonia salts now present are finally driven out by means of superheated
steam. Major’s apparatus is depicted in Fig. 11. A and B are chambers with
horizontal gratings upon which the lead lies; C is the furnace for burning
the charcoal, provided with a fan. The products of combustion from C pass
under the boiler D, their passage being regulated by the valve, a; they then
heat the boiler E, containing acetic acid, and enter the chambers, A and B,
through the flue, b, or are directed by the valve in b into the chimney, d. The
gases, after passing through the chamber, find an exit at e. The steam pipe,
f, conducts steam into the boiler E, through g, and also into the chambers, A
and B, thus heating them and providing the necessary moisture; h is the
funnel for filling E; the pipe, k, carries the acetic acid vapours into the
chambers. After the thin lead plates have been brought into the chambers
they are closed and the temperature raised to 49° to 60° C., steam and acetic

acid vapours are led in for 10 to 20 hours in order to form the basic acetate,
then the furnace gases are introduced at a temperature of 60° C. The white
lead obtained in this manner can be finished in the ordinary way by washing
and grinding, but it is better to remove the lead acetate by introducing
ammonia, and then hot air or superheated steam, as previously stated.

The process is complete in 2 to 4 weeks. Gartner, working according to
this process with 150 kilogrammes of lead in a chamber 1·26 metre long,
0·78 metre high and 0·78 metre wide, obtained good white lead in 28 days.
The apparatus designed by H. Kirberg for the manufacture of white lead
is illustrated in Figs. 12, 13 and 14. The lead plates are hung upon the laths,
a, in the chambers; the supports of the laths, b, go through the slits, c, in the
supports, d, and project through the walls at e. The carriers, b, hang from
bolts by brass wire. By striking the end of the laths, e, they are made to
swing so that the white lead loosely adhering to the lead plates is shaken off
and fresh surfaces of metal are exposed. The openings in the walls through
which the ends of the laths project are closed by indiarubber. In order to
prevent the production of dust when the chambers are emptied, water is
introduced in a fine spray upon the lead plates from the copper tubes, v,
placed under the roof of the chamber. The water washes off the remains of
the white lead from the plates.

Fig. 14.

In a similar manner, the white lead process may be carried out in a
shorter time when the gases enter the chamber under increased pressure; but
this is attended with difficulties, since continuous supervision of the
apparatus and of the tightness of the chamber is necessary.

Fig. 15.

The apparatus of W. Thompson (Fig. 15) consists of a chamber, A,
constructed with a false roof, B, to carry off the condensed vapours, and
provided at both sides with doors, through which the waggons, carrying the
lead, are introduced and removed. The pipes, n, which convey air and
carbonic acid, are provided with branches, e, which reach to the sides of the
chamber, and are joined to the perforated tubes, D. The pipes, n, are
provided with reservoirs, b, which prevent the too rapid entry of the air or
carbonic acid, and at the same time give a sufficient heating surface. Upon
the waggon, E, are the frames, v, which support the lead plates. The plates
may be solid or gratings; they are 3 to 12 millimetres thick, and are
arranged at intervals of about 25 millimetres in the frames. The troughs, F,
for the reception of the acetic acid are filled from the reservoir, Z. The
steam pipe, C, effects the evaporation of the acid; the steam pipe, d, heats

the chamber at the commencement of the process, and also when C is out of
use. When the chamber is filled with lead, the troughs, F, are filled with
acetic acid of 5 per cent. strength. The chambers are closed, and steam is
led through C and d until the temperature in the chamber reaches 25° to 50°
C., and the evaporation of the acid begins. Then air is forced in under a
small pressure through n, e, D during 3 to 4 days, after which follows a
mixture of equal parts of carbonic acid and air, continued until the
formation of white lead is complete. When plates 3 millimetres thick are
used, and the temperature is gradually raised from 25° to 50° C., about 12
days are required; but if the plates are 12 millimetres thick, 28 days are
necessary. For the proper carrying out of this process it is important that the
temperature of the chamber should be very gradually raised from 25° to 50°
C.
In the process of P. Rey molten lead is poured in a thin stream into
water, and the “granulated” metal then placed in vessels, in a layer 30
centimetres thick, upon a grating 5 centimetres above the bottom. In the
bottom are narrow tubes which, reaching above the lead, serve to admit air.
Acetic acid is allowed to flow over the lead from vessel to vessel. In order
to obtain the proper solution, a layer of lead, 2 metres thick, is necessary, so
that 6 to 7 vessels are arranged, one above the other. If lead acetate solution
be used, the layer of lead need only be 1·2 metre thick. The solution of
basic lead acetate is then treated with carbonic acid according to the
ordinary process.
In addition to the modifications of the German white lead process which
have been described, many others have been proposed. The principle of
many of these methods is, that finely divided lead is much more rapidly
converted into white lead than lead in the form of plates; finely divided lead
exposes an enormously greater surface to the action of the acetic acid and
other materials than do lead plates.
In Rostaing’s method the lead is changed into very small pellets, by
allowing the melted metal to flow on to a rapidly rotating iron disc; the
molten lead, in consequence of the centrifugal force due to the rapid
rotation, assumes the form of very small drops, which are thrown off the
disc and cooled in a vessel of cold water.

Torassa recommends that lead, obtained in pellets by pouring into cold
water, should be brought into a rapidly rotating vessel, whereby the greater
part will be converted into fine dust (?). This lead dust is said to be then
converted into white lead by simple exposure to air, lead oxide being first
formed, and from it basic lead carbonate. This process is not workable on a
large scale. The rapid oxidation of finely divided lead by air has been
applied by several inventors to the manufacture of white lead. The
processes of Woods, M’Cannel and Grüneberg are founded upon this
transformation. In essentials they are as follows: lead is finely divided in
iron or earthenware cylinders, and either carbonic acid and air, or a mixture
of these with acetic acid vapour, are introduced by means of the axle of the
cylinders.
If a special apparatus is not provided, as it should be, on the large scale,
to separate the unaltered lead from the white lead, the lumps coming from
the chambers must be subjected to a process of levigation, in which the
lead, being the heaviest body, will be first deposited; the last portions
deposited consist of the purest white lead, the lower layers of which will be
tinged more or less grey by an admixture of finely divided lead and lead
peroxide.
White lead manufactured by the German process occasionally exhibits
perceptible reddish or greyish tinges, the cause of which lies in the
defective execution of the process. The red tinge denotes the presence of
free lead oxide, caused by the use of an insufficient quantity of acetic acid.
A grey tint is due to the presence of metallic lead or an excess of lead
carbonate.

(c) French Process.

The French process for making white lead is based upon the reaction
which occurs when carbonic acid is passed into a solution of basic lead
acetate; basic lead carbonate is precipitated and neutral lead acetate remains
dissolved; the latter is again converted into basic acetate, from which
carbonic acid again separates white lead and so on. This process, at present
used on an enormous scale, is due to the French chemist Thénard, who first
put it into operation on the large scale at Clichy, near Paris. The method is
also known as the Clichy process.

The operations of this process are divided into the production of the
basic acetate and the treatment of its solution with pure carbonic acid,
whereby the basic carbonate is precipitated.
1. Preparation of the Solution of Basic Lead Acetate.—The
preparation of this compound has been already described; the following is
supplementary to what was previously given. If litharge be used, it is
dissolved in wooden tubs heated by steam. The acetic acid is brought nearly
to boiling by open steam, and the finely ground litharge gradually added. In
consequence of its high specific gravity, the litharge would quickly sink to
the bottom, so that it is advisable to keep the liquid in motion by means of a
stirrer, and to allow the litharge to fall in in a thin stream. The introduction
of the latter is continued until the specific gravity of the solution indicates
that the liquid contains three equivalents of lead oxide to one equivalent of
acetic acid.
In working with metallic lead, this must be used in a finely divided
form; it is cast, as in the Dutch process, into thin sheets or gratings, or into
flat wires or ribbons. These ribbons are easily made by melting the lead in a
pan provided with a delivery pipe with stop cock; beneath the latter is
brought a vessel, which can be moved backwards and forwards upon a tram
line, filled with water. When the melted lead is allowed to flow into this
vessel, which is being moved backwards and forwards, the metal forms
long, thin ribbons, which possess a large surface. A wooden tub is almost
completely filled with these lead ribbons, which are then covered with
acetic acid. After a short time the acid is run off, when, by the action of the
air, so energetic an oxidation of the lead takes place that the contents of the
tub become heated, and steam and acetic acid vapours begin to rise. When
this is seen, the original acetic acid is pumped back into the tub, and left
there for some hours in contact with the lead in order that it may dissolve
the lead oxide. When the solution has reached a specific gravity of 1·1326
to 1·1415, it is drawn off from the undissolved lead, which again, in a short
time, in consequence of the rapid oxidation, becomes warm, and is treated
with fresh acetic acid.
The lead ribbons become finally so thin that they fall together by their
own weight, and form tight masses, upon which air and acetic acid can no
longer act. These residues are then removed from the solution vessel and
new ribbons introduced. The residues have a velvety appearance; when they

are mixed up with water they make it dark, and from the turbid liquid a fine,
velvet-black powder soon separates, which consists of finely divided silver.
The liquid still remains turbid owing to the suspension of fine particles of
carbon. The lead ores, especially galena, often contain notable quantities of
silver; silver lead is generally desilverised before use; usually, however,
small quantities of silver remain in the lead. When the lead is dissolved in
acetic acid, this silver settles as a soft powder at the bottom of the vessels in
which the lead residues from the solution tubs are washed.
2. Preparation of the Carbonic Acid and Precipitation of the White
Lead.—The carbonic acid required for precipitating the basic carbonate is
obtained either by heating limestone in a small furnace, from which the gas
is drawn by a pump, or directly by burning charcoal. In the former case
very pure carbonic acid is obtained, and, as a by-product, valuable
quicklime; in the latter case precautions must be taken to produce pure
carbonic acid.

Fig. 16.

The furnace designed by Kindler for preparing carbonic acid (Fig. 16)
consists of a conical furnace, burning coal or coke upon the hearth, a. The
passage, c, is divided by a vertical wall, in order to avoid obstruction from
the piling up of fuel. The space, K, is filled with limestone, through which

the carbonic acid passes; the tanks, e, filled with water, cool the limestone,
and the gas, which then passes through the water in the washing vessel, D,
is drawn off by a pump.
In the old process at Clichy the apparatus depicted in Fig. 17 was used.
The basic lead acetate was made in the wooden tub, A, provided with the
stirrer, B C. The solution was run off from this vessel by means of the cock,
b, into the settling tank, E, in which the mechanical impurities separated
from the solution. The clear liquid ran into the decomposing vessel, a large
shallow covered tank, holding 9,000 to 10,000 litres. In this tank opened
800 copper tubes, given off from the large pipe, S. The small furnace, D, in
which limestone was burnt with coke, produced the carbonic acid; from the
pipe at the top of the furnace the carbonic acid was brought into the
Archimedean screw, h K, was washed with water, and pumped into the
solution of lead acetate. The introduction of the carbonic acid was
continued from 10 to 12 hours, after which the apparatus was left at rest
until the liquid in the decomposing tank had become quite clear, through
deposition of the white lead. The clear solution, now containing neutral lead
acetate, was run off into the receiver, m, from which the pump, P, carried it
back into the dissolving tub, A, where it was treated with fresh quantities of
litharge. The solution of neutral lead acetate drawn off from the white lead
had approximately a specific gravity of 1·0901. The white lead at the
bottom of the decomposing tank was a tolerably thick paste. It was
transferred to the tank, O, and washed several times with water. The first
wash waters, which contained small quantities of lead acetate, were
returned to the dissolving tub. The resulting white lead formed a very soft
powder; it was at once placed in the drying pans. The white lead prepared
by this process is a precipitate containing no coarse lumps, so that grinding
is unnecessary.

Fig. 17.

Theoretically, the quantity of acetic acid with which the process is
commenced is sufficient to form an unlimited quantity of white lead, since
all the acetic acid brought into the decomposing tank in the form of basic
acetate is returned to the dissolving tub as neutral acetate. In practice,
however, matters are somewhat different. Small quantities of acetic acid are
lost in the wash waters; each time the solution of lead acetate is pumped
back into the dissolving tub, a small quantity of acetic acid must be added
to make up the loss.

Fig. 18.

The method pursued by Ozouf, in France, is a considerable
improvement on Thénard’s process. Pure carbonic acid is used for the
precipitation, and white lead of similar composition to that produced by the
Dutch process is obtained, since the introduction of the carbonic acid can be
regulated according to the volume and strength of the lead solution, and
thus white lead of any desired composition can be produced. The most
elaborate precautions for the health of the work-people are taken.
The preparation of pure carbonic acid gas is based upon the absorption
of this gas from a mixture of gases by a solution of sodium carbonate, and
its evolution on heating the solution. The apparatus is shown in Figs. 18 and
19. The products of combustion obtained from the stove, A, are drawn by
the air pump, E, through the pipe, C, into the cooler, B, which is regularly
fed with cold water by D. The gases compressed in the receiver, E´, deposit
moisture there, and then proceed through 3 horizontal cylinders, F, of sheet-
iron, provided with agitators, in which the carbonic acid is absorbed by a
cold solution of sodium carbonate of 9° B. The unabsorbed gases escape

into the atmosphere through G (Fig. 19). The sodium bicarbonate solution is
received in the wooden tank, H, after passing through the 3 cylinders, F.
The pump, I, of the alternating pump, I I´, lifts the sodium bicarbonate
solution out of H and sends it through the pipe, K, into the tubular cylinder,
J, which stands upon a cylinder of larger diameter, J´, communicating with
it only by the vertical tubes. The bicarbonate solution rises between the
tubes in J, passes through the pipe, L, drops in a fine spray through the rose
forming its mouth, and by means of the vertical tubes passes into J´ and
thence into M, where it is heated by means of a steam coil to 100° C.
Carbonic acid is then evolved, and the residual solution of neutral sodium
carbonate, after cooling in the vessel, R, by means of the cold coil, is drawn
off by the pump, I´, again to enter the cylinders, F, by means of the pipe, K.
The carbonic acid evolved in M, together with steam, enters J´ through N,
and in rising in the tubes of the cylinder, J, is cooled by the falling spray of
bicarbonate solution. The cooling is completed in the coil, O, surrounded by
water; the vessel, P, separates the condensed water and passes the gas on
into the holder, Q. The pipe, S, connecting P with the suction pipe of the
pump, J´, serves to restore to the solution of sodium carbonate the water it
has lost, thus maintaining the proper concentration. The cost of 1 cubic
metre of carbonic acid is 10 centimes, of 1 kilogramme 5 centimes.

Fig. 19.

For the production of white lead, the carbonic acid, by means of the
pipe, U (Figs. 19 and 20), enters the cylinder, T, provided with an agitator
and containing a solution of basic lead acetate. By means of the pump, V,

the lead solution is fed into the cylinder, T, through W. The absorption of
the gas proceeds rapidly; the progress of the operation is followed by the
observation of a pointer moving over a scale; as the gas holder sinks the
pointer moves upwards. After the precipitation of white lead, the contents of
T are emptied into the tub, b, in which rotate rakes attached to a vertical
axis of iron plated with copper. When the white lead has settled, the
supernatant solution of neutral lead acetate is drawn off through the pipe, c,
by means of the pump, d, and conducted into the water-tight vessel, X,
containing a stirrer on the vertical axis, W, made of coppered steel. Here
litharge is added, and the resulting solution of basic lead acetate is
conveyed to the cylinders, T, by means of the pump, V, as already
described. The white lead in the tub, b, by putting the stirrer in motion, is
washed once with water which has been previously purified by a little lead
acetate. It then goes into another tub provided with stirrers, where it is
several times washed, sodium carbonate being added to the last wash water
until a sample of the white lead is not coloured by a drop of potassium
iodide solution. In this way the wash water is obtained free from lead, and
the product is said to be of better quality. This, however, is not in
accordance with the fact that good Dutch white lead generally contains
some lead acetate. The two-cylinder pump, h, which is in connection with
the gas holder, forces gas over the surface of the liquid in T in order to drive
it into tubs which are not in the position shown for b in the illustration, and
into which there is no direct flow. The washed white lead is brought into
bags which are pressed in a hydraulic press, dried, ground, sieved and
packed in casks. These troublesome and often dangerous operations have
been modified by Ozouf in the following manner. The pulp white lead runs
from the tub, b, into the hopper, g, where it is kept mixed by a small stirrer,
and from which it passes on to the cylinder, f, heated by gas from the inside.
In its rotation the cylinder carries along the white lead and dries it, it is then
removed by a knife below the hopper, and falls on to an inclined plane. The
hopper and cylinder are in a room provided with a good draught.

Fig. 20.

The lumps coming from the drying room are placed by workmen
wearing respirators in buckets on an endless chain, are carried to the mills,
ground and sieved; then, by means of an Archimedean screw, the white lead
is conveyed to a cask in which it is evenly pressed by means of a special
mechanism. A bell announces when a cask is full.
Manufacture of White Lead by means of Natural Carbonic Acid.—
In districts where currents of carbonic acid gas issue from the ground, they
can be used in the manufacture of white lead, and are actually utilised for
this purpose. Natural carbonic acid may, of course, be used for any of the
white lead processes.

(d) English Process.

In this process, now no longer in use, white lead was obtained by
mixing litharge to a stiff paste with a weak solution of lead acetate and
exposing the paste to the action of carbonic acid. By continually kneading
the mass by means of grooved rollers or of rotating cylinders, through the
hollow axis of which carbonic acid was led, the paste was thoroughly
brought into contact with the carbonic acid.
By this process a good product is only obtained when pure litharge,
entirely free from the oxides of iron and copper, is used. The copper oxide

may be removed from the litharge by means of ammonia if this can be
obtained at a low price; but oxide of iron cannot be removed, and very
small quantities of it are sufficient to impart a yellow tinge to the white
lead.

(e) Other Methods.

In Payen’s process the lead sulphate obtained in considerable quantities
as a by-product in calico printing is the raw material employed. By treating
this lead sulphate with a solution of ammonium or sodium carbonate, white
lead and ammonium or sodium sulphate are produced. The white lead is
then freed from the soluble salts by washing, mixed with a small quantity of
lead acetate, and pressed into the drying moulds.
By boiling lead sulphate with caustic soda and passing in carbonic acid
(Puissant’s process), a white lead is obtained which differs considerably in
composition from ordinary white lead.
Many methods have been proposed with the object of converting
insoluble lead salts, obtained as by-products or by an inexpensive process,
by treatment with alkaline or alkaline earth carbonates, into white lead. The
fact that none of these methods has obtained a permanent footing in the
industry shows that each must be accompanied by serious defects, or can
only be practicable under peculiar conditions.
Magnesium carbonate is used in Pattison’s process to decompose lead
chloride. Dolomite (magnesian limestone) is the raw material for the
magnesium carbonate. Coarsely powdered, it is heated at a low red heat,
when magnesia is formed, the calcium carbonate remaining almost entirely
unaltered, since it requires nearly a white heat for its decomposition. The
powder ground in water was, when treated with carbonic acid under a high
pressure, soluble, magnesium bicarbonate being formed, the saturated
solution of which contains 2·3 per cent. of magnesia, and has a specific
gravity of 1·028. The solution of lead chloride contains 1 part of the salt in
126 parts of water; it is mixed with a slight excess of the magnesium
carbonate solution as quickly as possible. The liquid is drawn off from the
mixing vessel into a large receiver in which a precipitate deposits,
consisting of white lead and a little oxychloride. After drying, the

precipitate is ground with a small quantity of caustic soda to decompose the
oxychloride. A few days afterwards the mass is washed to remove sodium
chloride and the product dried.
The process of Dale and Milner is similar to the above magnesia
process. Litharge, lead hydroxide or insoluble lead salts are mixed with
sodium bicarbonate solution, and, with repeated additions of water, ground
until the formation of white lead is completed. The lead compound and
sodium bicarbonate are used in equivalent proportions.
According to the process of P. Bronner (German patent 52,262), 3
molecules of freshly precipitated lead sulphate are heated with a solution of
2 molecules of caustic soda, when the basic sulphate 2 PbSO₄.Pb(OH)₂ is
formed according to the equation—
3 PbSO₄ + 2 NaOH = 2 PbSO₄.Pb(OH)₂ + Na₂SO₄.
Or 4 molecules of lead sulphate are decomposed by 2 molecules of caustic
soda, according to the equation—
4 PbSO₄ + 2 NaOH = 3 PbSO₄.Pb(OH)₂ + Na₂SO₄.
This transformation takes place at a temperature of 70° C. The resulting
basic sulphate, although pure white, cannot be used as a pigment on account
of its lack of covering power; but by heating with a solution of sodium
carbonate it is converted into white lead.
2 PbSO₄.Pb(OH)₂ + 2 Na₂CO₃ = 2 PbCO₃.Pb(OH)₂ + 2 Na₂SO₄.
3 PbSO₄.Pb(OH)₂ + 3 Na₂CO₃ = 3 PbCO₃.Pb(OH)₂ + 3 Na₂SO₄.
By this process, which is harmless to the workmen, the lead sulphate
obtained as a by-product in the preparation of mordants for calico printing,
can be converted into good saleable white lead. The lead sulphate may also
be obtained from litharge, lead acetate or nitrate.
It occasionally happens that white lead has a rose tint, which is clearly
perceptible by comparison with a pure white sample. This colouration
occurs in white lead made from argentiferous lead. A very small quantity of
silver is sufficient to produce the tinge of colour.
Occasionally white lead which has been ground in oil and used for
painting turns perceptibly yellow, the colouration being similar to that

observed on a surface painted with white lead from which light is almost
excluded. The yellow colouration is due to lead oxide. This has been proved
by suspending such a white lead in water and treating it with carbonic acid,
after which a surface painted with it remains permanently white.

(f) Oxychloride White Lead.

Under the name of white lead, but differing from it in composition,
various products are found which consist of lead oxychloride. This
compound is also known as Pattison’s white lead.
Pattison’s white lead can be much more cheaply manufactured than real
white lead, the raw material employed being the cheap galena. The finely-
powdered mineral is boiled with strong hydrochloric acid in closed lead
vessels. Sulphuretted hydrogen is evolved, which may be burnt to sulphur
dioxide and so used to make sulphuric acid. A hot saturated solution of lead
chloride remains, from which the salt separates in small crystals on cooling.
The crystals are drained in a basket and washed with cold water to remove
the acid. The pure lead chloride is then dissolved in hot water and mixed
with lime water. Pattison obtained lime water from dolomite by burning it,
treating with a little water to remove the easily soluble salts, and, after the
removal of this wash water, treating the residue repeatedly with water in
order to obtain a clear solution of pure hydrate of lime. When pure
limestone is used, it may be treated with water immediately after burning
without any preliminary preparation.
Two equivalents of lead chloride are used to one equivalent of calcium
hydroxide. Practical experience showed that the best product was obtained
when the precipitation was very rapidly brought about. With this object,
both solutions entered the precipitation tanks through pipes with narrow
slits at the side, so that the liquids met in a thin layer, in which the
precipitation of the pigment was instantaneous. It is also necessary that lead
chloride should be in excess throughout. The liquid is allowed to stand for
the precipitate to settle, which it does in a brief time on account of its high
specific gravity. The solution now contains the small excess of lead chloride
in addition to calcium chloride; lime water is added until the liquid turns red
litmus paper blue. From the alkaline solution all the lead soon separates as

lead hydroxide, which is dissolved in hydrochloric acid, and thus again
comes into the process.
In order to utilise the large quantities of hydrochloric acid obtained in
the manufacture of soda, Percy described a process in which galena is
ground with hydrochloric acid, whereby in 30 to 40 hours all the lead is
converted into lead chloride, whilst the stony admixtures are unattacked.
The lead chloride is then separated by levigation from the undissolved
minerals and washed until free from iron, when it is dissolved in hot water
and converted into oxychloride by means of lime water.
Lead Sulphite, PbSO₃, can be obtained by passing sulphur dioxide into
a solution of basic lead acetate; lead sulphite is precipitated and a solution
of neutral lead acetate remains. The process is similar to the French white
lead process, with the difference that sulphur dioxide is used instead of
carbon dioxide. Lead sulphite has no advantages over white lead, and is
more expensive; it has thus never found practical application.
Lewis and Bartlett’s White Lead Pigment.—In the lead works at
Zoplin, in Missouri, galena is smelted with limestone and coal, lead fume
being obtained in addition to metallic lead. The lead fume deposits are
ignited, and again worked for lead and lead fume. This last lead fume can at
once be used as a white pigment; it consists principally of lead sulphate,
lead oxide and zinc oxide.

White Lead-Antimony Pigments.

Lead antimonite and antimonate are both heavy, white powders which
can be used as pigments. They are dearer than white lead, to which they are
inferior in covering power, and which they do not exceed in permanence.
Lead Antimonite is obtained by heating 5 parts of finely powdered
antimony with 20 parts of sulphuric acid until a dry, white mass of
antimony sulphate is left. This is fused with soda ash, the melt is extracted
with water, and lead antimonite obtained by precipitating with lead acetate.
Lead Antimonate is formed by introducing in small quantities at a time
a mixture of 1 part of finely powdered stibnite (antimony trisulphide) with 5
parts of sodium nitrate into a red-hot crucible, boiling the mass with water
and precipitating the solution with lead acetate.

CHAPTER VIII.
ENAMEL WHITE.

Barium sulphate, known as permanent white, enamel white, blanc fixe,
barytes white, is the only white pigment which is absolutely unaltered by
exposure to the atmosphere. Lead pigments are discoloured in the course of
time, and in the end turn black; bismuth white behaves in the same manner;
zinc white is much more lasting, but not quite permanent.
Enamel white is really permanent; it deserves the greatest attention from
the colour manufacturer, especially as it can be made by a very simple and
cheap method. When sulphuric acid or a soluble sulphate is added to the
solution of a barium salt, all the barium is at once precipitated in the form
of barium sulphate.
When quite pure, barium sulphate forms an extremely soft, brilliantly
white powder, which offers complete resistance to the action of the
atmosphere, and also of strong acids and alkalis. It is extremely insoluble,
and can be precipitated from the most dilute solutions, and is then obtained
in so fine a state of division that it cannot be filtered from the liquid; it
passes through the closest filter together with the liquid. When the barium
solution is heated to boiling before precipitation, the precipitate is
somewhat coarser, and can be filtered off without difficulty.
Barium sulphate occurs ready formed in nature as the mineral barytes or
heavy spar. Finely ground barytes may be used alone as a pigment, but
more commonly is used for reducing white lead, for which purpose it is
particularly applicable on account of its high specific gravity. This
admixture must be regarded as diminishing the quality of the pigment,
because ground barytes has far less covering power than white lead.
Artificial barium sulphate is in a state of division which cannot be reached
by grinding barytes, consequently it considerably surpasses the latter in
covering power.
The raw material for the manufacture of enamel white is either barytes
or witherite (barium carbonate); the latter, however, occurs so rarely, in

comparison with barytes, that the greater quantity of all barium compounds
is obtained from barytes.
If witherite is obtainable in large quantity, enamel white can be prepared
from it by dissolving in hydrochloric acid and precipitating the solution of
barium chloride so obtained by sulphuric acid. If the witherite is very pure,
the process may be simplified by treating the mineral directly with
sulphuric acid, and separating the enamel white by a process of levigation
from the impurities. In this case it is, however, necessary to add a small
quantity of hydrochloric acid to the sulphuric acid, for the latter forms on
the surface of the witherite, at the commencement of the reaction, a thin
layer of barium sulphate, which is quite sufficient to prevent the further
action of the acid on the witherite lying below. The hydrochloric acid forms
barium chloride, which is at once decomposed by the sulphuric acid into
barium sulphate and free hydrochloric acid; this again dissolves a fresh
quantity of witherite, and this process is repeated until the mineral is
completely and quickly dissolved.
Enamel white is, however, generally prepared from barytes, which is
ground into a very fine powder and converted into barium sulphide by
heating with coal (see pages 41 and 42). Hydrochloric acid acting on the
sulphide produces barium chloride and sulphuretted hydrogen.
The covering power of a pigment is greater the finer its state of division,
so that it would appear advisable to precipitate a weak solution of barium
chloride by sulphuric acid at the ordinary temperature. When the barium
sulphate has been completely precipitated, a solution of pure hydrochloric
acid remains, which ought to be utilised; but when very dilute barium
chloride solution is used, the hydrochloric acid is so dilute as to be useless.
The barium chloride is, therefore, given in practice such a strength that it
has a specific gravity of about 1·198; when the barium sulphate has been
precipitated from this solution the residual hydrochloric acid has a specific
gravity of 1·043.
Water of considerable purity must be used to dissolve the barium
chloride. Experience has shown that water which contains appreciable
quantities of organic matter does not give a pure white product. The
presence of sulphate of lime in the water, which precipitates barium
sulphate, need not be regarded, because the barium sulphate is so finely

divided that it remains suspended in the liquid, and is carried down on
precipitation of the enamel white by sulphuric acid. Carbonate of lime in
the water causes the separation of barium carbonate; this may be avoided by
slightly acidifying the barium chloride solution, thus converting the calcium
carbonate into chloride.
According to C. A. F. Meissner, artificial barytes, suitable for use in oil
paints, is obtained by precipitating barium salts by soluble sulphates in
place of sulphuric acid, then quickly heating the washed and dried
precipitate in a muffle to a red heat and throwing into cold water.
As has been already stated, enamel white is the most permanent
pigment that exists; it appears destined in course of time to replace white
lead and all other white pigments, especially as its cost is generally lower
than that of the other white pigments. It costs, for example, only half as
much as white lead. At present, the principal uses of enamel white are found
in paper staining; it is not used to any extent in oil paints. On account of its
permanence, it should be used in the place of white lead and white zinc. It
also appears particularly suitable for obtaining pale shades; it can be mixed
with any other pigment in any quantity without altering it in the least. This
is, of course, only true when the enamel white is completely pure, and when
it has been freed from every trace of hydrochloric acid by careful washing.
Lithopone.
A white pigment is obtained, according to Orr’s process, by lixiviating
crude barium sulphide, obtained by igniting barytes with coal, with water
and dividing the solution into two parts. Zinc chloride is added to the first
portion, then zinc sulphate, and finally the second portion of barium
sulphide solution. The white precipitate obtained by this process contains
one equivalent of barium sulphate to two equivalents of zinc sulphide. It is
collected, quickly dried, heated in retorts to redness, and, whilst still hot,
thrown into cold water, by which its density and therefore covering power
are increased. The pigment is finally washed and ground. It is a good white,
but when mixed with lead pigments discolours them by reason of the
sulphide it contains.

CHAPTER IX.
WASHING APPARATUS.

In all colour works operating on a large scale, special apparatus is used,
in which are carried out the washing, pressing and drying of the pigments
obtained as precipitates. Only pigments prepared in small quantities are
filtered through filter paper. The treatment of enamel white and white lead
requires the use of apparatus for this purpose in a special degree. We insert
a short description here.
The preparation of enamel white takes place most conveniently in tubs
provided with a stirring apparatus. When the precipitate of barium sulphate
has once settled to the bottom, it is very difficult to again mix it up with
water by means of a hand stirrer, an operation which must be often
performed in washing. If vessels be used provided with a suitable
mechanical stirrer, the precipitates are rapidly and thoroughly washed. We
have already stated that vessels with a stirrer capable of being raised out of
the liquid were specially suitable for washing white lead. Such an
arrangement can with advantage be used in washing all precipitates. Many
of the mineral pigments have to be freed from admixed salts by washing. A
description follows of an effective washing apparatus with movable stirrer.
In the cylindrical vessel (Fig. 21), which may be of any size, is a vertical
iron shaft rotating upon a pin in the bottom of the vessel. On this axis is a
horizontal wooden crosspiece, the under surface of which is studded with
brushes; the disc, which this crosspiece carries, is united by means of two
bars with a second, in which is cut a screw moving on the screwed shaft.
The shaft is rotated by means of the cog-wheels shown in the illustration.
The handle fastened to the upper disc enables the crosspiece carrying the
brushes to be raised or lowered. When the handle is held fast, this
crosspiece rises or falls according to the direction of rotation of the axis.
The pipe shown at the side supplies the water for washing the precipitates.

Fig. 21.

In the preparation of colours liquids have often to be brought into the
precipitating vessel which would attack the iron of the stirring apparatus, so
that it is advisable to make the connecting rods between the two discs of
such a length that the screws may be above the vessel. All iron parts of the
apparatus dipping into the liquid should be protected by asphaltum varnish.
When a specially heavy precipitate is to be washed, such as enamel
white, chrome yellow or white lead, the stirrer is raised as high as possible
before the commencement of the operation. When the precipitate has
formed and settled, the liquid is run off, the water tap opened, and the stirrer
slowly brought down to the precipitate; the brushes fastened to the
crosspiece stir up first the top of the precipitate, then the next portions, and
so on until the whole of the precipitate has been stirred up into the liquid.
When this has been accomplished, the stirrer is kept going for some time, so
that the water may take up as much of the soluble materials as possible; it is
then raised out of the vessel, in which the precipitate again settles.

As a rule, two or three washings of permanent white, in an apparatus of
the construction described, are sufficient to render it quite free from acid.
Washing must be continued until the wash water leaves blue litmus paper
quite unchanged. When dry, precipitated barium sulphate is a very soft
powder of great whiteness, which, on account of the fineness of its
particles, can be readily ground with binding materials.
Enamel white loses, in a remarkable manner when completely dry, a
great portion of its covering power and of its valuable property of being
easily ground with oil or size to a homogeneous paste. It is not known
whether this alteration is caused by a molecular change of the barium
sulphate, as is not altogether improbable. In order to preserve the valuable
properties of enamel white it is not, as a rule, completely dried, but is
brought into the market in the form of pulp, which is obtained by bringing
the washed precipitate into strong linen bags and allowing the water to drop
through. This object is more quickly accomplished when the last wash
water is drawn off only to such a point that, when the precipitate is again
mixed up by means of the stirrer, a thin paste is formed. This paste is run
into a centrifugal machine, of which Fig. 22 is an illustration. In the drum,
B, provided with an outflow, R, the smaller removable drum, C, with
perforated walls, is caused to rotate by gearing, G, to which it is attached by
the screw, V. The drum, C, is lined by a tight linen bag.

Fig. 22.

When the drum is in rapid motion, the thick liquid in the washing vessel
is run in. In consequence of the centrifugal force due to the rapid rotation,
the whole mass is at once thrown on the sides of the drum, the liquid
penetrates the fine openings, is caught in the outer vessel, and runs away to
a receiver, in which it is kept until the finest particles of the precipitate,
which will penetrate even the closest fabric, have settled. The operation is
continued until the excess of water has been separated, when the bag
containing the precipitate is lifted out of the drum. Centrifugalised enamel
white is a fairly stiff, white paste, which should be packed in sacks lined
with oiled paper to prevent drying.

Fig. 23.

Filter Presses.
In recent years the use of filter presses for separating liquids from
precipitates and for washing the latter has become general. A filter press
consists, as shown in Fig. 23, of a number of frames, between which are
perforated plates and sheets of filter cloth, and which can be pressed tight
together by a screw. A powerful pump forces the liquid containing the
precipitate into the hollow spaces of the frames (known as chambers), in
which the solid body remains, whilst the liquid goes through the filter cloth.
By afterwards pumping fresh water through, the solid remaining in the
chambers is soon completely washed.

CHAPTER X.
ZINC WHITE.

Zinc oxide, known as a pigment under the name of zinc white, is one of
the most important white pigments. Although not absolutely permanent, yet
it has, in common with enamel white, the valuable property of preserving
its whiteness in air containing sulphuretted hydrogen. Its low price has
brought it into general use.
Although zinc white is a most important pigment, it is very seldom
made in colour works, because on account of its origin it is a product of
metallurgical processes. Zinc white in chemical composition is pure zinc
oxide; it is formed when zinc vapour burns in air. In zinc-smelting works
zinc white is obtained by putting zinc in tubes which are heated to
whiteness; the zinc vapours burn when they come in contact with the air,
and the zinc oxide is caught by special arrangements.
The retorts used for this purpose are similar to those employed in the
manufacture of coal gas. From 8 to 18 of these retorts are arranged in a
furnace, in two rows, one above the other. In the lid of the retorts is an
opening, which serves for charging and for carrying off the zinc vapours.
When the operation is commenced, the retorts being heated to a white heat,
two zinc plates are brought into each, the metal is soon volatilised, and the
vapours pass through the above-mentioned openings. A current of air,
heated to 300° C., is blown in to meet the zinc vapours, which take fire and
burn with a blinding white flame, producing a very fine white powder,
which is carried by the current of air through a series of chambers in which
it deposits.

Fig. 24.

C. Freitag recommends for the production of zinc white the use of
retorts of oval section, A, Fig. 24. These retorts, containing crude zinc, are
heated to a white heat, and then a mixture of generator gas from coke and
air is introduced by B and the pipe running through B. The zinc burns
completely in the flame of the generator gas, which contains excess of
oxygen. A product of faultless nature is said to be obtained in this way.
In zinc works zinc white is always made in the manner which has been
described. It may also be obtained as a by-product in another metallurgical
operation, the desilverisation of lead by Parkes’ process. In this process an
alloy of silver and zinc is obtained. By sending a current of superheated
steam over the molten alloy the zinc decomposes the steam, hydrogen and
zinc oxide being formed. The zinc white is carried by the current of gas into
chambers, in which it deposits.
The zinc white obtained by burning zinc is, as has been said, a very fine
pure white powder, which can at once be used for paint without further
preparation. The price of zinc white is rather higher than that of white lead,
but the difference is counterbalanced by the greater covering power of the

zinc white. Ten parts by weight of zinc white completely cover a surface for
which 13 parts of white lead are required.
Whilst white lead cannot be mixed with many pigments, such as those
which contain sulphur, zinc white may be mixed with all without fear of
alteration. Zinc white is even better than enamel white for producing pale
pigments from lakes; it has a lower specific gravity than enamel white, so
that the mixture with the light lake can be more easily made.
Zinc grey, which is produced by some works, is zinc oxide discoloured
by metallic zinc. Pure zinc white always has a pure white colour; if it is
tinged with grey it is contaminated by metallic zinc, whilst a brownish hue
denotes the presence of cadmium oxide. The latter impurity will be rarely
met with in commercial samples, since cadmium is worth much more than
zinc. Zinc oxide is used by the colour maker in the preparation of
Rinmann’s green; it is also used, as stated above, as an addition to other
pigments.
Griffith’s Zinc White.—This pigment, which is equal in covering
power to white lead, consists of zinc oxysulphide. It is obtained by
precipitating a zinc solution with a solution of barium sulphide, washing,
drying, igniting and grinding the precipitate. As it contains sulphur, it
should not be mixed with copper or lead pigments.
Tungsten White (Lead Tungstate), PbWO₄, is obtained as a heavy
powder by precipitating a solution of sodium tungstate with lead acetate and
treating the precipitate, which consists of basic lead tungstate, with dilute
acetic acid, by which lead oxide is dissolved and a salt of the above
composition left. This white pigment is dearer than other lead pigments, and
has no special advantages over them; it is, therefore, seldom used.

White Antimony Pigments.

Antimony forms a number of white compounds which can be made by a
simple and inexpensive process, and might, therefore, be used as pigments.
Two antimony compounds in particular are so used—antimony trioxide and
oxychloride (powder of algaroth).
Antimony Trioxide occurs ready formed in nature as white antimony or
antimony bloom. It is very simply prepared by burning the metal in air,

when it forms soft needles with a silvery lustre. It is only necessary to heat
melted antimony to a little above its melting point in a crucible placed in a
slanting position, when the metal takes fire and burns with a blue flame.
Nitric acid converts metallic antimony very quickly into antimony oxide
with a copious evolution of brown fumes.
Antimony trioxide may be more cheaply prepared from antimony
sulphide, artificial or natural (stibnite), by finely powdering, moistening
with water, and gently warming on plates. Oxidation takes place, the
sulphur is converted into sulphur dioxide and the antimony to trioxide. The
heating should not be carried too far, or the antimony takes up further
oxygen and forms the tetroxide.
Antimony Oxychloride, or Powder of Algaroth, is obtained by
dissolving stibnite in strong hydrochloric acid, the operation being
performed under a chimney to carry off the sulphuretted hydrogen. The
solution of antimony trichloride, when the impurities have settled, is poured
into a large vessel of water. At once a pure white precipitate is formed,
which quickly sinks; it is washed with water until the washings have no
acid reaction. Washing should not be continued too long or the oxychloride
will be further decomposed; by washing with hot water it is almost entirely
changed to antimony trioxide. The precipitate, after washing with cold
water, has generally the composition expressed by the formula SbOCl.
Antimony oxide and oxychloride are both very crystalline powders, and
have in consequence small covering power.
Bismuth White is not used as a painters’ pigment; it has no advantage
over the white pigments previously described, and is much more expensive.
Its only use as a pigment is for the preparation of white cosmetics, and even
for this purpose zinc white is now frequently used; it is cheaper and quite as
satisfactory.
Bismuth white is prepared by treating metallic bismuth with fuming
nitric acid. The white precipitate at first formed completely dissolves in an
excess of acid, and when the solution is poured into a large quantity of
water, basic bismuth nitrate—bismuth white—separates.
Pure bismuth white is a soft, heavy powder, brilliantly white; it must be
preserved in air-tight vessels as soon as it is dry, otherwise it acquires a

yellowish tinge. Bismuth compounds are, if possible, more susceptible to
the action of sulphuretted hydrogen than lead compounds. The yellow
colouration, turning to black in the course of time, is due to black bismuth
sulphide.
Tin White is used for earthenware enamels. It is obtained by treating
granulated metallic tin with very strong fuming nitric acid. The heavy,
white powder which is formed is separated from the undissolved tin by
floating. Tin white has no application as a pigment; when mixed with glazes
it gives them a handsome, milky appearance.
Manganese White.—When large quantities of a solution of impure
manganese chloride are at hand, such as are produced in the preparation of
chlorine, manganese carbonate may be obtained. A small quantity of soda
solution is first added and the liquid left for several days so that the oxide of
iron may separate. When this is the case an addition of soda gives a pure
white precipitate.
Magnesia White or Mineral White is obtained, according to T. H.
Cobley, by mixing a solution of magnesium sulphate with calcium chloride,
adding 10 per cent. of aluminium chloride to the mixture and stirring in
slaked lime so long as a precipitate is formed. A cheaper process is to
precipitate mixed solutions of magnesium and aluminium sulphates by
slaked lime.
Annaline.—A white pigment is recommended for use under this name;
it consists of dead-burnt gypsum, which has been converted into a fine
powder by grinding and levigating. (Dead-burnt gypsum has been so
strongly heated that it is not able to again unite with water.)
To obtain paler shades of certain colours, additions are made of natural
pigments, such as chalk, which has been converted into a very fine powder
by levigation. An addition of chalk to a heavy mineral pigment, such as
chrome yellow, is not advisable; it would, besides, seriously diminish its
covering power.

In the preceding pages, a large number of white pigments has been
enumerated. It would be easily possible to increase the list, but the result

would be of no practical interest, for other pigments, neither in respect of
quality nor price, can compete with the cheaper white pigments. Although
white lead has at the present time an enormous use, it is to be hoped that
this pigment, of good colour but little permanence, may be replaced entirely
in the course of time by zinc white, and for some purposes by enamel white.
White pigments, in addition to their use alone, are employed to produce
tints, which are obtained by mixing deeper colours with the white pigment.
By adding the proper quantity of white to a colour it is possible to produce
all paler shades of that colour. For example, the different varieties of the red
lakes which are found in commerce are obtained by mixing white pigments
with the deep red lakes.
The particular white pigment to be employed in these mixtures depends
on the nature of the colour and on its specific gravity. It should always be
remembered that white lead will not increase the permanence of a colour,
since it will be discoloured in a short time by the action of the atmosphere.
In the manufacture of fine colours for artists lead pigments should be
absolutely excluded.
To produce paler shades of colours which contain lead and therefore
have a high specific gravity, e.g., chrome yellow, white lead may be used;
for other colours, barytes or zinc white should be employed. In the case of
lakes enamel white is too heavy; zinc white or magnesia is suitable.

CHAPTER XI.
YELLOW MINERAL PIGMENTS.

As was the case with white pigments, so with yellow: of the large
number known very few are in extensive use. In former times the number of
yellow pigments employed in painting was far greater than at present;
several, formerly in general use, have dropped out, partially or entirely,
owing either to their poisonous character or to their replacement by others,
deeper and more handsome. Especially since the discovery of cadmium
yellow and the development of the manufacture of chrome yellows, many
colours once in general use have properly fallen into disuse.
Again, unfortunately, the most important of the yellow mineral
pigments contain lead, and have little stability; but a series of yellow
colours free from lead is known, and though some of them are inferior in
shade to the lead pigments they surpass the latter in permanence.
In addition to lead compounds, pigments derived from barium, zinc,
antimony and cadmium are in general use. The yellow lead pigments were
formerly preferred, and at present, so far as concerns beauty, they must be
regarded as superior to other mineral yellows. The endeavour to provide the
artist with permanent colours has resulted in the use of others, perhaps less
brilliant, but very lasting. Under the name of chrome yellow a single
pigment consisting of lead chromate was formerly understood; to-day,
under this title is comprised a series of pigments containing zinc or barium
in place of lead, but all commercially known as chrome yellow.

CHAPTER XII.
CHROME YELLOWS.

The chrome yellows are the lead, zinc or barium salts of chromic acid.
In describing the different methods used in making these colours, lead
chrome yellow will be taken first, since this is the one most largely used.

Lead Chrome Yellow.

Both neutral and basic lead salts of chromic acid are known. Neutral
lead chromate is found in nature as the somewhat rare mineral crocoisite,
which is found in very small but perfectly shaped crystals in many lead
mines.
Neutral Lead Chromate, PbCrO₄, is formed as a very heavy
precipitate of a fine deep yellow colour when a solution of potassium
chromate or bichromate is added to a solution of a lead salt in water. When
exactly equivalent quantities of potassium chromate and lead solution are
used, and the strength of the solutions is the same, the product has the same
shade each time the operation is performed. It is not immaterial whether the
one or the other salt is in excess, or whether strong or weak solutions are
used; all these conditions modify the shade of the chrome yellow produced.
Many colour makers are apparently of the opinion that some particular skill
of the workman is necessary to produce chrome yellow of a particular
shade. This is, however, not the case; manufacturers who know the simple
conditions which are important in making chrome yellows may produce any
desired shade without difficulty.
Neutral lead chromate readily parts with half the chromic acid it
contains. When treated with alkalis, such as lime or caustic soda, or even
when digested with finely ground litharge, the neutral salt gives up half of
its chromic acid, and is converted into the basic chromate or chrome red,
Pb₂CrO₅.

Basic lead chromate has, as the name chrome red indicates, a fine red
colour. If the quantity of lime or caustic soda used is sufficient to
decompose only a portion of the neutral lead chromate, a mixture of the
yellow neutral and the red basic chromate is formed, the shade of which
will incline to yellow or red according as it contains a preponderance of one
or the other compound. The pigment known as orange chrome is a mixture
of approximately equal parts of the neutral and basic lead chromates.
In order to brighten the deep yellow shade which distinguishes neutral
lead chromate, it is either mixed with a white pigment, or a white substance
(lead sulphate) is precipitated from the solution simultaneously with the
lead chromate. In this manner all the imaginable pale yellow shades, lemon
yellow, sulphur yellow, etc., can be obtained.
Just as the quantities of the solutions used and their strength influence
the shade of the chrome yellow they produce, so it also appears to be not
immaterial which lead salt is employed. Colour makers are generally agreed
that the finest product is obtained from neutral lead acetate. Any lead salt,
even insoluble in water, may be used for the preparation of chrome yellow.
The affinity of chromic acid for lead is so great that an interchange of
constituents occurs between the insoluble salt and the potassium chromate.
Chrome yellow may be made from lead acetate, chloride or sulphate; the
resulting substances are the same, but there is a considerable difference in
regard to the beauty of the product. The finest chrome yellow, which leaves
nothing to be desired in beauty of shade, is obtained by proceeding in the
following manner: Lead acetate is dissolved in water, the solution diluted
with an equal volume of water, and then mixed, under constant stirring,
with a similarly diluted solution of potassium chromate or bichromate. The
precipitate is immediately formed, and quickly sinks to the bottom in
consequence of its high specific gravity. It is washed with clean water so
long as this removes soluble salts. The precipitate is then drained on cloths
and dried in the air.
The finest product is obtained by working with the following
proportions:—
Sugar of lead 100
Potassium bichromate, or 50
Potassium chromate 40

If lead sulphate is used the following quantities are to be employed:—
Lead sulphate 100
Potassium bichromate 25
In the case of lead chloride the following is the proportion:—
Lead chloride 100
Potassium bichromate 27
The chrome yellows prepared from insoluble lead salts have no particular
beauty, but they may be used for mixed colours such as the spurious chrome
green.
Preparation of the Lead Solution.—Many makers of chrome yellow
do not use commercial lead acetate, but prepare its solution themselves. The
preparation of this solution requires neither much space nor labour, so that
considering the high price of lead acetate this procedure may be regarded as
advisable, but only when acetic acid is obtainable at a low rate.
The following is the method by which lead acetate solution is made.
Lead is granulated by pouring the molten metal from a height of several
yards into cold water, which is kept in rapid motion. The smaller the
particles of lead, the larger surface they will possess, and the more quickly
they will dissolve. For the solution of the lead small tubs are used, 50
centimetres in diameter and 90 to 100 centimetres in height, provided with a
tap immediately above the bottom to run off the liquid. Four of these
vessels are so placed, one above the other, that the contents of each may be
run into the one next below it. The lead in the top vessel is covered with
acetic acid; in a few minutes this is allowed to flow into the second vessel,
and similarly after a few minutes from the second to the third, from the
third to the fourth, from which it runs away into a receiver below. After this
treatment it contains but a small quantity of lead acetate; the object of the
operation is to start the oxidation of the lead, which quickly follows when
air has sufficient access, the lead particles lose their metallic appearance
and become covered with a white layer. When this is the case, the acetic
acid is pumped from the receiver back into the top vessel, where it is left
one to two hours in contact with the lead. It is then run off into the second,
and thence into the third and fourth, remaining in each vessel for about the
same time; the resulting liquid is an almost completely neutral solution of

lead acetate. The treatment with acetic acid is continued so long as lead
remains undissolved.
The solution of potassium chromate is prepared in a tub. The salt is
easily soluble, and warming is unnecessary if the chromate is placed in a
basket, lined with close linen cloth, hung in the liquid so that it is immersed
to half its depth. The salt rapidly dissolves, its solution has a greater density
than water, in consequence of which it sinks and fresh quantities of water
continually come in contact with the salt.
Precipitation of the Chrome Yellow.—Before the chrome yellow can
be precipitated it is necessary to estimate the quantity of lead acetate
contained in the solution, since upon this depends the quantity of potassium
chromate solution to be used. If the lead solution contained only acetate and
water, its strength could be simply found by means of the hydrometer. It
contains, however, varying quantities of acetic acid, on which account the
hydrometer would give very inaccurate results. The test by which the
relation between lead solution and potassium chromate solution is found is
performed in the following manner: The lead solution is measured off in a
cylinder divided into 100 divisions; the same volume of potassium
chromate solution is measured and placed in a high narrow vessel; the lead
solution is gradually added to the chromate solution so long as a precipitate
is formed. The precipitate settles rapidly, and there is no particular difficulty
with a little practice in finding with sufficient accuracy the quantities
required for the precipitation. In order to precipitate 100 litres of potassium
chromate solution, there are required as many litres of lead solution as were
used divisions of the cylinder.
The preparation of the chrome yellow is now a very simple matter.
Whilst steadily stirring, the measured quantity of lead solution is run into
the solution of potassium chromate; the precipitate is allowed to settle, is
well washed and dried. It does not make any difference to the colour
whether potassium chromate or bichromate is used; the same product is
obtained in each case.
It is stated by Dullo that chrome yellow prepared by the preceding
process alters in colour on long keeping. This is ascribed to the formation of
a basic compound. According to the same author, a chrome yellow free
from this objectionable property is obtained by using lead nitrate in place of

acetate and an excess of potassium chromate solution. The writer has kept
chrome yellow, made from lead acetate, for years without observing the
slightest alteration in the colour. On chemical grounds, it is
incomprehensible that a chrome yellow prepared from lead nitrate should
have different properties to the same substance prepared from another
soluble lead salt, and freed from foreign substances by sufficient washing.
The product of this process is that which par excellence is known as
chrome yellow, the chemist’s neutral lead chromate; it exhibits a
characteristic deep yellow colour, a shade which is known as chrome
yellow. Under the microscope chrome yellow is seen to be a crystalline
mass; it will possess greater covering power the smaller the crystals. Now,
the motion of a liquid in which crystals are forming prevents the production
of large crystals, thus the reason is clear for the rapid stirring of the
solutions in the preparation of this pigment.
According to C. O. Weber, who has published an exhaustive account of
chrome pigments (Dingler’s Journal, 282), the cost of the lead chrome
pigments varies greatly according to the raw materials employed. Assuming
that 100 kilogrammes of litharge cost 35 marks, 100 kilogrammes of 30 per
cent. acetic acid 25 marks, and 100 kilogrammes of 60 per cent. nitric acid
26 marks, Weber calculates that 100 kilogrammes of litharge, in a form
suitable for making chrome yellow, will cost as follows:—
From lead acetate at 56 marks per 100 kilogrammes 96 marks
” ” solution made in the works 80 ”
” lead nitrate at 50 marks per 100 kilogrammes 75 ”
” ” solution made in the works 64 ”
By the basic lead acetate method 51 ”
” ” chloride ” 40 ”
White lead method 55 ”
The Pale Chrome Yellows.—When the solution of the chromate used
for the precipitation of the lead solution is mixed with sulphuric acid, then a
mixture of lead sulphate and lead chromate is formed on precipitation. Lead
sulphate is white, so that the colour of the precipitate would be paler
according to the quantity of sulphuric acid added to the potassium chromate
solution. There are, however, compounds of lead chromate and lead

sulphate, of which we know two. Their composition is expressed by the
formulæ:—
PbCrO₄.PbSO₄ and PbCrO₄.2PbSO₄.
The former is a beautiful lemon yellow shade, the latter nearly
approaches sulphur yellow. By corresponding alterations in the quantity of
sulphuric acid added, all intermediate shades can be obtained. On the works
these shades are made in the following manner: buckets are used for taking
the potassium chromate solution out of the vessel in which it was made;
these buckets hold 12·5 litres. Now, if the solution of potassium chromate
has been made from 25 kilogrammes of the salt and 750 kilogrammes of
water, one of these buckets holds exactly 0·43 kilogramme of chromic acid.
In order to obtain the lemon yellow compound, 0·39 kilogramme of
sulphuric acid must be added to a bucketful of solution; 0·78 kilogramme
must be added to obtain the sulphur yellow chrome. These liquids are
prepared by pouring the sulphuric acid in a thin stream into the potassium
chromate solution made as above, the liquid must be stirred whilst the
sulphuric acid is being added. The lemon chrome has the peculiar property
of increasing considerably in volume soon after formation, a property to
which regard must be had in the manufacture. A description of the rational
preparation of the lemon and sulphur yellow shades of chrome yellow on
the manufacturing scale follows.
In making lemon chrome, the tub in which the precipitation is to take
place is two-thirds filled with water, the lead solution is stirred in, and then
the chromate solution, mixed with the proper quantity of sulphuric acid, is
run in; the liquid is well stirred so that the precipitate may form as quickly
as possible throughout the whole liquid. The precipitate is allowed to settle,
the liquid drawn off, and the colour washed twice with water as quickly as
possible. The paste is then removed from the tub and poured on a strong
linen strainer. At first, the fine precipitate goes through the strainer, the
liquid is poured back on the strainer until the size of its pores is so far
diminished by the precipitate itself that only clear liquid runs through. The
precipitate is left on the strainer until it forms a stiff mass, which can be
easily spread out upon boards by spatulas.
To obtain a good, that is, a loose product, it is necessary to carry out the
processes so quickly that the swelling mentioned above does not take place

while the colour is being strained, but when it has been spread out on the
boards. This swelling only takes place completely when the layer of
precipitate is fairly thin. Large boards should be used, upon which the
precipitate is spread out in a very thin layer. To prevent the mass—which is
still fairly fluid—from running off the boards, they are provided with raised
edges, and the paste is spread out smoothly in these flat trays. If the
operations have been properly performed, the precipitate at once begins to
swell and becomes of a loose nature. When it has acquired a buttery
consistency it is cut up, by means of a thin sheet of brass, into prisms,
which are placed near one another standing on the narrow side, and dried
first in the shade and then in the sun. It is necessary that the drying should
take place slowly at first, or the cakes will crack or even fall to pieces. The
precipitate cannot be washed completely in the tub, because it often begins
to swell on the strainers, consequently a crystalline crust covers the surface
of the cakes during drying. This layer must be removed by scraping the
cakes of colour, in which operation small quantities of chrome yellow dust
become suspended in the air. To protect the workman against poisoning by
this lead compound, precautions must be taken against breathing in the dust.
The simplest and most efficacious is to tie a wet sponge over mouth and
nose; this retains the particles of dust in the inspired air. The dust, scraped
off the cakes, is put into water, in which the salts dissolve, whilst the
chrome yellow sinks to the bottom. In this process for preparing chrome
yellow the solution of potassium acetate left after the precipitation contains
a considerable quantity of free acetic acid, which may be utilised to dissolve
lead.
When sulphur yellow chrome is to be made, the process is substantially
the same as for the preparation of the lemon shade, but with the difference
that everything is done to prevent the swelling of the precipitate. The
precipitation and washing of the precipitate are done as quickly as possible:
the washed precipitate is filled into press bags and strongly pressed. Care
must be taken in the pressing that the pressure is only gradually increased;
if powerful pressure is applied at once, even the strongest cloths will be
burst. The more thoroughly the precipitate is pressed, the closer will be the
fracture of the chrome, a property which is regarded as a sign of good
quality in this species of chrome yellow.

The manufacture of chrome yellow is intimately connected with that of
a number of colours, varying from orange to dark red, which are known
under the names of chrome orange or chrome red. In accordance with the
division adopted of pigments according to their colour, chrome orange and
chrome red will be considered among the red mineral pigments.

CHAPTER XIII.
LEAD OXIDE PIGMENTS.

Lead monoxide, PbO, exists in two different modifications; in the
crystalline form, as litharge, which is generally pale yellow with a reddish
tinge, and amorphous, as massicot, which is yellowish red.
Lead monoxide is a product of metallurgical works rather than of colour
works; still the preparation of the crude oxide falls in the domain of the
colour maker, since from litharge several pigments may be prepared by a
simple treatment.
Massicot is obtained by heating white lead, lead nitrate or red lead, and
also by heating melted lead in the air, with the precaution that the oxide
formed does not itself melt.
Litharge is obtained as a by-product in several metallurgical processes,
such as the cupellation of lead containing silver, in which the lead is melted
in a furnace with a shallow hearth and a powerful current of air blown over
the melted metal. The lead is oxidised, the oxide melts at the high
temperature approaching 1000° C., and flows through an orifice in the wall
of the furnace, whilst the silver remains on the cupel. The litharge is ground
and levigated and, according as it is pale yellow or reddish, sold under the
name of silver or gold litharge.
Both massicot and litharge have no particularly striking colour; they are
seldom used as pigments. Litharge has an extensive use in oil boiling and
for the manufacture of lead peroxide, which is used for matches.
Red Lead, Minium.—Lead forms a number of other oxygen
compounds, one of which, red lead, has the composition, Pb₃O₄. It is a
bright red powder, used as a pigment and as a constituent of certain cements
(for gas and water pipes).
Red lead is, like litharge and massicot, a metallurgical product, but, by
working on a small scale, products can be obtained of a much brighter
colour than the produce of the large scale.

Fig. 25.

Red lead is made in two ways—directly from metallic lead or by
heating easily decomposed lead salts. When it is made from the metal, the
following is the process: the lead, which must be very pure, is melted in a
reverberatory or calcining furnace, oxidised to massicot by the air passing
over it, and the massicot then, by careful heating, changed into red lead, a
process in which particular care must be taken that the mass is not melted.
By continued heating the massicot absorbs about 2 per cent. of oxygen, and
changes in colour to a bright red.
The art in making red lead by this process lies in maintaining the proper
temperature; the furnaces are constructed so that the temperature may be
regulated during the heating. A reverberatory furnace is used, in which is a
stirring apparatus, so that the heated mass may be continually turned over to
accelerate the oxidation. Muffle furnaces are also used, in which the
massicot is placed in crucibles on an iron plate, which can be pulled out of
the furnace for observation of the change of colour. Whatever method is
used, the temperature must be so regulated that over-heating of the material
is avoided, otherwise litharge is formed, which is only very slowly
converted into red lead.

Fig. 26.

Mercier states that the muffle furnaces are arranged as shown in Fig. 25.
The muffle, a, is 2·5 metres long and 2 metres wide; its bottom rests on an
iron plate. The passage, d, running under the muffle is 20 centimetres high;
it is divided by a partition, and at each end are two hearths, c, 70
centimetres long and wide. The products of combustion pass from the long
passages into side channels, f, provided with dampers, go round the muffle
and unite in the space, g. The flue, k, at the back of the muffle is provided
with dampers, m, which exactly regulate the current of air through the
muffle; n is the chamber in which are collected the particles of oxide carried
over by the draught. The furnaces used for manual labour consist, according
to Percy (Figs. 26 and 27) of a rectangular or circular hearth, a, of about 3
metres diameter, which is deeper in the middle and has two fireplaces, b.
The low arch surmounting the hearth is covered with sand in order to
prevent cooling.

Fig. 27.

When finished, the red lead is drawn out of the furnace and finely
ground under edge-runners, or occasionally levigated. The temperature
necessary in making red lead is that at which the angles of the muffles,
when these are used, begin to show a dark red glow.
Orange lead, which is a brighter variety of red lead, is prepared from
lead salts; white lead or lead nitrite is used for this purpose. The latter salt is
made by the process of Pischon, by heating 1 equivalent of lead nitrate with
4 equivalents of granulated lead and water at a temperature between 50°
and 60° C. After about 2 hours, the lead nitrite separates in the form of a
granular yellow mass. According to Burton’s process, lead carbonate is
oxidised by heating with 20 per cent. of sodium nitrate and extracting the
mass with water. There are also other methods by which red lead is obtained
from litharge by the use of potassium chlorate or saltpetre; but these
methods, without producing a finer product than those previously given, are

more expensive, and consequently have found no application on the large
scale.

CHAPTER XIV.
OTHER YELLOW PIGMENTS.

Cassel Yellow, also known as mineral or Veronese yellow, has now a
very restricted use; it has been replaced by the deeper and cheaper chrome
yellow. Much of the Cassel yellow of commerce is nothing but chrome
yellow shaded with barytes. As regards chemical composition, Cassel
yellow has the following formula: PbCl₂.7PbO. It is obtained by heating
litharge, red lead or white lead with ammonium chloride. To 10 parts of the
lead compound 1 part of ammonium chloride is used; on melting, ammonia
is set free, by which part of the lead oxide is decomposed, metallic lead
separating. The melted mass is poured off from the lead into iron moulds, in
which it solidifies to a very crystalline substance of a fine yellow colour. By
grinding and levigating, the Cassel yellow is prepared for use. Pale yellow
shades, obtained by admixtures of barytes, are occasionally encountered.
Montpellier Yellow consists, like the preceding pigment, of basic lead
chloride. It is obtained by gradually mixing 400 parts of powdered litharge
with a solution of 100 parts of common salt in 400 parts of water. After
each addition of salt solution the pasty mass must be thoroughly stirred, or
it will harden. When all the salt solution has been mixed with the litharge to
a homogeneous white mass, the latter is treated with water to remove excess
of salt, and the washed material dried and melted in earthenware crucibles.
The melt, which has a bright yellow colour, is ground and levigated, when it
forms a handsome pigment.
There are several other yellow pigments of similar composition, of
which one only need be mentioned, obtained by treating a solution of zinc
chloride with lead hydroxide.
Turner’s Yellow or English Yellow is prepared by two methods: either
by melting 7 parts of finely ground litharge with 1 part of common salt; or
by treating litharge with a solution of common salt and converting the white
oxychloride into a yellow pigment by melting.

Naples Yellow.—This handsome pigment, which is, unfortunately,
susceptible to the action of sulphuretted hydrogen, is known commercially
under different names. Naples yellow takes its names from the fact that it
was formerly exclusively made in Italy, where the method was kept secret, a
secret which disappeared with the advance of analytical chemistry. Naples
yellow is now known to be lead antimoniate.
Naples yellow is a handsome pigment. Its preparation is more tedious
than that of chrome yellow, hence it is now rarely employed. The author has
had practical experience that much of the so-called Naples yellow of
commerce is nothing but a suitably shaded chrome yellow.
Naples yellow can be prepared by different methods. According to the
oldest, given by Brunner, 1 part of pure tartar emetic is carefully and
thoroughly ground with 2 parts of lead nitrate and 4 parts of common salt.
The mixture is melted at a low heat in a Hessian crucible, and the fluid
mass poured on a cold iron plate. After cooling, it is boiled out with water,
when lead antimoniate remains as a powder of a more or less deep yellow
colour. It is not easy to obtain this favourable result with certainty in every
case. If a certain temperature is exceeded only by a little, a hard mass
results, which by long boiling does not become a fine powder, but a sandy
substance of little brilliance. Even when the operation succeeds, the product
often varies considerably in shade, sometimes a sulphur yellow, at other
times an orange pigment being formed. As a rule, the paler product is
obtained at a lower temperature; by stronger heating, darker products of a
red shade are obtained.
According to another recipe, 2 parts of tartar emetic are melted with 4
parts of lead nitrate and 8 parts of common salt. The mass is treated with
very dilute hydrochloric acid for a long time, which extracts some quantity
of lead oxide, a deeper product being thus obtained. Care is, however,
necessary in this treatment; acid of too great strength would spoil the whole
product.
The Paris method for Naples yellow is as follows: metallic antimony is
oxidised by melting in air; to 12 parts of antimony, 8 parts of red lead and 4
parts of zinc oxide are used, and the mixture is melted at a low red heat.
A cheap, but not particularly bright product, can be obtained from old
printer’s type. The metal, which is an alloy of antimony and lead, is

powdered, mixed with 3 parts of saltpetre and 4 parts of common salt,
melted, and the mass washed out with water.
Other formulæ which are said to yield a good result are as follows: 12
parts of white lead, 3 parts of antimony oxide, 1 part of ammonium
chloride, 1 part of alum. Or: 16 parts of stibnite, 24 parts of lead, 1 part of
common salt and 1 part of ammonium chloride. The intimate mixture of
these materials is first gently heated with access of air, then more strongly,
and the mass extracted with water. There are many other recipes for the
preparation of Naples yellow, the majority of which are distinguished by an
apparently arbitrary arrangement of the materials; for there is no scientific
reason. If it were possible to accurately obtain any desired high temperature
in a furnace, the manufacture of Naples yellow would no longer be a matter
of skill, but the same product could be obtained at every attempt. Since this
is not yet the case, the exact procedure for the preparation of this colour can
only be found by careful experiments.
Naples yellow is, as has been said, a handsome colour, and offers a
great resistance to varied reagents. It is only changed by one of them,
sulphuretted hydrogen, by the prolonged action of which it is turned
completely black.
Antimony Yellow is very similar in composition to Naples yellow. It
consists of a mixture of lead antimoniate with the oxides of lead and
bismuth. It is prepared by the process recommended by Meromé by
intimately mixing 3 parts of finely powdered bismuth with 24 parts of
powdered stibnite and 64 parts of saltpetre, melting the mixture and shaking
it whilst molten into water. The brittle mass is finely powdered, washed and
dried, then melted with 128 parts of litharge and 8 parts of sal ammoniac.
The mass obtained has a fine pale yellow colour; when powdered it is
antimony yellow. This pigment has almost fallen into disuse because of its
instability and the high price of bismuth.
Calcium Chrome Yellow.—Calcium forms a yellow pigment with
chromic acid, which, although far surpassed by the lead chromes in fineness
of shade, has the advantage over them of greater stability and cheapness.
For purposes for which cheap and at the same time permanent colours are
required calcium chrome yellow can be recommended. It is most simply
prepared from potassium chromate and calcium chloride, which, as a by-

product of many chemical operations, is obtainable at very low prices. The
deepest pigment is obtained when the precipitation is done with a boiling
solution of the chromate. Calcium chromate, in addition to its use alone,
may be employed instead of white pigments to produce pale shades from
deep lead chromes. This addition should not be carried to a great extent or
the chrome will be made too light, since calcium chromate has a much
lower specific gravity than lead chromate.
Barium Yellow, Yellow Ultramarine or Permanent Yellow.—This
pigment consists of barium chromate. The finest product is obtained when a
solution of a barium salt, generally barium chloride, is precipitated boiling
by a solution of potassium chromate. The very finely divided precipitate has
a pale yellow colour very similar to that of pale lead chromes. This
handsome pigment is distinguished by the valuable property of being
practically unaltered by the atmosphere; it is only attacked by strong acids
and alkalis. By long heating, the colour of this compound is gradually
changed to a handsome green, which consists of a compound of barium and
chromic oxides, and has occasional use as an artists’ colour. In order to
obtain this pigment, the heating must be intense and long continued.
According to the author’s experiments, it is not sufficient to heat for a short
time to a very high temperature; in that way a mass is obtained of very
unequal colour. The best result was obtained by spreading barium yellow in
a thin, even layer in a flat porcelain dish and heating to whiteness for 10
hours.
Zinc Chrome Yellow.—Zinc chromate is inferior to lead chromate in
beauty, but has the advantage of permanence. It does not blacken in an
atmosphere of pure sulphuretted hydrogen, and resists very well the action
of other agents. Zinc yellow may be prepared by the immediate
precipitation of a solution of zinc sulphate by a solution of potassium
chromate, both being boiling, but the very bright precipitate obtained in this
way is not stable; on washing, it gives up chromic acid continually to the
wash water, and only a pale yellow residue remains. A very fine colour is
obtained in the following manner: zinc sulphate is dissolved in water and
boiled for half an hour with 1 per cent. of white zinc whilst stirring. This
operation effects the separation of iron oxide and the neutralisation of the
free acid generally present in commercial zinc sulphate. When the solution
has cleared by standing, it is precipitated by a solution of potassium

chromate, the precipitate collected on a filter and allowed to drain
completely; it is then washed with very small quantities of water and finally
dried. A pure yellow precipitate is only obtained when all the iron oxide has
been removed by boiling the zinc sulphate solution with white zinc; if the
liquid contains only a very small quantity of iron, it has yet a very
considerable influence on the colour, the yellow is not pure, but has a
brownish tinge. Zinc yellow is used alone, and mixed with other pigments.
Chrome yellows of all possible shades may be obtained in this way. Chrome
yellows are often found in commerce which consist essentially of zinc
chromate.
Cadmium Chrome Yellow.—When a solution of cadmium sulphate, or
any other cadmium salt, is mixed with a solution of potassium chromate, a
precipitate of cadmium chromate, CdCrO₄, is formed. This pigment has a
beautiful, deep yellow colour, in no way inferior in shade to the finest lead
chromes, and having the great advantage over the latter of being entirely
unaltered by the atmosphere; it is thus to be highly recommended for
artistic purposes. The high price at which it is sold prevents its general use,
though now that cadmium compounds are to be obtained at so much lower
prices than formerly, the price of cadmium chromate appears to be
excessively high.
Cadmium Yellow is cadmium sulphide, CdS; in nature it occurs as the
somewhat rare mineral greenockite. Cadmium yellow is obtained by
dissolving metallic cadmium in sulphuric acid, and precipitating the
solution with sulphuretted hydrogen. The solution of cadmium sulphate
must be digested for some time with excess of cadmium, in order to
separate the foreign metals present as impurities; the colour is not so fine
when a quite pure cadmium solution is not used.
Cadmium yellow is a very bright yellow. Several shades are obtained
according as the solution of cadmium sulphate used in its precipitation is
neutral or acid. The reason of this difference in shade lies apparently in the
different size of the crystals of which the precipitate is composed. The deep,
pure yellow colour becomes still deeper by fusion, which takes place at a
white heat. Weak alkalis, acids and sulphuretted hydrogen do not alter
cadmium yellow; it is thus to be regarded as a durable artists’ colour. It can
be mixed with ultramarine without decomposition, when a fine green is

formed; but mixed colours cannot be made from cadmium yellow and blue
copper pigments, since these would blacken in the light.
Lead Iodide.—On precipitating a solution of lead nitrate with
potassium iodide, lead iodide is formed. This is but slightly soluble in
water, and, when dry, has a handsome, deep yellow colour. Unfortunately it
is not permanent, but is decomposed on exposure to light. It can be used for
bronzing, but other and cheaper pigments are available for this purpose.
On account of the great solubility of lead iodide in a solution of
potassium iodide, it is prepared in another way, and accurately weighed
quantities are used. Calcium iodide may be used instead of the potassium
salt; 100 parts of iodine, 15 parts of fine iron filings and 25 parts of lime are
mixed with sufficient water to form a thin paste, which is warmed until all
the iodine is dissolved, when water is added, the liquid filtered, and the
residue washed in order to extract all the calcium iodide. The solution and
wash waters are united, then a solution of 152 parts of lead acetate is added,
when all the iodine is precipitated as lead iodide.
A simpler method is to dissolve equal parts of lead nitrate and
potassium iodide separately, each in 20 parts of hot water, to mix the
solutions and cool quickly, when lead iodide separates in very small
crystals. When pure lead iodide is melted in the absence of air, and the
fused mass powdered, a product of yet finer colour is formed. It is
necessary to completely imbed the crucible in which the fusion is
performed in the fire. The action of air on the melted mass would produce a
basic iodide. The fine golden yellow colour of lead iodide adapts it
especially for the production of gold bronzes on wall papers and fabrics.
Mars Yellow, which is generally reckoned among the best artists’
colours, is usually a mixture of ferric oxide and calcium sulphate or
alumina. The pigment is prepared by mixing a solution of ferrous sulphate
with milk of lime, when ferrous oxide is precipitated, which becomes
yellowish brown on exposure to air, in consequence of the oxidation of the
ferrous oxide. By heating the precipitate, according to the temperature
different shades are obtained, varying between yellow and red. In addition
to Mars yellow, Mars orange and Mars red are found in commerce.
The manufacture of this pigment is very simple: 1 part of ferrous
sulphate is dissolved in 10 parts of water, and the solution mixed with milk

of lime made from 1 part of quicklime and 40 parts of water. If it is desired
to produce a darker shade, and especially a product to be afterwards
converted into Mars orange, the amount of ferrous sulphate is increased to 2
parts. When the mixture has been made, it must be stirred for a long time, in
order that the reacting substances may come thoroughly into contact. The
precipitate, which at first is greenish grey, soon acquires by the action of the
air the colour of ferric hydroxide, which becomes deeper on drying.
When dried and finely ground Mars yellow is heated in thin layers, it
changes to dark yellow, and finally to orange red, a similar alteration taking
place to that occurring when ferric hydroxide itself is heated.
A Mars yellow of a deeper shade, consisting of a mixture of ferric
hydroxide and alumina, is obtained by precipitating with caustic soda a
solution of ferrous sulphate and alum. The sodium sulphate which is formed
at the same time must be removed as completely as possible by washing
with boiling water.
By calcining Mars yellow for a long time at a high temperature, Mars
brown is produced, a fine brown pigment. The value of Mars yellow and the
pigments obtained from it lies not only in their fine shade, but in their
permanence, which distinguishes the majority of the iron colours.
Siderin Yellow.—This not very handsome yellow consists of ferric
chromate; it is obtained by adding a neutral solution of ferric chloride to a
strong boiling solution of potassium bichromate so long as a precipitate is
formed. Siderin yellow is said to be used both in oil and water, and to be
particularly adapted for use in sodium silicate paints, since in the course of
time it forms a stony mass with that substance.
The low price of iron salts would make it desirable to employ chromates
of iron, but it appears to be difficult to obtain a compound of constant
composition. In experiments with this object the author did not succeed in
obtaining products of the same shade. Others have probably been equally
unsuccessful, for siderin yellow has never been used in quantity, as it would
have been were there no difficulties in the way of its preparation.
Aureolin is a double nitrite of cobalt and potassium, Co(NO₂)₂.3
KNO₂. This pigment is prepared by adding excess of potassium nitrite to a
solution of cobalt nitrate acidified with acetic acid. As the liquid cools, a

deep lemon yellow crystalline powder separates, which, when dry, is known
as Indian yellow or aureolin. It is distinguished from other yellow pigments
by being unaffected by sulphuretted hydrogen.
The potassium nitrite required in the preparation of this pigment is most
easily made by melting saltpetre in a thick iron vessel and stirring in fine
iron filings in small quantities as soon as the saltpetre begins to decompose.
The iron glows brightly and burns to oxide, the saltpetre changes to
potassium nitrite. The mass is dissolved in a little hot water, the solution
filtered and cooled, when most of the undecomposed saltpetre crystallises
out, whilst the nitrite remains in solution. After further evaporation and
separation of another crop of potassium nitrate crystals, the solution can be
used to precipitate the aureolin.
It is advisable to use strong solutions in the precipitation of aureolin; the
finest precipitate is obtained in this way. If dilute solutions are used, the
precipitate forms gradually; it is then coarse and has little covering power.
According to the method of Hayes, aureolin is prepared by passing into
a solution of cobalt nitrate the vapours produced by pouring nitric acid over
copper and allowing air to enter. Caustic potash is added to the liquid from
time to time. In this way all the cobalt can be obtained in the form of a
yellow precipitate.
Tungsten Yellow.—Finely powdered tungsten is introduced in small
quantities into fused potassium carbonate so long as effervescence occurs.
After boiling with water and filtering, calcium tungstate is precipitated from
the filtrate by means of calcium chloride. The moist precipitate is added to
hot dilute nitric acid until the liquid is only slightly acid, when it is boiled
for half an hour and allowed to cool. The precipitate, after washing with a
little water and drying, is a deep lemon yellow powder.
Nickel Yellow consists of nickel phosphate. It is obtained by adding
sodium phosphate to a solution of nickel sulphate or nitrate, and heating the
pale green precipitate to redness. Nickel yellow has a pleasing shade, and is
distinguished by great permanence. Up to the present it has found little use
as an artists’ colour, but on account of its permanence, which does not
distinguish many yellow pigments, its use is to be recommended.

Mercury Yellow or Turpeth Mineral is a basic mercuric sulphate of
the formula Hg₃SO₆. It is obtained by heating 10 parts of mercury with 15
parts of sulphuric acid in a porcelain dish in a good draught, until a white
crystalline mass of neutral mercury sulphate remains. This salt, HgSO₄, is
decomposed in contact with water into free sulphuric acid and a basic salt
of the above composition. The decomposition is effected by treating the
finely powdered neutral sulphate with hot water so long as the washings are
acid, when a handsome lemon yellow substance remains. The wash waters
contain acid mercuric sulphate. They are allowed to stand with mercuric
oxide so long as this is dissolved, and the solution then used to prepare new
quantities of mercury yellow.
Turpeth mineral has a very bright shade and great covering power, but it
has little permanence. Sunlight soon turns it grey, and air containing
sulphuretted hydrogen in a short time turns it quite black, mercury sulphide
being formed.
Yellow Arsenic Pigments.—The extremely poisonous character of
arsenic pigments has practically banished these handsome and cheap
colours from use. In many countries their use is justly illegal. The majority
of arsenic pigments have, therefore, merely historic interest. The two
yellow arsenic pigments are found in nature as realgar and orpiment; though
these are not rare minerals, the artificial products were generally used as
pigments, and when they were in common use they were generally made in
metallurgical works, in which minerals containing arsenic were treated.
Realgar, As₂S₂, has an orange red colour, whilst orpiment, As₂S₃, is a
pure yellow. When these substances were used as pigments they had the
same drawbacks in regard to mixing with other pigments as other sulphur
compounds. King’s yellow is finely powdered natural or artificial orpiment.
Lead Arsenite is a permanent deep yellow, but extremely poisonous. It
can be made by fusing an intimate mixture of 100 parts of white arsenic
with 75 parts of gold litharge, grinding and levigating the mass. Cadmium
yellow, which has still more permanence and is less poisonous, replaces this
pigment.
Thallium Pigments.—Thallium is a metal which exhibits certain
similarities to lead. By precipitating a solution of a thallium salt with
potassium chromate or bichromate, according to the proportion between the

quantities of the two salts, precipitates are obtained of yellow, orange or
deep red colour, or, after fusion, brown. By the addition of a mixture of
potassium chromate and ferricyanide to a mixture of a thallium salt and
ferrous sulphate an olive green pigment is obtained. On account of the rarity
of thallium compounds, technical employment is out of the question, and
the sensitiveness of thallium pigments towards sulphuretted hydrogen
prevents their use for artistic purposes.

CHAPTER XV.
MOSAIC GOLD.

Mosaic gold consists of tin disulphide, SnS₂, fine scales of a golden
yellow colour, which sublime undecomposed at a fairly high temperature
and withstand the action of chemical reagents. It has a peculiar greasy
nature, and can be easily ground. It is, therefore, much used for bronzing
picture frames, as a pigment for painters and for wall papers.
Tin disulphide can be prepared either in the wet or the dry way; in the
wet way, by the action of sulphuretted hydrogen on a solution of tin
tetrachloride. The yellow precipitate so obtained has no handsome colour. A
far finer product is obtained in the dry way. The process is often regarded as
accompanied by particular difficulties, but in reality it is quite simple. It is
only necessary in preparing this pigment to take care not to raise the
temperature above a certain point, otherwise a great portion of the tin
disulphide will be decomposed into sulphur and tin monosulphide. To
prevent the temperature from rising too high, an addition of ammonium
chloride is made. This salt is volatile at a certain temperature; heat which
would otherwise raise the temperature above this point is used in
volatilising the ammonium chloride. With a little care it is easy to interrupt
the operation before all the ammonium chloride has been driven off. The
mosaic gold then obtained has a real golden glitter. If the temperature rises
too high, grey tin monosulphide is formed, which naturally considerably
diminishes the brilliancy of the product. Ammonium chloride may be
replaced by mercury or mercury compounds, which are volatile at a
temperature below that at which mosaic gold is decomposed. When
mercury compounds are used, both on account of their cost and poisonous
nature the heating must be conducted in glass retorts in order to recover the
mercury. This operation requires great care if loss due to the breakage of the
glass vessels at the high temperature is to be avoided. The process in which
metallic mercury is used gives the finest product of all, and is to be
recommended when a pigment is to be prepared which shall as nearly as
possible approximate to the appearance of gold.

In order to obviate the danger and loss associated with the use of glass
vessels, manufacturers who make mosaic gold in large quantity should use
an iron vessel. This will last a very long time. Such an apparatus consists of
an iron pan with a broad rim, upon which is fastened a head which has the
form of a retort neck; to this are connected short, wide tubes leading to a
chamber in which substances not condensed in the retort neck may deposit,
so that the use of this inexpensive apparatus will not only be without
danger, but will be accompanied by the recovery of almost the whole of the
volatilised substances. The pan is filled with the materials, the head placed
on, the joint tightly luted, and the retort neck connected with the chamber
by the wide iron tubes.
There are many formulæ for the preparation of mosaic gold. Some of
the most important are given, which in every case will yield a good result:
—
Tin filings 40 parts
Sulphur 35 ”
Ammonium chloride 25 ”

The tin filings, which must be very fine, are well mixed in a mortar with
the sulphur and ammonium chloride. The heating is gradual at first; when
the evolution of vapours has ceased the temperature is very slowly
increased to a dark red heat. The mosaic gold is found as a yellow mass at
the bottom of the vessel, but partly in crystalline scales on the walls and
head of the retort.
Other recipes are as follows:—
Tin dioxide 80 parts
Sulphur 60 ”
Ammonium chloride 30 ”

Tin filings 45 ”
Sulphur 35 ”
Ammonium chloride 25 ”

In all these cases the chief endeavour should be not to raise the
temperature too high; a dark red heat is sufficient to give a perfectly
satisfactory result.
When metallic mercury is used, it is employed in the form of an
amalgam with tin, which is then in such a finely divided condition that it
readily enters into chemical combination with the sulphur. The amalgam is

most simply obtained by heating 1 part of mercury almost to boiling, and
stirring 2 parts of tin filings into the hot metal; 18 parts of this amalgam
mixed with 7 parts of sulphur and 6 of ammonium chloride are heated
together.
The mosaic gold made after any of these methods may be used for
gilding gold frames or as a painter’s pigment. Much so-called gold paint
consists of mosaic gold ground with a thick gum solution.
Chrysean.—Wallach found that when a current of sulphuretted
hydrogen was passed through a saturated solution of potassium cyanide a
precipitate was formed, which had the formula C₄H₅N₃S₃. This substance,
chrysean, is similar in appearance to mosaic gold; its technical employment
is prevented by its extremely poisonous nature and its high cost as
compared with mosaic gold.

CHAPTER XVI.
RED MINERAL PIGMENTS.

Vermilion.
This beautiful scarlet red pigment, which has been used for so long a
time, consists of mercuric sulphide, HgS. The same compound occurs ready
formed in nature as cinnabar; picked pieces of this mineral come into the
market under the name of mountain vermilion (Bergzinnober). A far larger
quantity of vermilion is made artificially.
Mercuric sulphide exists in two forms—as a black non-crystalline
powder and in the crystalline form, which is used as a pigment. Each
modification may be transformed into the other by suitable treatment, and
each may pass into the other spontaneously under certain conditions. In the
manufacture of vermilion, the black form of mercuric sulphide plays an
important part; it is, therefore, necessary to give an account of the chemical
behaviour of the two modifications before proceeding to an account of the
method by which vermilion is made.
Black Mercuric Sulphide may be obtained either by the direct union of
metallic mercury with sulphur, or by precipitating the solution of a mercuric
salt with sulphuretted hydrogen. It is most simply formed by rubbing
together equal parts of sulphur and mercury moistened with water, until the
mixture is uniformly black. It is, however, difficult in this way to convert all
the mercury into sulphide. A better result is obtained when the mixture is
moistened by ammonium sulphide instead of water. In this case the time
required for the operation is shortened by warming the vessel. If the mortar
is placed in hot water it is generally sufficient to grind for about two hours
to bring about the combination of the mercury and sulphur.
This compound can also be easily obtained by heating mercury with
sulphur. In a vessel, placed under a chimney with a good draught, which is
necessary to carry away the poisonous mercury vapours, 6 parts of the
metal are heated nearly to boiling and 1 part of sulphur added. Combination
takes place at this temperature with a slight explosion, and pure mercuric

sulphide results when the heating is continued until the excess of sulphur is
driven off.
For the purpose of the manufacture of vermilion, black mercuric
sulphide is most simply made by filling a thick-walled vessel with equal
weights of mercury and sulphur, moistening the mixture with water and
shaking or rotating the vessel for several hours. This can be done either in a
rotating cylinder containing iron balls, or the vessel can be fastened to any
rotating object—to a water-wheel or to the fly-wheel of a steam-engine. The
vessel in which the combination is effected should of course not be quite
full. It has been found that a more jerky motion than that of rotation effects
the combination of the mercury and sulphur in a shorter time. For example,
an opportunity of fastening the vessel to a saw-mill would be of great
advantage.
The mercuric sulphide made by the above methods is a velvety black
mass, which, even when exactly equivalent weights of sulphur and mercury
have been used, is never quite pure. Carbon bisulphide will always extract a
certain quantity of uncombined sulphur. The most important property of the
black sulphide for the present purpose is that it is changed into the
crystalline modification by heating to the temperature at which it volatilises.
The sublimed mercuric sulphide has the well-known fiery scarlet colour
characteristic of vermilion.
Red Mercuric Sulphide, or vermilion, exhibits, for a sulphur
compound, considerable resistance to the action of chemical reagents; dilute
mineral acids do not decompose it. Unfortunately, vermilion has another
property which makes it quite unsuitable for the artist’s use: in the course of
time it gradually turns dull and at last is completely discoloured. This
alteration of colour can only be ascribed to a return of the red crystalline
modification into the black non-crystalline. When a white pigment is tinted
by vermilion it should not be a lead pigment, or in a brief time it will turn
black. A white pigment such as zinc white, which is not acted upon by
sulphur compounds, should be used.

CHAPTER XVII.
THE MANUFACTURE OF VERMILION.

The red modification of mercuric sulphide can be prepared in the wet or
the dry way. The latter was formerly in general use, but at present the wet
method is more generally employed, as it more easily and certainly
produces a handsome pigment. Each method has its advantages, and both
will be described.

(a) Dry Method.

The numerous prescriptions which have been given for the manufacture
of vermilion by the dry method are all founded on endeavours to convert
black mercuric sulphide into the red form. Many of these prescriptions
contain directions for the soundness of which no reason can be discovered,
and it is not going too far to say that none exists, and that operations
described as essential for the success of the process have been inserted
merely to give the recipe the appearance of novelty. It cannot be denied that
certain manipulations impart a greater brilliancy to the product, although it
is impossible to assign a physical or chemical reason; but the manufacturer
will quickly be able to differentiate the valuable from the worthless in these
processes. Two conditions have the greatest influence on the beauty of the
pigment—the temperature at which the black sulphide is sublimed, and the
complete freedom of the vermilion from excess of sulphur. Of less
importance is the repeated grinding of the vermilion; the oftener it is
ground, the smaller the crystals become and the paler the shade.
The operation of grinding the pigment in ordinary mills is known
practically as “preparing,” the extraction of excess of sulphur by boiling
with alkaline liquids as “refining,” the product.
The usual process in Holland, especially in the Amsterdam works, is as
follows: The black sulphide is made by heating 270 parts of mercury with
37·5 parts of sulphur in copper pans—the fire is so regulated that the
temperature is not high enough to ignite the sulphur. If properly prepared

the product has now a pure black colour. It is immediately finely ground
and preserved in earthenware bottles containing only 0·7 kilogramme each.
It is sublimed from hemispherical vessels, provided with iron covers
carefully joined to the rim of the sublimation vessel by a suitable fire-
resisting cement. Generally 3 such vessels are contained in one furnace. The
operation is commenced by heating them until the bottom shows a dark red
heat. The temperature should now be increased to such an extent that, when
the contents of one of the above-mentioned small bottles are poured into the
vessel, a small flame only appears; if on the contrary the contents burn
explosively, the temperature is too high, and further quantities of the black
sulphide must not be added until the vessels have cooled down to a certain
extent. If, on introducing the first quantity of mercuric sulphide, a flame
appears unaccompanied by an explosion, the contents of several of the
bottles may be introduced; the openings through which this addition is
made are immediately closed by a well-fitting iron plate. From time to time
this cover is raised and fresh quantities of mercuric sulphide added. The
operation lasts about 36 hours, when double the quantity of mercuric
sulphide, made from the above mixture of mercury and sulphur, will have
been introduced into the 3 vessels. For the complete success of the process,
the accurate regulation of the temperature to which the vessels are exposed
is particularly important; in practice, the temperature is judged by the height
of the flame which issues on removing the iron plates: if this reaches to 1
metre the fire is too fierce, but if the flame is very small the fire must be
increased. Towards the end of the sublimation the mass in the vessels is
stirred about every 15 minutes. As soon as the sublimation is finished the
fire is extinguished. The vessels are broken when completely cold; the
vermilion is then found in the upper portion as a sublimate of fibrous
character. The vermilion made by this process requires simply wet grinding
under ordinary mills and drying to be ready for market.
In the great mercury works at Idria, in Austria, vermilion is also made
from the black sulphide. The latter is made by mixing 84 parts of mercury
with 16 parts of finely powdered sulphur in rotating vessels driven by water
power, the operation lasting about 3 hours. This quantity of sulphur is larger
than is required to form mercuric sulphide; experience has shown that the
combination takes place more rapidly when more than the equivalent
quantity of sulphur is used. Heat is developed by the reaction and the
temperature of the mixture rises to over 30° C. The black sulphide is then

sublimed in cast-iron vessels, which are pear-shaped and built 6 together in
a furnace; each holds a charge of 315 kilogrammes of black.
Several periods are distinguished during the heating of the mass in the
sublimation vessels. The operation is commenced by heating 2 of the
vessels first. As soon as the vapours of sulphur, issuing from the neck of the
vessels, take fire with explosion, the fire is made to heat the adjacent
vessels. When the contents of all the vessels have exploded, the first part of
the process known as the “evaporation” is at an end. Earthenware heads are
then placed on the vessels and the fire is increased until the excess of
sulphur present begins to distil; its vapours take fire in the air with a slight
explosion. When this takes place, earthenware receivers are attached which
have only a small opening for the escape of uncondensed vapours. The
sulphur condenses in these. When sulphur vapours are no longer given off
in quantity, the intermediate period (stϋckperiode) is finished and the real
sublimation of the vermilion commences. The fire is now considerably
increased and the sublimed vermilion collects in the cooler parts of the
apparatus. When the sublimation is completely finished, small blue flames,
which quickly vanish, appear at the joints of the apparatus. The furnace is
allowed to get quite cold, when the various parts of the apparatus are taken
apart, the vermilion deposited in the tubes is carefully removed, so that they
may be again used, whilst the receivers and head are broken, so that the
vermilion they contain can be collected.
The larger pieces form lump vermilion; the fragments of the receivers
are cleaned with a wet brush to collect what adheres. The whole process of
sublimation from the introduction of the black to the end lasts about 7
hours.
The sublimed vermilion is ground in mills which differ little from
ordinary grinding mills. To prevent the formation of dust, water is added
and the stones are surrounded by wooden casings. The red paste from the
mills, which is now known as vermilion, is then refined.
The refining consists in extracting the excess of sulphur by means of
boiling potash solution; 300 kilogrammes of the ground vermilion are
stirred up with water in a tub, the water is drawn off and the wet mass
brought into an iron pan, in which it is heated with 22·5 kilogrammes of
potash lye for about 10 minutes. According to the composition of the crude

vermilion the lye has a strength of from 10° to 13° B. The smaller the
quantity of sulphur, the weaker is the potash solution; it is, or was, made in
Idria in a very primitive manner, by extracting wood ashes. For vermilion of
a bright red shade potash solution of 10°, for the dark red of 11°, and for
“Chinese vermilion” of 13° is used. The excess of sulphur, together with a
trace of mercuric sulphide, dissolves in the potash solution; the sulphur
chiefly forms potassium pentasulphide. When the boiling is finished, the
vermilion is carefully washed and dried in dishes placed in a heated
furnace. During drying, the vermilion agglomerates; finally the lumps are
broken and sieved.
Chinese Vermilion is in similar case to Indian ink. Both substances are
in common use in Europe, they far surpass in quality our own manufacture,
and in neither case do we know the exact method by which they are made.
Genuine Chinese vermilion so far surpasses European in brightness that it is
bought at five or six times the price. It is said on unauthenticated authority
to be made by subliming a mixture of 4 parts of quicksilver with 1 part of
sulphur in earthenware pots closed by an iron plate, which is kept
constantly wet and serves as a receiver, on which the vermilion deposits.
The sublimed masses adhering to the lid are sorted, ground and repeatedly
washed with water.
According to Callum’s description of the manufacture of vermilion at
Hong Kong, mercury and sulphur are heated in a large iron pan, with
continual stirring at about the melting point of sulphur (111° C.), until the
whole has changed to a black mass. After cooling, this is mixed with water
and mercury, the mixture thoroughly stirred, dried, placed in a
hemispherical dish and covered over with broken porcelain. A similar dish
is cemented on the top of the first, and the dishes are heated for 16 hours;
the vermilion adhering to the porcelain fragments is removed, wet ground
and dried.
A vermilion approaching Chinese in beauty is said to be obtained by
mixing ordinary vermilion with 1 per cent. of antimony sulphide and again
subliming the mixture. The dark grey sublimate produces a reddish brown
powder, which is repeatedly boiled with a solution of “liver of sulphur”
(potassium polysulphide), washed with water and digested for a long time
with hydrochloric acid. The author has repeatedly made vermilion by this

process, but could never obtain a product differing appreciably from the
original vermilion.

(b) Wet Methods.

The manufacture of vermilion in the wet way is founded on the
conversion of metallic mercury or its compounds into mercuric sulphide by
heating with alkaline sulphides, such as ammonium sulphide and liver of
sulphur. The product always contains uncombined sulphur, which is
eliminated by treatment with alkalis.
Apart from the danger to the health of the workmen caused by the
poisonous mercury vapours, which are present in rooms where mercury or
its compounds are heated in apparatus even of the best construction, and
which is not to be feared in a wet process, the vermilion prepared in the
latter manner has a finer and brighter shade than that prepared by a dry
process. The greater cost of the wet process is covered by the higher value
of the product.
The starting point of the wet method may be either metallic mercury,
black mercuric sulphide or another mercury salt. When metallic mercury is
used, according to an old process, 100 parts are ground with 23 parts of
flowers of sulphur and a little caustic potash solution to a homogeneous
mass, which is then heated with a solution of 53 parts of caustic potash in
an equal quantity of water, the evaporated water being continually replaced,
until the colour changes from brownish-red to the fiery red of vermilion.
When the colour approaches the desired shade a careful watch must be kept,
and immediately the proper shade is obtained the heating must be stopped.
If it is continued beyond this point for but a short time, the vermilion at
once loses its fire and cannot be again brightened. An interruption in the
heating is equally harmful, a dull shade being produced. When the proper
shade has appeared, the contents of the vessel are poured into a large
quantity of water. The vermilion is then washed with dilute caustic potash
and afterwards with water until the alkaline reaction disappears. It is finally
dried. The alkaline solutions obtained in this process contain considerable
quantities of mercury in solution. They are collected, and when a sufficient
quantity has accumulated the mercury is extracted.

Barff recommends the following method: Mercury is rubbed with ⅙ to
⅕ of its weight of sulphur until a uniform grey powder results. This is
heated in a porcelain dish with caustic potash solution (133 parts of potash
in 150 parts of water) at 45° C. until the powder has become bright red.
Heating above 45° C. is to be avoided; it would impart a brown tinge to the
vermilion.
A convenient process for making vermilion in the wet way is to first
produce the black sulphide in the dry way and then treat this with alkalis.
Brunner’s method is founded on this procedure: 100 parts of mercury and
38 parts of sulphur are used to prepare the black sulphide. This is heated
with a solution of 25 parts of caustic potash in 150 parts of water. The
operation is best conducted in a vessel placed in a water bath, the
temperature of which does not exceed 45° C. After heating 7 to 8 hours, the
mass begins to turn red, when the change proceeds more quickly, and the
greatest care must be taken not to exceed the point at which the colour has
reached its greatest brilliance. As soon as the desired shade has appeared,
the temperature of the water bath is lowered, but it is kept warm for several
hours. Caustic solutions of different strengths produce different shades.
Thus, in order always to obtain the same product from the same quantities,
it is necessary to replace the evaporated water at short intervals. In this
process some quantity of mercury remains uncombined; it is separated from
the vermilion by a process of levigation in the washing.
Firmenich’s Method.—Many wet methods for the manufacture of
vermilion differ but little from the preceding. In Firmenich’s method the
production of the black sulphide is united with the formation of vermilion
in one operation.
A solution of potassium sulphide of a certain strength is made by
igniting 20 parts of potassium sulphate with 6 parts of coal, boiling the
mixture with 1·5 time the quantity of rain water, cooling the solution,
separating from the potassium sulphate which crystallises out, and boiling
the solution with sulphur so long as it is dissolved. A mixture of 4·5 parts of
this solution, 100 parts of mercury, and 2 parts of sulphur is placed in
flasks, which are subjected for several hours to a shaking or rocking
motion. The liquid becomes greenish, and its temperature rises in
consequence of the combination of mercury with a portion of the sulphur of

the potassium sulphide. The latter immediately dissolves the free sulphur
present, and again gives it up to the mercury, which in the course of a few
hours is completely converted into the black sulphide. The shaking is
discontinued when the contents of the flasks have acquired a deep
brownish-red colour. The flasks are then placed in a room heated to 45° C.,
and their contents repeatedly shaken up. At this temperature the
transformation of the black into the red mercuric sulphide is completed. The
deposit in the flasks acquires more and more a scarlet shade. As soon as the
colour is found to have reached its greatest intensity the liquid is carefully
poured off, and the vermilion treated with caustic soda solution in order to
dissolve free sulphur; it is then very carefully washed.
In this process, the temperature at which the operation commences is
important. The cooler the mixture which is placed in the flasks before
shaking, the paler will be the colour of the vermilion obtained. It is to be
supposed that the reaction, which is but slow in the cold, forms in
consequence of its slowness a black sulphide of such character that in its
subsequent transformation into the red crystalline sulphide very small
crystals are produced.
The Gautier-Bouchart method for the preparation of vermilion from
mercury and ammonium sulphide is applied on the large scale as follows:
1,000 parts of mercury are shaken for 7 hours with 200 parts of flowers of
sulphur and 400 parts of ammonium sulphide saturated with sulphur; the
dark-coloured mixture is exposed for several days to a temperature of 60°
C., when the colour changes to red. In addition to the usual washings, the
mass is further treated with nitric acid. The purpose of this operation is to
oxidise all the free sulphur to sulphuric acid.
For a colour works in which, in addition to vermilion, are made other
pigments, especially such as are sensitive to the action of sulphuretted
hydrogen, this process, which produces vermilion of a good shade, although
not particularly stable, is not to be recommended on account of the
evolution of sulphuretted hydrogen, which cannot be completely prevented
even by the greatest care.
Liebig’s Process.—The starting point in the manufacture of vermilion
by this process is the compound known as white precipitate (see below). It
is only necessary to heat white precipitate with a solution of ammonium

sulphide saturated with sulphur, at 40° to 50° C., for a long time. The
operation may be conducted in well-closed flasks in a place the temperature
of which is, as nearly as possible, 45° to 50° C., such as the flues leading
from a furnace in constant use. The change of colour is gradual; this is an
advantage, since it is easier to obtain the correct shade. By treating the
product with weak caustic potash solution it may be made still brighter.
After washing and drying at a gentle heat the pigment is finished.
This method is especially adapted for manufacturers not exclusively
engaged in making vermilion. No special arrangement of apparatus is
necessary; the apparatus for making ammonium sulphide and a number of
glass flasks are the only essentials. There is another considerable advantage,
that the product cannot be completely spoiled. The process of formation of
the vermilion is tolerably slow; a careful observation of the progress of the
earlier operations is sufficient to determine the time required. In order to be
able to do this with certainty, it is necessary to work always under exactly
the same conditions; the same quantity of white precipitate must always be
used and the solution of ammonium sulphide must always have the same
strength.
Mercuric Ammonium Chloride, Infusible White Precipitate,
HgClNH₂.—This compound, which is required in the last-mentioned
process for the manufacture of vermilion, is most cheaply made in the
following way: to a solution of 1 part of common salt in 32 parts of water, 2
parts of dry mercuric sulphate are added in small quantities, whilst
thoroughly stirring. It is absolutely necessary to work in this way, because
mercuric sulphate is decomposed by water into free sulphuric acid and a
basic salt, which is much more slowly converted into white precipitate than
the neutral salt. The liquid now contains mercuric chloride; on the addition
of ammonia to alkaline reaction it gives a heavy white precipitate. The
liquid is poured off and the precipitate washed with water containing a little
ammonia, until the washings give only a slight turbidity with barium
chloride. This process may also be commenced with mercuric chloride, but
the above method is cheaper. White precipitate should volatilise without
fusing when heated on platinum foil and should keep its white colour when
treated with ammonia.
Electrolytic Process.—In a wooden vessel 1 metre in diameter and 2
metres deep are placed, at the sides, dishes 15 centimetres wide, containing

a layer of mercury 1 centimetre deep. These dishes are connected with the
positive pole of a dynamo. The negative pole is connected to an iron plate,
electrolytically coated by copper, placed at the bottom of the vessel, which
is filled with a solution containing 8 per cent. of ammonium nitrate and 8
per cent. of sodium nitrate. A regular current of sulphur dioxide is
introduced through a perforated coil. The excess of gas escapes by a tube in
the cover. When the current is passed a precipitate of red mercuric sulphide
is at once formed. Attempts have been made to dispense with the current of
sulphur dioxide. The bath then contains 100 litres of water and 8
kilogrammes each of ammonium nitrate, sodium nitrate, sodium sulphide
and sulphur. Under these conditions it is only necessary to add sulphur and
mercury in order to obtain, at the end of the operation, vermilion which is in
no way inferior to that made by the first process.
Vermilion is frequently grossly adulterated by cheaper pigments.
Substances are often found under the name of vermilion which contain no
trace of mercury, but consist of bright orange lead mixed with a few per
cent. of ferric oxide, and having a deceptive similarity in colour to the best
vermilion.
Mercuric Iodide.—When corrosive sublimate solution is precipitated
with exactly the necessary quantity of potassium iodide, mercuric iodide is
obtained as a scarlet precipitate which surpasses in beauty even the best
samples of vermilion. Unfortunately, this substance cannot be used as an
artists’ colour. Exposure to light soon turns it brown and finally black. It
appears to be unaltered in the dark; the author possesses a sample which has
been so kept for 30 years without losing its shade in any way.

CHAPTER XVIII.
ANTIMONY VERMILION.

Antimony vermilion is a red pigment which will bear comparison in
fineness of shade with mercury vermilion, over which it has the advantage
of cheapness. In composition it is antimony trisulphide, Sb₂S₃. This
compound is obtained by precipitating a solution of antimony trichloride
with sulphuretted hydrogen. However, the precipitate, which is a very fine
red whilst wet, loses its colour in drying, and the product is almost
worthless as a pigment.
In another way it can be obtained in such a condition that it loses
nothing of its beauty in drying, but retains its brilliance. Böttger gives the
following process: a solution of antimony trichloride is mixed with a
solution of sodium hyposulphite (thiosulphate) and the liquid heated so long
as a precipitate forms, which is then washed on a filter with water
containing acetic acid. If pure water were used for washing, the antimony
chloride still present would be decomposed, forming the white oxychloride,
which would detract from the shade of the antimony vermilion. In this
process particular regard is to be paid to the use of exact quantities of
materials. The finest product is obtained when 2 parts of a solution of
antimony trichloride, which has exactly the specific gravity 1·35, are mixed
with a solution of 3 parts of sodium hyposulphite in 6 parts of water.
According to R. Wagner, antimony vermilion is obtained by dissolving
4 parts of tartar emetic and 3 parts of tartaric acid in 18 parts of water,
heating to 60° C., mixing with a solution of sodium hyposulphite
(thiosulphate) and heating to 90° C. The precipitate is then carefully washed
and dried.
Pure antimony vermilion closely approaches, as we have said, ordinary
vermilion in shade, and for a sulphur compound shows a remarkable
resistance towards chemical reagents. By dilute acids, ammonia and
alkaline carbonates, it is attacked only on long continued contact, but it is
easily decomposed by very dilute hydrochloric acid and by caustic alkalis.
A mixture with white lead keeps for a long time, but there can be no

question of the permanence of such a mixture, in consequence of the oft-
repeated properties of lead pigments. Antimony vermilion is well adapted
for oil painting. When ground with oil it exhibits a red of a brilliance in no
way inferior to that of genuine vermilion. It may also be used as a water
colour, but is not adapted for fresco work, since it is quickly decomposed
by lime.
In spite of these favourable properties antimony vermilion has so far
been little used. Considering the small cost of its preparation, especially
when calcium hyposulphite, which gives an equally good result, is used in
place of the sodium salt, its use is to be recommended in the place of
mercury vermilion. It appears as if the high price demanded for this
substance by several manufacturers has prevented its general employment.
Appendix. Antimony Blue.—This fine blue pigment can be prepared
by the addition of a dilute solution of potassium ferrocyanide to a clear
solution of antimony in aqua regia. According to Krauss, it contains no
antimony as colouring principle, but is a Prussian blue obtained from the
ferrocyanide, which is decomposed by the strong acid, with evolution of
hydrocyanic acid.

CHAPTER XIX.
FERRIC OXIDE PIGMENTS.

The pigments composed of ferric oxide are used in enormous quantity.
They are distinguished by a high degree of permanence. Large deposits of
ferric oxide occur in nature, and in places where it is found in considerable
quantity iron is manufactured from it on the spot. Several varieties of
natural ferric oxide are distinguished: specular iron ore forms crystalline
masses of brilliant lustre; another variety in small crystals is called iron
glance; micaceous iron ore consists of shining scales; red hæmatite has the
appearance of bundles of fibres; an earthy variety of hæmatite is also
common.
The compound of ferric oxide with water, ferric hydroxide, is still more
abundant than hæmatite; brown hæmatite, limonite and other iron ores
consist essentially of this compound. The pigment known as ochre is also
ferric hydrate.
Very pure red hæmatite has so fine a red colour that it may often be used
as a pigment after grinding or levigating. The famous red of Pompeii, which
has been found on the ruined walls of the town, still shows, after eighteen
centuries’ exposure to damp, the brightest shade, certainly a striking proof
of the extraordinary permanence of this pigment. Considering its great
permanence, its easy preparation, and its low price, it is surprising that
ferric oxide is not more extensively used by artists than it is at present. It is
used, however, extensively in colouring earthenware, for which purpose its
stability at high temperatures makes it suitable. Principally on account of its
cheapness it is largely used in ordinary painting, but for artistic purposes it
is not used to the extent it deserves.
Every colour maker well knows that artists justly complain that the
pigments offered to them have generally but a small degree of permanence.
They are surprised that the paintings of the old masters show now, after the
lapse of centuries, their colours unaltered, whilst the pigments
manufactured at the present day, instead of corresponding to the high
standard of chemical knowledge, are often discoloured within a few months

after use. But it was just the restricted knowledge of chemistry which the
ancients possessed which compelled them to make extensive use of the
permanent earth pigments, to which class of permanent colours ferric oxide
belongs. The advances of science have succeeded in preparing ferric oxide,
not only as a red pigment, but in the different shades of red, from yellow to
brown and deep violet, all consisting entirely of pure ferric oxide. Ferric
oxide has the property of altering its molecular condition on protracted
heating; this change is accompanied by an alteration in colour. If ferric
oxide is heated for a very long time at the highest temperatures its colour
changes at last to black.
Ferric oxide can be prepared by different methods for artists’ purposes.
The process by which it is made is not unimportant. Either ferrous or ferric
salts can be employed as the raw material. With the latter, pure ferric oxide
is at once formed, or hydroxide, which is converted into oxide by heating.
The ferrous salts are generally cheaper than ferric salts; they are therefore
commonly used for the preparation of ferric oxide, as well as of the other
iron pigments. Even in combination with the strongest acids, ferrous oxide
has but little stability; when separated from its salts as ferrous hydroxide,
the greatest precautions must be taken to obtain it pure; in contact with the
air it at once takes up oxygen and changes to ferric hydroxide. Ferrous
carbonate shows this same degree of instability; the natural substance,
occurring in large crystals as spathic ironstone, is no exception: on exposure
to the air it is gradually changed to ferric oxide.
There is another reason for the advisability of using ferrous salts to
prepare ferric oxide. When ferric hydroxide is made by precipitating the
solution of a ferric salt with ammonia or caustic potash, the least excess of
the precipitant unites with the hydroxide to form a compound which is only
decomposed by long washing with water. The precipitate is, however,
gelatinous, and consequently very difficult to wash.
In order to prepare ferric oxide suitable for an artists’ pigment the
following process may be used: 17 parts of soda are dissolved in 68 parts of
water; the solution is boiled in an iron pan, and 10 parts of crystallised
ferrous sulphate are added in small quantities with continual stirring. The
boiling and stirring are continued until the green vitriol has completely
dissolved, when the greenish white precipitate is allowed to settle, washed
several times with water, and then exposed to the air in thin layers. The

precipitate, which begins to turn yellow during washing, becomes in a short
time ochre yellow in the air, being changed into ferric hydroxide. After
drying and calcining, a fine red powder of pure ferric oxide is formed. The
shade depends on the temperature at which the substance is calcined: the
higher the temperature and the longer the heating is continued, the darker is
the product.
Vogel’s Iron Red.—This preparation, which is particularly brilliant and
therefore highly suitable for an artists’ colour, is made by adding a saturated
solution of oxalic acid to a boiling solution of green vitriol. The greenish
yellow precipitate of ferrous oxalate is collected on a filter and well washed
with water. After drying, the precipitate is heated in a shallow iron dish to a
temperature of 200° C., at which the ferrous oxalate decomposes and is
converted into a soft fiery red powder consisting of pure ferric oxide. By
igniting this powder in covered crucibles the different shades of ferric oxide
can be obtained.
Macay’s English Red.—Seven hundred and four parts of ferrous
sulphate, 1,000 parts of copper chloride, and 1,678 parts of common salt are
dissolved, the solution boiled and the precipitate ignited.
In the manufacture of fuming or Nordhausen sulphuric acid, ferric oxide
is obtained as a residue. It is then known under the names of English red,
caput mortuum, colcothar, rouge and Indian red as a very cheap pigment. It
is also used as a polishing material. Fuming sulphuric acid is made by
heating green vitriol at a white heat in retorts placed in furnaces; sulphur
dioxide and trioxide are evolved, whilst in the retorts there remains a
residue of almost pure ferric oxide, containing small quantities of basic
ferric sulphate, which can only be decomposed by long continued violent
heating. The vapours of sulphur trioxide are caught in receivers containing
oil of vitriol, in which they dissolve and produce fuming sulphuric acid.
The residue in the retorts, which has only a low commercial value, can
be converted into a good pigment without the expenditure of much money
or labour. It is ground in mills as finely as possible, and, if necessary,
afterwards levigated. The fine powder is mixed with varying quantities of
common salt, the object of which is to prevent the temperature from rising
too high in the calcining process. Common salt is volatile at a temperature
approaching a strong red heat; when the temperature has once risen so far, a

further rise is prevented by the heat taken up in volatilising the salt. In order
to make ferric oxide of a yellow tinge, 2 per cent. of salt are added, and the
mixture heated with a moderate fire for 1 hour. To obtain the deeper shades,
the addition of common salt is increased even to 6 per cent. For a brownish-
red oxide, 4 per cent. of salt are added and the mixture heated for 4 hours;
for a dark violet oxide, 6 per cent. of salt are used, and the mixture is heated
for 6 hours with the fiercest fire.
It has been observed that the shade of the product is finer the more
completely air is excluded from the glowing mass and the more quickly the
product, after sufficient heating, is cooled down to the ordinary
temperature.
Fire-clay tubes are used for heating the ferric oxide, which are similar to
gas retorts, and are built one above the other in furnaces. The number in one
furnace may reach 60. Each retort is closed by a well-fitting lid, which is
luted with clay after the retorts are filled, a small opening being left through
which the heated air may escape. If the retorts were closed completely air-
tight they would burst on heating.
Ferric oxide pigments are made in very large quantity by several works,
which bring up to 20 different shades, varying from reddish yellow to dark
violet, into the market.
On account of their great resistance to the action of the atmosphere and
of chemical agents, the ferric oxide pigments are particularly suitable for
painting iron and other metals which are exposed to air or water. They are
also suited to fresco work.
Lower qualities of iron reds are made by calcining ochres, large deposits
of which occur in nature, and also from the residues of basic ferric sulphate
obtained in the alum manufacture. These varieties are at the best usable for
ordinary painting, but never for artists’ pigments.
According to the method of Steinau, iron pigments can be made from
wrought-iron scraps, turnings, etc., by causing them to rust through
alternate contact with air and water. The resulting ferric hydrate is either at
once used as a pigment or converted into other shades by calcining. By
heating in the air a red pigment is formed; by heating with coal in the
absence of air a black; and from mixtures of red and black different shades

of brown. If the iron turnings can be obtained cheap, this process should be
well adapted to the manufacture of fine iron pigments.
Indian Red consists of ferric oxide. It was originally obtained from a
very pure hæmatite, occurring in India, by grinding and levigating. It can,
however, also be artificially made, in shades varying between bright red and
dark brownish-red, by heating pure ferric oxide. This valuable pigment is
extremely brilliant and durable.
The darkest brownish-red shades can be obtained from Indian red by
mixing it with varying quantities of litharge, and heating the mixture very
strongly in a covered crucible.

CHAPTER XX.
OTHER RED MINERAL PIGMENTS.

Chrome Red or Chrome Vermilion.—Under the most varied names,
chrome orange, Persian red, Derby red, Chinese red, Indian red, chrome
vermilion, etc., there occur in commerce numerous pigments, orange to
dark red in colour, which can all be made from neutral lead chromate. When
this salt, i.e., ordinary chrome yellow, is treated with smaller or larger
quantities of a strong base, such as caustic potash or caustic soda, “basic”
lead chromates are formed, which exhibit a more intense red shade the more
lead they contain. These compounds can be made either by adding caustic
potash to the solution of potassium chromate used for making the chrome,
or the precipitated chrome yellow may be treated with caustic potash. The
latter process readily yields good results, so that it is generally followed.
Recent researches have shown that differences in the shade of chrome
red are due to the varying size of the single microscopic crystals of which it
consists. When chrome reds of the most different shades are ground the
powder has almost the same colour. The larger are the crystals the deeper is
the shade of the red, so that the art of making deep chrome red lies in
working so as to produce large crystals.
As was said in describing the preparation of chrome yellow, the lead
liquors used in its manufacture contain varying quantities of acetic acid,
which affect the shade of the product. The quantities of caustic potash
necessary to turn a certain quantity of chrome yellow into chrome red of a
certain shade can, therefore, only be estimated from time to time in an
empirical manner. This estimation may be performed by the method of
Habich with great certainty. The precipitate of chrome yellow is made in the
ordinary way, and well washed 6 to 8 times. Equal portions of the paste are
brought into vessels of equal height and diameter. To the first of these
portions caustic soda solution of a certain strength is added; to the second
rather stronger caustic soda solution; in each succeeding vessel the strength
of the solution is increased to a known extent. After the addition of caustic
soda the vessels are energetically shaken. They are then set aside for the

precipitate to settle in a place completely free from vibration. In one of the
vessels, an orange or red shade will be seen very similar to that it is desired
to make. Now, it is known how much caustic soda solution of a certain
strength has been added to this quantity of colour, so that the quantity
necessary for the whole of the chrome yellow precipitate may be found by a
simple calculation. In working on the large scale, the process is exactly the
same as in the preliminary test; the calculated amount of caustic soda
solution is added to the precipitate in the tub, it is well stirred, and left at
rest for some hours. When the proper shade appears, the liquid above the
precipitate, which contains sodium chromate in proportion to the strength of
the caustic soda solution used, can be drawn off and used to precipitate
fresh lead acetate solution. The precipitate is washed first in the tub and
then on the strainers.
Since, as has been said, the depth of colour depends upon the size of the
single crystals, care should be taken in washing the precipitate not to agitate
it too vigorously, or the pigment will lose in depth by the breakage of a
large number of crystals.
Special formulæ for the manufacture of chrome red are abundant, but if
the above process be followed they are superfluous, and it is only for the
sake of completeness that one or two are given. Deep chrome red is made
by the action of 25 parts of caustic soda on 100 parts of pure chrome
yellow.
A noteworthy process is due to Liebig. Saltpetre is melted in a crucible,
and heated to a temperature below that at which oxygen is evolved. Dry
chrome yellow is introduced so long as effervescence occurs; the fused
mass appears, whilst liquid, deep black. As soon as it is in quiet fusion it is
poured on a cold plate. In consequence of the rapid cooling it becomes
brittle and can be easily powdered. The hot mass is broken up and boiled
with water, potassium chromate going into solution. The chrome red made
by this process is little inferior in brilliance to vermilion.
According to Prinveault a handsome chrome red is obtained by treating
25 grammes of neutral lead carbonate with 10 grammes of yellow
potassium chromate dissolved in 5 litres of water, boiling for half an hour,
washing the violet precipitate, and finally treating with 1 gramme of
sulphuric acid diluted with 100 grammes of water.

Cobalt Red consists of cobalt phosphate. It was first recommended by
Salvetat. It is obtained by precipitating a solution of a cobalt salt by sodium
phosphate. After drying at the ordinary temperature the precipitate has a
beautiful rose-red colour. By careful heating, the shade becomes more
violet according to the temperature used.
Cobalt-Magnesia Red.—When magnesia is moistened with the
solution of a cobalt salt and strongly heated, a rose-red mass is obtained
which can be used as an extremely durable artists’ colour. It probably
consists of a compound of cobalt oxide with magnesia. The method of
preparation is as follows: Magnesium carbonate is mixed to a thin paste
with a dilute solution of cobalt nitrate; this is heated and stirred until quite
dry, when the residue is strongly heated in a covered crucible. In the
manufacture of this and other cobalt pigments it is of particular importance
to prevent the entry of the fire gases into the crucible. Unless this is done a
product of good colour is not obtained.
Cobalt Arsenate.—By precipitating a cobalt salt with sodium arsenate,
a violet-red precipitate is obtained which leaves a fine durable red on
heating. The mineral erythrite has a similar composition and colour. This
pigment has a bright shade, is permanent, and although it contains arsenic is
not particularly poisonous, since after fusion it is little soluble.
Chromium Stannate.—This artists’ pigment, which may be used in oil
and porcelain painting, is made in the following manner (Gentele): 1
kilogramme of tin is converted by strong nitric acid into metastannic acid,
50 grammes of potassium chromate are dissolved in 1 litre of water, the
solution mixed with 2 kilogrammes of chalk and 1 kilogramme of powdered
quartz, and finally with the metastannic acid. The mixture is dried and
strongly heated. The temperature must be raised to a white heat, at which
the mass sinters and acquires a dark rose-red colour. It is then completely
extracted with boiling water, when a fine powder is left.
This pigment, which is known in the market as “pink colour,” may be
used with advantage in oil painting in the place of rose madder lake, over
which it has the advantage in durability.
Silver Chromate.—A solution of silver nitrate gives, with potassium
chromate, a deep red precipitate of silver chromate, which is occasionally
used as an artists’ pigment under the name of “purple red”. Not only is the

price of this pigment high, but it has little durability, since, like all silver
compounds, it is acted upon by sulphuretted hydrogen and turned dark,
owing to the formation of black silver sulphide.

CHAPTER XXI.
PURPLE OF CASSIUS.

Purple of Cassius, or gold purple, is not a pigment for painters’ use; it is
employed for colouring glass, for porcelain painting and for coloured
glazes. For these last purposes we know nothing which can completely
replace purple of Cassius, so that its preparation will be described here. Its
costliness prevents its general use.
As regards the chemical constitution of purple of Cassius very diverse
views are held. Chemists have not yet succeeded in explaining it; some
regard it as aurous stannate, others as stannic acid, in which a particular
form of gold of a red colour is contained in an extremely finely divided
condition. There are reasonable grounds for both opinions. Theoretical
considerations are of little interest to the manufacturer, who is chiefly
interested in the method by which he can prepare a product which satisfies
the demands of the consumer.
Purple of Cassius has been known for nearly two hundred years, hence
there are many formulas for its preparation. Before these are enumerated it
is necessary to give an account of the conditions under which this substance
is formed. In order to obtain gold purple, a solution is required which
contains both stannous and stannic chlorides; if this solution is mixed with a
very dilute solution of gold chloride a precipitate is obtained which is
generally brownish red, and only acquires its fine red shade on igniting. The
colour shown by the purple after precipitation is no guide to the shade of
the finished product; a purple which shows a very fine colour whilst wet
often produces a pigment of much less beauty than an actually ugly
precipitate. These differences are to be ascribed to the varying molecular
condition of the purple, for the different varieties show very small
differences in chemical composition.
According to the directions of Fuchs, a fine purple is obtained by
mixing a solution of stannous chloride with sufficient ferric chloride
solution to give the mixture a green colour. Part of the ferric salt is then
reduced to ferrous salt, whilst the stannous chloride is partially oxidised. At

the same time a solution of gold chloride is made; this must be quite free
from nitric acid and contain 1 part of the salt in 400 to 500 parts of water.
The tin solution is added drop by drop to the gold solution with constant
stirring; the mixture becomes turbid, but the precipitate requires a long time
to settle on account of its fine state of division. The depth of colour shown
by the purple varies according to the strength of the solutions.
M. Müller states that the following process gives the best results: The
quantity of stannic chloride equivalent to 9 grammes of stannic acid is
dissolved in 200 cubic centimetres of water; potassium carbonate is added
to alkaline reaction, then 1 gramme of gold in the form of chloride. Grape
sugar is added to the mixture, which is made up to 300 cubic centimetres by
water, and warmed until the brightest shade is reached. When a very
gelatinous mass is formed after the addition of the potassium carbonate it is
heated for a short time before adding the gold solution and grape sugar.
Wächter gives directions for obtaining pale and deep shades. To obtain
the pale purple 5 grammes of tin are dissolved in aqua regia, the solution
evaporated to dryness on the water bath, the solid residue mixed with a
solution of 2 grammes of stannous chloride and dissolved in 10 litres of
water. To this liquid is added a solution of 0·5 gramme of gold in aqua
regia, and immediately 50 grammes of ammonia to neutralise free acid. The
precipitate separates spontaneously from the dark red liquid; its formation
may be hastened by adding sulphuric acid. The washed precipitate is mixed
whilst moist by means of a silver spatula with 20 grammes of lead flux, 2
grammes of red lead, 1 gramme of quartz sand, and 1 gramme of calcined
borax. The mixture is then dried; it produces a purple red colour. If 3
grammes of silver carbonate are added, a clean, pale purple pigment results.
The deep purple is made by mixing a solution of 0·5 gramme of gold in
10 litres of water with 7·5 grammes of stannous chloride solution (specific
gravity, 1·7) whilst stirring, and adding a few drops of sulphuric acid. The
washed precipitate is mixed with 10 grammes of lead flux and 0·5 gramme
of silver carbonate.
A rose-red shade of gold purple is obtained by dissolving 1 gramme of
gold and mixing the solution simultaneously with solutions of 50 grammes
of alum in 20 litres of water, and of 1·5 gramme of stannous chloride
(specific gravity, 1·7). Ammonia is now added until all the alumina is

precipitated. In order to prepare from the dried precipitate the mixture
which will produce the colour on fusion, it is mixed with 50 grammes of
lead flux and 2·5 grammes of silver carbonate.
Magnesia Gold Purple.—According to M. Müller, the colour of gold
purple is produced by covering the particles of a very finely divided white
substance with metallic gold. Thus, when calcined magnesia is stirred up
with water, gold chloride solution added, and the mixture warmed to 100°
C., the gold is precipitated upon the magnesia; the wet, yellow powder
acquires a reddish shade on drying, which at a red heat becomes so
beautiful a carmine-red that it surpasses purple of Cassius in fineness of
shade. A purple which contains 20 grammes of gold as chloride to 84
grammes of magnesia has the most pure carmine tint; the shade varies with
the proportion of gold to magnesia. The different shades of purple contain
the following percentages of gold:—
Percentage of Gold. Shade of the Purple.
33·5 Brownish-red (excess of gold).
25·0 Deep carmine-red.
20·0 Medium carmine-red.
10·0 Pale carmine.
5·0 A good rose-red.
3·0 Rose-red.
1·0 Pale rose.
0·2 Delicate rose.
0·1 Appreciable red tint.

Alumina Gold Purple is obtained, according to Müller, by adding gold
chloride and excess of potassium carbonate to alum solution and boiling.
The purple obtained in this way is as deep in colour with 10 per cent. of
gold as a magnesia purple with 20 per cent. of gold, but has a more bluish
violet shade. If, in the preparation of purple, the alumina is precipitated by
potassium carbonate and the gold chloride reduced by grape sugar, a
different shade of purple is obtained to that in the precipitation of which
ammonia is used. The alumina gold purple is specially adapted for the
production of porcelain enamels.

CHAPTER XXII.
BLUE MINERAL PIGMENTS—
IRON-CYANOGEN PIGMENTS.

The pigments occurring in commerce under the names of Prussian,
Chinese, Berlin or Paris blue consist, when pure, of ferric ferrocyanide,
Fe₄[Fe(CN)₆]₃. This compound is obtained as a deep blue precipitate when
yellow prussiate (potassium ferrocyanide) is mixed with the solution of a
ferric salt.
The precipitate shows certain differences in composition according to
the exact method by which it is obtained. A different result is obtained when
the solution of the ferric salt is poured into that of a ferrocyanide, to that
produced by the reverse procedure, the precipitate produced in the one case
has properties which it does not possess in the other. Similar differences
often occur in precipitating pigments.
If the solution of the ferrocyanide is poured into the iron solution, the
latter being in excess, an insoluble precipitate is formed; but if the iron
solution is poured into the ferrocyanide solution of which an excess is
present, the blue precipitate is formed, but it is soluble in pure water, though
not in water containing salts. Thus when this precipitate is separated from
the liquid and washed, the washings are at first quite colourless and remain
so whilst salts are present, but when these have been completely washed
away, a fine blue solution of the ferric ferrocyanide is formed, from which
the addition of a salt solution again separates the dissolved Prussian blue.
Before proceeding to the manufacture of Prussian blue on the large
scale, it is necessary to consider the behaviour of ferrous salts towards
yellow prussiate, since these are commonly used instead of ferric salts.
When a solution of potassium ferrocyanide is mixed with a solution of a
ferrous salt, there is formed a pure white precipitate of potassium ferrous
ferrocyanide, K₂FeFe(CN)₆. In order to obtain a completely white
precipitate, it is necessary that the ferrous salt should be completely free
from ferric salts and that the solutions should contain no dissolved oxygen;

the solutions would therefore have to be boiled before mixing. If one of the
solutions contains the smallest quantity of oxygen a bluish precipitate is
formed. When the white precipitate of potassium ferrous ferrocyanide is
exposed to the air it immediately acquires at the surface a dark blue colour,
and is completely changed to Prussian blue by sufficiently long contact with
air.
In accordance with this behaviour of ferrous and ferric salts different
methods may be used to obtain Prussian blue. Either the process is
commenced with a ferric salt from which a precipitate of Prussian blue is at
once obtained, or the solution of a ferrous salt is converted into ferric salt
by a powerful oxidising agent, such as chlorine or nitric acid, or a solution
of a ferrous salt is precipitated with yellow prussiate and the white or bluish
precipitate, which consists chiefly of potassium ferrous ferrocyanide,
converted into ferric ferrocyanide or Prussian blue by treatment with nitric
acid.
Prussian blue is also known, as we have said, as Chinese blue, Berlin
blue and Paris blue; Brunswick blue or mineral blue is another form.
Generally the product under the name of Chinese or Paris blue, which has
an intense dark blue colour, is quite pure, whilst under the names of
Prussian blue and Berlin blue mixtures with starch or alumina are included,
which consequently have a more or less pale colour. The pigments known
as mineral or Brunswick blue are the least valuable, they have generally a
less deep colour and are quite without the metallic lustre which
distinguishes Chinese blue and the better varieties of Prussian blue. Mineral
blue often contains very large quantities of alumina, chalk, or even barytes;
the addition of the last is to be regarded as irrational, since it makes the
pigment conspicuously heavy.
Chinese Blue.—Pure Chinese blue appears in the form of deep blue
masses, characterised by a peculiar metallic lustre, which is especially
marked when the surface of a lump is rubbed with the finger nail. This
metallic lustre is accompanied by a copper-red shimmer which is similar to
that exhibited by fine indigo. In large pieces pure Chinese blue appears very
dark by artificial light; it possesses enormous strength of colour. The
following test serves for the recognition of the complete purity of Chinese
blue: A small quantity is powdered and rubbed in a thin layer on white
paper; if the metallic lustre shows in undiminished strength the blue may be

regarded as quite pure, for Chinese blue quickly loses this property by
additions of other substances.
From what has been said above concerning the different behaviour of
ferrous and ferric salts, the simplest method of making Chinese blue would
be to precipitate the solution of a ferric salt with yellow prussiate, keeping
the ferric salt in excess, to wash and dry the precipitate. Ferric salts are,
however, dearer than ferrous salts, so that the latter are generally used.
In making Chinese blue from a ferrous salt ferrous sulphate or green
vitriol is generally taken. This salt is dissolved in water; it is advisable to
add a small quantity of sulphuric acid, since the water will contain
carbonates, which produce ferrous carbonate. Air oxidises the latter to ferric
hydroxide, which is brown, and if present even in small quantity will spoil
the shade of the blue. This addition of sulphuric acid is of special
importance when the white precipitate produced by the prussiate solution is
to be oxidised by the air.
A solution of yellow prussiate is added to the iron solution in such
quantity that a very small excess of iron salt is left; there results a white or
rather, since the ferrous sulphate always contains small quantities of ferric
oxide, a pale blue precipitate. This is allowed to settle, the liquid drawn off
as completely as possible from the precipitate, and nitric acid then added, or
a solution of bleaching powder, followed by the amount of sulphuric acid
required to decompose it. The nitric acid or the chlorine liberated from the
bleaching powder speedily effects the oxidation, changing the pale blue
colour to the deep blue of Chinese blue. The precipitate should be left for
several days in contact with the liquid in order to complete the oxidation; it
is then well washed and dried. Pieces of definite shape and size bearing the
trade mark of the firm are often pressed from the blue when it has acquired
a stiff consistency.
There are many formulæ for the preparation of Chinese blue, differing
in regard to the quantities of green vitriol, yellow prussiate, and nitric acid
or bleaching powder to be used. It is, however, clear that only one formula
can be correct, that in which the quantities of materials are equivalent, since
the reactions take place between equivalent quantities. In practice the
equivalent quantities would not be weighed to the tenth of a gramme—there
would be no object in such a proceeding, since in works chemically pure

substances are not used; but manufacturers should work according to
equivalent quantities in order to use up their materials as completely as
possible. No increase in labour is involved, the same labour weighs one or
another quantity.
The proportion of materials in which the least loss is involved is here
given. Nine kilogrammes of green vitriol are dissolved in 100 litres of
water, 15 kilogrammes of strong sulphuric acid are added, and then a
solution of 15 kilogrammes of yellow prussiate in 100 litres of water. The
solutions are kept in constant motion during the mixing. As soon as the
decomposition is finished, without waiting for the precipitate to settle,
steam is led in and 20 kilogrammes of nitric acid of 1·3298 specific gravity
added in small portions. The heating by direct steam is continued until red
vapours are no longer evolved from the liquid, which is a sign that the
oxidation is finished.
Although the use of direct steam is very convenient for heating, since
wooden vessels can be used and the heating quickly accomplished, it may
be dispensed with, and the solutions heated in pans or even not at all. In this
case the oxidation lasts considerably longer than when the liquid is heated.
In the process recommended by Gentele 109 parts of yellow prussiate are
used to 20 parts of green vitriol, each dissolved in much water. The
precipitate is heated for a short time with 51 parts of nitric acid of 1·2285
specific gravity and 16 parts of sulphuric acid. The mixture is allowed to
stand for several days to complete the oxidation before the precipitate is
separated from the liquid.
In Hochstätter’s method, solutions of 6 parts of yellow prussiate in 15
parts of water and of 6 parts of green vitriol in 5 parts of water are mixed.
To the precipitate 24 parts of strong hydrochloric acid and 1 part of
sulphuric acid are added, and then a solution of bleaching powder in 80
parts of water until the liquid smells distinctly of chlorine. Apart from the
fact that this method does not employ equivalent quantities of iron salt and
yellow prussiate, it is also to be regarded as unsuitable, since the addition of
sulphuric acid produces calcium sulphate, which, being soluble with great
difficulty, will be precipitated along with the blue, making the shade lighter.
The addition of sulphuric acid should be omitted and only hydrochloric acid
used, the calcium chloride formed by the decomposition of the bleaching
powder being very soluble in water. Many formulæ given for the

manufacture of Chinese blue are said to produce a pigment of special
beauty. This, however, is incorrect. Chinese blue is a compound of fixed
composition, which is always formed when equivalent quantities of the salts
are used and the precipitate converted into the blue compound by one of the
oxidising agents given. It is most simple to use nitric acid as the oxidising
agent; for this purpose the presence of chlorine in the acid is harmless.
When sufficient space and time are allowed, a special oxidising agent may
be omitted and the oxidation of the white precipitate accomplished by
atmospheric air.
In working in this manner the precipitate is washed several times with
water, and spread in a thin layer upon boards which are placed in rows one
above the other. In a short time the mass turns blue on the surface, but the
pale colour remains underneath much longer, since the access of oxygen to
the interior is difficult. The boards must be left until the mass has a uniform
colour throughout. In order that the paste shall not dry too thoroughly whilst
on the boards it is desirable to make it very fluid. If the precipitate has been
completely washed before the oxidation the product is finished after drying,
but if a portion when completely dried shows a crystalline incrustation, salts
are still present, which must be removed by careful washing.
Prussian Blue.—As we have already stated, Prussian blue is identical
in chemical nature with Chinese blue, from which it differs only by certain
additions which are made in order to reduce the price. Prussian blue should
therefore be regarded as a more or less impure Chinese blue; less pure
materials are used in its preparation than for the fine Chinese blue. Instead
of pure potassium ferrocyanide, the crude salt, which is sold by the
prussiate makers at a lower price, is used for Prussian blue; in addition to
the ferrocyanide this crude salt contains a considerable proportion of other
salts, including a quantity of potassium carbonate.
The ferrous sulphate used for Prussian blue is mixed with alum, which
neutralises the potassium carbonate, precipitating alumina, by which the
precipitate formed from the yellow prussiate and ferric salt is made paler. If
the pale blue precipitate were treated with acids they would first dissolve
out the alumina; this should be avoided, since the alum has been added with
the intention of increasing the quantity of precipitate and making the
pigment paler. The precipitate can therefore only be changed into the blue
pigment by spontaneous oxidation; this is accomplished by the oxygen of

the air in the manner previously described. The oxidation is more rapid
according to the quantity of ferric salt contained in the green vitriol at the
commencement; it is convenient to prepare a large quantity of green vitriol
solution and expose it in shallow vessels to the air so that the oxidation may
proceed. But if the solution of green vitriol were exposed alone to the action
of the air a rust-coloured precipitate of basic salt would be deposited at the
bottom of the vessels. To prevent the formation of this basic salt, sulphuric
acid is added in small quantity, and, as the oxidation proceeds, further small
quantities of acid are added. In this way a solution of ferric sulphate is
slowly formed, which gives with yellow prussiate a blue precipitate
requiring little further oxidation.
Mineral Blue, Brunswick Blue.—This pigment, the most reduced of
the colours of this class, is obtained by mixing white pigments, barytes,
pipe clay, etc., with the already impure Prussian blue.
Soluble Prussian Blue.—The modification of ferric ferrocyanide easily
soluble in water is obtained by adding a solution of a ferric salt to a solution
of yellow prussiate, the latter being in considerable excess. Brücke gives the
following method: a solution is prepared of 217 grammes of yellow
prussiate in 11 kilogrammes of water. Another solution contains 100
grammes of ferric chloride in 1 litre, 1 litre of which is mixed with 2 litres
of a saturated solution of Glauber’s salt and the mixture added to the
prussiate solution so long as a blue precipitate forms. The Glauber’s salt
plays no part in the formation of the colour; it is added to prevent the
solution of the precipitate, which is insoluble in salt solutions. The
precipitate is brought on to a filter and washed with water until the
washings begin to be blue, which is a sign that the salts have been almost
completely removed. The precipitate cannot be further washed without the
loss of a large quantity which would be dissolved by the water; without
further washing it is slowly dried in the air.
Soluble Prussian blue has but a limited application; anatomists use its
solution to inject specimens, and it was formerly used for blue writing inks,
for which purpose aniline dyes are now preferred. It may also be remarked
that ordinary Prussian blue readily dissolves in a solution of oxalic acid; the
solution was also used as a writing ink, but since it attacks steel pens, it has
now been replaced by solutions of dyes which have no action on steel.

Special Processes for the Manufacture of Chinese Blue.—In place of
chlorine or nitric acid manganic chloride can be used to oxidise the white
precipitate obtained from yellow prussiate and green vitriol. This process is
based upon the property of manganic chloride to readily give up part of its
chlorine and be changed into manganous chloride (the ordinary chloride of
manganese). The conversion of the white precipitate into the blue is thus
also in this case indirectly effected by chlorine.
Many varieties of pyrolusite contain considerable quantities of
manganic oxide which are of little use in the manufacture of chlorine, for
which pyrolusite is chiefly employed. If such pyrolusite be digested with
crude hydrochloric acid for several days, the manganic oxide is dissolved
with the production of manganic chloride.
The oxidation of the white precipitate is performed in the usual way;
after running off the liquid an excess of manganic chloride solution is
added, as soon as two tests taken at an interval of a few minutes show no
further increase in the depth of the colour, the reaction is ended and the
solution of manganous chloride is drawn off.
Any powerful oxidising agent may be used to oxidise the white
precipitate; chromic acid has been proposed for this purpose, but up to the
present the chromates are too dear to be used in place of the cheap oxidising
agents previously mentioned.
In regard to the present endeavours to utilise by-products of the
chemical industries, a method for preparing this pigment may be mentioned
which would certainly be very profitable on the large scale. Coal gas before
purification contains some quantity of ammonium cyanide, which is
decomposed by ferric oxide with the production of Prussian blue. This
pigment might be made by passing coal gas over very finely divided ferric
oxide, until it was completely converted into Prussian blue. The process is,
however, not practicable on the large scale: the ferric oxide would have to
be exposed in an extremely thin layer to be completely converted into
Prussian blue, and the transformation would require a long time. A better
process is to make yellow prussiate instead of Prussian blue. A mixture of
ferric oxide with coarse sawdust is spread out in thin layers, over which the
impure coal gas passes. When the mass has acquired a blue colour owing to
the Prussian blue produced, it is treated with a mixture of quicklime and

potassium sulphate. Potassium ferrocyanide is formed; it is obtained in
crystals by the evaporation of the solution and can be used in the
preparation of Prussian blue.
The “Laming’s mass,” now generally used in gas works, contains, after
use, a considerable quantity of cyanogen compounds, and it would only
require a short trial to decide whether it was more advantageous to make
this mixture by exposure to air again suitable for the purification of coal
gas, or to use it to obtain yellow prussiate.
Turnbull’s Blue.—When a solution of a ferrous salt is precipitated by
potassium ferrocyanide, a fine blue precipitate is formed, identical in
physical properties with Chinese blue, but differing in chemical
constitution. It is ferrous ferrocyanide, Fe₃[Fe(CN)₆]₂. The manufacture of
Turnbull’s blue is expensive; the pigment has no specially advantageous
properties and is therefore seldom made.
In preparing this pigment, a solution of yellow prussiate is transformed
into red prussiate by passing chlorine through it so long as it is absorbed by
the liquid; the mother liquors from which red prussiate has been crystallised
may be used with advantage. When to the solution of red prussiate green
vitriol solution is added so that the former remains in excess, a soluble
Turnbull’s blue is obtained, but when excess of green vitriol is used the
insoluble form is produced.
Chinese blue and Turnbull’s blue are used both as oil and water colours,
and these handsome but not particularly durable pigments are also
employed in colouring wall papers.
Antwerp Blue is a mixture of Prussian blue with zinc cyanogen
compounds. It is obtained by precipitating a solution of 2 parts of zinc
sulphate and 1 to 2 parts of green vitriol (according as pale or deep blue is
required) by a dilute solution of yellow prussiate.

CHAPTER XXIII.
ULTRAMARINE.

At the present time blue, green, red and violet pigments come into the
market under the name of ultramarine. The green and blue have been
commercial articles for about seventy years; the violet and red were
introduced about the year 1860.
At first the name ultramarine was restricted to a natural blue pigment
obtained from lapis lazuli, which was extremely costly. Accounts of the
payments of Italian artists, still extant, show the expensive nature of the
ultramarine blue used in their paintings. At that time, when artists were
compelled themselves to make the majority of their pigments, ultramarine
was made in a most laborious manner from lapis lazuli, for which
incredibly high prices were paid. In order to make the mineral easier to
powder the lumps were heated, and, whilst hot, thrown into water. They
were then powdered as finely as possible. The powder was mixed with
melted resin, and the mixture kneaded under water for a long time. The
ultramarine suspended in the water by this crude method of levigation was
obtained by allowing the wash waters to settle. Few places are known at
which lapis lazuli occurs in quantity. It is chiefly obtained in China and
Thibet, and, considering the little intercourse between Europe and these
distant countries at that time, it is no wonder that the price of ultramarine
was fabulously high. One ounce cost about £8—a price which is explained
by the small yield of ultramarine from the best lapis lazuli. By the most
careful work not more than from 2 to 3 per cent. of the mineral was
obtained, the residue consisted of foreign minerals.
The enormously high price of this pigment was the stimulus for the
endeavours to make it artificially. The attempts are to be regarded not only
as completely successful, but it must be allowed that science has gone a
considerable step further than nature, since the researches have made it
known that there is not only a blue ultramarine, but also a green, and,
according to the latest researches, there exist in addition violet, red and
white compounds, also to be described as ultramarines. With the discovery

of the methods by which ultramarine can be made artificially, the
preparation of this pigment from lapis lazuli came to an end, and has now
but historic interest. The discovery of artificial ultramarine is due to the
French chemist Guimet and to the great German chemist Gmelin.
Ultramarine was first manufactured in Germany in the year 1828 by A.
Köttig, as a branch of the porcelain works at Meissen in Saxony, where the
manufacture was continued for about fifty years.
The discovery of the aniline dyes is rightly called a triumph of human
intellect. The artificial manufacture of ultramarine deserves the same
description in no less degree, although it has not effected so great a
revolution as the former.
Attempts to make ultramarine artificially would naturally be based on
the analysis of the natural product. The following comparison of the
compositions of natural and artificial ultramarines shows how nearly the
artificial product approaches the natural:—

NATURAL ULTRAMARINE.
Clément and Desormes. Gmelin.
Silica 35·8 per cent. 47·31 per cent.
Alumina 34·8 ” 22·00 ”
Soda 23·2 ” 12·06 ”
Lime 3·1 ” 1·55 ”
Sulphuric acid — ” 4·68 ”
Sulphur 3·1 ” 0·19 ”
Water and organic matter — ” 12·21 ”

ARTIFICIAL ULTRAMARINE.
Blue. Green.
Observer. Warrentrapp. Elfen. Brunner. Pohl. Gentele.
Silica 45·60 40·0 2·54 36·67
47·31
Alumina 23·31 29·5 25·25 32·12
Soda 21·47 23·0 16·91 21·45 39·93
(sodium silicate)
Potash 1·75 — — — 3·92
Lime 0·02 — 2·38 — 1·13
Sulphuric Acid 3·83 3·4 — 2·08 —
Sulphur 1·69 4·1 11·63 7·22 6·62
Iron 1·06 1·0 2·25 trace 1·95
(ferric silicate)

Blue. Green.
Observer. Warrentrapp. Elfen. Brunner. Pohl. Gentele.
Water — — — — —
Oxygen — — 9·04 0·58 —

I. Szilasi found three samples of green ultramarine to have the following
composition:—
Water 2·20 1·20 1·19
Aluminous residue 1·80 1·42 1·41
Silica 16·73 17·18 16·74
Aluminium 15·92 15·87 16·15
Sodium 18·42 18·18 18·10
Sulphur 7·19 6·97 6·85

There are many other analyses in addition to those we have given and
agreeing with them, so that there is no doubt as to the composition of
ultramarine, but as to the manner in which the elements are grouped nothing
is definitely known. Some chemists are of the opinion that the colouring
principle of ultramarine is a sulphur compound of iron, whilst others oppose
this view and consider that the colour is due to the combination of a double
silicate of alumina and soda with an unknown sulphide of sodium. Although
no blue or green compound is known of corresponding composition, the
majority of chemists incline to the latter view. Experience has shown that
the presence of iron in any of the materials used in the manufacture of
artificial ultramarine is very dangerous to the success of the operation, and
at the least considerably injures the beauty of the product.
Although the manufacture of ultramarine is now very well known, it
cannot be denied that some works produce a pigment of a shade which
cannot be obtained by others. These works keep their method very secret, so
that it is not possible to say with certainty whether they have introduced a
process varying from that commonly known, or whether, by carefully
watching the process, they have achieved great technical dexterity in the
manufacture of this product. The latter appears the most probable, for in
order to obtain a good result, many experiments, and an accurate knowledge
of the raw materials, are necessary.
The raw materials used are as follows: pure aluminium silicate, sodium
sulphate, soda, sulphur, coal. The aluminium silicate is used in the form of
fine china clay or kaolin, sodium sulphate and soda must be used in the

anhydrous form, the sulphur is the ordinary commercial substance purified
by distillation. Charcoal or coal containing little ash can be used.
Whilst the remaining raw materials are always of similar composition,
the china clay from different localities possesses a very varying
composition. This substance must be carefully chosen. There is hardly any
kaolin which is naturally of sufficient purity to be used without purification;
it is well known that the china clay used for porcelain is subjected to a
thorough preparation before it is used. Kaolin, like all clays, has been
produced by the decomposition of feldspar; when the aluminium silicate so
formed was able to deposit without foreign admixtures, that mineral was
formed which is the purest of all clays and is called kaolin. The more
foreign substances are mixed with the aluminium silicate the further is the
clay removed from kaolin. The impurities which generally accompany the
aluminium silicate are quartz sand, chalk and ferric oxide. We distinguish
accordingly between kaolin, white clay or pipe clay, clay, and lastly marl, a
clay containing much chalk.
Even the purest kaolin contains certain impurities, of which quartz sand
is the principal and the least harmful. Before kaolin can be used in the
ultramarine manufacture it must always be purified by levigation; it is then
ignited at a low temperature and powdered under stamps or in mills. The
other materials required are generally produced by the chemical works in a
condition of such purity that they can be at once used.
Occasionally Glauber’s salt contains iron, which would spoil the shade
of the ultramarine. The iron can be easily removed by dissolving the
crystallised salt in water, adding a little milk of lime and leaving the liquid
for several days, stirring frequently. The lime neutralises every trace of free
acid, and at the same time produces an ochre-yellow precipitate of ferric
hydrate. The solution of Glauber’s salt thus freed from iron is evaporated in
reverberatory furnaces, in which the salt is then calcined. This operation is
not only simpler than evaporating the solution in iron pans to crystallisation
and subjecting the dried salt alone to calcination, but it also guards against
fresh contamination by iron, which might be caused by the use of iron
evaporating pans.
The sulphur and coal are employed in a soft powder, which is most
readily obtained by placing the coarsely powdered materials in rotating

drums containing a number of iron balls. By continued rotation the
materials are converted to any desired degree of fineness without the
production of dust. The powder is then put through fine sieves by which the
larger particles are retained.
The proportions in which the raw materials are mixed vary within
certain limits; fixed quantities can only be given for a kaolin of definite
composition. Definite formulæ are known by which ultramarine is made,
but these can only be regarded as approximate. The proportions employed
by French manufacturers differ considerably from those usual in German
works. This variation is chiefly due to the difference in the composition of
the kaolin employed in the two countries.
From the composition of the different mixtures one conclusion may be
drawn with certainty—sufficient sodium is always used to neutralise half
the silicic acid of the kaolin and to form some quantity of sodium sulphur
compounds. In the successful process of the German makers a portion of
the soda unites with the silica during heating. By the action of the coal on
the Glauber’s salt it is reduced to sodium sulphide, which, since sulphur is
present, unites with a further quantity of that element. The sodium sulphur
compounds then unite with the silicates of aluminium and sodium to form a
green compound, which is converted into blue ultramarine by a further
treatment with sulphur in the presence of air.
Instead of using Glauber’s salt, which must always be decomposed in
the first process, the sodium sulphide may be formed by the action of
sulphur on soda in the presence of coal. This procedure is adopted in the
French process. The proportions of the mixture used in different works vary.
If the kaolin employed is assumed to be bisilicate of alumina—a somewhat
arbitrary assumption—the following mixture can be successfully used:—
Anhydrous kaolin 100
” Glauber’s salt 42
” soda 42
Sulphur 60
Coal 13

In working by the French method the following formula is suitable:—
Anhydrous kaolin 100
” soda 100
Sulphur 62
Coal 14

These formulæ are not to be regarded as unalterable. In the different
works such varied mixtures are used that it may be said with truth that each
works has its particular formula for the mixture, the composition of which
depends on the nature of the clay used in the works. The composition of the
mixtures used in works is kept secret as far as possible. The formulæ given
above refer to a clay approximating in composition to the bisilicate of
alumina.
It will appear from the description of the manufacture of ultramarine
that certain quantities of sodium sulphide are produced. The process is,
however, conducted so that sodium sulphide shall be formed; thus the soda
and Glauber’s salt used in the mixture may be replaced by the sodium
sulphide produced in previous operations. The liquors in which the sodium
sulphide is contained are evaporated to dryness, and as much of the residue
added to the clay as corresponds to the quantity obtained from the usual
amounts of soda and Glauber’s salt. Assuming that these latter materials are
pure, 80 parts of the sodium sulphide residue correspond to 100 parts of
soda, and 60 parts to 100 parts of Glauber’s salt.

CHAPTER XXIV.
THE MANUFACTURE OF ULTRAMARINE.

We have thought it necessary to an understanding of the manufacture of
ultramarine to discuss the production of this important pigment in some
detail, for it is only possible to properly conduct an operation regulated by
chemical laws when the processes which take place are accurately known.
The mixture of the materials used in making ultramarine must be most
intimate in order that the constituents may act chemically upon one another.
In some works the mixing is performed in a very laborious manner—upon a
heap of the dried, levigated clay the remaining materials are thrown, and the
whole shovelled about till it is completely homogeneous. Naturally, instead
of this primitive and costly operation, mechanical mixers may be used, but
the mixture can also be effected in a simpler manner, which almost
dispenses with the necessity for a mechanical process. Of the substances to
be mixed only kaolin and sulphur are insoluble in water; the others and also
the by-products of previous processes are easily soluble. The soluble and
insoluble constituents of the charge may be mixed in a very simple way by
bringing the levigated kaolin in the form of paste, without drying, into a
pan, adding the solutions of the salts and the powdered sulphur.
The pans are most conveniently heated by the fire gases from the
ultramarine furnaces. After mixing the solutions with the clay to a thick
paste the heating is commenced, the solid substances are prevented from
sinking to the bottom by continual stirring, the mixture is slowly
evaporated, and an extremely intimate and complete incorporation of the
constituents occurs. The heating is continued until a dry mass is obtained,
which may be used for the second operation without powdering.
The mixture of raw materials is heated for a long time at a clear red
heat, or, in some works, at a white heat. The operation demands some care;
air must be excluded and the whole mass must acquire a uniform
temperature. If both these conditions are not obtained in heating it is very
difficult to obtain a product of uniform colour.

In different works different arrangements are used for the calcining
process. In the older methods crucibles or dishes of fire-clay were used, of
such a shape that the bottom of one crucible formed the lid of the crucible
beneath, just as is the case with the saggers used in porcelain kilns. The
mixture is closely pressed into the crucibles, of which piles are built in the
furnaces. These must be so arranged that each crucible is surrounded by the
fire. The furnaces are very similar to porcelain kilns.
This method of calcining is obviously attended with many drawbacks; a
large number of costly crucibles is required, a certain proportion of which is
lost at each burning, either by breakage in the fire or in filling. The pressing
of the charge into the small crucibles requires much labour, as also the
placing of the crucibles in the furnace and their removal; at the end of the
nine to ten hours’ calcining, it is necessary to wait some time to take the
crucibles out of the furnace, or considerable loss will be caused by breakage
of the crucibles due to rapid cooling.
For these reasons in most works crucibles have been abandoned in
favour of muffles, a large number of which are built in one furnace. The
charge is placed in the muffles; after completion of the calcining it is
quickly raked out, and the still hot muffle at once recharged, thus the losses
of time and heat are reduced to a minimum. The muffles are generally about
1 metre long, 1 metre wide and 30 to 40 centimetres high. They are made of
fire-clay, and, when kept in uninterrupted use, last a long time. The muffles
have a small opening at the back which communicates with the furnace, so
that the gases evolved have a free outlet without reaching the working
place. The front of the muffle is closed by an iron plate in which are small
openings, through which the contents can be examined without the entry of
much air.
Three muffles are generally built in a furnace, but, by suitable
alterations in the construction, a considerably larger number may be heated
in one furnace. Whatever the number of muffles, the furnace must be so
arranged that the fire can be controlled at will. This is best done by a good
damper; all the muffles must be uniformly heated.
J. Curtius recommends cast-iron cylinders, lined with a thin layer of
fire-clay. For this purpose a covering of fire-proof cement is used to protect
the retorts. This covering, as it is gradually destroyed, may be renewed by

smearing over the damaged places with fire-proof cement, mixed with
water or some binding material. Aluminium silicate, graphite and coke may
also be used. The retorts, a (Fig. 28), project through the wall of the furnace
at one end. They rest on fire-proof stone bridges, and can be put into
communication with the air by the pipe, m, which can be closed air-tight.
Short pipes, k and d, which can be closed, connect the interior of the retort
with the cooling and collecting chambers, P and g, the latter of which is
connected by r with the vessel, h, where the gaseous products are absorbed.
The porous plate, f, prevents the charge in the hinder part of the retort from
becoming open and falling to pieces during stoking. In order to obtain a
regular heating of the retorts the fire gases passing through the space, q, are
led away at two opposite places by flues into the main flue, F. The intimate
mixture of finely-powdered materials is introduced into the hinder part of
the retort, the porous plate, f, placed in position, the cover, t, closed up air-
tight, and the connection with the chamber, P, closed. On heating, the
volatile products pass through the porous plate and the pipe, d, into the
chamber, g, the more volatile portions, without mixing with the furnace
gases, passing direct to the absorption vessel, h, or to a lead chamber, whilst
the sulphur which distils over remains in g. When the reaction is finished,
by opening the cover, t, and the tube, m, the crude green ultramarine in the
retort can be rapidly oxidised to blue by aspirating air at m, or after closing
the opening, d, and removing the plate, f, the green ultramarine can be raked
into the collecting chamber, P, and there oxidised. The retorts are then
immediately re-filled with the mixture and the temperature raised.

Fig. 28.

The heating of a charge lasts nine to ten hours; the larger the quantity of
sodium sulphide in the mixture the shorter is the time required. The mixture
is spread out in the muffles in a layer 7 to 9 centimetres thick, and brought
to a moderate red heat. When the whole mass is uniformly hot it is more
strongly heated; the change of the constituents then becomes visible. The
mixture takes at first a brownish colour, and is somewhat similar in
appearance to liver of sulphur. The colour soon begins to incline to green,
and finally changes to a tolerably pure green, but always has a yellow tinge.
Those parts of the hot mass which come into contact with air are more blue,
and occasionally change to a pure blue. When crucibles are used the upper
layer of the contents is always bluer than the lower, and when crucibles
crack during the heating the portions near the cracks always show distinctly
a more or less blue colour.
When the calcination is finished the hot mass drawn out of the muffles
is at once brought into washing vessels, in which it is treated with water so
long as soluble materials are extracted. The compounds dissolved in the
water are chiefly sodium sulphide and sulphate and a little alumina
dissolved in caustic soda. The presence of a larger quantity of caustic soda
in the wash waters indicates too violent heating. In this case the mass will
not mix up easily in water; it contains sintered lumps. The washed material

is strained, spread out on boards or linen stretched across frames, and
completely dried by artificial heat.
At this stage the product may be described as green ultramarine and
sold as such, or it may be converted into blue ultramarine. For the latter
purpose it is ground to a moderately fine meal, but green ultramarine must
be ground to a very fine powder; the finer the powder the brighter the
shade.
In order to obtain blue ultramarine from the washed and ground
substance, it is subjected for a short time to a moderate red heat with
unrestricted access of air. The heating by which the blue colour is produced
must be continued until the proper shade is obtained. This operation is
conducted in special muffles; in many works it is customary to sprinkle
powdered sulphur upon the heated mass during the roasting. The sulphur
burns, forming sulphur dioxide, which escapes into the furnace through an
opening at the back of the muffle. The object of the addition of sulphur may
be to keep the temperature from rising beyond a certain point, or to prevent
a possible reduction of the sulphur compounds in the ultramarine.
The muffles used for finishing the ultramarine are similar to those used
in making green ultramarine; they are generally 50 centimetres wide and
100 to 120 centimetres long. A muffle of this size will contain 6 to 18
kilogrammes of green ultramarine. The temperature is gradually raised to a
gentle red heat, each muffle being provided in front with an iron plate
which prevents the cooling of the contents by the external air. When the
charge in the muffles is carefully observed, it is noticed that the blue colour
first appears at the surface and the edges, i.e., at those places where the
oxygen of the air has unrestricted access. The mass is frequently turned
over with an iron rake, in order to expose all parts to the action of the air;
samples are frequently taken in order to observe the moment at which the
colour has reached its greatest intensity, when the heating is at once
stopped. By proper treatment about half an hour is required to convert the
charge of a muffle of the above dimensions into ultramarine of the deepest
blue. The hot mass is drawn out of the muffles and spread out upon flags so
that it may cool quickly. During the cooling the colour is often observed to
become considerably darker, which is a sign that the ultramarine has not
been heated for a sufficient length of time.

Mechanical arrangements are often used to continually turn over the
ultramarine in the muffles, which are then made in the form of cylinders in
which the ultramarine is continually turned by a stirrer provided with
wings. The whole arrangement is very similar to the apparatus used for
roasting coffee.
The ultramarine is then carefully ground to a very fine meal; it is
tolerably hard, and granite stones should be used, ordinary stones would be
too quickly worn down. It is not possible, by the most careful grinding, to
convert ultramarine into that condition of fine division which is necessary if
it is to be ground into paint; grinding must be followed by levigation. The
various qualities and shades brought into the market differ only in regard to
the fineness of their particles; they do not vary in chemical nature, for the
substance has always the same composition.
It occasionally happens in ultramarine works that a charge does not turn
out a bright blue; the product is then sold as inferior quality at a lower price,
or is mixed with a larger quantity of good material. In working by a settled
process the different shades of ultramarine vary in composition between
narrow limits; the difference is caused by variations in the raw materials.
It has been already remarked that there is no agreement as to the
composition of the colouring principle of ultramarine; some maintain that it
is an iron compound, whilst others regard the iron found in ultramarine as
an accidental impurity, which has no connection with the colour. Recently
this point appears to have been decided from the results of many most
accurate analyses of ultramarine. It has been found that its chemical
composition shows the greatest similarity with the mineral nepheline.
Nepheline is a double silicate, its composition is expressed by the
following formula: Na₂O.SiO₂ + Al₂O₃.2SiO₂. By a comparison of the
analyses of green and blue ultramarine with that of nepheline, it is seen that
blue ultramarine may be regarded as nepheline combined with sodium
pentasulphide, whilst green ultramarine is nepheline combined with sodium
bisulphide. If this is the correct view, the two species of ultramarine have
the following formulæ:—
2{Na₂O.SiO₂ + Al₂O₃.2SiO₂} + Na₂S₅.
2{Na₂O.SiO₂ + Al₂O₃.2SiO₂} + Na₂S₂.

Ultramarine is one of the most permanent pigments. It is not altered by
the substances with which a pigment generally comes in contact. Being a
sulphur compound, it retains its colour completely in air containing
sulphuretted hydrogen. By acids, even weak organic acids such as malic,
citric or tartaric, it is rapidly decomposed, sulphuretted hydrogen being
evolved, and a greyish white residue left. It is sometimes found that the
sugar used in making lemonade produces a perceptible smell of
sulphuretted hydrogen. This is due to ultramarine, which has been added to
the sugar to hide its yellow colour.
From the preceding account of the manufacture of ultramarine it will be
understood that there exist only two species of ultramarine, green and blue.
In commerce a large number of different shades are found, which are not to
be regarded as pure ultramarine; they owe their particular shade to
additions. By mixing in a white pigment, such as barytes or starch, the paler
shades are obtained; by adding a small quantity of a pure red pigment a
colour with a violet tinge is produced. In a similar manner any number of
shades of ultramarine can be obtained.
The following account is taken from a new comprehensive work by J.
Wunder, on the different ultramarines.
Preparation of Mixtures for Ultramarine.—The following points are
to be observed: Chemically pure soda and ammonia soda react with
difficulty. The presence of caustic soda in the soda facilitates the formation
of sodium sulphide, and gives a finer product. It is also of advantage to
sprinkle the soda with a strong solution of sodium sulphide. The more silica
the mixture contains the more difficult is the transformation into
ultramarine, but the product is deeper and better in shade, and has more
resistance to alum and weak acids. The sodium sulphide must react with
silica and alumina. When oxygen enters during the burning, silica is re-
formed, which with soda and alumina produces slags. In order to exclude
oxygen many manufacturers burn with restricted access of air; then the
carbon bisulphide and sulphur gases evolved are not burnt, but, together
with tarry materials from the coal, form an evil-smelling smoke, which
renders the neighbourhood objectionable. The coal consumption in burning
is also greater. The varieties rich in silica and stable towards alum are
generally dark reddish-blue. In order to obtain pure blue shades from
mixtures rich in silica they are burnt in closed crucibles to produce the

green, which is powdered and heated with restricted air supply, and the
admission of a little steam, to 160° to 180° C., when pure blue or greenish-
blue shades are obtained as required. In this way a mixture rich in silica can
be burnt to a good blue, containing 69·32 parts of silica to 30·67 parts of
alumina, or 1 equivalent of Al₂O₃ to 3·84 equivalents of SiO₂; whilst
ultramarine rich in silica, made in the ordinary way, contains at the most
66·7 parts of silica to 33·3 parts of alumina.
Ultramarine Violet was first introduced into commerce about the year
1859 by the Nuremberg Ultramarine Works, under the direction of Leykauf.
By the reaction of moist calcium chloride on ultramarine blue in the warm
chambers above the furnaces the blue was changed to violet. By the action
of air a portion of the moist calcium chloride is decomposed into lime and
hydrochloric acid, the latter of which, together with air, reacts on the blue.
In 1872 Wunder, intending to apply to the estimation of sulphur in
ultramarine blue a method which gives good results with metallic sulphides,
passed chlorine over heated ultramarine. The pigment was converted into a
brownish-red substance, which on washing combined with water and turned
violet. The red substance is not produced at every temperature; 300° C. is
the most favourable. On washing sodium chloride is dissolved; the washed
violet is free from chlorine, but has taken up water, which is expelled on
heating, when the violet turns to blue. A similar brownish-red body is
formed when sulphur trioxide is led over heated ultramarine blue, and when
sulphur chloride acts on warm ultramarine blue. When the brownish-red
chlorine compound is introduced into absolute alcohol, a reddish-violet
compound is produced, containing organic matter. Ethyl chloride is formed
at the same time. When ammonia gas is passed over the heated chlorine
compound it is absorbed; a violet ultramarine is produced, from which the
ammonia is not removed by washing with water, but only by heating nearly
to redness, or, better, by fusing with caustic alkalis. Aniline combines in a
similar manner. In order to obtain a good violet the brownish-red chlorine
compound must not be formed; the violet is produced by leading chlorine
and steam over ultramarine blue at 160° to 200° C. Blue rich in silica is
most suitable for transformation into violet. The violet so obtained is not
decomposed by lime.
C. Mahla produced the chlorine in the blue itself by the reaction: NH₄Cl
+ 2NH₄NO₃ = 6H₂O + 5N + Cl. By heating a mixture of ultramarine blue

with ammonium chloride and nitrate in crucibles at 200° C. a fine violet is
produced. It contains ammonia, which cannot be removed by washing, but
only by strongly heating or heating with concentrated alkalis. This violet is
decomposed by the prolonged action of lime, in three days it is changed to a
grey green. In the course of the manufacture it was noticed that less nitrate
was required in the mixture according to the length of time it was exposed
in porous crucibles to the action of air and heat; by heating for a sufficient
length of time with access of air, a good violet is obtained with ammonium
chloride alone. In the manufacture of violet by means of moist chlorine
hydrochloric acid is formed, which is also the active material in its
formation by means of ammonium chloride. It was therefore to be expected
that chlorine gas might be replaced by hydrochloric acid, if it were
accompanied by sufficient air. This has been found to be the case. The
temperature must be maintained between 180° and 230° C. Below 150° C.
the blue is decomposed by moist hydrochloric acid gas; at temperatures
above 230° C. it is unchanged. Hydrochloric acid gas and air, without
steam, give with ultramarine blue on heating a brownish red substance
similar to that produced by dry chlorine; it also is changed to violet by
washing with water. The violet obtained by means of moist chlorine differs
from the blue from which it was formed, in that the latter has lost one-sixth
of its sodium and combined with water and much oxygen. The following is
the approximate formula for a blue:—
Na₆Al₄Si₆S₄O₂₁,
and for the violet obtained from it
Na₅HAl₄Si₆S₄O₂₄ + H₂O.
The violet contains much thiosulphate. If it is decomposed by nitric acid
and silver nitrate added to the filtered solution a precipitate is obtained
which changes in colour from white to yellow, orange and brown, just as
the precipitate given by silver nitrate in solutions of thiosulphates
containing nitric acid.
Chlorine and Steam Process.—Chlorine does not attack iron at 150° to
250° C. The reaction is carried out in heated iron boxes 1 metre wide, 2
metres long and 65 centimetres high. The blue is spread out in a layer 2
centimetres thick on earthenware plates, which stand one above another at a

distance of 5 centimetres, each supported on three feet. The plates are
introduced by means of iron tongs through large openings in the top of the
box; after filling iron plates are screwed on to the openings, through them
pass sheet-iron tubes reaching to the bottom, in which thermometers can be
lowered on wires. The iron boxes stand in heated chambers, which have
openings corresponding with the openings in the boxes, and shut off from
the interior of the furnace. Chlorine and steam are led into the iron boxes at
both ends through lead tubes reaching to the bottom; the parts of these tubes
in the furnace and the iron boxes are protected by wide sheet-iron tubes, the
space between being filled with clay. The gases evolved, hydrochloric acid,
sulphur chloride and steam, pass from the covers through earthenware pipes
into boxes filled with limestone, upon which water drops, and thence to the
chimney. After filling, the boxes are heated to 280° C. and steam introduced
to remove sulphur; they are then allowed to cool to 160° C., when chlorine
and steam are led in for three hours. The violet is then finished, and the
residual gases are blown out by a fan.
Hydrochloric Acid and Air Process.—The operation is conducted in
the same iron boxes, upon the bottoms of which, beneath the openings, are
earthenware dishes into which hydrochloric acid can be poured through
earthenware pipes. The chimney draught must be strong enough to draw
sufficient air through these pipes into the boxes. After the temperature has
been maintained for seven hours at 220° to 230° C., hydrochloric acid being
poured in from time to time, the blue is changed to a dull violet, which
becomes brighter when hydrochloric acid is repeatedly added at
diminishing temperatures, 210°, 200°, 180° and 160° C. More recently the
iron boxes have been replaced by stone chests or by chambers above the
ultramarine furnaces, the temperature in this case being kept at about 200°
C.
Ammonium Chloride Process.—A mixture of ultramarine blue with 5
per cent. of ammonium chloride is heated during fourteen days in porous
crucibles placed in the upper chambers of the ultramarine furnaces, when
the contents of the crucibles become a handsome violet throughout. If
sodium nitrate is used together with ammonium chloride, the violet is
formed in a much shorter time. After washing, the violet still contains
nitrogen; on ignition or heating with strong alkalis it loses ammonia.
Unfortunately, this fine violet is decomposed by the continued action of

moist slaked lime. The violets made by the first two processes absorb
ammonia when this gas is led over them at 180° to 200° C., and it cannot be
removed by washing.
Pale Blue Ultramarine.—If the violet is heated in hydrogen at 280° to
290° C., it is converted into a pure, bright, pale blue. This has an absorption
spectrum in which the red is not absorbed, but appears more brilliant than in
the spectrum of ultramarine rich in alumina. Pale blue is turned violet blue
by heating at 300° C., and at a red heat a dull blue. It is not yet made on a
commercial scale, but on account of its great purity of shade it appears to be
valuable for many purposes; perhaps it may replace alumina cobalt blue.
The composition of pale blue is:—
Calculated for Found.
Na₅ 12·4 per cent. 11·9 per cent.
H₅ 0·54 ” 0·62 ”
Al₄ 11·7 ” 13·1 ”
Si₆ 18·2 ” 19·7 ”
S₄ 13·9 ” 12·7 ”
O₂₅ 43·3 ” 42·0 ” (by difference).

By a comparison of the composition of the violet and pale blue
ultramarines, it is seen that the chief difference is an increase of hydrogen in
the latter.
Ultramarine Red.—Since ultramarine violet increases in brightness
and redness of shade in the air, Wunder erroneously believed that this was
due to oxidation, and that, consequently, the violet could be converted into a
red by oxidising agents. Nitric acid vapours led over ultramarine violet at
170° to 200° C. do not act upon it, but where drops of nitric acid are spirted
over, the violet is changed to red. Wunder then reduced the temperature to
135° to 145° C. and obtained the first ultramarine red. Iron is attacked by
nitric acid at lower temperatures, but not at 135° C.; the iron boxes
previously described could therefore be used. It was afterwards found that
at a sufficiently low temperature hydrochloric acid gas converts ultramarine
violet into red. The iron boxes cannot be used for this operation, as they are
attacked at the temperature; the stone chests are used instead. Other acids
also act on ultramarine violet; boric acid gives a reddish-violet.
The violet is spread out on the dishes standing on three feet mentioned
before, and heated to 128° to 132° C. At higher temperatures the violet is
unaltered, whilst below 100° C. it is decomposed. The hydrochloric acid is

poured in from time to time through earthenware tubes into dishes in which
it evaporates.
A mixture of red and blue would appear violet, but would behave
towards reagents in a different manner to real ultramarine violet.
Ultramarine blue is decomposed at 128° to 132° C. by hydrochloric acid to
a gelatinous mass, whilst at this temperature ultramarine violet is changed
into a bright red. From the blue no violet can be obtained by nitric acid, but
the violet gives a red at 135° to 145° C. Analysis would also indicate the
difference. Ultramarine red has the following composition:—
Calculated for Found.
Na₃ 7·9 per cent. 8·1 per cent.
H₅ 0·57 ” 0·72 ”
Al₄ 12·3 ” 13·3 ”
Si₁₆ 19·1 ” 19·3 ”
S₄ 14·6 ” 15·2 ”
O₂₅ 45·6 ” 43·4 ” (by difference).

It appears that in the red two more equivalents of sodium have been
replaced by hydrogen. The violet is apparently a sodium salt of which the
red is the acid. The violet made by means of ammonium chloride is also
converted by hydrochloric acid gas at 128° to 132° C. into a handsome red
containing nitrogen, and the red is changed by hydrogen at 280° to 290° C.
into a lighter pale blue.

CHAPTER XXV.
BLUE COPPER PIGMENTS.

Bremen Blue and Green.—The pigment known as Bremen blue or
Bremen green consists of copper hydroxide. It possesses all the
disadvantages of copper compounds; it is very sensitive towards
sulphuretted hydrogen and sulphur dioxide; in contact with air containing
the former gas it is quickly turned black. It has also other disadvantages; it
cannot be used in oil, for when ground in oil it is quickly discoloured, on
account of the formation of copper oleate. If a wall which has not
completely dried be covered with Bremen green, in a short time it will be
covered with spots. In spite of this small stability, Bremen green is still
largely used by artists on account of its low price, although it could be
replaced by other pigments almost as cheap and considerably more durable.
Copper hydroxide comes into the market under many names. The
pigments known as blue verditer, lime blue and mountain blue contain
essentially copper hydroxide. The preparation of pure copper hydroxide on
the large scale will be first described. Copper sulphate is the usual raw
material. In order to obtain a colour of a pure blue shade it must be free
from iron, the presence of which would result in the production of a
discoloured precipitate.
A fairly concentrated solution of copper sulphate is warmed in a pan to
about 30° C.; weak caustic potash solution is added until the liquid is
slightly alkaline. A green precipitate separates, which consists of copper
sulphate and hydroxide. In order to remove the copper sulphate from the
precipitate, and impart to it the correct bluish-green colour, it is left on a
strainer until it has acquired a pasty consistency, when it is brought into a
tub and mixed with weak caustic potash solution. Care must be taken that
this potash solution is not too concentrated, or it will withdraw water from
the copper hydroxide and produce black copper oxide. In order to prevent
this result, which would render the whole mass useless, a portion of the
precipitate is tested with the caustic potash solution. If it is blackened, the
solution must be diluted with the necessary quantity of water before it is

used. Instead of diluting with water, potassium carbonate solution is often
added. The addition of the caustic liquor is continued until the colour of a
test portion of the precipitate is not deepened by further additions. When
this point is reached the colour must be at once withdrawn from the action
of the alkali. It is brought into a large tub filled with water, in which it is
most carefully washed with many waters.
Basic copper chloride may also be used as the raw material for this
pigment. It is obtained by treating scrap copper with 60 per cent. of
common salt and 30 per cent. of sulphuric acid. The pale green basic
chloride is then treated with caustic potash solution. This method is now
rarely used; copper sulphate is considerably lower in price than formerly
and copper hydroxide is prepared from it with the least trouble.
Neuberg Blue is a mixture of copper blue with Chinese blue. The larger
the quantity of the latter component the deeper the colour. Neuberg blue can
be more easily ground with oil than pure copper blue. It should not be used
as a distemper colour, for the Chinese blue is decomposed by the action of
lime, and the colour will quickly be spoiled by the separated ferric oxide.
Lime Blue is ordinary copper blue, mixed with varying quantities of
gypsum. It is paler than pure copper blue; different shades are obtained by
increasing the quantity of gypsum.
This pigment is made by dissolving 15 parts of copper sulphate in 1,300
parts of water, adding a strong solution of 121 parts of ammonium chloride,
and then milk of lime prepared from 30 parts of quicklime. The precipitate
is well washed, strained, ground, and finally dried.
Lime blue is brought into the market in several forms; in cubes obtained
by cutting up the stiff paste and then completely drying, and in irregular
lumps or a coarse powder. The pigments containing copper hydroxide as
their colouring principle are used less as artists’ colours than for
distempering.
Payen’s Mountain Blue is a mixture of copper hydroxide with varying
quantities of calcium carbonate. It is prepared by adding calcium chloride to
a solution of copper sulphate and stirring in dilute milk of lime. A green
precipitate of basic copper chloride is formed; potassium carbonate solution
mixed with milk of lime is then added. The latter mixture contains caustic

potash, which decomposes the copper chloride, copper hydroxide and
calcium carbonate being simultaneously precipitated. Different shades may
be obtained; by increasing the quantity of lime the product is paler. This
pigment commonly comes into the market in the paste form. When it is
dried the operation must be conducted at a low temperature; copper
hydroxide is converted into copper oxide at relatively low temperatures.
Oil Blue.—The pigment known under this name consists of copper
sulphide (CuS). It can be made in several ways. The simplest process,
which also gives a good product, is here described.
Sulphur is boiled in a glass flask with a long neck. When the heavy
reddish brown vapours begin to fill the neck, copper turnings or wire free
from oxide are thrown in. The copper burns in the sulphur vapour with a red
flame to cupric sulphide. When it has once commenced to burn, the flask
requires little heating, so much heat is produced by the combustion of the
copper that the sulphur continues to boil. The introduction of copper is
continued until only a small excess of sulphur is left, the flask is then closed
and allowed to cool. When the contents have reached the temperature of the
air the flask is broken, and the mass boiled with caustic potash solution in
order to remove excess of sulphur.
The product obtained by this process has a handsome violet blue colour,
which must be shown by the whole mass. If portions taken out of the flask
are black, an excess of copper has been used. In order to improve the colour
of a faulty batch, it is mixed with sulphur and quickly heated to the boiling
point of sulphur, air being excluded. The excess of sulphur is then removed
by boiling with caustic potash solution.
Oil blue is a handsome but not very durable pigment. When used in
varnish, which protects it from the action of air, it is fairly permanent.
Copper Hydroxide.—By precipitating carefully a solution of a copper
salt with caustic soda, a blue precipitate of copper hydroxide is obtained,
which, though not distinguished by any particular beauty, is adapted for use
as a distemper colour and for similar purposes. The operation is conducted
at ordinary temperatures; on account of its voluminous nature the pale blue
precipitate requires a long time for washing. Hot solutions must not be
used; at a temperature near the boiling point of water copper hydroxide is
completely decomposed into black copper oxide.

CHAPTER XXVI.
BLUE COBALT PIGMENTS.

Cobalt Blue, Thénard’s Blue, Cobalt Ultramarine, King’s Blue,
Leyden Blue.—The pigment known under these names is a compound of
cobalt oxide with alumina, and is prepared in a similar manner to
Rinmann’s green. Of all the blue pigments used in painting, cobalt blue is
the most handsome and most durable. It is completely unaltered by the
atmosphere. The most powerful chemical reagents have but a slight action
upon it, and their action is less the higher the temperature used in the
manufacture. Cobalt blue is made by precipitating mixed solutions of a
cobalt salt and alum, washing, drying, and igniting the precipitate. A
product of pure blue colour is only obtained when alum absolutely free
from iron is used. If but very small quantities of iron are present, the red
ferric oxide has a very considerable influence on the shade of the product.
The depth of colour varies according to the proportion of cobalt salt
used. The variation is not so considerable as in the case of cobalt green; as a
rule, 50 to 100 parts of cobalt sulphate or nitrate are used to 100 parts of
alum. The temperature at which the precipitate is ignited must be much
higher than that used for cobalt green; it should be raised to a white heat.
The crucible should be provided with a well-fitting lid—the fire gases
damage the shade of the colour.
When cobalt sulphate is ignited with ammonia alum, cobalt blue is
obtained; but this method is not to be recommended. The ignition must be
of long duration to completely drive off the sulphuric acid. The author has
made a cobalt blue of remarkable beauty in the following manner: Alumina
obtained by precipitating electrolytically an alum solution (page 49) was
mixed with a solution of pure cobalt chloride to a thin pulp, which was
quickly dried in a shallow porcelain dish. The mixture, whilst still hot, was
pressed into a crucible placed in a larger one filled with magnesia; the
crucible was heated to a good white heat, at which it was maintained for an
hour, when it was allowed to cool. The cobalt blue made by this method has
the purest blue colour, being without that admixture of red which is not

rarely to be perceived in this pigment. To avoid the appearance of this red
tint, certain additions are made; a small quantity of a zinc salt is most
frequently employed, according to the quantity of which more or less pale
shades are produced.
Cæruleum is a pale blue pigment consisting of a compound of the
oxides of cobalt and tin. It is made by converting tin by the action of nitric
acid into white stannic oxide, adding a solution of cobalt nitrate and
evaporating to dryness in a porcelain dish whilst stirring; whilst still warm
the powder is strongly ignited. In cæruleum the chemical combination is
less firm than in the compounds of cobalt oxide with zinc oxide or alumina,
strong hydrochloric and dilute sulphuric acids decompose it, continued
treatment with nitric acid extracts all the cobalt. Cæruleum resists
completely all atmospheric influences.
The circumstance that all the cobalt in this compound may be removed
by treatment with nitric acid leads to the conclusion that the pigments
composed of cobalt oxide and another oxide are not chemical compounds of
the two oxides, but that the alumina, zinc oxide and tin oxide serve simply
as bases for the blue modification of cobalt oxide. That a chemical
compound cannot be present follows from the fact that the quantities of the
oxides uniting together may be very considerably altered and a coloured
product still obtained. The author has shown in 1868 that the blue
modification of cobalt oxide is obtained by heating the red. The change of
the red colour of cobalt chloride into the blue is not caused, as was formerly
thought, by loss of water, but by the transformation of the cobalt oxide
contained in the compound from the red into the blue modification. Strong
bases are able to retain the cobalt oxide in the blue form, and strongly basic
oxides are therefore used in the manufacture of this series of pigments.
Cobalt Zinc Phosphate.—This pigment is made by precipitating a
solution of zinc sulphate, free from iron, by sodium phosphate, adding a
solution of cobalt sulphate and again sodium phosphate so long as a
precipitate is produced. Excess of sodium phosphate is to be avoided,
otherwise a considerable quantity of cobalt remains unprecipitated. In the
directions given by Gentele it is stated that the sodium phosphate must be in
excess, and that the liquid above the precipitate must possess a reddish
colour. The author’s experiments have shown that a pigment in every
respect equal to that produced by Gentele’s process is obtained when all the

cobalt is precipitated. The precipitate, which consists of variable quantities
of zinc and cobalt phosphates, is dried and ignited, when it acquires a very
deep blue colour.

CHAPTER XXVII.
SMALTS.

This blue pigment is a very finely powdered glass, coloured by cobalt
oxide, according to the quantity of which products of very different shades
are obtained. If a very small amount of cobalt oxide is used the smalts has a
beautiful pale blue colour, but by the use of larger quantities an almost
black mass can be obtained.
Although the proportion of cobalt oxide contained in smalts is so
variable, yet it is probably present as a compound of constant composition
—the cobalt potassium double silicate of the formula CoO. 2SiO₂ + K₂O.
2SiO₂. The numerous published analyses of smalts lead to this conclusion.
The small quantities of other metallic oxides which are found in smalts are
to be regarded as chance impurities. Being a glass free from lime smalts is
easily decomposed; in this respect it shows great similarity with water-
glass, which is completely soluble in boiling water.
Smalts comes into commerce in the form of powder. In the ”Blue
Colour Works” in which smalts is made a number of qualities are
distinguished; these are separated according to the fineness of the powder.
Smalts of good quality is composed of particles of the same size and has a
pure sky-blue colour. A shade inclining to red is generally to be ascribed to
the presence of iron; material of this nature is of little value.
Smalts is not easily attacked by chemical reagents. It has been above
stated to belong to the easily decomposed glasses, and it is attacked by
reagents which ordinary glass completely resists. Finely ground smalts is
strongly attacked by boiling sulphuric acid, and, after fusion with soda, is
soluble in acids. Water has some action upon smalts; by the long continued
action of a large quantity the shade is injured, the colour acquires a greenish
tinge.
In the manufacture of smalts there are three principal operations:—

1. Preparation of the charge.

2. Fusion of the charge.
3. Grinding the fused mass.

These processes are principally mechanical. Only as regards the
composition of the charge is chemical knowledge required.
1. Preparation of the Charge.—The cobalt is supplied by substances
of different origin—as a rule, roasted speiss cobalt and cobalt glance are
used; occasionally ores are employed without previous preparation. When
pure cobalt salts (sulphate and nitrate) are used, a product of very fine shade
is produced, but in practice these salts are not applicable on account of their
cost. There remain therefore only the compounds of cobalt occurring in
nature. On account of the foreign substances they contain, these require a
special treatment before they can be used in the manufacture of smalts; the
treatment varies according to the other elements present in the cobalt ores.
Ores containing bismuth are first treated in order to obtain this valuable
metal; ores containing arsenic are roasted in furnaces of special
construction, which are connected with a series of stone or wooden
passages in order to condense the arsenic trioxide completely.
If ores are employed which contain only a small quantity of cobalt, the
same quantity of glass is melted time after time with fresh quantities of the
poor ore. At each operation cobalt is taken up. In place of glass a mixture of
potash and very fine quartz sand may be used with advantage. It produces
that species of glass which most easily takes up cobalt oxide. The quartz
sand is generally made by igniting flint and quenching in water to facilitate
the grinding; it is then powdered. River sand is generally so impure that it
would produce a brown or green coloured glass, which, when fused with
cobalt ores, would give smalts of a bad shade.
2. Fusion of the Charge.—This process is very similar to the fusion of
ordinary glass. The materials, in very fine powder, are carefully mixed and
then ”fritted,” that is, heated until the mass sinters together without melting.
The real fusion is accomplished in glass furnaces. The fritted mass is fused
in small ”glass pots” made of fire-clay, in a furnace similar in principle to
the ordinary glass furnace. The glass pots are made rather small, to hold
about 50 kilogrammes, in order to obtain as homogeneous a product as
possible. They are placed to the number of six in a furnace heated by coal,
in which a very high temperature can be obtained. This is essential, because

the charge has a high melting point and must be made quite fluid in order
that impurities may settle.
The furnace must be slowly heated at first to prevent the pots from
cracking. When they are once heated the temperature is raised to fuse the
charge, which is repeatedly stirred so that the residues may sink to the
bottom. These residues consist chiefly of metallic sulphides and arsenides.
After six to seven hours’ fusion the stirring is discontinued, and the
temperature raised for a short time to the highest it is possible to attain, so
that the smalts may be as fluid as possible. When this is the case it can be
drawn out into long thin threads. The melted glass is then taken out and
poured into cold water. The rapid cooling makes it extremely brittle, and
considerably facilitates the powdering.
When new pots are used for the fusion, their walls are considerably
attacked by the alkalis contained in the glass. In order to prevent this a
small quantity of common smalts is fused in them before the introduction of
the real charge; the inner surface then becomes covered with a protective
glaze.
3. Grinding the Fused Mass.—The smalts is next powdered under
stamps. These only differ from the ordinary construction in that both stamp
and trough are made of granite. The coarse powder from the stamps then
goes to mills of ordinary construction, in which it is ground as finely as
possible in a current of water. A blue liquid runs from the mills into tubs.
The coarser particles which settle in these vessels constitute the article
known commercially as strewing smalts, which is of small value. The
quantity of this quality is fairly considerable; it cannot all be sold; part is
fused in a later operation with a fresh quantity of the fritted mixture. The
liquid remaining in the tubs then gives the different qualities of smalts.
After having deposited the coarsest particles in the first vessel it is brought
into another, in which it stays only a few minutes, when it is run off into
another vessel, in which also it remains but a short time. The liquid goes in
this manner through from three to five vessels, and in the process deposits
all the coarser particles. The oftener the liquid is brought into fresh vessels
the finer are the particles still suspended in it, and the longer it should
remain before being drawn off into another.

When the liquid has run through three or four vessels, as the case may
be, it still retains the finest particles in suspension. It is now run into a tank,
in which it remains until it is perfectly clear and has deposited all the blue.
The deposit in this tank has a dirty bluish-grey colour; it is utilised by being
added to a later fusion. It would be expected that this last deposit would
have the best shade, since it consists of smalts in the finest state of division,
which would no doubt be the case if the smalts remained unchanged. Smalts
is, however, a glass which is rather easily decomposed. In grinding, the
smalts comes in contact with large quantities of water, and in the
succeeding levigation the water decomposes it; the final residue is thus
produced. In view of the easy decomposition of smalts by water, it is
necessary to carry out all processes in which it is in contact with water as
rapidly as possible, in order to obtain the largest yield of smalts.
The smalts deposited in the successive vessels have a different degree of
fineness, and are sold under particular marks. The Saxony works, which
produce the greater part of the smalts used in Germany, distinguish the
qualities by letters. The better qualities are distinguished by the same letter,
the fineness of the quality being indicated by the repetition of this letter.
Thus FFFE, FFE, and FE indicate qualities which are of lower value the
less often the letter F occurs in the mark. The qualities following those
marked F are distinguished by M; the lowest quality is marked OE. F
indicates fine; M, medium; and O, ordinary.
The qualities generally designated smalts are those deposited in the
third and fourth vessels; they are also known as king’s blue or azure blue.
The processes used in different works apparently vary. The essentials of
the processes are, however, as they have just been described; the time
during which the liquid is left in the various depositing vessels is different.
Smalts, being a glass, well resists the action of the weather, although on
account of the absence of lime it has not the great chemical indifference of
ordinary glass. Smalts is not altered by atmospheric influences, but from
analogy with the behaviour of water-glass, to which smalts is closely related
in chemical composition, we should conclude that, although smalts resists
the action of the air for a tolerably long time, it cannot be regarded as
absolutely permanent; in the course of time potassium carbonate and silica
will be formed.

Since the introduction of the manufacture of ultramarine, the use of
smalts has considerably decreased. Ultramarine is incomparably the more
handsome pigment, but it cannot be exposed to high temperatures without
undergoing change of shade. For purposes in which the pigment is
subjected to a high temperature, as in porcelain painting or earthware
glazing, smalts has maintained its position. Smalts is also used in fresco
painting, for colouring wax and tinting writing paper. For the latter purpose
it should be rejected. It has no influence on the durability of the paper, but it
spoils steel pens. Any one who has written for a long time on paper
coloured with smalts must have observed this; the best steel pens are soon
worn out by such paper. The very fine splinters of glass present in large
number in the paper act upon the steel as a very fine but very hard file, and
quickly wear it down.
When smalts are used for painting earthenware, the outlines of the
design are quite obliterated, the cause of which is that smalts, being itself a
glass, fuses with the glaze (also a glass) of the vessel. When clean outlines
are required smalts should not be used; some other cobalt pigment resisting
heat should be substituted.
In addition to the above blue pigments which are in common use, there
are two others to be mentioned, rarely used and very costly—tungsten blue
and molybdenum blue.
Tungsten Blue is made by adding excess of ammonium chloride
solution to a hot solution of potassium tungstate, collecting the precipitate
resulting when the liquid has completely cooled, drying and heating to
redness in a crucible, through the cover of which a porcelain tube reaches to
the bottom. When the crucible begins to glow, hydrogen is passed through
the tube; the crucible is then heated to a full red heat for fifteen minutes, the
current of hydrogen being continued. The pigment obtained in this way is a
deep blue powder of velvety appearance.
Tessié du Motay’s Blue.—The following is the method of preparation:
10 parts of sodium tungstate, 8 parts of tin crystals, 5 parts of yellow
prussiate, and 1 part of ferric chloride are dissolved separately and the
solutions mixed. The precipitate is washed and exposed to light in thin
layers. The blue colour develops in the course of several days. According to
the discoverer, this pigment consists of a compound of tungsten oxide with

a double cyanide of iron and tin; in physical properties it is similar to good
Prussian blue, but is distinguished by greater durability on exposure to light
and also by a much higher cost. Up to the present this pigment has not been
made commercially.
Molybdenum Blue.—This fine but costly pigment is obtained by
mixing a solution of sodium molybdate with a solution of stannous
chloride. A blue precipitate is formed, which, after washing and drying, may
be used as an artists’ colour. It is distinguished by great durability. A good
shade of molybdenum blue can be obtained with greater certainty by adding
finely-powdered tin and a few drops of hydrochloric acid to a solution of
pure molybdic acid. In composition it is a mixture of stannic molybdate and
the blue modification of molybdenum oxide.

CHAPTER XXVIII.
GREEN MINERAL PIGMENTS—
GREEN COPPER PIGMENTS.

Copper Carbonate.—In nature two salts occur containing copper,
carbonic acid, and water in different proportions—one, the mineral
malachite, has a fine emerald green colour; the other, azurite, is sky blue.
Both minerals are ground and levigated, and brought into commerce as
mountain green and mountain blue.
Compounds of copper with carbonic acid, made artificially, are used
under different names as pigments. The terms mountain blue and mineral
blue are applied indiscriminately to various copper compounds; as
mountain blue, or Brunswick green, pigments come into commerce
containing varying quantities of copper carbonate (made by precipitating a
copper salt with soda) mixed with barytes, zinc white, or another white
pigment, in order to obtain paler and deeper shades.
The poisonous Scheele’s green is also given the same name as that
under which copper carbonate is sold, viz., mineral green, so that the
composition of this colour can only be decided by analysis.
A green pigment, similar in composition to malachite, is obtained by
precipitating copper sulphate with soda in the cold and washing quickly. If
the precipitate is left in contact with the liquid, its colour changes and it is
converted into a blue mass similar in properties to ground malachite. A
green precipitate is obtained with more certainty by precipitating a boiling
solution of copper sulphate with soda solution.
Both green and blue copper carbonate are pigments of little depth; as a
rule ground and levigated malachite and azurite have a greater intensity of
colour than the artificial pigments.
Copper Arsenite.—The compounds of arsenious acid with copper are
among the most handsome and bright colours which exist. It is thus to be
regretted that these pigments are disappearing more and more, and indeed

with every reason, from the pigments permitted by law. In the end they
must fall into complete disuse. It is a matter of common report that these
pigments, and particularly emerald green, have frequently caused arsenical
poisoning. Numbers of people have been repeatedly made ill by the dust of
emerald green fixed on light fabrics, such as are used for ball dresses,
simply by albumin or dextrine, and brought into the air by the movements
of the dancers. It has been further shown that the air of rooms papered with
hangings coloured by emerald green contains small quantities of the most
poisonous of all arsenic compounds, arseniuretted hydrogen.
Although it is in the interest of colour makers to produce the brightest
possible colours, yet, on account of their dangerous properties, pigments
containing arsenic should be quite excluded. In several countries their use is
forbidden by law. Since there are several pigments which can compete in
brightness with the arsenic colours, it is to be hoped that the latter will
shortly disappear from commerce.
Scheele’s Green, Swedish Green.—A solution of 32 parts of potash is
boiled with 11 parts of white arsenic (arsenious acid) until the latter is
completely dissolved. This solution of potassium arsenite is mixed with a
hot solution of 32 parts of copper sulphate free from iron. The beautiful
green precipitate is washed with hot water and dried. These directions for
the preparation of copper arsenite are due to Scheele, the discoverer of the
pigment. It can be prepared by several other methods.
When white arsenic and copper sulphate are dissolved together, and
caustic soda added to the boiling solution, Scheele’s green is precipitated.
When made by this method the pigment must be very carefully washed; if
the excess of caustic soda is not completely removed it is very hard when
dry, and is difficult to powder. In order to obtain darker greens the quantity
of copper sulphate is increased.
A particularly bright green is obtained by dissolving copper sulphate in
water, with the addition of 12 to 15 per cent. of white arsenic, and
precipitating by a solution of potassium zincate, which is obtained by
boiling caustic potash solution with zinc filings, hydrogen being evolved.
The shade of Scheele’s green depends on the relative quantities of
arsenious acid and copper sulphate employed. If the arsenious acid
predominates, a pale product is obtained; when excess of copper sulphate is

used, the colour is pure green. The pigment made according to the
foregoing directions possesses only small covering power. A product better
in this respect is obtained by precipitating a solution of 100 parts of copper
sulphate by a solution of 90 parts of potassium carbonate, in which 66 parts
of arsenious acid have been dissolved.
Scheele’s green is a bright green pigment, which resists the action of the
atmosphere tolerably well. It is easily decomposed by dilute acids; when
strongly heated arsenious acid is set free, and copper arsenate and arsenide
are produced. Copper arsenite is the essential constituent of the pigments
sold under the name of mineral green, which is generally a mixture of
copper arsenite with varying quantities of mountain green.
Brunswick Green, Green Verditer, is a mixture of copper arsenite,
copper hydroxide and gypsum. It is prepared by dissolving 100 parts of
copper sulphate in water, adding a solution of ¼ part of arsenious acid and
10 parts of anhydrous potassium carbonate, and finally precipitating with
milk of lime made from 20 parts of quicklime. The pale to bluish-green
precipitate is washed and pressed into flat cakes, which are brought into the
market and used as lime colours. This pigment is not adapted for use in oil;
when applied in that medium it darkens considerably.
Neuwied Green is made in almost exactly the same manner as
Brunswick green, the chief difference being that a larger quantity of arsenic
is used, and thus a product obtained inclining more to green than to blue. As
a rule 2 to 2½ parts of arsenious acid are used to 100 parts of copper
sulphate. The colours found in commerce under this name very often
consist of mixtures of emerald green and finely ground barytes.
Copper Oxychloride was at one time largely used. At present it is
scarcely employed; it has been replaced by handsomer and cheaper
pigments. Copper oxychloride is made by exposing a wet mixture of
copper, copper sulphate and common salt to the action of the air. To 111
parts of copper sulphate 112·5 parts of copper and the same quantity of
common salt are employed. The mixture is made into heaps, which are
wetted and from time to time shovelled about, so that the air may come in
contact with all portions of the mass, and the copper chloride be converted
into basic chloride. It is advisable to allow the heaps to dry frequently after
they have been turned over, so that the air may more readily penetrate into

the interior. Copper oxychloride has a pale green colour of little brilliance.
On this account it is hardly ever used now; it forms the raw material for the
preparation of Bremen green.

CHAPTER XXIX.
EMERALD GREEN.

There are many versions as to the discovery of this pigment. According
to one it was first made in Schweinfurt by Russ and Sattler; another version
gives it that Mitis of Vienna first made it on the large scale. It is possible, as
has been the case with many chemical products, that this pigment was
simultaneously discovered by both. The method of making the pigment was
at first a secret possessed by few. Not until the publication of a method by
Liebig in 1822 was the preparation of emerald green generally known. In
recent times the industry has supplied other colours which are less
poisonous. The time should not be far distant in which this handsome but
highly poisonous pigment shall no longer be found in commerce.
The circumstance that the poisonous nature of the pigment and cautions
as to its use were given great publicity, brought about the discovery of the
countless names under which it has been sold. It was endeavoured by re-
naming the pigment to pass it off as a different substance. All the pigments
which have been sold under the names of Mitis, moss, patent, new, king’s
green, etc., are either pure emerald green or mixtures of it with barytes or
gypsum, which additions were intended to alter the shade of the green and
make it appear to the buyer as a new pigment.
In chemical composition emerald green is a compound of copper
arsenite and copper acetate. It has the following formula:—
3(CuO.As₂O₃) + Cu(C₂H₃O₂)₂.
This compound rarely comes into commerce in the pure state; it is generally
mixed with chrome yellow or lead sulphate, which are added with the
object of producing shades between yellowish-green and dark green.
Emerald green is a compound of constant composition. When made by
different processes it has the same properties. Under the microscope it is
seen to consist of crystals; when the pure substance is ground it becomes
paler, the intensity of the colour being diminished by the breakage of the
crystals. The colour of emerald green is not altered by artificial light. This is

a most valuable property; the majority of green pigments appear yellowish-
green in artificial light.
Emerald green can be made on the large scale by two processes; these
differ according to the copper salt used, which is either the acetate
(verdigris) or the sulphate. The latter is by far cheaper than verdigris, which
is now seldom used for this purpose.
Manufacture of Emerald Green from Verdigris.—By boiling for
several hours, 70 parts of ordinary verdigris are dissolved in water; in a
second vessel 100 parts of arsenious acid are dissolved in 1,500 parts of hot
water; the verdigris solution is filtered through a cloth into a larger wooden
vessel and mixed with two-thirds of the arsenic solution. The mixture is
well stirred and allowed to stand for some hours. A precipitate of dull green
colour is formed. This consists of copper arsenite. After three to four hours
the remaining one-third of the arsenic solution is added; the precipitate then
changes into the double salt of which emerald green consists.
The use of such dilute solutions as are given in these directions is
essential when a product of particularly good colour is required. Emerald
green is a crystalline substance which is deeper in colour in proportion to
the size of its crystals. Large crystals can only be formed from very dilute
solutions; when strong solutions are used, the emerald green has never the
deep colour possessed by the product of dilute solutions. In the place of
ordinary (basic verdigris) the so-called distilled verdigris (neutral copper
acetate) can be used, of which seventy-six parts are required for the quantity
of arsenious acid given. The emerald green made from ”distilled” verdigris
is of a yet deeper colour than that made from ordinary verdigris, but is
considerably dearer. Such emerald green is commercially described, very
incorrectly, as distilled emerald green.
The method given by Liebig is as follows: 4 parts of basic verdigris and
3 parts of arsenious acid are separately dissolved in boiling vinegar; the
liquids are mixed and evaporated until a precipitate of emerald green
separates.
According to Ehrmann and Kastner, 70 parts of verdigris and 56 to 63
parts of arsenious acid are separately dissolved, the boiling solutions mixed
and boiled until the dirty green precipitate changes into a bright green. In
consequence of the rapid formation of the precipitate due to the boiling, the

colour is pale green; if a dark-coloured product is required, the solutions are
allowed to cool before mixing. After several days a very dark-coloured
precipitate separates from the liquid; it consists of comparatively very large
crystals of emerald green.
Manufacture of Emerald Green from Copper Sulphate.—There are
two processes. Either the solution of copper sulphate is mixed with an
acetate, by which it is converted into copper acetate, and then with
arsenious acid, or copper arsenite is precipitated from the solution of copper
sulphate and then converted into emerald green by treatment with acetic
acid. If acetic acid can be cheaply obtained, the latter process is to be
recommended.
Braconnet gives the following directions: 3 parts of copper sulphate are
dissolved in a little boiling water, the hot solution is mixed with a hot
solution of 4 parts of arsenious acid and four parts of potassium carbonate;
the precipitate is then treated with 3 parts of pyroligneous acid. The dull-
coloured precipitate at first formed is changed by the acetic acid to a bright
green. The colour is more quickly produced when the liquid is heated nearly
to boiling for several hours. When the desired shade appears the boiling is
stopped and the precipitate at once separated from the liquid, by long
contact with which it would give up arsenious acid and thus lose in shade.
According to Fuchs, 5 kilogrammes of lime and 25 kilogrammes of
copper sulphate are dissolved in acetic acid, then a boiling solution of 25
kilogrammes of arsenious acid is added. The precipitate is at once formed;
it is immediately separated from the liquid, washed, and dried; the top
liquor, with the addition of arsenious acid, can be again used to precipitate
the copper lime solution.
Emerald green has an extremely bright colour which surpasses other
green mineral pigments in beauty. The larger are the microscopic crystals of
which it consists the deeper is the colour. In order to obtain the desired deep
shade of emerald green it is necessary to use very dilute solutions; from
strong solutions the precipitate is instantaneously produced, in which case it
is impossible to obtain large crystals. The covering power of emerald green,
in consequence of its crystalline nature, is less the deeper the shade. It
cannot be regarded as a permanent colour: very dilute alkalis and acids
attack it. By the action of sulphuretted hydrogen it is discoloured, owing to

the formation of black copper sulphide. It ought not to be used upon lime,
which withdraws acetic acid and produces yellowish-green copper arsenite.
This pigment should not be used for distempering or for colouring wall
papers. By the action of the atmosphere on the arsenic it contains,
arseniuretted hydrogen is formed—an extremely poisonous gas, very small
quantities of which are sufficient to cause serious illness in the case of
susceptible people. In spite of its poisonous nature, emerald green is still
largely used. All possible shades of green can be produced from it,
yellowish-green or pale green, by admixtures of yellow pigments, such as
chrome yellow, or white pigments, such as barytes or white lead. Many
qualities of green known as palette green, Basle green, etc., are generally
composed of such mixtures.
Mitis Green or Vienna Green is obtained by dissolving verdigris in
acetic acid and adding a boiling solution of white arsenic in water; acetic
acid is then added until the precipitate dissolves. On boiling the clear
solution a green precipitate forms, which, when dry, is a deep bluish-green
of characteristic shade. This pigment is now practically disused because of
its poisonous nature, which is equal to that of emerald green.
Copper Stannate.—This green pigment, also known as Gentele’s
green, can be made by two methods. Gentele gave the following process: 59
parts of tin are converted into stannic chloride by solution in aqua regia; a
solution of 125 parts of copper sulphate is added and copper stannate is
precipitated from the mixture by caustic soda, on washing and igniting it
acquires a pretty green colour. It may also be made by fusing 59 parts of tin
with saltpetre; the potassium stannate is then dissolved in dilute caustic
soda and copper sulphate solution added; the precipitate is washed and
ignited.
Copper stannate is tolerably durable, in this respect it considerably
surpasses emerald green. Only sulphuretted hydrogen has any considerable
action upon it, turning it to a dirty brownish-green hue.
Kuhlmann’s Green is a basic chloride of copper, obtained by heating 2
equivalents of lime with a solution of 3 equivalents of copper chloride. It is
important that the copper salt should be present in excess. In shade
Kuhlmann’s green is very similar to emerald green, with which it agrees in
retaining its colour by artificial light. It is a somewhat less pure green than

emerald green; the difference is only perceived when the two pigments are
directly compared. When, however, it is considered that it is far more
permanent than emerald green and cheaper, a more extended use is
indicated than is yet the case.
Elsner’s Green is a species of lake, made by mixing a solution of
copper sulphate with a decoction of fustic, adding a small quantity of
stannous chloride and precipitating with caustic soda. To 100 parts of
copper sulphate 10 to 14 parts of stannous chloride are used. According as
the copper salt or fustic extract predominates, the colour of the precipitate
inclines to blue or yellow.
Elsner’s green is also sold under the name of ”non-poisonous green”.
This description is incorrect; the pigment is indeed free from arsenic, but is
poisonous on account of the copper it contains.
Casselmann’s Green approaches emerald green in brightness. It
consists of a compound of copper sulphate with copper hydroxide and
water, CuSO₄.3Cu(OH)₂.½H₂O. This pigment is obtained by mixing
solutions of 4 equivalents of copper sulphate and 3 equivalents of sodium
acetate at a certain temperature. The author’s experiments have shown that
the best results are obtained when the solutions are mixed at a temperature
of about 100° C. With this object the solutions are placed in vessels
standing in a pan of boiling water; when their temperatures have risen to
about 100° C. they are quickly mixed, the mixture stirred, and the
precipitate allowed to settle. When the precipitate is cautiously treated with
very weak caustic soda solution a rather deeper colour is obtained. This
treatment should not be continued too long, or the precipitate may acquire
an ugly bluish shade.
Lime Green is a mixture of copper arsenite and calcium sulphate. It is
thus a pigment which ought no longer to be used, on account of its
poisonous properties. It is made by boiling milk of lime with excess of
arsenious acid so long as the latter is dissolved. To this solution of calcium
arsenite, copper sulphate solution is added so long as a precipitate is
formed. A mixture of copper arsenite and calcium sulphate is precipitated.
Patent Green is similar in composition to lime green. A solution of
calcium acetate is made by adding quicklime or pure powdered limestone to
acetic acid; copper sulphate solution is then added, when gypsum is

precipitated and copper acetate remains in solution. A hot solution of
arsenious acid is next added, and the precipitated copper salt mixed with the
gypsum at the bottom of the vessel.
Copper Borate.—A pale green precipitate of copper borate is obtained
by adding a solution of 3 parts of borax to a solution of 2 parts of copper
sulphate. It must be dried at a very moderate temperature, or it will
decompose. When perfectly dry the precipitate can be heated to a red heat
without decomposition; according to the temperature employed different
shades are obtained. It is most convenient to take tests out of the crucible
from time to time during the ignition, and to quickly cool the crucible when
the desired shade is obtained. When levigated this pigment may be used in
oil, or as a porcelain colour.
Copper Silicate (Egyptian Blue).—When a solution of water-glass is
added to a solution of copper sulphate a pale green precipitate of copper
silicate is obtained, which can be heated to redness without alteration.
When 70 parts of white quartz sand, 15 parts of copper oxide, 25 parts of
chalk and 6 parts of soda are fused together a glass is produced which, after
pouring into water, grinding and levigating, exhibits a pretty blue and very
permanent colour. It appears from the examination of the colours of the
Egyptian mural paintings that this pigment was already known to the
ancient Egyptians.

CHAPTER XXX.
VERDIGRIS.

Verdigris is little used as a pigment, but is important to the colour
maker, because copper colours are made from it. In wine-growing countries
it is made from the marc at small expense and without much labour.
In commerce several kinds of verdigris are distinguished; they differ in
physical and chemical properties. All verdigris consists of copper acetate,
either alone or combined with varying quantities of copper hydroxide.
Blue Verdigris is made in large quantities in France and is commonly
known as French verdigris (vert de gris naturel). It has the following
formula:—
Cu(OH)(C₂H₃O₂).2½H₂O.
The process adopted in southern France, especially in the neighbourhood of
Montpellier, is as follows: copper plates are placed in layers in heaps of the
freshly pressed grape residues, which always contain a certain quantity of
must even when the most powerful presses are used. The process is
conducted either in large heaps or in pots; in the first case care must be
taken that the heaps are in a room whose temperature does not lie below 15°
C. It is important to maintain this temperature, because the formation of
acetic acid takes place with sufficient rapidity only at temperatures not
below 12° to 15° C. Too high a rise of temperature in the heaps must also be
avoided or a considerable quantity of acetic acid will be volatilised. The
heaps should therefore not be made too large, otherwise the temperature
cannot be kept within the proper limits.
The residues contain a considerable quantity of sugar, which can be
transformed by fermentation into alcohol. If air has free access, the alcohol
is at once oxidised to acetic acid, which is recognised by the acid smell
given off. In order to make possible the entry of air into the interior of the
heaps, rectangular wooden bars are introduced in piling up the heaps. These
are carefully withdrawn when the heap is finished, so that it is traversed by

passages through which air can enter. By the simultaneous action of air and
acetic acid copper acetate is formed, and since copper is always in excess,
the acetate produced is a basic salt. In consequence of the chemical
processes, the temperature of the heap rises to 35° to 40° C.; it is
endeavoured to attain this temperature by artificial heat. It has already been
stated that too great a rise of temperature is accompanied by a considerable
evaporation of acetic acid. The process is then finished in a short time, but
the loss of acetic acid is remarkable. When the temperature is so regulated
that it varies between 25° and 30° C. the process takes a normal course—
four to five days may be regarded as its duration.
The grape residues are effective on account of the sugar, which is
transformed into alcohol and then into acetic acid. Residues which have
been extracted with water after pressing should not be used in the
manufacture of verdigris; they contain but little sugar and hence undergo a
very feeble acetic fermentation. When such residues are used the verdigris
is often accompanied by black spots of copper sulphide upon the plates; this
is due to incipient decomposition of the residues.
As a practical test for the termination of the process a strip of copper is
plunged into the heap and left for some hours. It should be covered by a
uniform coating of verdigris; if it is covered with small drops that is a sign
that the process is not completed and the heaps must be left some time
longer. The course of the process can be simply and safely followed by
means of a thermometer. This is placed in a perforated copper tube and
plunged into the interior of the heap. A continuous rise of temperature
indicates a steady increase of chemical action; the temperature of the room
is regulated in accordance. When the temperature in the interior of the mass
begins to fall the process is nearing the end. More heat is now supplied to
assist the chemical action. The process is at an end when, in spite of
external heat, the temperature of the heap decreases and approaches the
temperature of the air of the room.
Verdigris can also be made by placing sheet copper and grape residues
in layers in pots, which are deposited in a room of fairly uniform
temperature, such as a cellar. This method has the advantage that the
formation of the verdigris is finished in a shorter time, which is more than
counterbalanced by the labour and expense of filling the pots.

The copper plates, when removed from the heaps or pots, are covered
by a crust of thin needle-shaped crystals of a bright green colour. They are
shaken to free them from the adhering grape skins and seeds, and then
treated with weak vinegar, in which they are dipped; they are then left
standing on edge for several days. This treatment with vinegar has the
object of converting the neutral salt formed on the plates into basic salt. The
dipping into vinegar and exposure to air are repeated six to eight times,
when the originally pure green crust on the plates is gradually changed into
bluish-green verdigris. These processes are continued until the plates are
covered uniformly by a layer of verdigris about 5 centimetres thick. The
crust is then scraped off by copper knives and stirred with water to a paste,
which is pressed in leather bags in rectangular moulds. The lumps are then
slowly dried in the air.
Plates which have been once used give a larger quantity of verdigris in a
second operation, which is to be ascribed to the fact that they possess a
larger surface than unused plates. New plates are made more susceptible to
the further action of the acid by dipping in strong vinegar, by which they
become covered with a layer of copper acetate.
The verdigris obtained by this method consists of crystalline scales of a
pale bluish-green colour, which produce a pale blue powder. In crude
verdigris grape seeds are often found, and occasionally stalks and pieces of
metallic copper. These admixtures are a consequence of the method of
manufacture and cannot be regarded as adulterants. Gypsum is to be
regarded as an adulterant. Verdigris often contains basic copper carbonate.
Verdigris behaves in a peculiar manner with water. In contact with a
small quantity it swells to a bluish-green mass, which becomes quite warm.
Neutral copper acetate and the basic salts 2Cu(C₂H₃O₂)₂.Cu(OH)₂.5H₂O
and Cu(C₂H₃O₂)₂.2CuO.1½H₂O are formed, the latter of which is
insoluble.
By the addition of a larger quantity of water the basic compounds are
decomposed, neutral copper acetate dissolves, and a mixture remains of the
compound Cu(C₂H₃O₂)₂.2CuO.1½H₂O and a brown basic acetate
containing still less acetic acid. On account of this peculiar behaviour of
verdigris towards water some care is required when it is used as a water
colour. A very dilute solution is not green, but has an indefinite shade.

Distilled or Crystallised Verdigris consists of neutral copper acetate,
Cu(C₂H₃O₂)₂.H₂O. It can be made from blue verdigris by treating with the
amount of acetic acid required to completely neutralise the copper oxide, or
by decomposing copper sulphate with the acetate of a metal which forms an
insoluble or difficultly soluble sulphate.
Crystallised verdigris is very simply made from ordinary verdigris; the
latter, whilst still moist, but containing only a small quantity of water, or the
acetic acid would be too largely diluted, is brought into a pan and strong
acetic acid poured over. Strong pyroligneous acid may be used, its
empyreumatic odour is without influence on the quality of the product. The
pan is heated until the contents almost, but not quite, boil. They are
frequently stirred so that the particles at the bottom are brought in contact
with the acid. A dark green solution is formed; when its colour no longer
increases in depth it is allowed to stand until suspended solids sink to the
bottom. The clear solution is then drawn off and rapidly evaporated in
shallow copper pans. It is important that the liquid should not contain an
appreciable quantity of free acid. When a crystalline skin begins to form on
the surface of the solution it is drawn off into the crystallising vessels.
These are made of glazed earthenware; wooden rods are placed in them, on
which the crystallisation takes place. In order that large crystals shall be
obtained, it is necessary to maintain a regular temperature in the room in
which the crystallising vessels are placed. It is heated at the
commencement, and the temperature is allowed to sink a little towards the
end of the operation. From twelve to fourteen days are required to obtain
large crystals. The mass of verdigris crystals adhering to each rod weighs
2·5 to 3 kilogrammes. The residual liquid is a saturated solution of
verdigris, and is used in the next operation. It is only necessary to heat it
with the residue from the first operation and some acetic acid in order to
obtain a crystallisable solution of verdigris.
The residue in the pan contains metallic copper, grape stems and seeds,
and basic copper acetate. The copper is extracted by moistening with acetic
acid and exposing to the air, when verdigris is formed, which is added to a
later batch.
It is often advisable to make verdigris from a soluble copper salt; the
process varies somewhat according to the salt employed. From copper
sulphate verdigris is obtained by the action of calcium acetate. When

solutions of these compounds are mixed, calcium sulphate separates as a
white precipitate, whilst a solution of the easily soluble copper acetate is
left. It is only necessary to mix the solutions in equivalent proportions,
separate the liquid from the precipitated gypsum and evaporate. Gypsum is
somewhat soluble in water, so that when the solution is evaporated a double
salt separates out first. This consists of copper calcium acetate; it may be
used as a pigment, but is of less value than verdigris. It is advisable to
prevent the formation of this double salt, which is accomplished by adding
copper sulphate to a slightly acid solution of calcium acetate until a
precipitate is no longer produced. The solution of copper acetate is boiled
for several hours to effect the separation of the dissolved gypsum; at the
same time the iron contained as impurity in the copper sulphate is also
precipitated. The purified solution of verdigris is concentrated and allowed
to cool, when a further quantity of gypsum separates; the solution is then
evaporated to crystallisation.
If barium salts and lead acetate are cheap, barium acetate can be
obtained by double decomposition. When copper sulphate is added to the
solution of barium acetate so long as a precipitate is formed, a solution of
verdigris is produced, which only requires evaporation to crystallise. The
precipitate is barium sulphate, and can be used as enamel white. It
obstinately retains a quantity of copper, which, though small, is sufficient to
give it a greenish tint. By several washings with acetic acid this copper is
removed; the acid can be used to obtain fresh quantities of verdigris, so that
none is lost, and the copper retained in the precipitate is regained. The
precipitate, after treatment with acetic acid, only requires thorough washing
with water to produce permanent white, satisfactory in every respect.
Ammonia may also be used in the manufacture of verdigris from copper
sulphate. Strong ammonia is left in contact with copper sulphate in a
covered vessel for several hours; gentle warming accelerates the formation
of copper ammonium sulphate. The liquid has then a fine blue colour; it
should not contain sufficient ammonia to be perceptible by the smell. It is
then gently warmed and acetic acid gradually added. On continued heating
small green crystals separate from the solution, which is boiled with
continual additions of acetic acid in small quantities until small crystals
appear on the surface, when it is allowed to cool and the crystals are
strained off.

”Distilled” verdigris forms deep bluish-green crystals, which effloresce
slightly in the air; they dissolve in 5 parts of boiling water and in 13·4 parts
at 20° C. When the solution is boiled for some time the tribasic acetate
separates; the liquid becomes brown and acquires an acid reaction.
German Verdigris does not differ essentially from ordinary verdigris. It
is a compound of the following basic acetates: 2Cu(C₂H₃O₂)₂.CuO and
Cu(C₂H₃O₂)₂.2CuO. The method is similar to that in which grape residues
are used; where acetic acid is cheap the process is especially appropriate. In
Sweden, where pyroligneous acid is made in large quantities and copper is
also cheap, verdigris is made by arranging copper plates and flannel in
alternate layers. The plates are frequently turned over and the flannel kept
saturated with acetic acid. The same chemical reactions occur as in the
process in which grape residues are used. When a layer of verdigris has
formed on the copper plates they are taken apart, exposed to the air, and
frequently wetted. The formation of the verdigris is thus completed. The
difference between this process and that adopted in wine-producing
countries lies simply in the use of pure acetic acid; the product is free from
the mechanical impurities introduced by the grape residues. In properties
German or Swedish verdigris is completely identical with blue verdigris.

CHAPTER XXXI.
CHROMIUM OXIDE.

Chrome Green.—Although very different substances are commercially
known as chrome green, this name is usually applied to the very valuable
pigment which consists of pure chromium oxide. There is hardly another
pigment for the preparation of which such varied directions have been given
and whose shade varies so much according to the manner of production.
The cheapest process is to ignite potassium bichromate with sulphur,
extract the mass with very dilute sulphuric acid and wash the residue. The
chromic acid is reduced by the sulphur; when the mass is treated with
sulphuric acid sulphur dioxide is evolved, potassium sulphide and sulphate
are dissolved, whilst pure chromium oxide remains. The larger the quantity
of sulphur used the paler is the chromium oxide. The purity of the
potassium bichromate has considerable influence on the shade of the
pigment; if iron is present in any quantity a discoloured product always
results. Convenient proportions are: 19 parts of potassium bichromate and 4
parts of sulphur, which produce 9·33 parts of chromium oxide. If potassium
bichromate free from iron cannot be obtained, the shade of the product is
somewhat improved by treatment with dilute hydrochloric acid, in which
ferric oxide dissolves more easily than chromium oxide. When the latter has
been strongly ignited it is soluble with very great difficulty.
There are many formulæ for the preparation of chromium oxide by
means of sulphur: in all the above statement holds good, that the more
sulphur the paler the product.
According to A. Casali, a chrome green which satisfies every
requirement is obtained by strongly igniting 1 part of potassium bichromate
with 3 parts of burnt gypsum. After ignition the mass is boiled with very
dilute hydrochloric acid. In this process the chromic oxide is produced
according to the following equation:—
2K₂C₂O₇ + CaSO₄ = 2Cr₂O₃ + 2K₂SO₄ + 2CaO + 3O₂.

On boiling with hydrochloric acid the lime is dissolved; when the liquid
remains distinctly acid after long boiling it is poured off and the chromium
oxide washed with hot water and dried.
In another method ammonium chromate is very gradually heated. At a
certain temperature the salt suddenly becomes incandescent and is
converted into a dark green, almost black, mass which has a very similar
appearance to dry tea-leaves. The green obtained after washing and
powdering is handsomer the lower the temperature of the decomposition.
Chromium oxide may also be made in the wet way, but the product does
not compare in beauty with that obtained in the dry way. When soda
solution is added to a solution of chrome alum, a greyish green precipitate
of chromium hydroxide is formed; on ignition of the washed precipitate
pure chromium oxide remains.
In a similar manner chromium oxide is produced by precipitating with
soda solution the solution of chromium chloride obtained by adding
hydrochloric acid to a solution of potassium bichromate, and then alcohol in
small quantities, so long as a reaction occurs and the green liquid becomes
deeper in colour.
A handsome and bright chrome green is only obtained when the
potassium bichromate is quite free from iron, a very small quantity of which
has a very harmful effect upon the brightness of the pigment. The author
has found that there is no great difficulty in rendering commercial
potassium bichromate tolerably free from iron by recrystallisation. As much
of the salt is dissolved in boiling water as it will take up, the boiling
solution is quickly filtered and rapidly cooled with continual stirring; the
fine crystalline meal is left on a strainer until no more liquid filters through,
and then washed with a little cold water to remove the mother liquor. The
salt obtained by this simple operation is of great purity and always produces
chrome green of a good shade.
When chrome green contains ferric oxide it shows a blackish colour.
From such chrome green, which has little value, a product of more pure
colour can be obtained with a little care. By treatment with dilute
hydrochloric acid the ferric oxide dissolves, even when it has been
somewhat strongly ignited; chromium oxide loses almost all solubility in
dilute hydrochloric acid after heating at a relatively low temperature. To

facilitate the solution of the ferric oxide, the green to be treated is finely
powdered; it is then covered with a mixture of equal parts of hydrochloric
acid and water. After several days the mass is strained and washed with
pure water until the acid reaction disappears. Comparative analyses have
shown that almost the whole of the ferric oxide can be removed from
chrome green in this manner; the brightness of the colour is considerably
increased. It is, however, advisable to employ materials free from iron in the
manufacture of chrome green.
Chromium oxide can be obtained as a colour of particular brightness,
according to the process of Leune, by precipitating it very slowly from a
solution in which it is contained in the green modification. For this purpose
a solution of chrome alum is boiled until the violet colour has changed to
green; the green modification of chromium oxide is now contained in the
solution. It is cooled to about 10° C., and either freshly precipitated alumina
or zinc carbonate added, very gradually and in small quantities. A fine green
shade of chromium oxide separates; after washing and drying it is obtained
in a condition in which it can be used as a pigment.
Chromium oxide can indeed be obtained by this process, but it is not
distinguished by particular beauty; the author has never been able to obtain,
even by using the precipitant in very small quantities, a chromium oxide
surpassing in shade that obtained by the direct precipitation of a chromium
salt with an alkali.
The emerald green described in the following chapter is obtained in a
similar manner by precipitation with zinc oxide.

CHAPTER XXXII.
OTHER GREEN CHROMIUM PIGMENTS.

Guignet’s Green is a chromium oxide made in the dry way; it is
obtained by grinding 1 part of potassium bichromate with 3 parts of pure
boric acid and water, drying and heating to a dark red heat with access of
air. The hot mass is brought into water; by boiling repeatedly with water it
is freed from the boric acid which it obstinately retains. In order to remove
the last portions of boric acid it is necessary to boil with sulphuric acid and
then with caustic soda; this treatment may be omitted for technical
purposes, since the small quantities of boric acid contained in the pigment
are not harmful.
Guignet’s green is distinguished by the great resistance it offers to
chemical reagents; it is largely used in painting and calico printing. Pale
shades are obtained by additions of barytes.
Emerald Green.—This pigment must not be confounded with the more
important copper compound previously described, to which the name
emerald green is usually applied. It consists of chromium hydroxide
obtained by precipitating a solution of the green modification of a
chromium salt by zinc hydroxide. After careful washing it has a dull green
shade; it is a very durable pigment.
Chrome Green Lake is a mixture of chromium oxide with alumina; it
is obtained by precipitating a solution of alum and a chromium salt by a
soda solution. The precipitate, which contains aluminium and chromium
hydroxides, on ignition takes a paler shade in proportion to the quantity of
alumina it contains. For the chromium salt a solution of potassium
bichromate, which has been allowed to stand with sulphuric acid and
alcohol until if has acquired a pure green colour, may be used.
Turkish Green.—This pigment, which retains its fine green colour in
artificial light, is prepared in a peculiar manner: 40 parts of alumina free
from iron, 30 parts of cobalt carbonate and 20 parts of chromium oxide are
thoroughly ground in a mortar. The mixture is placed in a porcelain tube,

which is exposed to a strong white heat whilst pure oxygen is led through.
The oxygen may be replaced by air if it is previously heated and under
pressure.
Another recipe for Turkish green is to intimately mix 4 parts of freshly
precipitated alumina with 3 parts of cobalt carbonate and 2 parts of
chromium oxide, heat the mixture in a crucible at a white heat, powder and
levigate. Turkish green has a characteristic bluish-green colour; the shade is
inclined to blue or green by increasing the quantity of cobalt carbonate or
chromium oxide.
Leaf Green is a pale green, very durable pigment similar to chrome
green lake. It is obtained by igniting mixtures of chromium oxide with pure
aluminium hydrate; the paleness of the shade is in proportion to the amount
of alumina.
Chromium Phosphate Pigments.—Several chromium phosphates are
used as pigments; the more important are described here.
Arnaudan’s Green is chromium metaphosphate; it is obtained by
intimately mixing 128 parts of neutral ammonium phosphate with 149 parts
of potassium bichromate by long grinding, and carefully heating the
mixture at 170° to 180° C., but not higher, until the mass is pure green. It is
then brought into hot water and thoroughly washed; it has a very handsome
green colour, which is not altered in artificial light.
Ammonium arsenate may be used in the place of ammonium phosphate;
a handsomer green is then obtained, which is, however, very poisonous.
Plessy’s Green is essentially a chromium phosphate mixed with
variable amounts of chromium oxide and calcium phosphate. It is obtained
by boiling a solution of 1 part of potassium bichromate in 10 parts of water
with 3 parts of a solution of acid calcium phosphate and 1 part of sugar,
until the whole has become deep green. The chromic acid is reduced by the
sugar, so that the precipitate contains chromium phosphate, chromium
oxide and neutral calcium phosphate.
These two chromium phosphate greens are very stable towards chemical
reagents, and very durable towards atmospheric influences.
Schnitzer’s Green.—Thirty-six parts of sodium phosphate are melted
in its water of crystallisation, 15 parts of potassium bichromate and 14 parts

of Rochelle salt are then added. A large dish must be used for the fusion,
since the mass effervesces. The colour changes gradually from yellow to
green. When it is pure green the heating is stopped, and, after cooling
somewhat, as much hydrochloric acid is added as the mass will absorb;
then, after standing some time, it is washed with cold and finally with
boiling water. On account of their great durability, the chromium phosphate
pigments are well adapted for paperhangings, calico printing and oil
painting.
Chromaventurine is a glass coloured green by chromium oxide. It is
hardly used as a painters’ colour in the ordinary sense of the term; it is
applied in porcelain painting as an under-glaze colour, and is largely used
for colouring glass.
Chromaventurine is most simply made by the process of Pelouze: 250
parts of quartz sand, 100 parts of soda, 50 parts of calcium carbonate and 40
parts of potassium bichromate are fused together. The presence of iron
would have a very harmful effect on the shade of the product. Quartz sand
generally contains a small quantity of ferric oxide, which should be
extracted by treatment with strong hydrochloric acid when a product of the
best colour is required. The calcium carbonate should also, as far as
possible, be free from ferric oxide.
Chrome Blue (Garnier).—A mixture of 48·62 parts of potassium
chromate with 65 parts of fluorspar and 157 parts of silica is fused in a
crucible lined with coal-dust.

CHAPTER XXXIII.
GREEN COBALT PIGMENTS.

From mixtures of chromium oxide, cobalt carbonate and alumina in
different proportions a series of pigments can be obtained varying in shade
between pale blue and bluish-green. On account of the great stability of
these colours at the highest temperatures they are of great importance in
porcelain painting, for which they are principally used. It is necessary that
the porcelain should be free from iron, since ferric oxide forms a black
compound with chromium oxide, very small quantities of which would be
sufficient to considerably damage the fineness of the colour.
Cobalt Green, which is also known as Rinmann’s green or zinc green,
is a compound of the oxides of cobalt and zinc. It is always produced when
a cobalt compound is ignited with zinc oxide. Rinmann’s green has not so
deep a colour as the poisonous emerald green, but it is distinguished by
extraordinary durability, and deserves to be more largely used than it is at
present. This pigment is most simply made by precipitating the mixed
solutions of a zinc salt and a cobalt salt. It is also formed by moistening
pure zinc oxide with a cobalt solution and igniting.
In precipitating the mixed solutions of zinc and cobalt salts products of
different shades are obtained, according to the proportions in which the salts
are used. If equivalent quantities are used, an almost black product is
obtained, quite useless as an artists’ colour. The best result is obtained by
mixing intimately pure precipitated cobalt carbonate with zinc oxide and
igniting the mixture; a mixture of 9 to 10 parts of zinc oxide with 1 to 1·5
part of cobalt carbonate gives colours between pale green and dark green.
Especially fine Rinmann’s green is obtained by igniting cobalt arsenate
with zinc oxide and arsenious acid. The addition of the last-named
substance can only have the object of preventing the temperature from
rising too high, which might injure the beauty of the colour; arsenious acid
is volatile at a fairly low temperature.

Cobalt green is also obtained by evaporating a solution of cobalt nitrate
and zinc nitrate and igniting the residue, or by igniting a mixture of the
sulphates. In the latter case a tolerably high temperature is necessary in
order that the sulphates may be decomposed.
The author has found that a particularly handsome product is obtained
by mixing pure zinc oxide with a dilute solution of cobalt chloride, drying
and then slowly heating to redness in a crucible with a well-fitting lid.
Towards the end of the operation the heat is considerably increased and
maintained for a short time, after which the mass is rapidly cooled.

CHAPTER XXXIV.
GREEN MANGANESE PIGMENTS.

Manganese Green, Rosenstiehl’s Green.—This pigment, which is
handsome but difficult to produce, consists of barium manganate. It can be
obtained by several methods, all of which produce a good green, but with
very unequal properties. Manganese green made from barium nitrate has
little durability; when made from barium hydrate it is more costly, but very
durable.
Barium manganate is most easily obtained by precipitating a boiling
solution of potassium manganate by barium chloride. The almost blue
precipitate becomes nearly white when washed and dried, but when it is
gradually heated on a porcelain plate to a dark red heat it acquires a fine
green colour. The heating must be carefully conducted; if the temperature
rises too high, the colour changes to a dirty greyish brown, owing to the
reduction of the manganic acid.
This pigment is also made by heating 14 parts of manganic oxide, 80
parts of barium nitrate and 6 parts of barytes with access of air until the
desired shade appears. The mass is then ground in a continuous current of
water until it is changed into a very fine powder and nothing more is
dissolved by the water. The best result is obtained by Rosenstiehl’s process:
4 parts of barium hydrate, 2 parts of finely-powdered barium nitrate and 0·5
part of artificial manganese dioxide are rapidly mixed; the mixture is
moistened and heated to a dark red heat. The fused mass is boiled out with
water and the residue dried under a bell jar, under which are dishes of
sulphuric acid and caustic potash, the former of which absorbs the water,
whilst the latter keeps the air free from carbonic acid, which would injure
the shade of the moist substance.
Manganese green is amongst the pigments more recently discovered.
On account of its high price it has found but limited use up to the present. It
may be used in any vehicle, and is distinguished by great permanence.

In regard to the preparation of this colour by Rosenstiehl’s method,
which of all gives the best results, it should be observed that the shade
largely depends on the quantity of barium hydrate used; the greater this is
the more the shade inclines to blue. If it is desired to produce a colour of a
greener tone than is produced by the fusion, this can be accomplished by
boiling for a very long time with very weak hydrochloric acid, which
extracts a portion of the base from the compound and produces a deeper
shade.
Böttger’s Barium Green.—A beautiful green is obtained by the
process given by Böttger, as the author has found. It consists of barium
manganate. The process is, however, somewhat costly, so that the pigment
would only be available for artists’ purposes.
A solution of potassium manganate is first made by gradually adding 2
parts of very pure pyrolusite, finely powdered, to a fused mixture of 2 parts
of caustic potash and 1 part of potassium chlorate, bringing the mass to a
low red heat after the introduction of the whole of the pyrolusite, and finally
extracting with water, in which the potassium manganate dissolves to a fine
emerald green solution. This process is unattended by difficulty if the
pyrolusite contains a sufficient quantity of manganese peroxide and is in a
sufficiently fine powder, which is most important. Potassium manganate is
a very unstable substance. Cold water must be used to dissolve it, and the
solution should not long stand exposed to the air, but should at once be used
to obtain barium manganate.
When the solution of potassium manganate is mixed with a solution of a
barium salt, a handsome violet precipitate immediately forms. This is
washed with water and quickly ground with ¾ to 1 part of its weight of
barium hydrate. The mixture is heated in a copper dish with constant
stirring to a low red heat; the colour changes gradually to a very fine green.
When the mass has reached the proper shade it is treated with cold water to
remove excess of barium hydrate, until the washings show no trace of
alkaline reaction.
Manganous Oxide is occasionally used as a green pigment, especially
for painting metal work; it is obtained in the following manner: Manganese
sulphate solution is precipitated by soda solution, and the manganous
carbonate obtained strongly ignited in a crucible. In order to avoid

oxidation of the manganous oxide, which would readily occur in the
process, it is necessary to prevent air from reaching the substance. With this
object the crucible in which the heating is conducted is covered by another
from which the bottom has been removed and which is filled with coal. The
air entering the crucible on cooling must pass through the layer of glowing
coals, by which it is deprived of oxygen.
Manganese Blue is obtained, according to G. Bong, by igniting a
mixture of 3 parts of quartz, 6 parts of soda ash, 5 parts of limestone and 3
parts of manganese oxide, or of 3 parts of quartz, 8 parts of barium nitrate
and 3 parts of manganese oxide, air being admitted and reducing gases
excluded. All materials must be free from iron. The quantity of manganese
oxide regulates the depth of the colour, but not its shade; by increasing the
soda a greener, by increasing the quartz a more violet, shade is obtained.

CHAPTER XXXV.
COMPOUNDED GREEN PIGMENTS.

Mixtures of a yellow and a blue pigment produce a green; according as
one or the other predominates colours are obtained inclining to yellow or
blue.
In some cases the mixing can be accomplished in the actual production
of the pigments, so that a green precipitate is directly produced. This is,
however, rarely the case; generally the compound pigments are obtained by
simply mixing the two colours. The mixing can be done either in the dry or
the wet way; colours in the wet state are more easily mixed than when dry,
so that even when dry colours are employed the mixing is done with the
addition of water. Although the mixture then requires a second drying, this
method is still to be recommended, in the first place, because the mixing
can be more quickly done in consequence of the greater mobility of the
mass, and in the second, because the formation of poisonous dust is
completely avoided.
The mixing is generally accomplished by mechanical arrangements. If
dry colours are to be mixed, rotating cylinders may be used; these are filled
with the materials to be mixed, well closed and rotated about the axis as
long as may be necessary. It should be observed that when colours which
have a very different specific gravity, such as chrome yellow and Prussian
blue, are to be mixed, the cylinders must be rotated for a much longer time
than when colours of approximately equal specific gravity are to be united.
In working in the wet way, which is to be preferred, sufficient water is
added to the colours to make a paste thin enough to be mixed by spatulas;
the mixture is thoroughly stirred and the pulp ground in ordinary mills until
the mixture is quite uniform. Already whilst on the mills the pulp becomes
thicker in consequence of evaporation; when the grinding is finished it is
spread out in thin layers so that it may dry as quickly as possible. This is
important in the case of mixtures whose constituents have very different
specific gravities, otherwise the heavier pigment may sink to the bottom of
the paste and the mass thus lose its uniformity.

The compound green pigments come into the market under most varied
names, which are often entirely without connection with their chemical
composition. Such names are: mineral green, English green, oil green,
green vermilion; the most common are chrome green and Brunswick green.
It should be noted that these names are more strictly applied to simple
colours previously described.
Chrome Green is generally made by mixing deep chrome yellow and
Prussian blue; it can naturally be obtained in all possible shades. The
brightness of this already handsome pigment can be considerably increased
by the addition of a small quantity of indigo carmine. In this case the
mixing is best done by adding a solution of the indigo carmine to the stiff
paste and again sending the mixture through the mill.
By additions of white substances paler shades of chrome green are
obtained; levigated terra alba or white pipe clay can be advantageously
employed.
Elsner’s Chrome Green is obtained by preparing a solution of yellow
prussiate of potash and potassium chromate and another solution of lead
acetate and ferric chloride. The two liquids are mixed with vigorous
stirring; according to the proportions of the materials a blue or a yellow
shade of green is produced.
Silk Green.—Forty-one parts of lead nitrate are dissolved in 20 to 30
times the weight of water, the solution is boiled in a copper pan, and,
according to the shade required, 10 to 30 parts of fine Chinese blue are
added. After well stirring, a solution of 10 parts of potassium bichromate
and 1 part of nitric acid is poured into the boiling liquid, the mixture is
again well stirred, the precipitate allowed to settle, washed and dried. The
green pigment obtained in this way has a peculiar silky lustre, hence the
name ”silk green”.
Natural Green is a mixture of Guignet’s green with picric acid; it is
used in making artificial flowers instead of emerald green.
Non-arsenical Green was proposed as a substitute for emerald green,
which, however, it does not equal in brilliance. It is obtained by mixing
copper blue (basic copper carbonate) with chrome yellow, chalk and ferric

oxide, in somewhat variable proportions. It usually contains 80 to 82 per
cent. of the copper compound and 13 to 15 per cent. of chrome yellow.

In making mixtures of pigments it is to be remembered that chemical
actions may occur in the mixture; colours which act upon one another
should not be mixed: the mixture would contain the cause of its own
destruction. For example, a lead pigment should not be mixed with one
containing sulphur in the form of a sulphide or sulphate. By following this
important rule the number of pigments which may be mixed together is
considerably diminished; the advantage is that durable colours are
produced, which do not change in a short time to an unrecognisable shade.

CHAPTER XXXVI.
VIOLET MINERAL PIGMENTS.

Chromic Chloride.—The violet modification of chromic chloride is a
compound which is little used, but deserves the highest attention on account
of its durability and beauty of shade. Up to the present chromic chloride has
been almost exclusively used for colouring paper hangings under the name
of chrome bronze, which it has received in consequence of its property of
imparting a peculiar metallic lustre to paper upon which it is rubbed. It is
also possible to fix this substance upon fabrics, which thus acquire a similar
metallic lustre.
Pure chromic chloride forms beautiful peach-blossom coloured scales,
which can only be obtained by treating chromium oxide in a certain manner
with chlorine. The pure substance is practically insoluble in water, but if the
water contains but a trace of chromous chloride the chromic chloride
readily dissolves to a green solution. In the preparation of this compound it
is necessary to avoid the least trace of chromous chloride, otherwise the
product will begin to change when it comes into contact with moist air.
Pure chromic chloride is obtained by proceeding according to the
method first given by Wöhler: the simple apparatus necessary is depicted in
Fig. 29. Pure chromic oxide is first made by any suitable process; this is
made into a paste with charcoal, starch paste and water, and formed into
small balls which are heated in a crucible to a white heat. The residual
intimate mixture of chromic oxide and carbon is loosely placed in a crucible
which stands in a furnace; a porcelain tube is cemented into the bottom of
the crucible, it passes through the ashpit to connect with a chlorine
apparatus. Upon the crucible another larger one is placed, which has a small
opening in the bottom and serves as a receiver for the sublimed chromic
chloride. The heating and current of chlorine are started at the same time,
the fire is regulated so that the lower crucible glows brightly. This is
continued for an hour, during which a rapid current of chlorine must pass
into the crucible; the fire is then removed and a slow current of chlorine
passed through the apparatus until it is quite cold.

Fig. 29.

If the operation has succeeded the lower crucible is found empty and the
interior of the upper covered with the magnificent peach-blossom crystals
of chromic chloride. A small quantity of the chromic chloride is tested as to
its behaviour towards water; if it remains unaltered the whole may be
washed, but if it dissolves to a green solution it must be again ignited in a
current of chlorine. The washing is necessary to remove small quantities of
aluminium chloride formed by the action of chlorine on the clay of the
crucible.
Manganese Violet, or Nuremberg Violet, consists of manganic
phosphate. It is obtained by fusing glacial phosphoric acid with pure
pyrolusite, boiling the melt with ammonium carbonate, filtering the
solution, evaporating to dryness, and again fusing the residue. After boiling
with water this forms a handsome violet powder; it is suitable for use as a
very durable artists’ colour.
Shades of this pigment inclining to blue are obtained when a ferric
compound is added in the first fusion; the more iron ore is used the more

intense is the blue tone.
Tin Violet or Mineral Lake is obtained as a violet mass by igniting an
intimate mixture of 100 parts of tin dioxide with 2 parts of chromic oxide.
This pigment is completely permanent in air; it is suitable for printing paper
hangings and for colouring faience.
Copper Violet, Guyard’s Violet.—According to J. Depierre, this
pigment is obtained by precipitating a solution of copper ammonium
sulphate with potassium ferrocyanide, washing and drying the precipitate
and heating in a porcelain dish. At 170° C. cyanogen and ammonia are
evolved, the mass takes up oxygen and becomes violet; it then resists the
action of dilute acids and alkalis, and as a pigment has great covering
power. On heating to 200° C. it turns blue, and at 240° to 250° C. becomes
greenish.

CHAPTER XXXVII.
BROWN MINERAL PIGMENTS.

Lead Brown.—When red lead is treated with nitric acid, lead monoxide
is dissolved, whilst deep brown lead peroxide remains. When the action of
the nitric acid is finished the residue is well washed and dried. Lead
peroxide is now extensively used for lucifer matches; it readily gives up
oxygen on heating, and thus accelerates the ignition of the composition.
Brown match-heads owe their colour to lead peroxide.
Manganese Brown consists of manganic oxide; it occurs in nature, but
seldom in a state of sufficient purity to be used as a pigment; it is, therefore,
generally made artificially. The process is very simple. A solution of
manganese sulphate is precipitated by caustic soda solution, the manganous
hydroxide produced is quickly changed by the action of the air into
manganic hydroxide. The colour is most rapidly developed when the
precipitate is spread out in thin layers; it is well washed after it has changed
to brown.
Pyrolusite Brown is manganese peroxide in a state of fine division. It
can be made from the residual liquors of the preparation of chlorine by
adding sodium hypochlorite. A brown precipitate results, which is kept in
contact with the liquid until it no longer changes in colour. It is then washed
with water slightly acidified with sulphuric acid, and finally with pure
water. When finely powdered, manganese peroxide has a very fine brown
colour. It is quite unaltered by the air and thus, in consideration of its
cheapness, deserves a much greater use than it has yet found.
Prussian Brown.—When Prussian blue is heated in air it soon begins
to glow, and is converted into a brown mass, the shade of which depends on
the amount of impurity in the Prussian blue. It is desirable to use Prussian
blue quite free from impurities, so that a brown of the same shade is always
obtained. The brown obtained can be shaded by additions of indifferent
substances.