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Title: A text-book of assaying
for the use of those connected with mines.

Author: C. Beringer
J. J. Beringer

Release date: July 3, 2006 [eBook #18751]

Language: English

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Transcriber's Note:
Parentheses have been added to clarify fractions. Letters in brackets with a = sign before it
means that the letters have a macron over them, e.g. H[=A=c] signifies that the Ac has a
macron over it.

Minor typographical errors have been corrected. Footnotes have been moved to the end of
the chapter, and all advertisements have been moved to the end of the book.

A TEXT-BOOK OF ASSAYING:
FOR THE USE OF THOSE CONNECTED WITH MINES.

BY

C. AND J. J. BERINGER.
REVISED BY

J. J. BERINGER,
ASSOC. OF THE ROYAL SCHOOL OF MINES; FELLOW OF THE CHEMICAL
SOCIETY AND OF THE INST. OF CHEMISTRY; PRINCIPAL OF THE CAMBORNE
MINING SCHOOL; AND LATE PUBLIC ANALYST FOR THE COUNTY OF
CORNWALL.

With numerous Diagrams and Tables.

NINTH EDITION.

LONDON:
CHARLES GRIFFIN AND COMPANY, Limited,
EXETER STREET, STRAND.

1904.

[All rights reserved.]

PUBLISHER'S NOTE TO THE NINTH EDITION
The continued popularity of the present work, the last edition of which was published only a
little over a year ago, continues to be a source of gratification to the publishers, who have
much pleasure in issuing the present edition.
January 1904.

PREFACE TO THE SIXTH EDITION
The principal changes in this edition are additions to the articles on Gold, Cyanides, and
Nickel, and a much enlarged Index. The additional matter covers more than forty pages.
J. J. BERINGER.

Camborne,
January 1900.

PREFACE.
The Text-book now offered to the public has been prepared to meet the existing want of a
practical "handy book" for the Assayer.
To mining men the word "assaying" conveys a sufficiently clear meaning, but it is difficult
to define. Some writers limit it to the determination of silver and gold, and others imagine
that it has only to do with "furnace-work." These limitations are not recognised in practice.
In fact, assaying is becoming wider in its scope, and the distinction between "assayers" and
"analysts" will in time be difficult to detect. We have endeavoured rather to give what will
be of use to the assayer than to cover the ground within the limits of a faulty definition.
At first our intention was to supply a description of those substances only which have a
commercial value, but on consideration we have added short accounts of the rarer elements,
since they are frequently met with, and occasionally affect the accuracy of an assay.
Under the more important methods we have given the results of a series of experiments
showing the effect of varying conditions on the accuracy of the process. Such experiments
are often made by assayers, but seldom recorded. Statements like those generally made—
that "this or that substance interferes"—are insufficient. It is necessary to know under what
conditions and to what extent.
Students learning any particular process cannot do better than repeat such a series of
experiments. By this means they will, at the same time, acquire the skill necessary for
performing an assay and a confidence in their results based upon work under different
conditions.
The electrolytic method of copper assaying given under Copper is a modification of
Luckow's; it was introduced by us into the offices of the Rio Tinto Copper Company, and
has been in use for many years with success. This modification is now employed in copper-
works in Spain, Germany, and England, and is used in place of the dry assay for the
commercial valuation of copper ores.
We have adhered to the gram and the "c.c." as the units of weight and volume. Those who
prefer working with grains and grain-measures can use the figures given, multiplied by ten.
For example:—When 1 gram is mentioned, 10 grains should be used, and 10 grain-
measures take the place of 1 "c.c." It is not advisable to mix the two systems, as by using
gram weights and grain-measures.
We have intentionally to a large extent omitted to mention the names of those who have
originated or modified the various processes. The practice of naming a process after its
discoverer has developed of late years, and is becoming objectionable. It is a graceful thing
to name a gas-burner after Bunsen, or a condenser after Liebig; but when the practice has
developed so far that one is directed to "Finkenerise" a residue, or to use the "Reichert-
Meissl-Wollny" process, it is time to stop.

We are indebted to the standard works of Allen, Crookes, Fresenius, Lunge, Michell, Percy,
and Sutton, and wish to express our sense of special indebtedness to Mr. Richard Smith, of
the Royal School of Mines. One or two of the illustrations are taken from Mr. Sexton's
excellent little book on Qualitative Analysis. Our obligation to some others is mentioned in
the text.
Finally, we have to thank for assistance in the experimental work Messrs. Bailey, Beswick,
Clarke, Grant, Higgins, and Smith.
THE AUTHORS.
Camborne, Nov. 1889.

CONTENTS.
PART I.

CHAPTER I.
INTRODUCTORY.
Page
Object of assaying 1
Sampling 1
Drying: determination of moisture 5
Calculation and statement of results 7
Laboratory books and report forms 9
Quantity to be taken for an assay 11
Exercises 14

CHAPTER II.
METHODS OF ASSAYING.—DRY GRAVIMETRIC METHODS.
Methods of assaying 15
Gravimetric methods 15
Mechanical separations 16
Dry assays 16
(a) Fluxes 16
(b) Reducing agents 21
(c) Oxidising agents 22
(d) Apparatus 24

CHAPTER III.
WET GRAVIMETRIC METHODS.
Wet gravimetric methods 27
(a) Solution 29
(b) Precipitation 30
(c) Filtration 31
(d) Drying and igniting 32

CHAPTER IV.
VOLUMETRIC ASSAYS.
Titrometric assays 35
(a) Standard solutions 36
(b) Standardising 37

(c) Methods of working 42
(d) Indirect titration 43
Colorimetric assays 44
Gasometric assays 44

CHAPTER V.
WEIGHING AND MEASURING.
Weighing 47
Measuring liquids 49
(a) Graduated flasks 49
(b) Pipettes 50
(c) Burettes 51
Measuring gases 52

CHAPTER VI.
REAGENTS.
Acids, &c. 54
Bases, salts, &c. 59

CHAPTER VII.
Formulæ, equations, &c. 68

CHAPTER VIII.
SPECIFIC GRAVITY.
Introductory 75
Determination of specific gravity—
(a) Hydrometers 76
(b) Specific gravity bottles 78
Calculations depending on specific gravity 84

PART II.

CHAPTER IX.
SILVER, GOLD, PLATINUM, CYANIDES, MERCURY.
Silver—Detection 87
Dry assay 87
(1) Scorification 88
(2) Pot assays, average ores 90
" ores with metallic oxides 91
" ores with metallic sulphides 91

Explanatory notes on the fusion 93
The effect of charcoal, flour, &c. 94
The effect of nitre 95
The effect of mineral sulphides 95
(3) Cupellation 98
The loss of silver 101
Condition affecting the loss 102
Methods of correction 103
Lead required for cupellation 105
(4) Calculation of the results in ounces to the ton of 2240 lbs. Table 107
Ores with metallic particles 108
(5) Explanatory notes 110
(6) Examples of dry silver assays 113
Wet assays 116
Gravimetric method 117
Gay-Lussac's method 119
Volhard's method 121
A modified Gay-Lussac 123
Volhard's method applied to arsenic 124
Gold—Detection 126
Amalgamation assay 126
Dry assay 127
(1) Size of charges 127
(2) Sampling 127
(3) Assay tons 131
(4) Small buttons, weighing 131
" " measuring 133
(5) Concentration in lead 136
Quartz ores 136
Ores with oxide of iron 138
Ores with metallic sulphides 139
(6) Cyanide charges, residues, &c. 140
(7) Cupellation 142
Cupels 142
Cupellation temperature 143
Cupellation loss 145
(8) Inquartation 146
(9) Flatting 149
(10) Parting, in flasks 151
" in test tubes 152
" in glazed crucibles 153

" Loss, &c. 154
(11) Check assays, surcharge 154
(12) Bullion assays in special apparatus 156
Silver, &c., in gold bullion 157
(13) Sampling of base bullion, &c. 157
Cyanides—Commercial cyanides 160
Double cyanides 161
Prussic acid 162
Gold-dissolving power of cyanide liquor 162
Assay for cyanide strength 163, 165
Assay of commercial cyanide 167
Alkalinity of cyanides 167
Acidity of ores 168
Metals in cyanide liquors 169
Cyanicides 169
Platinum 170
Iridium 171
Mercury 171
Dry assay 172
Wet method 173

CHAPTER X.
COPPER, LEAD, THALLIUM, BISMUTH, ANTIMONY.
Copper—Introductory 175
Dry assay 176
Valuation of copper ores 181
Wet methods 183
(1) Electrolytic assay 184
Volumetric methods 194
(1) Cyanide method 194
(2) Iodide method 199
(3) Colorimetric method 203
Examination of commercial copper 205
Lead 211
Dry assay 211
Wet assay 213
(1) Gravimetric method 213
(2) Volumetric method 214
(3) Colorimetric method 218
Thallium 219
Bismuth 220

Dry assay 221
Wet method 221
(1) Gravimetric determination 222
(2) Colorimetric assay 223
Antimony 225
Dry assay 225
Wet method 227
(1) Gravimetric assay 228
(2) Volumetric method 229

CHAPTER XI.
IRON, NICKEL, COBALT, ZINC, CADMIUM.
Iron 231
Gravimetric determination 233
Permanganate and bichromate methods 234
Stannous chloride method 244
Colorimetric determination 247
Nickel 251
Dry assay 251
Electrolytic assay 254
Titration by cyanide 255
Cobalt 259
Zinc 261
Gravimetric method 262
Volumetric method 263
Gasometric method 266
Cadmium 269

CHAPTER XII.
TIN, TUNGSTEN, TITANIUM.
Tin 271
Vanning 273
Dry assay 276
Detection, &c. 279
Gravimetric determination 281
Volumetric determination 282
Examples 284
Titanium 292
Tungsten 295
Niobic and Tantalic Oxides 297

CHAPTER XIII.
MANGANESE, CHROMIUM, ETC.
Manganese 298
Gravimetric determination 300
Volumetric determination 300
Ferrous sulphate assay 301
Iodine assay 302
Colorimetric determination 306
Chromium 307
Vanadium 310
Molybdenum 311
Uranium 312

CHAPTER XIV.
EARTHS, ALKALINE EARTHS, ALKALIES.
Alumina 314
Thoria 317
Zirconia 317
Cerium 318
Lanthanum and Didymium 319
Yttria 319
Beryllia 319
Lime 320
Strontia 324
Baryta 326
Magnesia 328
The Alkalies 330
Sodium 334
Potassium 336
Lithium 338
Cæsium 339
Rubidium 340
Ammonium 340

PART III.

CHAPTER XV.
OXYGEN AND OXIDES—THE HALOGENS.
Oxygen 344
Oxides 345

Water 350
The Halogens 358
Chlorine 359
Bromine 361
Iodine 362
Fluorine 363

CHAPTER XVI.
SULPHUR AND SULPHATES.
Sulphur 367
Gravimetric determination 369
Volumetric determination 370
Sulphates 377
Selenium 379
Tellurium 379

CHAPTER XVII.
ARSENIC, PHOSPHORUS, NITROGEN.
Arsenic 381
Gravimetric determination 383
Volumetric method, "iodine" 384
" " "uranic acetate" 389
Phosphorus 394
Gravimetric determination 396
Volumetric determination 397
Nitrogen and Nitrates 400

CHAPTER XVIII.
SILICON, CARBON, BORON.
Silicon and Silicates 405
Carbon and Carbonates 414
Coals 418
Shales 420
Carbonates 424
Boron and Borates 429

APPENDIX A.
Table of atomic weights and other constants 433
Table for converting degrees of the centigrade thermometer
into degrees of Fahrenheit's scale 435
Tables showing strengths of aqueous solutions of nitric and hydrochloric acids,

of ammonia and of sulphuric acid 436

APPENDIX B.
Estimation of small quantities of gold 440
Practical notes on the iodide process of copper assaying 441
Method of separating cobalt and nickel 442

APPENDIX C.
A lecture on the theory of sampling 444

Index 450

A TEXT-BOOK OF ASSAYING.

CHAPTER I.
INTRODUCTORY.

Assaying has for its object the determination of the quantities of those constituents of a
material which add to or detract from its value in the arts and manufactures. The methods of
assaying are mainly those of analytical chemistry, and are limited by various practical
considerations to the determination of the constituents of a small parcel, which is frequently
only a few grains, and rarely more than a few ounces, in weight. From these determinations
calculations are made, which have reference to a mass of material of, perhaps, hundreds of
tons. But in all cases, whether the mass under consideration be large or small, whether the
material be obtained by mining, grown, or manufactured, the assayer is supposed to receive
a small quantity, called "the sample," which is, or ought to be, the exact counterpart of the
mass of material that is being dealt with. The taking and making of this sample is termed
"sampling"; and the men whose special work it is to select such samples are "the samplers."
But although "sampling" is thus distinct from "assaying," the assayer should be familiar
with the principles of sampling, and rigorous in the application of these principles in the
selecting, from the sample sent him, that smaller portion upon which he performs his
operations.
Sampling.—In the case of gases, there is absolutely no trouble in mixing. The only
difficulty is in drawing off a fair sample where, as in flues, the body of the gas is in motion,
and varies a little in composition from time to time. In this case, care must be taken to draw
off uniformly a sufficient volume of the gas during a prolonged period; any portion of this
larger volume may then be taken for the analytical operation.
In the case of liquids, which mix more or less easily—and this class includes metals, &c., in
the state of fusion—more or less severe agitation, followed by the immediate withdrawal of
a portion, will yield a fairly representative sample.
In the case of solids, the whole mass must be crushed, and, if not already of fairly uniform
quality, mixed, before sampling can take place. Most of the material which a sampler is
called upon to deal with, is, however, in a more or less divided state and fairly uniform. In
practice it is assumed that 5 per cent. of the whole (= 1/20th), if taken in portions of equal
weight and at frequent and regular intervals, will represent the mass from which it was
taken. Taking a heap of ore, A, and selecting one out of every twenty spade-, bag-, barrow-,
or wagon-fuls, according to the quantity of stuff in the heap, there is obtained a second
heap, B, containing one-twentieth of the stuff of the heap A. If we crush the stuff in B until
this heap contains approximately the same number of stones as A did—which means,
crushing every stone in B into about twenty pieces—B will become the counterpart of A.
Selecting in the same manner 5 per cent. of B, there is got a third heap, C. This alternate

reduction and pulverising must be carried on until a sample of suitable size is obtained. This
may be expressed very clearly thus:—
A = 1000 tons of rocks and lumpy ore.
B = 50 " " rough stones, 1/20th of A.
C = 2.5 " " small stones, 1/20th of B.
D = 0.125 " " coarse powder, 1/20th of C.

If the material to be sampled is already a dry powder, 5 per cent. of it should be heaped in a
cone; each lot being added on the apex of the cone already formed, so that it may distribute
itself by falling evenly in all directions. When the cone is completed, convert it into a low
frustrum of a cone by drawing stuff uniformly and in a direct line from the centre to the
circumference. Draw two diameters at right angles to each other, and reserving any two
alternate quarters, reject the others. Mix; and form another cone, and proceed until a sample
is got of the bulk required.
This is the usual plan, and all samples should be treated in this way when the stuff is fine
enough to fall evenly down the sides of a cone.
Samples as they reach the assay office are seldom in a fit state for the work of the assayer;
they are generally too coarse, and ought always to be more than he wants for any particular
determination. The portion he requires should never be taken at hap-hazard; the sample
must be reduced systematically to the quantity required.
1. If the sample is a liquid: it is sufficient to shake the bottle, and take out a measured or
weighed quantity for the assay.
2. If a liquid with a solid in suspension: measure the whole of it. Filter. Make up the filtrate
with the wash-water or water to the original bulk. Assay it. Dry and weigh the residue, and
make a separate assay of it.
3. If of a creamy consistency, free from heavy particles: mix well; spread out evenly on a
glazed tile. Take up equal portions at equal distances. Mix and assay.

4. If a mud of coarse and fine particles, or of particles of unequal density: weigh and
transfer to a porcelain dish, or weigh in the dish. Dry at 100° C., weigh. Treat the residue as
a solid capable of being powdered.
5. If a solid capable of being powdered, or already powdered: heap up into a cone; flatten
with a spatula; divide along two diameters at right angles, and carefully reject the whole of
two alternate quarters, brushing away any fine powder. Mix the other quarters, and repeat (if
necessary). For small quantities a fine state of division is essential.
6. If a solid with metallic particles: powder and pass through a sieve; the metallic particles
will not pass through. Weigh both portions and assay separately. Sifting should be followed
by a very thorough mixing.
7. If a metal or alloy in bar or ingot: clean the upper surface of the bar, and bore through
the bar. Use the borings. If the ingot or bar is small, cut it through and file the section.
Filings must be freed from fragments of the file by means of a magnet; and from oil, if any
be present, by washing with a suitable solvent.[1] Where practicable, metals and alloys are
best sampled by melting and granulating. The student must carefully avoid any chance of
mixing dirt or particles of other samples with the particular sample which he is preparing.
One ore should be done at a time, and when finished, it should be labelled and wrapped up,
or bottled, before starting on a fresh sample.
When an ore requires to be very finely ground in an agate mortar, it is often advisable to
mix with a little pure alcohol and rub until free from grit; dry at 100° C. and mix well
before weighing.
When an assay is required of a quantity of ore made up of parcels of different weight and
quality, each parcel should be separately sampled and parts of each sample, bearing to each
other the same proportion by weight as the original parcels, should be taken and mixed. For
example, a lot of ore is made up of one parcel of A, 570 tons, one of B, 180 tons, and
another of C, 50 tons; a sample representing the whole may be got by mixing 57 parts of a
sample of A with 18 parts of a sample of B, and 5 parts of a sample of C.

A bruising plate, like that in fig. 2, is convenient for general office work. The slab is of cast
iron, about an inch thick. It is firmly supported on a solid block of wood, and pivoted for
convenience in emptying. The bruising-hammer is steel-faced, about 4 inches square, and 1-
1/2 inch thick. The block is firmly fixed to a small table or tressel, so that the slab is about 2
feet 6 inches from the ground. The slab is cleaned, and the sample collected with the help of
a stiff-haired brush.
Drying: Determination of Moisture.—In practice, the moisture is generally determined by
the samplers, and the proportion is specified in grains per pound on the label attached to the
sample when it reaches the assay office. The method adopted is usually to dry 1 lb. = 7000
grs. of the ore in a frying-pan heated over a gas flame, or in an ordinary oven, until a cold
bright piece of metal or glass is no longer damped when held over it. The loss of weight in
grains = moisture.
Properly, however, this work should be done by the assayer, if only for the following
reason. It is assumed that the dry ore of the sampler and of the assayer are the same thing;
according to the nature of the ore, this may or may not be the case. The assayer, however,
uses the sample which he has dried for his moisture-determination, as the dry ore on which
he makes his other assays, and no variation in moisture would influence the other and more
important determinations. Some ores are sent to the smelter with from 5 to 15 per cent. of
adherent water. In these cases it is best to spread out the sample, and taking equal portions
fairly at regular intervals, weigh into a Berlin dish 20 grams. This should then be dried over
a sand-bath, or if the ore is likely to be injured by excess of heat, over a water-bath until the
weight is constant. The loss of weight multiplied by 5 gives the percentage of water present.
Example:—

Weight of dish + wolfram 32.67 grms.
" " dish 12.67 "
———
" " wolfram 20.00 "
" " dish + wolfram 32.67 "
" " " dried 30.15 "
———
" " water 2.52 "
2.52 × 5 = 12.6 12.6%.

There are other ores which are not apparently wet, but in the state called "air-dried." It is
easier to take fair samples of these, and, consequently, it is not necessary to use so large a
quantity as 20 grams. But with a smaller quantity, extra precautions must be taken. All dry
solids at ordinary temperatures absorb moisture from the air. The amount varies with the
nature of the material and with the quantity of surface exposed. Light bulky powders absorb
more than heavy ones, because of the greater condensing surface. It is on this account that it
is well to weigh substances, which have been dried, between close-fitting watch-glasses.
The method of determining moisture is to weigh out into the glasses 5 grams of ore, and dry

in the water-oven until there is no further loss of weight. On taking the glasses out of the
oven, they should be at once closed, the clip put on, and after cooling in a desiccator
weighed. If after a second trial the loss is the same, or only increased by a milligram, the
determination is finished.
Example:—

Weight of glasses + pyrites 31.0470 grms.
" " glasses 26.0470 "
———
" " pyrites 5.0000 "
" " glasses + pyrites, dried 1 hour 30.8965 "
" " " " dried 1-1/2 " 30.8957 "
" " " " 31.0470 "
" " " " dried 30.8957 "
———
" " moisture 0.1513 "
0.1513 × 20 = 3.026 3.02%.

Sometimes it may be advisable to dry 10 grams, in which case multiplying the loss by 10
will give the percentage. The dried ore should be transferred to a weighing-tube (fig. 3), and
reserved for the subsequent determinations. The weighing-tube with the ore must be
marked, and kept in a desiccator.
Most ores and inorganic substances can be dried, and their moisture determined by the loss
in this way. When, however, the substance contains another somewhat volatile ingredient, it

is exposed over sulphuric acid in a desiccator for two days (if in vacuo, all the better), and
the loss determined. Moisture in dynamite should be determined in this way.
When water is simply mechanically mixed with a substance it presents but little difficulty.
The combined water is a different matter. Slaked lime, even when perfectly dry, contains
much water; and if the water of soda crystals were separated and frozen, it would occupy a
volume equal to that of the original crystals. Perfectly dry substances may contain much
water, and this combined water is retained by different materials with very unequal vigour.
Sodium sulphate and sodium phosphate crystals lose water even when exposed under
ordinary conditions to dry air. Soda crystals when heated melt, and at a moderate
temperature give off their water with ebullition. The temperature at which all the water is
given up varies with each particular salt; the actual determination of the water in each case
will require somewhat different treatment. Such determinations, however, are seldom
required; and from a practical point of view this combined water causes no trouble.
In assaying ores, we term "moisture" all water which is lost by exposure in a water-oven at
100° C., and the "dry ore" is the ore which has been dried at this temperature. No
advantage, but rather endless confusion, would be caused by varying the temperature with
the object of estimating the whole of the water which a hydrated salt may contain. The
results of the assay of the other components should be calculated on the "dry ore." One
advantage of this is obvious:—The dry ore has a constant composition, and the results of all
assays of it will be the same, no matter when made; the moisture, however, may vary from
day to day, and would be influenced by a passing shower of rain. It is well to limit this
variability to the moisture by considering it apart, and thus avoid having the percentage, say,
of copper rising and falling under the influence of the weather.
In the case of certain salts, however, such as soda crystals and hydrated sulphate of copper
(when these constitute the bulk of the substance to be assayed), it is as well to perform the
assay on the moist, or at any rate air-dried, substance.[2] It would be equally convenient to
calculate on the substance dried at 100° C.; but in this case it would be well, in order to
avoid a somewhat shallow criticism, to replace the term "moisture" by the longer but
equivalent phrase "water lost at 100° C."
Calculation and Statement of Results.—By far the most generally convenient method of
stating the results of an assay is that of the percentage or parts in a hundred, and to avoid a
needlessly troublesome calculation it is well to take such a quantity of ore for each assay as
by a simple multiplication will yield the percentage. In these calculations decimals are
freely employed, and students should make themselves familiar with the methods of using
them.
Other methods of statement are in use, and have advantages in certain special cases. With
bullion the parts in a thousand are given, and in those cases in which the percentage is very
small, as in water analysis, it is convenient to report on parts in 100,000, or even on parts
per 1,000,000. These are easily got from the corresponding percentages by shifting the
decimal point one, three, or four places to the right. Thus 92.5 per cent. is 925 per thousand;
and 0.0036 per cent. is 3.6 per 100,000, or 36 per million.

With ores of tin, silver, and gold, the result is stated as so many cwts., lbs., or ozs., in the
ton. With dressed tin ores as they are sent to the smelter, the produce is given in cwts. and
quarters to the ton. The corresponding percentage may be obtained by multiplying by five;
or, inversely, if the percentage is given, the produce may be got by dividing by five. A
produce of 13-1/2 equals a percentage of 13.5 × 5 = 67.5; and a percentage of 70.0 equals a
produce of 70 / 5 = 14. With tin ores as raised (in which the percentage is small) the
reduction must be carried to pounds per ton. One per cent. equals 22.4 lbs. to the ton;
consequently, if we multiply the percentage by 22.4, the produce will be given. Thus, if an
ore contains 6.7 per cent. of oxide of tin, the produce is 6.7 × 22.4 = 150 lbs. (or 1 cwt., 1
quarter, and 10 lbs.) to the ton. With gold and silver ores, the proportion of precious metal is
small, and it is necessary to carry the reduction to ozs. and dwts. to the ton; and since gold
and silver are sold by troy weight, whilst the ton is avoirdupois, it is of importance to
remember that the ounces in the two systems are not the same. A ton contains 15,680,000
grains, which equal 653,333.3 dwts. or 32,666.6 ozs. (troy). The following rules are useful:
—
To get ozs. (troy) per ton, multiply parts per 100,000 by 0.327;
To get dwts. per ton, multiply parts per 100,000 by 6.53;
To get grains per ton, multiply parts per 100,000 by 156.8.
Where liquids are being assayed, cubic centimetres are held to be equivalent to grams, and
the usual method of statement is, "so many parts by weight in so many by measure." Where
the statement is made as grams per litre or grains per gallon, there can be no doubt as to
what is meant; and even if it be expressed in parts per 100,000, parts by weight in a
measured volume must be understood unless the contrary is expressly stated.
In some cases, where the density of the solution differs greatly from that of water, the
percentage by weight may be given; and in others, mixtures of two or more liquids, the
percentages may be given by volume or by weight; as so many c.c. in 100 c.c., or as so
many grams in 100 grams, or even as so many grams in 100 c.c. In such cases it must be
distinctly shown which method of statement is adopted.
One grain per gallon means 1 grain in 70,000 grain-measures, or one part in 70,000.
Dividing by 7 and multiplying by 10 will convert grains per gallon into parts per 100,000.
Inversely, dividing by 10 and multiplying by 7, will convert parts per 100,000 into grains
per gallon.
Grams per litre are parts per 1000; multiplying by 100 will give parts per 100,000, and
multiplying by 70 will give grains per gallon.
Among foreign systems of weights, the French is by far the best. Kilograms (2.205 lbs.) per
quintal (220.5 lbs.) are parts per cent.; and grams (15.43 grs.) per quintal are parts per
100,000. From the rule already given, grams per quintal may be converted into ounces to
the ton by multiplying by 0.327.
The German loths per centner (1/2 oz. (avoirdupois) to 100 lbs.) equal parts per 3200; they
are converted into parts per cent. by dividing by 32, or into ounces (troy) per ton by

multiplying by 10.208.
In the United States, as a sort of compromise between the avoirdupois and metric systems, a
ton is taken as 2000 lbs. There, too, the custom is adopted of reporting the gold and silver
contents of an ore as so many dollars and cents to the ton. In the case of gold, an ounce is
considered to be worth 20.6718 dollars. With silver, the nominal value is 1.2929 dollars per
ounce, but frequently in assay reports it is taken as one dollar. The practice is objectionable.
The prices of metals vary with the fluctuations of the market, and if the assayer fixed the
price, the date of his report would be all important; if, on the other hand, he takes a fixed
price which does not at all times agree with the market one, it leaves a path open for the
deception of those unacquainted with the custom. American "dollars on the ton of 2000 lbs."
may be converted into "ounces in the ton of 2240 lbs." by dividing by 1.1544 in the case of
silver, and by 18.457 in the case of gold.
Laboratory Books and Report Forms.—The record which the assayer makes of his work
must be clear and neat, so that reference, even after an interval of years, should be certain
and easy. One method should be adopted and adhered to. Where there are a large number of
samples, three books are required.
Sample Book.—This contains particulars of the samples (marks, &c.), which are entered by
the office-clerk as they arrive. He at the same time puts on each sample the distinguishing
number.
Example of Page of Sample Book.

Date. Number. Sample. Remarks.
Feb. 1482 Tough Copper For Arsenic.
" 2X Piece of Metal For Ni and Cu.
" 483 Tough Copper.
" 73 Silver Precipitate, 4 casks, 24 cwt. 1 qr. With Letter.
" 494 Purple Ore, 200 tons.
" 1 J.T. Lead Ore, 1 J.T. From Corsica.
" 2 J.T. " 2 J.T.

Laboratory Book. This is the Assayer's note-book, in which he enters clearly the particulars
of his work—the results obtained, as well as how these results were arrived at. The
calculations should be done on scrap-paper, and should not be entered, although, of course,
detail enough must be shown to enable the results to be recalculated.
Example of Page of Laboratory Book.

____________________________________________________________

Purple Ore 5 grams
19/10/89 0.0042 grm.
0.0021 "
———

Colorimetric 0.0063 × 20 = 0.13% Copper
______________________________________________________________

482
Tough Copper 10 grams
Feb. 1/89 10.5 c.c. Uranium.
= 0.52% Arsenic
______________________________________________________________

2082
Tough Copper 10 grams
12.7 c.c. Uranium.
= 0.63% Arsenic
______________________________________________________________

491 10 grams
Tough Copper 13.7 c.c. Uranium
Feb. 1/89
= 0.68% Arsenic
______________________________________________________________

Standard of Uranium acetate.
0.150 gram As2O3 = 23.3 c.c. Uranium.
∴ 100 cc. Uranium = 0.5 gram As.
______________________________________________________________

10071 5 grams
Tin Ore Cruc. and SnO2 9.6065 grms.
Feb. 3/89 Cruc. and Ash 9.4235 "
———
SnO2 = 0.1830 = 2.88% Tin
______________________________________________________________

The Assay Book.—This is the Official book, and is a combination of the Sample and
Laboratory books. It corresponds with the report-forms. Without being loaded with detail, it
should contain sufficient to characterise each sample.
Example of Page of Assay Book.
Description of Sample.

Water Lost Assay on the Date
Date. Material. Weight. No.
at 100° C. Dry Material. Reported.
1889 ton cwt qrs lbs
Feb. Tough cake
... ... ... ... 482 ... Arsenic, 0.52% 7
1 copper
Tough cake
" ... ... ... ... 2082 ... Arsenic, 0.63% 7
copper
Tough cake
" ... ... ... ... 491 ... Arsenic, 0.68% 7
copper
Feb. Nickel disc for
... ... ... ... X ... Copper, 73.75 7
2 C.R.

Nickel, 24.34
Iron, 2.18
———
100.27
———
Silver
" precipitate, 4 ... 24 1 0 73 Not det. Silver, 4.851 10
casks
Gold, 0.0215 ...
Lead, 19.37 ...
Zinc, 2.00 ...
Silver, 1584.7
...
ozs. per ton
Gold, 7.0 ozs.
...
per ton
Copper,
" Purple ore ... 200... ... ... 494 Not det. 11
0.13%
Sulphur
...
0.15%

When the number of samples is small, the Sample Book may be omitted, and the entries
made in the Assay Book as the samples arrive.
Report-forms. These should entail as little writing as possible in making out the report. For
general purposes the form given on p. 12 is useful.
The quantity of substance to be taken for any particular assay depends largely upon the
method of assay adopted. There are, however, some general considerations which should be
remembered, and some devices for simplifying the calculations which should be discussed.
The smaller the percentage of the substance to be determined, the larger should be the
amount of the ore taken. The following table will give a general idea as to this:—

Percentage of the substance to be determined. Amount of ore, &c. to be weighed.
100-10 1 gram.
10-5 2 grams.
5-1 5 "
1-0.1 10 "
0.1-0.01 20 "

ASSAY NOTE
The rougher the method of assay adopted, the larger should be the quantity of ore taken. If
the degree of accuracy attainable with the methods and instruments at the assayer's service
is known, it is easy to calculate what quantity should be taken for any particular case. If the
results are good within 0.001 gram, then, taking 1 gram of ore we can report within 0.1 per
cent., or if they are good within 0.0002 gram, taking 20 grams of ore, we can report within 1
part per 100,000, or very closely within 6-1/2 dwt. to the ton. If it is wished to be yet more
particular in reporting, larger quantities must be taken. The difficulty of manipulating very
small or very large precipitates, &c., must be borne in mind. So, too, must the fact that the
greater the weight of the final product of an assay, the less, as a rule, is the percentage error.
The distinction between absolute and percentage error, often overlooked, is important. If 0.5
gram of silver be cupelled with 20 grams of lead, there may be obtained a button of 0.495
gram; the absolute loss is 0.005 gram, and this equals 1 per cent. of the silver present.
Similarly, cupelling 0.1 gram, the resulting button may be 0.098; the absolute loss is only
0.002 gram, but this equals 2 per cent. of the silver present. In the same way the student
should see that the two results, 91.5 per cent. and 92.0 per cent., are really more concordant
than the results 9.1 per cent. and 9.2 per cent.
A device often adopted in practice where a large number of assays of one kind are made,
and the report is given as so many ounces or pounds to the ton, is that known as the assay
ton. The assay ton may be any arbitrary and convenient weight, but its subdivisions must
bear to it the same relations as pounds and ounces bear to the actual ton. On the other hand,
in a laboratory where many kinds of work are performed, different sets of weights of this
kind would only tend to confusion, even if they were not unnecessary. With a set of gram
weights and its subdivisions anything may be done. If it is desired to report as pounds to the
ton, then, since there are 2240 lbs. to the ton, a weight of 2.240 grams may be taken as the
assay ton, and each 0.001 gram yielded will equal 1 lb., or 22.4 grams may represent the
ton, and each 0.01 gram a pound. Similarly, since there are 32,666.6 ozs. troy to the ton; if
we take 32.6667 grams as the assay ton, each 0.001 gram will equal 1 oz. to the ton. In
some cases it may be convenient to have, in addition to the usual gram weights, one or other
of the "assay tons" mentioned above, but generally it is better to work on a purely decimal

system, and convert when required into ounces per ton, &c., either by actual calculation or
by reference to a set of tables.
Practical Exercises.
The student should practise such calculations as the following:—
1. Calculate the percentages in the following cases:—
(a) Ore taken, 2 grams; copper found, 0.2155.
(b) " 1.5 gram; iron found, 0.8340.
(c) " 30 grams; lead found, 23.2.

2. Calculate the parts per thousand in the following:—
(a) Bullion taken, 1.1 gram; silver found, 1.017.
(b) " 1.14 gram; silver found, 1.026.
(c) " 0.6 gram; gold found, 0.5500.

3. Calculate parts per 100,000 in the following:—
(a) Ore taken, 20 grams; silver found, 0.0075.
(b) " 50 grams; gold found, 0.0026.
(c) Water taken, 500 c.c.; solids found, 0.1205.

4. Calculate cwts. to the ton in the following:—
(a) Ore taken, 5 grams; tin found, 2.816.
(b) " 5 grams; tin found, 3.128.
(c) An ore with 68.2 per cent. of tin.

5. Calculate lbs. to the ton in the following:—
(a) An ore with 3.28 per cent. oxide of tin.
(b) Ore taken, 20 grams; oxide of tin found, 1.67.

6. Calculate ozs. (troy) to the ton in the following:—
(a) Ore taken, 50 grams; gold found, 0.0035.
(b) " 20 grams; silver found, 0.0287.
(c) " 25 grains; silver found, 0.0164.

7. Calculate in grains per gallon:—
(a) 0.51 gram per litre.
(b) 24.6 parts per 100,000.
(c) Solution taken, 100 c.c.; copper found, 0.0045 gram.
(d) " 50 c.c.; iron found, 0.165 gram.

8. Convert into ozs. (troy) per ton:—
(a) 7 loths per centner.
(b) 30 grams per quintal.
(c) 15 parts per 100,000.

FOOTNOTES:
[1] Ether or carbon bisulphide.
[2] Such substances are best dried by pressing between folds of dry filter-paper.

CHAPTER II.
METHODS OF ASSAYING.—DRY GRAVIMETRIC METHODS.

The methods of assaying are best classed under two heads, Gravimetric and Volumetric, in
the former of which the final results are weighed, whilst in the latter they are measured. A
commoner and older division is expressed in the terms much used in practice—wet assays
and dry assays. Wet assays include all those in which solvents, &c. (liquid at the ordinary
temperature), are mainly used; and dry assays, those in which solid re-agents are almost
exclusively employed. Dry assays form a branch of gravimetric work, and we shall include
under this head all those assays requiring the help of a wind furnace. Wet assays, as
generally understood, would include not only those which we class as wet gravimetric
assays, but also all the volumetric processes.
Gravimetric Methods aim at the separation of the substance from the other matters present
in the ore, so that it may be weighed; and, therefore, they must yield the whole of the
substance in a pure state. It is not necessary that a metal should be weighed as metal; it may
be weighed in the form of a compound of definite and well known composition. For
example, one part by weight of silver chloride contains (and, if pure, always contains)
0.7527 part of silver; and a quantity of this metal can be as exactly determined by weighing
it as chloride as by weighing it in the metallic state. But in either case the metal or its
chloride must be pure.
Exact purity and complete separation are not easily obtained; and methods are used which
are defective in one or both of these respects. It is well to note that an impure product
increases the result, whilst a loss of the substance decreases it; so that if both defects exist in
a process they tend to neutralise each other. Of dry methods generally, it may be said that
they neither give the whole of the substance nor give it pure; so that they are only calculated
to show the amount of metal that can be extracted on a manufacturing scale, and not the
actual quantity of it present. Their determinations are generally rough and always low. The
gold and silver determinations, however, will compare very favourably with any of the other
processes for the estimation of these metals in their ores.
The calculation of the results of a gravimetric assay has already been referred to. If the
result is to be stated as percentage, it may always be done by the following rule:—Multiply
the weight of the substance got by the percentage of metal it contains, and divide by the
weight of ore taken.
Gravimetric methods are divided into three groups: (1) mechanical separations; (2) dry
methods; and (3) wet methods.
Mechanical Separations.—Under this head are classed the method of assaying tin ores,
known as vanning, and the amalgamation assay for gold. A set of sieves to determine the

relative proportion of powders of different degrees of fineness is sometimes useful. A set
with 10, 20, 40 and 80 meshes to the inch is convenient.
Dry Assays.—An important distinction between wet and dry methods of assaying is, that in
the former the substance is got into the liquid state by solution, whilst in the latter fusion is
taken advantage of.
The difference between solution and fusion is easily illustrated: a lump of sugar heated over
a candle-flame melts or fuses; suspended in water it dissolves. Many substances which are
insoluble or infusible of themselves, become soluble or fusible when mixed with certain
others; thus, in this way, solution is got with the aid of reagents, and fusion with the help of
fluxes. For example, lead is insoluble in water, but if nitric acid be added, the metal rapidly
disappears. It is convenient, but somewhat inaccurate, to say that the acid dissolves the lead.
If the lead be acted on by nitric acid alone, without water, it is converted into a white
powder, which does not dissolve until water is added; in this case it is obvious that the water
is the solvent. The function of the acid is to convert the lead into a soluble compound.
Fluxes may act as true solvents. Fused carbonate of soda dissolves baric carbonate, and
perhaps in many slags true solution occurs; but in the great majority of cases a flux is a
solid reagent added for the purpose of forming a fusible compound with the earthy or stony
minerals of the ore. Few of the minerals which occur in the gangue of an ore are fusible;
and still fewer are sufficiently fusible for the purposes of the assayer, consequently the
subject is one of importance, and it ought to be treated on chemical principles. An idea of
the composition of some of the more frequently occurring rocks may be gathered from the
following table, which represents rough averages:—

Oxide of Lime and
Silica. Alumina. Alkalies.
iron Magnesia.
% % % % %
80-
Sandstone, grit, quartzite, &c. — — — —
100
Granite, gneiss, quartz- 5-8 Less in
70-75 13-20 2 2
porphyry, fire-clay, &c. fire-clay.
Mica-schist 65 18 5 3 3
Trachyte, syenite 60 17 7 4-7 6-9
Clay-slate 60 18 10 8 3
Diorite 54 17 12 9 3-4
Horneblende-rock 50 18 15 12 3-4
Brick-clay 50 34 8 6 —
China-clay 47 39 — — —
Basalt, dolerite, &c. 50 15 15 16 3
Serpentine 44 — — 44 —
Chalk, limestone, dolomite, &c. — — — 45-55 —

Silica itself, and the silicates of alumina, of lime, and of magnesia, are practically infusible;
the silicates of soda, of potash, and of iron are easily fusible if the base (soda, potash, or
oxide of iron) be present in sufficient quantity, and if, in the case of the iron, it is present
mainly as lower oxide (ferrous silicate). The addition of lime, oxide of iron, or alkali to
silicate of alumina results in the formation of a double silicate of alumina and lime, or of
alumina and iron, &c., all of which are easily fusible. Similarly, if to a silicate of lime we
add oxide of iron, or soda, or even alumina, a fusible double silicate will be formed. Thus
lime, soda, oxide of iron, and clay, are fluxes when properly used; but since lime, clay (and
oxide of iron if there be any tendency to form peroxide), are of themselves infusible, any
excess of these fluxes would tend to stiffen and render pasty the resulting slag. So, too,
soda, which is a very strong base, may act prejudicially if it be in sufficient excess to set
free notable quantities of lime and magnesia, which but for that excess would exist in
combination as complex fusible silicates. There are many minerals which with but little
soda form a glass, but with more yield a lumpy scoriacious mass. There are many minerals,
too, which are already basic (for example, calcite), and which, when present, demand either
a less basic or an acid flux according to the proportions in which they exist. For purposes of
this kind borax, or glass, or clay with more or less soda may be used, and of these borax is
by far the most generally useful. An objection to too basic a slag (and a very important one)
is the speed with which it corrodes ordinary crucibles. These crucibles, consisting of quartz
and clay, are rapidly attacked by lime, soda and bases generally.

In considering what is and what is not a good slag, certain chemical properties are of
importance. If a mixture of many substances be fused and allowed to solidify in a crucible,
there will be found some or all of the following. At the bottom of the crucible (fig. 4) a
button of metal, resting on this a speise; then a regulus, next a slag made up of silicates and
borates and metallic oxides, and lastly, on the top another layer of slag, mainly made up of
fusible chlorides and sulphates. In assaying operations the object is generally to concentrate
the metal sought for in a button of metal, speise or regulus, and to leave the earthy and other
impurities as far as possible in the slag; whether there be one or two layers of slag is a
matter of indifference;[3] but the chemical action of the lower layer upon the speise, or
regulus, or metal, is of great importance.

A regulus is a compound of one or more of the metals with sulphur; it is usually brittle,
often crystalline, and of a dull somewhat greasy lustre. It is essential that the slag, when
solid, shall be so much more brittle than the regulus, that it shall be easy to crumble, and
remove it without breaking the latter; and it must not be basic. The effect of fusing a regulus
with a basic slag is well seen when sulphide of lead is fused with carbonate of soda; the
result is a button of metal (more or less pure), and a slag containing sulphides of lead and
sodium; and again, if sulphide of lead be fused with an excess of oxide of lead, a button of
lead will be got, and a slag which is simply oxide of lead (with whatever it may have taken
up from the crucible), or if a sufficient excess has not been used, oxide of lead mixed with
some sulphide. When (as is most frequently the case) the desire is to prevent the formation
of regulus, these reactions may be taken advantage of, but otherwise the use of a flux
having any such tendency must be avoided. A good slag (from which a regulus may be
easily separated) may be obtained by fusing, say, 20 grams of ore with borax 15 grams,
powdered glass 15 grams, fluor spar, 20 grams, and lime 20 grams; by quenching the slag in
water as soon as it has solidified, it is rendered very brittle.
Sulphide of iron formed during an assay will remain diffused through the slag, instead of
fusing into a button of regulus, if the slag contain sulphide of sodium. The same is true of
other sulphides if not present in too great a quantity, and if the temperature is not too high.
Speises are compounds of a metal or metals with arsenic. They are chiefly of interest in the
metallurgy of nickel, cobalt, and tin. They are formed by heating the metal or ore in covered
crucibles with arsenic and, if necessary, a reducing agent. The product is fused with more
arsenic under a slag, consisting mainly of borax. They are very fusible, brittle compounds.
On exposure to the air at a red heat the arsenic and the metal simultaneously oxidize. When
iron, cobalt, nickel, and copper are present in the same speise, they are eliminated in the
order mentioned.
Slags from which metals are to be separated should not be too acid; at least, in those cases
in which the metal is to be reduced from a compound, as well as separated from earthy
impurities. Where the object is simply to get a button of metal from a substance in which it
is already in the metallic state, but mixed with dross (made up of metallic oxides, such as
those of zinc or iron), from which it is desired to separate it, an acid flux like borax is best;
or, if the metal is easily fusible, and there would be danger of loss of metal by oxidation or
volatilising, it may be melted under a layer of resin or fat. Common salt is sometimes used
with a similar object, and is often useful. Under certain conditions, however, it has a
tendency to cause the formation of volatile chlorides with a consequent loss of metal.
In the great majority of cases, the fusion of the metal is accompanied by reduction from the
state of oxide; in these the slag should be basic. It is not easy to reduce the whole of a
reducible oxide (say oxide of copper or of iron) from a slag in which it exists as a borate or
silicate; there should be at least enough soda present to liberate it. When the object is to
separate one metal, say copper, without reducing an unnecessary amount of another (iron) at
the same time, a slag with a good deal of borax is a distinct advantage. The slag then will
probably not be free from copper, so that it will be necessary to powder and mix the slag
with some soda and a reducing agent, and to again fuse the slag in order to separate this

residual metal. In all those cases in which the slag retains an oxide of a heavy metal, this
cleaning of the slag is advisable, and in the case of rich ores necessary. Slags containing
sulphides are especially apt to retain the more easily reducible metals.
The following are the ordinary and most useful fluxes:—
Soda.—The powdered bicarbonate, sold by druggists as "carbonate of soda," is generally
used. It gives off its water and excess of carbonic acid readily and without fusion. Where
the melting down is performed rapidly, the escaping gas is apt to cause trouble by frothing,
and so causing waste of the material. Ordinary carbonate of soda, when hydrated (soda
crystals), melts easily, and gives off its water with ebullition. It is unfit for use in assaying,
but when dried it can be used instead of the bicarbonate. One part of the dried carbonate is
equivalent to rather more than one and a half parts of the bicarbonate. From two to four
parts of the flux are amply sufficient to yield a fluid slag with one part of earthy matter. This
statement is also true of the fluxes which follow.
Borax is a hydrated biborate of soda, containing nearly half its weight of water. When
heated it swells up, loses its water, and fuses into a glass. The swelling up may become a
source of loss in the assay by pushing some of the contents out of the crucible. To avoid
this, fused or dried borax may be used, in which case a little more than half the amount of
borax indicated will suffice. Borax will flux almost anything, but it is especially valuable in
fluxing lime, &c., and metallic oxides; as also in those cases in which it is desired to keep
certain of the latter in the slag and out of the button of metal.
Oxide of Lead, in the form of red lead or litharge, is a valuable flux; it easily dissolves
those metallic oxides which are either infusible or difficultly fusible of themselves, such as
oxides of iron or copper. The resulting slag is strongly basic and very corrosive; no crucible
will long withstand the attack of a fused mixture of oxides of lead and copper. With
silicates, also, it forms very fusible double silicates; but in the absence of silicates and
borates it has no action upon lime or magnesia. Whether the lead be added as litharge or as
red lead, it will exist in the slag as monoxide (litharge); the excess of oxygen of the red lead
is thus available for oxidising purposes. If this oxidising power is prejudicial, it may be
neutralised by mixing the red lead with 1 per cent. of charcoal.
Glass: broken beakers and flasks, cleaned, dried, and powdered will do. It should be free
from lead.
Fluor: fluor-spar as free as possible from other minerals, powdered. It helps to flux
phosphate of lime, &c., and infusible silicates.
Lime: should be fresh and powdered. It must not be slaked. Powdered white marble
(carbonate of lime) will do; but nearly double the quantity must be taken. One part of lime
produces the same effect as 1.8 parts of the carbonate of lime.
Tartar and "black flux," are reducing agents as well as fluxes. The "black flux," which may
be obtained by heating tartar, is a mixture of carbonate of potash and charcoal.

Reducing Agents.—The distinction between reducing agents and fluxes (too often ignored)
is an important one. Fluxes yield slags; reducing agents give buttons of regulus or of metal.
The action of a reducing agent is the separation of the oxygen or sulphur from the metal
with which it is combined. For example, the mineral anglesite (lead sulphate) is a
compound of lead, sulphur, and oxygen; by carefully heating it with charcoal the oxygen is
taken away by the charcoal, and a regulus of lead sulphide remains. If the regulus be then
fused with metallic iron the sulphur is removed by the iron, and metallic lead is left. The
charcoal and the iron are reducing agents. But in defining a reducing agent as one which
removes oxygen, or sulphur, from a metallic compound so as to set the metal free, it must
be remembered that sulphur itself will reduce metallic lead from fused litharge, and that
oxygen will similarly set free the metal in fused lead sulphide. There is no impropriety in
describing sulphur as a reducing agent; but it is absurd to call oxygen one. Some confusion
will be avoided if these substances and those which are opposite to them in property be
classed as oxidising and de-oxidising, sulphurising, and de-sulphurising agents. Most
oxidising agents also act as de-sulphurisers.
The de-oxidising agents most in use are the following:—
Charcoal.—Powdered wood charcoal; it contains more or less hygroscopic moisture and
about 3 or 4 per cent. of ash. The rest may be considered carbon. Carbon heated with
metallic oxides takes the oxygen; at low temperatures it forms carbon dioxide, and at higher
ones, carbon monoxide. Other conditions besides that of temperature have an influence in
producing these results; and as the quantity of charcoal required to complete a definite
reaction varies with these, it should be calculated from the results of immediate experience
rather than from theoretical considerations.
Flour.—Ordinary wheat flour is convenient in use. On being heated it gives off
inflammable gases which have a certain reducing effect, and a residue of finely divided
carbon is left. It is likely to vary in the quantity of moisture it contains. Two parts of flour
should be used where one part of charcoal would be otherwise required.
Tartar.—This is crude hydric potassic tartrate; the purified salt, cream of tartar, may be
used. On being heated it gives off inflammable gases, and leaves a residue formed of
potassic carbonate mixed with finely divided carbon. Five parts of tartar should be used in
the place of one of charcoal.
Anthracite or Culm is a kind of coal containing 90 per cent. or more of carbon. It gives off
no inflammable gas. It is denser, and takes longer in burning, than charcoal. Its reducing
effect is little inferior to that of charcoal. Almost any organic substance can be used as a
reducing agent, but it is well not to select one which melts, swells up, or gives off much
water and gas when heated in the furnace.
Potassic Cyanide is an easily fusible and somewhat volatile salt, which, when fused,
readily removes oxygen and sulphur from metallic compounds, and forms potassic cyanate
or sulphocyanate as the case may be. Commercial samples vary much in purity; some
contain less than 50 per cent. of the salt. For assaying, only the better qualities should be
used.

Iron is a de-sulphurising rather than a de-oxidising agent. Iron is used in the form of rods,
1/2-inch in diameter, or of nails, or of hoop iron. In the last case it should be thin enough to
be bent without difficulty. Wrought iron crucibles are very useful in the processes required
for making galena assays.
The chief oxidising agents (which are also de-sulphurisers) are the following:—
Nitre, or Potassic Nitrate.—This salt fuses very easily to a watery liquid. It oxidises most
combustible substances with deflagration, and thereby converts sulphides into sulphates,
arsenides into arsenates, and most metals into oxides. In the presence of strong bases, such
as soda, the whole of the sulphur is fully oxidised; but in many cases some arsenic is apt to
escape, and to give rise to a peculiar garlic-like odour. The sulphates of soda and potash are
thus formed, and float as a watery liquid on the surface of the slag.
Red lead is an oxide of lead. About one-quarter of its oxygen is very loosely held, and,
hence, is available for oxidising purposes, without any separation of metallic lead. The rest
of the oxygen is also available; but for each part of oxygen given off, about 13 parts of
metallic lead are deposited. In silver assays this power of readily giving up oxygen is made
use of. The residual oxide (litharge) acts as a flux.
Hot air is the oxidising agent in roasting operations. The sulphur and arsenic of such
minerals as mispickel and pyrites are oxidised by the hot air and pass off as sulphur dioxide
and "white arsenic." The metals generally remain in the form of oxide, mixed with more or
less sulphate and arsenate. The residue may remain as a powdery substance (a calx), in
which case the process of roasting is termed calcination; or it may be a pasty mass or liquid.
In the calcination of somewhat fusible minerals, the roasting should be done at a low
temperature to avoid clotting; arsenic and sulphur being with difficulty burnt off from the
clotted mineral. A low temperature, however, favours the formation of sulphates; and these
(if not removed) would reappear in a subsequent reduction as sulphides. These sulphates
may be decomposed by a higher temperature towards the end of the operation; their
removal is rendered more certain by rubbing up the calx with some culm and re-roasting, or
by strongly heating the calx after the addition of solid ammonic carbonate. In roasting
operations, as large a surface of the substance as possible should be exposed to the air. If
done in a crucible, the crucible should be of the Cornish type, short and open, not long and
narrow. For calcinations, roasting dishes are useful: these are broad and shallow, not unlike
saucers, but unglazed. In those cases in which the products of the roasting are liquid at the
temperature used, a scorifier (fig. 38) is suitable if it is desired to keep the liquid; but if the
liquid is best drained off as quickly as it is formed, a cupel (fig. 5) should be used.

A scorifier is essentially a roasting dish sufficiently thick to resist, for a time, the corrosive
action of the fused metallic oxides it is to contain. The essential property of a cupel is, that it
is sufficiently porous to allow the fused oxide to drain into it as fast as it is formed. It should
be large enough to absorb the whole of the liquid; and of course must be made of a material
upon which the liquid has no corrosive action. Cupels do not bear transport well; hence the
assayer generally has to make them, or to supervise their making. A quantity of bone ash is
carefully mixed with water so that no lumps are formed, and the mixture is then worked up
by rubbing between the hands. The bone ash is sufficiently wet when its cohesion is such
that it can be pressed into a lump, and yet be easily crumbled into powder. Cupel moulds
should be purchased. They are generally made of turned iron or brass. They consist of three
parts (1) a hollow cylinder; (2) a disc of metal; and (3) a piston for compressing the bone
ash and shaping the top of the cupel. The disc forms a false bottom for the cylinder. This is
put in its place, and the cylinder filled (or nearly so) with the moistened bone ash. The bone
ash is then pressed into shape with the piston, and the cupel finished with the help of three
or four smart blows from a mallet. Before removing the piston, turn it half-way round upon
its axis so as to loosen and smooth the face of the cupel. The cupel is got out by pressing up
the disc of metal forming the false bottom; the removal is more easily effected if the mould
is somewhat conical, instead of cylindrical, in form. The cupels are put in a warm place to
dry for two or three days. A conveniently sized cupel is 1-1/4 inches in diameter and about
3/4 inch high. The cavity of the cupel is about 1/4 inch deep, and something of the shape
shown in fig. 5.

There are two kinds of furnaces required, the "wind" and "muffle" furnaces. These are built
of brick, fire-brick, of course, being used for the lining. They are connected with a chimney
that will provide a good draught. Figure 6 shows a section of the wind furnace, fig. 7 a
section of the muffle furnace, and fig. 8 a general view of a group comprising a muffle and
two wind furnaces suitable for general work. When in operation, the furnaces are covered
with iron-bound tiles. The opening under the door of the muffle is closed with a loosely
fitting brick. The floor of the muffle is protected with a layer of bone-ash, which absorbs
any oxide of lead that may be accidentally spilt. The fire bars should be easily removable.
Few tools are wanted; the most important are some cast-iron moulds, tongs (fig. 9), stirrers
for calcining (fig. 10), and light tongs of a special form for handling scorifiers and cupels
(see Silver).

The coke used should be of good quality; the formation of a fused ash (clinker), in any
quantity, causes ceaseless trouble, and requires frequent removal. The coke should be
broken into lumps of a uniform size (about 2 in. across) before being brought into the office.
The furnace should be well packed by stirring, raising the coke and not ramming it, and it
should be uniformly heated, not hot below and cold above. In lighting a furnace, a start is
made with wood and charcoal, this readily ignites and sets fire to the coke, which of itself
does not kindle easily.

In commencing work, add (if necessary) fresh coke, and mix well; make hollows, and into
these put old crucibles; pack around with coke, so that the surface shall be concave, sloping
upwards from the mouths of the crucibles to the sides of the furnace; close the furnace, and,
when uniformly heated, substitute for the empty crucibles those which contain the assays. It
is rarely advisable to have a very hot fire at first, because with a gradual heat the gases and
steam quietly escape through the unfused mass, while with too strong a heat these might
make some of the matter in the crucible overflow. Moreover, if the heat should be too strong
at first, the flux might melt and run to the bottom of the crucible, leaving the quartz, &c., as
a pasty mass above; with a gentler heat combination is completed, and the subsequent
fiercer heat simply melts the fusible compound into homogeneous slag.
The fused material may be left in the crucible and separated from it by breaking when cold.
It is generally more convenient to pour it into cast-iron moulds. These moulds should be dry
and smooth. They act best when warmed and oiled or black-leaded.
Air entering through the fire-bars of a furnace and coming in contact with hot coke
combines with it, forming a very hot mixture of carbonic acid and nitrogen; this ascending,
comes in contact with more coke, and the carbonic acid is reduced to carbonic oxide; at the
top of the furnace, or in the flue, the carbonic oxide meeting fresh air, combines with the
oxygen therein and re-forms carbonic acid. In the first and third of these reactions, much
heat is evolved; in the second, the furnace is cooled a little. It must always be remembered,
that the carbonic oxide of the furnace gases is a reducing agent. When these gases are likely
to exert a prejudicial effect, and a strongly oxidising atmosphere is required, the work is
best done in a muffle.

FOOTNOTES:
[3] There is an exception to this, as when the slag is liable to be acted on when exposed to the air
and to the gases of the furnace. In this case a layer of fused common salt floating on the slag, so
as to protect it from the air and furnace gases, is a distinct advantage.

CHAPTER III.
WET GRAVIMETRIC METHODS.

In dry assays the metal is almost always separated and weighed as metal; in wet gravimetric
assays the metal is more usually weighed in the form of a definite compound of known
composition. The general methods of working resemble those of ordinary chemical
analysis, and their successful working is greatly helped by a knowledge of, at any rate,
those compounds of the metal which enable it to be separated, and of those which are the
most convenient forms in which it can be weighed. But the work of the assayer differs from
that of the analyst, inasmuch as the bulk of his estimations are made upon material of
practically the same kind, varying only in richness; consequently in assaying, it is possible
(and necessary) to work on such a definite plan as will involve the least amount of labour in
weighing and calculating.
The assayer connected with mining has generally two classes of material to deal with: those
comparatively rich and those comparatively poor. For example, silver in bullion and in ores;
copper precipitates or regulus, and copper ores and slags; and "black tin" and tin ores. He is
only occasionally called on to assay the intermediate products. It is indispensable that he
should have an approximate knowledge of the substance to be determined. With new ores
this information is best got by a qualitative testing. Knowing that only certain bodies are
present, it is evident that the number of separations can be reduced, and that simple methods
can be devised for arriving at the results sought for. The best method is that which involves
the least number of separations. The reactions must be sharp and complete, and yet not be
liable to error under varying conditions.
To bring the richer and poorer materials under the same conditions for the assay, a small
weight, say 1 gram of the richer, and a larger weight (5 or 10 grams) of the poorer,
substance is weighed up. A method is then adopted which will concentrate the whole of the
metal (either during or after solution) in a product which need not necessarily be pure. The
work on this product is comparatively easy. In separating small quantities of a substance
from a large bulk of impurities, the group separations must not as a rule be too much relied
on. Very large precipitates carry down small quantities of bodies not belonging to the group,
more especially when there is a tendency to form weak double compounds. The re-
dissolving and re-precipitating of bulky precipitates should be avoided.
When a large number of assays of the same kind have to be carried out, a plan something
like the following is adopted:—The samples, after having been dried, are placed in order on
a table at the left hand of the assayer. He takes the first, marks it with a number, samples
and weighs up the quantity required, and transfers it to a flask, which is similarly marked.
As the weighings are finished, the samples are placed in the same order on his right hand.
The assistant takes the flasks in batches of four or five at a time to the fume cupboard,
where he adds a measured quantity of acid. When solution has been effected, dilution with a

measured volume is generally necessary. The assayer sees to this and (whilst the funnels
and filters are being prepared) makes any separation that is necessary. The filters are
arranged in order on a rack (fig. 11), and need not be marked unless the precipitates or
residues have subsequently to be dried. The filters are washed with hot water, and if the
filtrates are wanted flasks are placed beneath, if not, the solution is drained off down the
sink. Precipitation or reduction (or whatever it may be) is now made; the assistant filters the
prepared samples, one at a time, whilst the assayer is engaged with the others. The same
style of work is continued until the assays are completed. If one should be spoiled, it is
better to allow it to stand over for assaying along with the next batch. If one filters slowly or
is in any way less forward than the rest, it may lessen the accuracy of the other assays,
owing to oxidation, &c., it should, therefore, be put on one side. The assays are dealt with in
batches of ten or twenty, so that a large quantity of work can be quickly finished.

When the assays are finished, it is the duty of the assistant to clean the apparatus (with
reagents, if necessary), and to put the vessels in the place set apart for them. Flasks are best
kept inverted on a rack, so that they may be dry and clean by the next morning. Berlin
crucibles must be cleaned and ignited.
The amount of apparatus employed should be as little as is feasible. The assay should be
carried out as much as possible in the same flask. The bench must be clean, and altogether
free from apparatus not in actual use. Crucibles and dishes in which weighings are made
should be marked with numbers or letters; and their weights recorded, together with the date
of weighing, in a small ledger, which is kept in the drawer of the balance. By this means a
record of the "wear" of each piece of apparatus is obtained, and, what is more important,
much weighing is saved, and increased confidence is gained. The weight of each piece of
apparatus need not be taken daily. It will be seen from the record in the book and a
knowledge of the use it has been put to how often a checking of the weight is necessary.
The entries are made in black lead as follows:—

Dish, A. Feb. 3 9.4210 grams.
5 9.4225
6 9.4230
7 9.4200

Platinum vessels and apparatus lose, and porcelain ones slightly gain, weight
with continued use.
The special details of the work is given under each assay; certain general
instructions will be given here.
Solution.—It is not always necessary to get the whole of the mineral in solution,
provided the body sought for is either completely dissolved or altogether left in
the residue. It is often only by a qualitative examination of the solution (or
residue, as the case may be) that the assayer can satisfy himself that it is free
from the substance sought. But previous experience with the same kind of ore
will show to what extent this testing is necessary.
Solution is generally best effected in flasks; but where the resulting liquid has
afterwards to be evaporated to dryness and ignited, evaporating dishes (fig. 12)
are used. With them clock glasses are used as covers during solution to avoid
loss through effervescence. Evaporating dishes are also best when an insoluble
residue has to be collected, since it is difficult to wash out most residues from a
flask. Bumping occurs less frequently in dishes than in flasks.

After the addition of the acid, and mixing by agitation, the vessel containing the
substance is heated. This is best done on the "hot plate" (fig. 13). This consists
of a slab of cast iron about half or three-quarters of an inch thick, supported on
loose fire bricks, and heated by two or three ring burners (figs. 14 and 15). The

burners are connected to the gas supply by means of lead tubing, to which they
are soldered. Flasks and dishes after being put on the plate are not further
handled until solution is complete or the evaporation is carried to dryness. The
hot plate is contained in a cupboard so as to be out of the reach of cold draughts.

The action of the acids and other solvents is described in the chapter on
Reagents.
Precipitation.—In precipitating add sufficient of the reagent to complete the
reaction. The student must be on his guard against adding a very large excess,

which is the commoner error. In some reactions the finishing point is obvious
enough; either no more precipitate is formed, or a precipitate is completely
dissolved, or some well-marked colour or odour is developed or removed.
In those cases in which there is no such indication, theoretical considerations
should keep the use of reagents within reasonable limits. The solutions of the
reagents (see Reagents) are generally of five or ten per cent. strength. A small
excess over that demanded by theory should be sufficient.

Filtration.—Solutions are best filtered hot whenever the assay allows of this
being so done. The precipitate should be allowed to settle, and the clear liquid
decanted on the filter with the aid of a glass rod if necessary. The filter-paper
must not be too large, but at the same time it must not be overloaded with the
precipitate. There should be ample room for washing. For general use three sizes
of filter-paper are sufficient. Common quick filtering-paper (English) is best for
most work in assaying. The specially prepared paper (Swedish or Rhenish) is
used for collecting those precipitates which have to be weighed. The papers are
folded as shown in fig. 16, and should not project above the funnel. The filter-
paper works better if damped with hot water. In special cases filtering is
hastened by means of an air-pump. The apparatus used consists of a water-jet
(fig. 17), which is connected with the tap, as also with a bottle fitted as shown in
fig. 18. The pump draws the air out from the bottle, and atmospheric pressure
forces the liquid through the filter-paper. The bottom of the funnel is provided

with a platinum cone, which supports the filter-paper, and prevents its breaking.
The pump is only used in exceptional cases; nearly all the filtrations required by
the assayer can be made without it. The usual methods of supporting the funnel
during filtration are shown in fig. 19. Where the filtrate is not wanted, pickle
bottles make convenient supports. After the precipitate has been thrown on the
filter, it is washed. In washing, several washings with a small quantity of water
are more effective than a few with a larger quantity of that fluid. The upper edge
of the filter-paper is specially liable to escape complete washing. Excessive
washing must be avoided; the point at which the washing is complete is found
by collecting a little of the filtrate and testing it. The precipitate is removed from
the filter-paper for further treatment by opening out the paper and by washing
the precipitate with a jet of water from a wash-bottle into a beaker, or back
through the funnel into the flask. In some cases, when the precipitate has to be
dissolved in anything in which it is readily soluble, solution is effected in the
filter itself allowing the liquid to run through as it is formed.
Drying and Igniting.—Precipitates, as a rule, require drying before being
ignited. With small precipitates the filter-paper may be opened out, and placed
on a warm asbestos slab till dry; or the funnel and the filter with the precipitate
is placed in a warm place, and supported by any convenient means. The heat
must never be sufficient to char the paper. Some precipitates must be dried at a
temperature not higher than 100°C. These are placed in the water-oven (fig. 20),
and, when apparently dry, they are taken from the funnel, placed between
glasses, and then left in the oven till they cease to lose weight. Such precipitates
are collected on tared filters. Those precipitates which will stand a higher
temperature are dried in the hot-air oven at a temperature of from 120° to 150°.
The drying is continued until they appear to be free from moisture, and until the
precipitate ceases to adhere to the filter. In drying sulphides the heat must not be
raised to the melting point of sulphur, since, if there is any free sulphur present,
it fuses and filters through.

The precipitate, having been dried, is transferred to a watch-glass. The filter-
paper is opened out over a sheet of note-paper, and, with a camel-hair brush, the
precipitate is gently brought into the glass. Most precipitates come away easily,
and the transfer can be made without apparent loss. The watch-glass is covered
by the funnel, and the filter-paper (folded into a quadrant) held by the tweezers
and set fire to with the flame of a Bunsen burner. It is allowed to burn over the
crucible, into which the black bulky ash is allowed to drop, and two or three
drops of nitric acid are then added. The crucible is placed on a pipe-stem
triangle (fig. 21), supported on a tripod. It is at first heated gently with a Bunsen
burner, and afterwards more strongly, until the residue is free from carbon. It is
cooled, and treated with any acid necessary to convert the small amount of
precipitate into the state in which it is to be weighed; heated again, and cooled.
The main precipitate is transferred to the crucible, and the heating repeated very
gently at first, but more strongly towards the end of the operation. It is next
placed in the muffle, and, after two or three minutes at a red heat, it is removed
and allowed to cool in the desiccator before weighing. This is for bodies that
will bear a red heat; for those compounds that require a lower temperature the
heating in the muffle is omitted. The muffle used for this purpose must not be
used at the same time for cupelling; a gas muffle (fig. 22), such as one of

Fletcher's, is best. A desiccator (fig. 23) is an air-tight vessel which prevents
access of moisture, &c., to the substance. Usually the air in it is kept dry by
means of a basin containing sulphuric acid.

The crucible is removed from the muffle with the tongs and carried to the
desiccator. It is best, in an office, to have a large desiccator permanently fixed
alongside the balance, into which all substances may be put before being
weighed. The substance is removed from the bench or muffle in the small hand
apparatus generally sold, and carried to the balance room to be transferred to the
large desiccator, where it is allowed to become thoroughly cold before being
weighed. Twenty minutes is generally the time allowed after ignition before it is
advisable to weigh. Bodies allowed to cool in the air after they have been ignited
will absorb moisture, and hot bodies placed in the balance-pan will disturb the
equilibrium and show false results. Compounds that absorb moisture must be
weighed quickly; they should, therefore, be weighed in covered vessels. Such
compounds are detected by their continually-increasing weight. They should be
ignited and weighed again in a well-covered dish.
Substances that have been washed with alcohol, ether, or any readily volatile
liquid are dried in the water oven. They quickly dry if there is no water present,
and are generally fit for weighing in less than one hour. Sometimes drying for a
few minutes only will be sufficient.
The weight of the crucible and precipitate having been obtained, the weight of
the crucible and ash is deducted; for example—

Crucible and precipitate 10.183grams.
Crucible and ash 9.527 "
———
0.656 "

The weight of the ash is best added to that of the crucible. The amount of ash in
filter-papers must not be neglected, although papers are now made almost free
from ash, and the amount to be deducted is found by taking eight or ten papers
and burning them until they become white, and then weighing the ash. The
amount varies from 0.004 to 0.0005 gram for different papers. Having
determined the ash, place in the balance-drawer three of the filter-papers pinned
together, with the weights marked on them in the way shown in fig. 24, so as to
be readily seen when there is occasion to refer to them.

It must be remembered that the determination of small quantities of substances
generally involves the use of reagents which are often contaminated, as an
impurity, with the body sought for. Thus, in assaying silver, the oxide of lead or
metallic lead used is rarely free from silver; and in the case of arsenic, the acids,
zinc or ferric chloride are sure to contain arsenic. The same observation applies
to the precipitation of lead by zinc, &c. The errors caused by these impurities are
more marked in the determination of material having small quantities of metal
than in that of ores which contain larger quantities. Errors of this kind are
counteracted or neutralised by "blank" or "blind" determinations. These consist

in carrying out by the side of and during the assay a duplicate experiment with
the reagents only, which are thereby subjected to the same processes of solution,
evaporation, filtration, &c. The final result thus obtained is deducted from that
given by the assay, the difference gives the corrected result. In some cases,
where it is desired or necessary to have a tangible residue or precipitate, some
pure inert material is added.

CHAPTER IV.
VOLUMETRIC ASSAYS.

These have been already described as those in which the results are got by
measuring, either—(1) the volume of a reagent required to complete some
reaction, or (2) the volume of the resulting product. For example, if a
permanganate of potash solution be added to a solution containing a weighed
amount of iron, dissolved in sulphuric acid, the strong colour of the
permanganate of potash will be removed until a certain quantity of it has been
added. Repeating the experiment, it will be found that the same amount of iron
decolorises the same volume of the permanganate solution within certain narrow
limits of variation, known as "error of experiment." This error is due to variation
in the method of working and to slight differences in the weighings and
measurings; it is present in all experimental methods, although the limits of
variation are wider in some than in others. Apart from this error of experiment,
however, it is certain that a given volume of the permanganate of potash solution
corresponds to a definite weight of iron, so that if either is known the other may
be calculated. Similarly, if a known weight of zinc (or of carbonate of lime) be
dissolved in hydrochloric acid, a gas will be given off which can be measured,
and so long as the conditions of the experiment do not vary, the same weight of
zinc (or of carbonate of lime) gives off the same volume of gas. The weight of
the one can be determined from the volume of the other.
Or, again, the quantity of some substances may be measured by the colour of
their solutions, on the principle that, other things being equal, the colour of a
solution depends upon the quantity of colouring matter present. So that if two
solutions of the same substance are equally coloured they are of equal strength.
In this way an unknown may be compared with a known strength, and a fairly
accurate determination may be made. These three illustrations serve as types of
the three chief classes of volumetric assays—titrometric, gasometric, and
colorimetric.
Titrometric Assays.—Within the limits of the error of experiment, a definite
volume of a solution or gas represents a certain weight of metal or other
substance, hence the exact weight may be determined by experiment. The error
of experiment may be reduced to insignificant dimensions by repeating the

experiment, and taking the mean of three or four determinations. This will at the
same time show the amount of variation. Thus, if 0.5 gram of iron were
dissolved and found to require 50.3 cubic centimetres of the solution of
permanganate of potash, and if on repeating, 50.4, 50.2, and 50.3 c.c. were
required, the experimenter would be justified in saying that 50.3 c.c. of the
permanganate solution represent 0.5 gram of iron, and that his results were good
within 0.2 c.c. of the permanganate solution. So that if in an unknown solution
of iron, 50.5 c.c. of the permanganate solution were used up, he could state with
confidence that it contained a little more than 0.5 gram of iron. With a larger
experience the confidence would increase, and with practice the experimental
error will diminish.
But supposing that the unknown solution required, say, 100.5 instead of 50.5
c.c., he would not be justified in saying that, since 50.3 c.c. are equivalent to 0.5
gram, 100.6 c.c. are equivalent to twice that amount; and that, consequently, the
unknown solution contained a little less than 1 gram of iron; or, at least, he could
not say it except he (or some one else) had determined it by experiment. But if
on dissolving 1 gram of iron, he found it to require 100.6 c.c. of the solution,
and in another experiment with 0.8 gram of iron that 80.5 c.c. of the solution
were required, he would be justified in stating that the volume of solution
required is proportional to the quantity of metal present. There are a large
number of volumetric assays of which this is true, but that it is true in any
particular case can only be proved by experiment. Even where true it is well not
to rest too much weight upon it, and in all cases the quantity of metal taken, to
determine the strength of the solution used, should not differ widely from that
present in the assay. There are certain terms which should be explained here.
When the solution of a reagent is applied under such conditions that the volume
added can be correctly determined, the operation is called "titrating," the
solution of the reagent used the "standard solution," and the process of
determining the strength of the standard solution is "standardising." The
"standard" is the quantity of metal equivalent to 100 c.c. of the standard
solution.
Standard Solutions.—In making these the salt is accurately weighed and
transferred to a litre flask, or to the graduated cylinder, and dissolved. The
method of dissolving it varies in special cases, and instructions for these will be
found under the respective assays. Generally it is dissolved in a small quantity of
liquid, and then diluted to the mark. For those substances that require the aid of
heat, the solution is made in a pint flask, cooled, and transferred; after which the
flask is well washed out. After dilution, the liquids in the measuring vessel must

be thoroughly mixed by shaking. This is more easily and better done in the
cylinder than in the litre flask. The solution is next transferred to a dry
"Winchester" bottle and labelled. The label may be rendered permanent by
waxing it.
Standard solutions should not be kept in a place exposed to direct sunlight.
Oxidising and reducing solutions, such as those of permanganate of potash,
ferrous sulphate, iodine, hyposulphite of soda, &c., gradually weaken in
strength; the solutions of other salts are more stable; while those of potassium
bichromate and baric chloride are almost permanent. Solutions of potassium
permanganate may be kept for a month or so without much change. The
solutions of hyposulphite of soda and of iodine should be examined weekly.
Ferrous sulphate solutions, if acidulated with sulphuric acid, may be depended
on for two or three weeks without fresh standardising. Before filling the burette,
the "Winchester" bottle should be well shaken and a portion of about 50 or 100
c.c. poured into a dry beaker or test-glass. Besides the standard solutions, which
are required for titrating an assay, permanent solutions of the metal or acid of
equivalent strength are very useful. When the finishing point of a titration has
been overstepped (i.e., the assay has been "overdone"), a measured volume, say
5 or 10 c.c., of a solution containing the same metal may be added. The titration
can then be continued, but more cautiously, and the value in "c.c." for the
quantity added be deducted from the final reading.
Standardising.—Suppose the object is to standardise a solution of
permanganate similar to that referred to above. A convenient quantity of iron
(say 0.5 gram) would be weighed out, dissolved in dilute sulphuric acid, and the
solution titrated. Suppose 49.6 c.c. of the permanganate solution are required,
then
49.6 : 0.5 :: 100 : x
x = 1.008 gram.
This result, 1.008 gram, is the "standard." When a gas is measured, the standard
may be calculated in the same way. For example: with 0.224 gram of zinc, 75.8
c.c. of gas were obtained. Then the quantity of zinc equivalent to 100 c.c. of the
gas is got by the proportion.
75.8 : 0.224 :: 100 : x
x = 0.2955 gram.
Using the term "standard" in this sense, the following rules hold good:—

To find the weight of metal in a given substance:—Multiply the standard by the
number of c.c. used and divide by 100. For example: a piece of zinc was
dissolved and the gas evolved measured 73.9 c.c. Then by the rule, 0.2955 ×
73.9 / 100 should give the weight of the piece of zinc. This gives 0.2184 gram.
To find the percentage of metal in a given substance:—Multiply the standard by
the number of c.c. used and divide by the weight of substance taken. For
example: if 2 grams of a mineral were taken, and if on titrating with the
permanganate solution (standard 1.008) 60.4 c.c. were required, then
(1.008×60.4)/2 = 30.44. This is the percentage.
If the standard is exactly 1 gram, and 1 gram of ore is always taken, these
calculations become very simple. The "c.c." used give at once the percentage, or
divided by 100 give the weight of metal.
If it is desired to have a solution with a standard exactly 1.0 gram, it is best first
to make one rather stronger than this, and then to standardise carefully. Divide
1000 by the standard thus obtained and the result will be the number of c.c.
which must be taken and be diluted with water to 1 litre. For example: suppose
the standard is 1.008, then 1000/1.008 gives 992, and if 992 c.c. be taken and
diluted with water to 1000 c.c. a solution of the desired strength will be
obtained. The standard of this should be confirmed. A simpler calculation for the
same purpose is to multiply the standard by 1000; this will give the number of
c.c. to which 1 litre of the solution should be diluted. In the above example a
litre should be diluted to 1008 c.c.
It has been assumed in these rules that the titration has yielded proportional
results; but these are not always obtained. There can be no doubt that in any
actual re-action the proportion between any two re-agents is a fixed one, and that
if we double one of these then exactly twice as much of the other will enter into
the re-action; but in the working it may very well be that no re-action at all will
take place until after a certain excess of one or of both of the re-agents is
present. In titrating lead with a chromate of potash solution, for example, it is
possible that at the end of the titration a small quantity of the lead may remain
unacted on; and it is certain that a small excess of the chromate is present in the
solution. So, too, in precipitating a solution of silver with a standard solution of
common salt, a point is reached at which a small quantity of each remains in
solution; a further addition either of silver or of salt will cause a precipitate, and
a similar phenomenon has been observed in precipitating a hydrochloric acid
solution of a sulphate with baric chloride. The excess of one or other of the re-
agents may be large or small; or, in some cases, they may neutralise each other.

Considerations like these emphasise the necessity for uniformity in the mode of
working. Whether a process yields proportional results, or not, will be seen from
a series of standardisings. Having obtained these, the results should be arranged
as in the table, placing the quantities of metal used in the order of weight in the
first column, the volumes measured in the second, and the standards calculated
in the third. If the results are proportional, these standards will vary more or less,
according to the delicacy of the process, but there will be no apparent order in
the variation. The average of the standards should then be taken.

Weight. Volume found.Standard
0.2160 gram72.9 c.c. 0.2963
0.2185 " 73.9 " 0.2957
0.2365 " 79.9 " 0.2959
0.2440 " 82.3 " 0.2964
0.2555 " 85.9 " 0.2974

Any inclination that may be felt for obtaining an appearance of greater accuracy
by ignoring the last result must be resisted. For, although it would make no
practical difference whether the mean standard is taken as 0.2961 or 0.2963, it is
well not to ignore the possibility that an error of 0.4 c.c. may arise. A result
should only be ignored when the cause of its variation is known.
In this series the results are proportional, but the range of weights (0.216-0.2555
gram) is small. All processes yield fairly proportional results if the quantities
vary within narrow limits.
As to results which are not proportional, it is best to take some imaginary
examples, and then to apply the lesson to an actual one. A series of titrations of a
copper solution by means of a solution of potassic cyanide gave the following
results:—

Copper taken.Cyanide used. Standard.
0.1 gram 11.9 c.c. 0.8403
0.2 " 23.7 " 0.8438
0.3 " 35.6 " 0.8426
0.4 " 47.6 " 0.8403

These are proportional, but by using a larger quantity of acid and ammonia in
the work preliminary to titration, we might have had to use 1 c.c. of cyanide

solution more in each case before the finishing point was reached. The results
would then have been:

Copper taken.Cyanide used. Standard.
0.1 gram 12.9 c.c. 0.7752
0.2 " 24.7 " 0.8097
0.3 " 36.6 " 0.8191
0.4 " 48.6 " 0.8230

It will be noted that the value of the standard increases with the weight of metal
used; and calculations from the mean standard will be incorrect.
By subtracting the lowest standardising from the highest, a third result is got free
from any error common to the other two; thus:—

0.4 gram= 48.6 c.c. "cyanide."
0.1 " = 12.9 " "
—— ——
0.3 " = 35.7 " "

And the standard calculated from this corrected result is 0.8404. Further, if 0.3
gram requires 35.7 c.c., then 0.1 gram should require 11.9 c.c., or 1.0 c.c. less
than that actually found.
We may therefore use the following rules for working processes which do not
yield proportional results. Make a series of two or three titrations, using very
different quantities of metal in each. Subtract the lowest of these from the
highest, and calculate the standard with the remainder. Calculate the volume
required by this standard in any case, and find the excess or deficit, as the case
may be. If an excess, subtract it from the result of each titration; if a deficit, add
it; and use the standard in the usual way. The following table shows an actual
example:—

Chalk taken. Gas obtained. Standard.
0.0873 gram17.8 c.c. 0.4904
0.1305 " 27.3 " 0.4780
0.1690 " 35.8 " 0.4721
0.1905 " 40.4 " 0.4715
0.2460 " 52.5 " 0.4686

0.3000 " 64.0 " 0.4687

It will be seen that the standard decreases as the quantity of chalk increases; this
points to a deficiency in the quantity of gas evolved.
Then

0.3000 = 64.0 c.c.
0.0873 = 17.8 "
———= ——
0.2127 = 46.2 "

and 0.2127×100/46.2 = 0.4604. Then, multiplying the weight of chalk taken by
100, and dividing by 0.4604, we get the calculated results of the following table:
—

Chalk taken. Gas found. Gas calculated. Difference.
0.0873 gram17.8 c.c. 18.9 c.c. -1.1 c.c.
0.1305 " 27.3 " 28.3 " -1.0 "
0.1690 " 35.8 " 36.7 " -0.9 "
0.1905 " 40.4 " 41.4 " -1.0 "
0.2460 " 52.5 " 53.4 " -0.9 "
0.3000 " 64.0 " 65.1 " -1.1 "

By adding 1 c.c. to the quantity of gas obtained, and taking 0.4604 as the
standard, the calculated results will agree with those found with a variation of
0.1 c.c. When a large number of assays of the same kind are being made, this
method of calculation is convenient; when, however, only one or two
determinations are in question, it is easier to make a couple of standardisings,
taking quantities as nearly as possible the same as those present in the assays.
Sometimes it is necessary to draw up a table which will show, without
calculation, the weight of substance equivalent to a given volume of gas or of
solution. The substance used for standardising should be, whenever possible, a
pure sample of the substance to be determined—that is, for copper assays pure
copper should be used, for iron assays pure iron, and so on; but when this cannot
be got an impure substance may be used, provided it contains a known
percentage of the metal, and that the impurities present are not such as will
interfere with the accuracy of the assay. Including compounds with these, the
standard may be calculated by multiplying the standard got in the usual way, by

the percentage of metal in the compound or impure substance, and dividing by
100. If, for example, the standard 1.008 gram was obtained by using a sample of
iron containing 99.7 per cent. of metal, the corrected standard would be
1.008×99.7/100 = 1.005.
In volumetric analysis the change brought about must be one in which the end of
the reaction is rendered prominent either by a change of colour or by the
presence or absence of a precipitate. If the end of the reaction or finishing-point
is not of itself visible, then it must be rendered visible by the use of a third
reagent called an indicator.
For example, the action of sulphuric acid upon soda results in nothing which
makes the action conspicuous; if, however, litmus or phenolphthalein be added
the change from blue to red in the first case, or from red to colourless in the
second, renders the finishing-point evident. Some indicators cannot be added to
the assay solution without spoiling the result; in which case portions of the assay
solution must be withdrawn from time to time and tested. This withdrawal of
portions of the assay solution, if rashly done, must result in loss; if, however, the
solution is not concentrated, and if the portions are only withdrawn towards the
end of the titration, the loss is very trifling, and will not show-up on the result.
The usual plan adopted is to have a solution of the indicator placed in drops at
fairly equal intervals distributed over a clean and dry white porcelain-plate: a
drop or two of the solution to be tested is then brought in contact with one of
these and the effect noted. Another plan is to have thin blotting-paper, moistened
with a solution of the indicator and dried; a drop of the solution to be tested
placed on this shows the characteristic change. When the assay solution contains
a suspended solid which interferes with the test, a prepared paper covered with
an ordinary filter-paper answers very well; a drop of the solution to be tested is
placed on the filter-paper, and, sinking through, shows its effect on the paper
below.
Except when otherwise stated, all titrations should be made at the ordinary
temperature; cooling, if necessary, by holding the flask under the tap. When a
titration is directed to be made in a boiling solution, it must be remembered that
the standard solution is cold, and that every addition lowers the temperature of
the assay.
On running the solution from the burette into the assay, do not let it run down
the side of the flask. If a portion of the assay has to be withdrawn for testing,
shake the flask to ensure mixing, and then take out a drop with the test-rod; the
neglect of these precautions may give a finishing-point too early. This is

generally indicated by a sudden finish, in which case on shaking the flask and
again testing no reaction is got. Do not remove the drop on the point of the
burette with the test-rod; let it remain where it is or drop it into the solution by
carefully opening the clip.
Generally the methods of working are as follows:—
(1) When the finishing-point depends on a change of colour in the solution.—
Increase the bulk of the assay up to from 100 to 150 c.c. with water. Boil or
cool, as the case may be. Run in the standard solution from a burette speedily,
until the re-agent appears to have a slower action, and shake or stir all the time.
Then run 1 c.c. or so at a time, still stirring, and finally add drops until the
colour change is got.
(2) When an outside-indicator is used.—Pour the standard solution from a
burette into the assay until 5 or 6 c.c. from the finishing-point; then run in 1 c.c.
at a time (stirring and testing on the plate between each) until the indicator
shows the change wanted, and deduct 0.5 c.c. for excess. When greater accuracy
is sought for a duplicate assay is made. In this case the standard solution is run
in close up to the end, and the operation is finished off with a few drops at a
time.
(3) Where the finishing-point depends upon the absence of a precipitate and no
outside-indicator is used.—As in the last case, run in the standard solution up to
within a few c.c. of the end, then run in 1 c.c. at a time until a precipitate is no
longer formed, but here 1.5 c.c. must be deducted for excess, since it is evident
that the whole of the last "c.c." must have been, and a portion of the previous
one may have been, in excess.
Indirect Titration.—The action of permanganate of potash upon a ferrous
solution is one of oxidation, hence it is evident that if any other oxidising agent
is present it will count as permanganate. In such a case the titration can be used
(indirectly) to estimate the quantity of such oxidising agent, by determining how
much less of the permanganate is used. For example, suppose that 1 gram of iron
dissolved in sulphuric acid requires 100 c.c. of standard permanganate to fully
oxidise it, but that the same amount of iron only requires 35.6 c.c. of the same
standard permanganate if it has been previously heated with 0.5 gram of black
oxide of manganese. Here it is evident that 0.5 gram of black oxide does the
work of 64.4 c.c.[4] of the permanganate solution, and that these quantities are
equivalent; moreover, if 64.4 c.c. correspond with 0.5 gram, then 100 c.c.
correspond with 0.7764 which is the standard. On theoretical grounds, and by a

method of calculation which will be explained further on (under the heading
"Calculations from Formulæ"), it can be found that if the standard for iron is 1
gram, that for the black oxide will be 0.7764 gram.
The principles of these indirect titrations become clearer when expressed in a
condensed form. Thus, in the example selected, and using the formulæ Fe =
Iron, KMnO4 = permanganate of potash, and MnO2 = oxide of manganese, we
have:—
(1) 1 gram Fe = 100 c.c. KMnO4

(2) 1 gram Fe = 35.6 c.c. KMnO4 + 0.5 gram MnO2
∴ 100 c.c. KMnO4 = 35.6 c.c. KMnO4 + 0.5 gram MnO2
(100 - 35.6) c.c. KMnO4 = 0.5 gram MnO2
64.4 c.c. KMnO4 = 0.5 gram MnO2

The iron does not enter into the calculation if the same quantity is present in the
two experiments.
An indirect titration thus requires three determinations, but if more than one
assay is to be carried on, two of these need not be repeated. The standard is
calculated in the usual way.
Colorimetric Assays.—These are assays in which the colour imparted to a
solution by some compound of the metal to be determined is taken advantage of;
the depth of colour depending on the quantity of metal present. They are
generally used for the determination of such small quantities as are too minute to
be weighed. The method of working is as follows:—A measured portion of the
assay solution (generally 2/3, 1/2, 1/3, or 1/4 of the whole), coloured by the
substance to be estimated, is placed in a white glass cylinder standing on a sheet
of white paper or glazed porcelain. Into an exactly similar cylinder is placed the
same amount of re-agents, &c., as the portion of the assay solution contains, and
then water is added until the solutions are of nearly equal bulk. Next, a standard
solution of the metal being estimated is run in from a burette, the mixture being
stirred after each addition until the colour approaches that of the assay. The bulk
of the two solutions is equalised by adding water. Then more standard solution is
added until the tints are very nearly alike. Next, the amount added is read off
from the burette, still more is poured in until the colour is slightly darker than
that of the assay, and the burette read off again. The mean of the readings is
taken, and gives the quantity of metal added. It equals the quantity of metal in

the portion of the assay. If this portion was one-half of the whole, multiply by
two; if one-third, multiply by three, and so on. When the quantity of metal in
very dilute solutions is to be determined, it is sometimes necessary to
concentrate the solutions by boiling them down before applying the re-agent
which produces the coloured compound. Such concentration does not affect the
calculations.
Gasometric Assays.—Gasometric methods are not much used by assayers, and,
therefore, those students who wish to study them more fully than the limits of
this work will permit, are recommended to consult Winkler and Lunge's text-
book on the subject. The methods are without doubt capable of a more extended
application. In measuring liquids, ordinary variations of temperature have but
little effect, and variations of atmospheric pressure have none at all, whereas
with gases it is different. Thus, 100 c.c. of an ordinary aqueous solution would,
if heated from 10° C. to 20° C., expand to about 100.15 c.c. 100 c.c. of a gas
similarly warmed would expand to about 103.5 c.c., and a fall of one inch in the
barometer would have a very similar effect. And in measuring gases we have not
only to take into account variations in volume due to changes in temperature and
atmospheric pressure, but also that which is observed when a gas is measured
wet and dry. Water gives off vapour at all temperatures, but the amount of
vapour is larger as the temperature increases.
By ignoring these considerations, errors of 3 or 4 per cent. are easily made; but,
fortunately, the corrections are simple, and it is easy to construct a piece of
apparatus by means of which they may be reduced to a simple calculation by the
rule of three.
The volume of a gas is, in practice, usually reduced to that which it would be at
a temperature of 0° C., when the column of mercury in the barometer is 760
mm. high. But, although convenient, this practice is not always necessary. The
only thing required is some way of checking the variations in volume, and of
calculating what the corrected volume would be under certain fixed conditions.
Suppose that at the time a series of standardisings is being made, 100 c.c. of air
were confined in a graduated tube over moist mercury. These 100 c.c. would
vary in volume from day to day, but it would always be true of them that they
would measure 100 c.c. under the same conditions as those under which the
standardisings were made. If, then, in making an actual assay, 35.4 c.c. of gas
were obtained, and the air in the tube measured 105 c.c., we should be justified
in saying, that if the conditions had been those of the standardising, the 105 c.c.
would have measured 100 c.c., and the 35.4 c.c. would have been 33.7; for 105:

100:: 35.4: 33.7. The rule for using such a piece of apparatus for correcting
volumes is:—Multiply the c.c. of gas obtained by 100, and divide by the number
of c.c. of air in the apparatus.
If it is desired to calculate the volumes under standard conditions (that is, the gas
dry, at 0° C. and 760 mm. barometric pressure) the calculations are easily
performed, but the temperature and pressure must be known.
Correction for Moisture.—The "vapour tension" of water has been accurately
determined for various temperatures, and it may be looked upon as
counteracting the barometric pressure. For example, at 15° C. the vapour tension
equals 12.7 millimetres of mercury; if the barometer stood at 750 mm., the
correction for moisture would be made by subtracting 12.7 from 750, and taking
737.3 mm. to be the true barometric pressure.
The vapour tensions for temperatures from 0° C. to 20° C. are as follows:—

Temp. Tension. Temp. Tension. Temp. Tension.
0° 4.6 mm. 7° 7.5 mm. 14° 11.9 mm.
1° 4.9 mm. 8° 8.0 mm. 15° 12.7 mm.
2° 5.3 mm. 9° 8.6 mm. 16° 13.5 mm.
3° 5.7 mm. 10° 9.2 mm. 17° 14.4 mm.
4° 6.1 mm. 11° 9.8 mm. 18° 15.3 mm.
5° 6.5 mm. 12° 10.5 mm. 19° 16.3 mm.
6° 7.0 mm. 13° 11.2 mm. 20° 17.4 mm.

The correction for pressure is:—Multiply the volume by the actual pressure and
divide by 760.
The correction for temperature:—Multiply the volume by 273 and divide by the
temperature (in degrees Centigrade) added to 273.
For all three corrections the following rules hold good. To reduce to 0° C. and
760 mm. dry.
Volume × 0.3592 × (Pressure-tension)
Corrected volume = ———————————————————
Temperature + 273
To find the volume, which a given volume under standard conditions would
assume, if those conditions are altered.

Volume × 2.784 × (Temperature + 273)
Resulting volume = ——————————————————
Pressure - tension
As an example, we will suppose that it is desired to enclose in the apparatus
referred to on p. 45, a volume of air, which, when dry (at 0° C. and 760 mm.),
shall measure 100 c.c., whilst the actual temperature is 15° C., and the pressure
750 mm.
The second formula is the one to be used, and we get 108.7 c.c.
100 c.c.×2.784×288
Required volume = ———————————
750-12.7

80179.2
= ————
737.3

= 108.7 c.c.

FOOTNOTES:
[4] 100-35.6 = 64.4.

CHAPTER V.
WEIGHING AND MEASURING.

Weighing.—The system of weights and measures which we have adopted is the
French or metric system; in this the gram (15.43 grains) is the unit of weight; the
only other weight frequently referred to is the milligram, which is 0.001, or
1/1000 gram. The unit of volume is the cubic centimetre, which is
approximately the volume of 1 gram of water, and which thus bears to the gram
the same relation as grain-measures bear to grains. It is usual to write and even
pronounce cubic centimetre shortly as c.c., and the only other denomination of
volume we shall have occasion to use is the "litre," which measures 1000 c.c.,
and is roughly 1-3/4 pints.
The weights used are kept in boxes in a definite order, so that the weights on the
balance can be counted as well by noting those which are absent from the box as
by counting those present on the scale-pan. The weights run 50, 20, 10, 10, 5, 2,
1, 1 and 1 grams, and are formed of brass. The fractions of the gram are
generally made of platinum or of aluminium, and are arranged in the following
order:—0.5, 0.2, 0.1, 0.1, and 0.05, 0.02, 0.01, 0.01. These may be marked in
this way, or they may be marked 500, 200, 100, 100, 50, 20, 10, 10; the 500
meaning 500 milligrams.
Some makers send out weights in the series 50, 20, 20, 10, &c.
Weights of less than 0.01 gram are generally present in a box, but it is much
more convenient to work with a rider. This is a piece of wire which in the pan
weighs 0.01 gram; it is made in such a form that it will ride on the beam, and its
effective weight decreases as it approaches the centre. If the arm of the beam is
divided into tenths, then each tenth counting from the centre outward equals
0.001 gram or 1 milligram, and if these tenths be further subdivided the fractions
of a milligram are obtained; and these give figures in the fourth place of
decimals. A fairly good balance should be sensitive to 0.0001 gram. The weights
must never be touched with the fingers, and the forceps for moving them is used
for no other purpose. When not in actual use the box is kept closed. The weights
must not be allowed to remain on the pan of the balance. The balance-case must
not be open without some reason. It must be fixed level, and, once fixed, must

not be needlessly moved. The bench on which it stands should be used for no
other purpose, and no one should be allowed to lean upon it.

When using a balance sit directly in front of it. Ordinarily the substance to be
weighed is best put on the pan to the user's left; the weights and the rider are
then easily manipulated. Powders, &c., should not be weighed directly on the
balance; a counterpoised watch-glass or metal scoop (fig. 25) should be used. In
some cases it is advisable to use a weighing-bottle. This is a light, well-
stoppered bottle (fig. 3) containing the powdered ore. It is first filled and
weighed; then some of the substance is carefully poured from it into a beaker or
other vessel, and it is weighed again; the difference in the two weighings gives
the weight of substance taken. A substance must always be cold when weighed,
and large glass vessels should be allowed to stand in the balance-box a little
while before being weighed. Always have the balance at rest when putting on or
taking off anything from the pans. Put the weights on systematically. In using
the rider (except you have a reason to the contrary), put it on at the 5; if this is
too much, then try it at the 3; if then the weights are too little, try at the 4, if still
not enough, the correct weight must be between the 4 and 5; try half-way
between.
It is best to work with the balance vibrating; equilibrium is established when the
vibration to the left is the mean of the preceding and succeeding vibrations to the
right. For example, if it vibrates 6 divisions to the right on one swing, and 5
divisions on the next, the intermediate vibration to the left should have been 5-
1/2.
Note whether the substance increases in weight whilst on the balance. If it does
it may be because it was put on warm, and is cooling, or it may be because it is
taking up moisture from the air. Substances which take up moisture rapidly
should be weighed in clipped watch-glasses or in light-weighing bottles or tubes.
Students, in recording the weights, should first read off those missing from the
box, writing down each order of figures as determined; first tens, then units, and
so on. Remember that the first four platinum weights give the figures of the first

place of decimals, the second four give the second place, and that the third and
fourth places are given by the rider. Having taken down the figures, confirm
them by reading off the weights as you put them back into the box. Do not rest a
weight on the palm of your hand for convenience in reading the mark upon it.
Remember one weight lost from a box spoils the set. Do not take it for granted
that the balance is in equilibrium before you start weighing: try it.

Measuring Liquids.—For coarse work, such as measuring acids for dissolving
ores, graduated glasses similar to those used by druggists may be used. It is well
to have two sizes—a smaller graduated into divisions of 5 c.c. (fig. 26), and a
larger with divisions equal to 10 c.c. No measurement of importance should be
made in a vessel of this kind, as a slight variation in level causes a serious error.
Graduated flasks must be used when anything has to be made up to a definite
bulk, or when a fixed volume has to be collected. If, for example, a certain
weight of substance has to be dissolved and diluted to a litre, or if the first 50
c.c. of a distillate has to be collected, a flask should be used. Each flask is
graduated for one particular quantity; the most useful sizes are 1000 c.c., 500
c.c., 200 c.c., 100 c.c., and 50 c.c. The mark should be in the narrowest part of
the neck, and should be tangential to the curved surface of the liquid when the

flask contains the exact volume specified. The level of a curved surface of liquid
is at first somewhat difficult to read: the beginner is in doubt whether the surface
should be taken at a, b, or c (fig. 27). It is best to take the lowest reading c. In
some lights it is difficult to find this; in such cases a piece of white paper or card
held behind and a little below, so as to throw light up and against the curved
surface, will render it clear. In reading, one should look neither up at nor down
upon the surface, but the eye should be on the same level with it. It must be kept
in mind that flasks contain the quantity specified, but deliver less than this by
the amount remaining in them and damping the sides. If it is desired to transfer
the contents say of a 100 c.c. flask to a beaker, it will be necessary to complete
the transfer by rinsing out the flask and adding the washings; otherwise there
will be a sensible loss. Graduated cylinders (fig. 28) are convenient for
preparing standard solutions.

Pipettes and burettes are graduated to deliver the quantities specified. The
principle of the pipette, and the advantages and disadvantages of its various
forms, may be understood by considering the first form shown in fig. 29. It is
essentially a bulbed tube drawn out to a jet at its lower end, and having on each

side of the bulb a mark so placed that when the surface of the liquid falls from
the upper to the lower mark the instrument shall deliver exactly 100 c.c. The
bore of the jet should be of such a size as will allow the level of the liquid to fall
at the rate of about one foot in two minutes. If it runs more quickly than this, an
appreciable error arises from the varying amount of liquid remaining, and
damping the sides of the bulb. The flow of liquid from a pipette must not be
hastened by blowing into it. The lower tube or nose of the pipette should be long
enough to reach into the bottle or flask containing the liquid about to be
measured. The pipette is filled by sucking at the open end with the mouth; this
method of filling renders the use of the instrument dangerous for such liquids as
strong acids, ammonia, and such poisonous solutions as that of potassic cyanide.
One attempt with a fairly strong solution of ammonia will teach the beginner a
very useful lesson. As soon as the liquid rises above the upper mark in the
pipette, the mouth is withdrawn, and the pipette quickly closed by pressing the
upper aperture with the index finger of the right hand; it is well to have the
finger slightly moist, but not damp. The neck of the pipette should be long
enough to allow its being firmly grasped by the fingers and thumb of the right
hand without inconvenience. The pipette is first held in a vertical position long
enough to allow any moisture outside the tube to run down, and then the liquid
is allowed to run out to the level of the upper mark; this is easily effected by
lessening the pressure. If the finger is wet, the flow will be jerky, and good work
impossible. The pipette is next held over the vessel into which the 100 c.c. are to
be put, and the liquid allowed to run out. When the bulb is nearly empty, the
flow should be checked by replacing the finger, and the liquid allowed to escape
slowly until the lower mark is reached. The pipette is then withdrawn; it is in the
withdrawing that the disadvantage of this particular form[5] makes itself felt. It
must be withdrawn very steadily, as the slightest shock causes the remaining
column of liquid to vibrate, whereby air is drawn in and the liquid is forced out.
This disadvantage is got rid of by making the mouth of the jet the lower limit,
or, in other words, allowing the instrument to empty itself. There are two forms
of such pipettes; in the one generally recommended in Gay-Lussac's silver assay
(the last shown in fig. 29) the nose is replaced by a jet. This is most conveniently
filled by stopping the jet with the finger, and allowing the liquid to flow in a fine
stream into the neck until the pipette is filled, and then working as just
described. The other form is the one in general use; in fact, a long nose to a
pipette is so convenient that it may almost be said to be necessary. But the
accuracy is slightly diminished; a long narrow tube makes a poor measuring
instrument because of the amount of liquid it finally retains. A defect possessed
by both forms is the retention of a drop of varying size in the nozzle. Whatever

method is adopted for removing this drop must be always adhered to. The most
convenient form is the one last described, and the most useful sizes are 100 c.c.,
50 c.c., 20 c.c., 10 c.c., and 5 c.c. Ten c.c. pipettes graduated into tenths of a
cubic centimetre are very useful: those are best in which the graduation stops
short of the bottom.
All measurements should be made at the ordinary temperature; and, before being
used, the pipette should be rinsed out with a cubic centimetre or so of the
solution to be measured. After using, it should be washed out with water.
Burettes differ mainly from pipettes in having the flow of liquid controlled from
below instead of from above. The best form is that known as Mohr's, one kind of
which is provided with a glass stopcock, while the other has a piece of india-
rubber tube compressed by a clip. The latter cannot be used for solutions of
permanganate of potash or of iodine, or of any substance which acts on india-
rubber; but in other respects there is little to choose between the two kinds. A
burette delivering 100 c.c., and graduated into fifths (i.e., each division = 0.2
c.c.), is a very convenient size. For some kinds of work, 50 c.c. divided into
tenths (i.e., each division = 0.1 c.c.) may be selected.
Burettes may be fixed in any convenient stand; they must be vertical and should
be so placed that the assayer can read any part of the graduated scale without
straining. When not in use, they should be kept full of water. When using a
burette, the water must be run out; the burette is next rinsed with some of the
solution to be used, and drained; and then it is filled with the solution. Next
squeeze the india-rubber tube so as to disentangle air-bubbles and, by smartly
opening the clip, allow the tube and jet to be filled; see that no bubbles of air are
left. Then run out cautiously until the level of the liquid in the burette stands at
zero. In reading the level with very dark-coloured liquids it is convenient to read
from the level a (fig. 27), and, provided it is done in each reading, there is no
objection to this. The accuracy of the reading of a burette is sensibly increased
by the use of an Erdmann float. This is an elongated bulb, weighted with
mercury, and fitting (somewhat loosely) the tube of the burette. It floats in the
solution, and is marked with a horizontal line; this line is taken as the level of
the liquid. If the burette is filled from the top, the float rises with aggravating
slowness, and this is its chief disadvantage. The float must come to rest before
any reading is made.

A convenient plan for filling a burette from below is shown in fig. 30. The
diagram explains itself. The bottle containing the standard solution is connected
with the burette by a syphon arrangement through the glass tube and T-piece.
The flow of liquid into the burette is controlled by the clip. When this clip is
opened, the burette fills; and when it is closed, the burette is ready for use in the
ordinary way.
Measuring Gases.—Lange's nitrometer (fig. 69) is a very convenient
instrument for many gasometric methods. It requires the use of a fair quantity of
mercury. In fig. 31, there is a representation of a piece of apparatus easily fitted
up from the ordinary material of a laboratory. It is one which will serve some
useful purposes. It consists of a wide-mouthed bottle fitted (by preference) with
a rubber cork. The cork is perforated, and in the perforation is placed a glass
tube which communicates with the burette. The burette is connected by a rubber
tube and a Y-piece, either with another burette or with a piece of ordinary
combustion-tube of about the same size. The wide-mouthed bottle contains
either a short test-tube or an ordinary phial with its neck cut off. In working the
apparatus the weighed substance is put in the bottle and the re-agent which is to
act on it, in the test-tube; the cork is then inserted. The liquid in the two burettes

is next brought to the same level, either by pouring it in at a or running it out at
b. The level of the liquid in the apparatus for correcting variation in volume is
then read and noted. Next, after seeing that the level of the liquid in the burette
has not changed, turn the bottle over on its side so that the re-agent in the test-
tube shall be upset into the bottle. Then, as the volume of the gas increases,
lower the liquid in the burette by running it out at b, and at the same time keep
the level in a half an inch or so lower than that in the burette. When the action
has finished bring the liquid in the two vessels to the same level and read off the
burette. This part of the work must always be done in the same manner.

The volume corrector for gas analysis is a graduated glass tube of 120 c.c.
capacity inverted over a narrow glass cylinder of mercury. It contains 0.2 or 0.3
c.c. of water and a volume of air, which, if dry and under standard conditions,
would measure 100 c.c. The actual volume varies from day to day, and is read
off at any time by bringing the mercury inside and outside to the same level.
This is done by raising or lowering the tube, as may be required. Any volume of
gas obtained in an assay can be corrected to standard temperature and pressure
by multiplying by 100 and dividing by the number of c.c. in the corrector at the
time the assay is made.

FOOTNOTES:
[5] It is best to use this form with a glass stopcock, or with an india-rubber tube and
clip, after the manner of a Mohr's burette.

CHAPTER VI.
RE-AGENTS.—ACIDS, ETC.

Acetic Acid, H[=A=c] or C2H4O2. (sp. gr. 1.044, containing 33 per cent. real
acid).—An organic acid, forming a class of salts, acetates, which are for the
most part soluble in water, and which, on ignition, leave the oxide or carbonate
of the metal. It is almost always used in those cases where mineral acids are
objectionable. To convert, for example, a solution of a substance in hydrochloric
acid into a solution of the same in acetic acid, alkali should be added in excess
and then acetic acid. Many compounds are insoluble in acetic acid, which are
soluble in mineral acids, such as ferric phosphate, ferric arsenate, zinc sulphide,
calcium oxalate, &c., so that the use of acetic acid is valuable in some
separations. The commercial acid is strong enough for most purposes, and is
used without dilution.
"Aqua Regia" is a mixture of 1 part by measure of nitric acid and 3 parts of
hydrochloric acid. The acids react forming what is practically a solution of
chlorine.[6] The mixture is best made when wanted, and is chiefly used for the
solution of gold and platinum and for "opening up" sulphides. When solutions in
aqua regia are evaporated, chlorides are left.
Bromine, Br. (sp. gr. 3.0). Practically pure bromine.—It is a heavy reddish-
brown liquid and very volatile. It boils at 60° C., and, consequently, must be
kept in a cool place. It gives off brown irritating vapours, which render its use
very objectionable. Generally it answers the same purpose as aqua regia, and is
employed where the addition of nitric acid to a solution has to be specially
avoided. It is also used for dissolving metals only from ores which contain
metallic oxides not desired in the solution.
"Bromine Water" is simply bromine shaken up with water till no more is
dissolved.
Carbonic Acid, CO2.—A heavy gas, somewhat soluble in water; it is mainly
used for providing an atmosphere in which substances may be dissolved,
titrated, &c., without fear of oxidation. It is also used in titrating arsenic assays
with "iodine" when a feeble acid is required to prevent the absorption of iodine

by the alkaline carbonate. It is prepared when wanted in solution, by adding a
gram or so of bicarbonate of soda and then as much acid as will decompose the
bicarbonate mentioned. When a quantity of the gas is wanted, it is prepared, in
an apparatus like that used for sulphuretted hydrogen, by acting on fragments of
marble or limestone with dilute hydrochloric acid.
Citric Acid (H3[=C=i] or C6H8O7.H2O) is an organic acid which occurs in
colourless crystals, soluble in less than their weight of water. The solution must
be freshly prepared, as it gets mouldy when kept. It forms a comparatively
unimportant class of salts (citrates). It is used in the determination of phosphoric
acid, chiefly for the purpose of preventing the precipitation of phosphates of iron
and alumina by ammonia, and in a few similar cases. The commercial crystals
are used; they should be free from sulphuric acid and leave no ash on ignition.
Hydrochloric Acid, HCl in water, (sp. gr. 1.16. It contains 32 per cent. of
hydrogen chloride).—It is sometimes called "muriatic acid," and when impure,
"spirit of salt." The acid solution should be colourless and free from arsenic,
iron, and sulphuric acid. It forms an important family of salts, the chlorides. It is
the best acid for dissolving metallic oxides and carbonates, and is always used
by the assayer when oxidising agents are to be avoided. The acid is used without
dilution when no directions are expressly given to dilute it. It has no action on
the following metals: gold, platinum, arsenic, and mercury; it very slightly
attacks antimony, bismuth, lead, silver, and copper. Tin is more soluble in it, but
with difficulty; whilst iron, zinc, nickel, cobalt, cadmium, and aluminium easily
dissolve with evolution of hydrogen and the formation of the lower chloride if
the metal forms more than one class of salts. All the metallic oxides, except a
few of the native and rarer oxides, are dissolved by it with the formation of
chlorides of the metal and water.
Dilute Hydrochloric Acid is made by diluting the strong acid with an equal
volume of water. This is used for dissolving precipitates obtained in the general
course of analysis and the more easily soluble metals.
Hydrofluoric Acid, HF.—A solution in water may be purchased in gutta-percha
or lead bottles. It is of variable strength and doubtful purity. It must always be
examined quantitatively for the residue left on evaporation. It is used
occasionally for the examination of silicates. It attacks silica, forming fluoride of
silicon, which is a gas. When the introduction of another base will not interfere
with the assay, the substance may be mixed in the platinum dish with fluoride of
ammonium, or of potassium, or of calcium, and hydrochloric acid, instead of
treating it with the commercial acid. It is only required in special work. The

fumes and acid are dangerous, and, of course, glass or porcelain vessels cannot
be used with it.
Iodine, I.—This can be obtained in commerce quite pure, and is often used for
standardising. It is very slightly soluble in water, but readily dissolves in
potassium iodide solution. It closely resembles chlorine and bromine in its
properties, and can be used for dissolving metals without, at the same time,
attacking any oxide which may be present. It is chiefly used as an oxidizing
agent in volumetric work, being sharp in its reactions and easily detected in
minute quantities. It cannot be used in alkaline solutions, since it reacts with the
hydrates, and even with the carbonates, to form iodides and iodates. Iodine is
soluble in alcohol.
Nitric Acid, HNO3. (Sp. gr. 1.42; boiling point 121° C.; contains 70 per cent. by
weight of hydrogen nitrate).—It is convenient to remember that one c.c. of this
contains 1 gram of real acid. It combines the properties of an acid and of an
oxidising agent. One c.c. contains 0.76 gram of oxygen, most of which is very
loosely held, and easily given up to metals and other oxidisable substances.
Consequently it will dissolve many metals, &c., upon which hydrochloric acid
has no action. All sulphides (that of mercury excepted) are attacked by it, and for
the most part rendered soluble. It has no action on gold or platinum, and very
little on aluminium. The strong acid at the ordinary temperature does not act on
iron or tin; and in most cases it acts better when diluted. Some nitrates being
insoluble in nitric acid, form a protecting coat to the metal which hinders further
action. Where the strong acid does act the action is very violent, so that
generally it is better to use the dilute acid. When iron has been immersed in
strong nitric acid it not only remains unacted on, but assumes a passive state; so
that if, after being wiped, it is then placed in the dilute acid, it will not dissolve.
Tin and antimony are converted into insoluble oxides, while the other metals
(with the exception of those already mentioned) dissolve as nitrates. During the
solution of the metal red fumes are given off, which mainly consist of nitrogen
peroxide. The solution is often coloured brown or green because of dissolved
oxides of nitrogen, which must be got rid of by boiling. Generally some
ammonium nitrate is formed, especially in the cases of zinc, iron, and tin, when
these are acted on by cold dilute acid. Sulphur, phosphorus, and arsenic are
converted into sulphuric, phosphoric, and arsenic acids respectively, when
boiled with the strong acid.
Dilute Nitric Acid.—Dilute 1 volume of the strong acid with 2 of water.

Oxalic Acid, H2O or (H2C2O4.2H2O.)—This is an organic acid in colourless
crystals. It forms a family of salts—the oxalates. It is used in standardising;
being a crystallised and permanent acid, it can be readily weighed. It is also used
in separations, many of the oxalates being insoluble. For general use make a 10
per cent. solution. Use the commercially pure acid. On ignition the acid should
leave no residue.

Sulphuretted Hydrogen. Hydrosulphuric acid, SH2.—A gas largely used in
assaying, since by its action it allows of the metals being conveniently classed
into groups. It is soluble in water, this liquid dissolving at the ordinary
temperature about three times its volume of the gas. The solution is only useful
for testing. In separations, a current of the gas must always be used. It is best
prepared in an apparatus like that shown in fig. 32, by acting on ferrous sulphide
with dilute hydrochloric acid. When iron has to be subsequently determined in
the assay solution, the gas should be washed by bubbling it through water in the
smaller bottle; but for most purposes washing can be dispensed with. The gas is
very objectionable, and operations with it must be carried out in a cupboard with
a good draught. When the precipitation has been completed, the apparatus
should always be washed out. The effect of this acid on solutions of the metals is
to form sulphides. All the metallic sulphides are insoluble in water; but some are
soluble in alkaline, and some in acid, solutions. If sulphuretted hydrogen is
passed through an acid solution containing the metals till no further precipitation
takes place, a precipitate will be formed containing sulphides insoluble in the
acid. On filtering, adding ammonia (to render the filtrate alkaline), and again
passing the gas, a further precipitate will be obtained, consisting of sulphides

insoluble in an alkaline solution, but not precipitable in an acid one; the filtrate
may also contain sulphides not precipitable in an acid solution, which are
soluble in an alkaline one; these will be thrown down on neutralising. Again, the
metals precipitated in the acid solution form sulphides which may be divided
into groups, the one consisting of those which are soluble, and the other of those
which are not soluble, in alkalies. This classification is shown in the following
summary:—
1. Precipitable in an acid solution.

(a) Soluble in Alkalies.—Sulphides of As, Sb, Sn, Au, Pt, Ir, Mo, Te, and Se.

(b) Insoluble in Alkalies.—Sulphides of Ag, Pb, Hg, Bi, Cu, Cd, Pd, Rh, Os,
and Ru.

2. Not precipitated in an acid solution, but thrown down in an alkaline one.

Sulphides of Mn, Zn, Fe, Ni, Co, In, Tl, and Ga.
These can again be divided into those which are dissolved by dilute acids and
those which are not.
3. Not precipitated in an acid or alkaline solution, but thrown
down on neutralising the latter.

Sulphides of V and W.
Sulphuretted hydrogen is a strong reducing agent. Ferric salts are thereby
quickly reduced to ferrous; in hot solutions nitric acid is decomposed. These
changes are marked by a precipitation of sulphur, and the student must be
careful to pass the gas sufficiently long, and not be too hasty in concluding that
no sulphide will form because it does not at once make its appearance. The best
indication that it has been passed long enough is the smell of the gas in the
solution after shaking.
Sulphurous Acid, H2SO3.—The reagent used may be regarded as a saturated
solution of sulphur dioxide in water. It may be purchased, and keeps for a long
time. It may be made by heating copper with sulphuric acid and passing the gas
formed into water. The heat should be withdrawn when the gas is coming off
freely. It is used as a reducing agent, and should not be diluted.

Sulphuric Acid, H2SO4. (Sp. gr. 1.84, containing 96 per cent. of real acid,
H2SO4.)—This acid forms insoluble sulphates with salts of lead, strontium, and
barium. It has a high boiling point, 290° C., and, when evaporated with salts of
the more volatile acids, converts them into sulphates. When nitrates or chlorides
are objectionable in a solution, evaporation with sulphuric acid removes them.
In working with this acid caution is necessary, since, on mixing with water, great
heat is evolved; and, if either the acid or water has been previously heated, a
serious accident may result. In diluting the acid it should be poured into cold
water. Glass vessels containing boiling sulphuric acid should be handled as little
as possible, and should not be cooled under the tap. The action of diluted
sulphuric acid on metals closely resembles that of dilute hydrochloric acid.
Magnesium, aluminium, iron, zinc, nickel, cobalt, manganese, and cadmium
dissolve, with evolution of hydrogen, in the cold acid, or when warmed. The
action of hot and strong sulphuric acid is altogether different; it acts as an
oxidising agent, and is itself reduced to sulphur dioxide or even to sulphur. The
following metals are attacked in this way:—copper, bismuth, mercury, silver,
antimony, tin, and lead. Gold, platinum, and arsenic are not affected. This
property is made use of in parting silver from gold and platinum. Metallic
sulphides are similarly attacked; but this method of opening up minerals has the
disadvantage of giving rise to the formation of anhydrous sulphates of iron, &c.,
which are not readily dissolved when afterwards diluted. The use of sulphuric
acid in assaying is (for these reasons) to be avoided. Its chief use is as a drying
agent, since it has a strong affinity for water. Air under a bell jar may be kept dry
by means of a basin of sulphuric acid, and gases bubbled through it are freed
from water-vapour.
Dilute Sulphuric Acid.—This is made by diluting 1 volume of the strong acid
with 4 of water.
Tartaric Acid, H2[=T] or C4H6O6.—A crystallised organic acid, soluble in less
than its own weight of water, or in less than three parts of alcohol. It is used for
the same purposes as citric acid is. The solution is made when required.

BASES, SALTS, &c.

Alcohol, C2H6O. (Commercial alcohol of sp. gr. 0.838; it contains 84 per cent.
by weight of alcohol.)—It should burn with a non-luminous flame and leave no
residue. It is used for washing precipitates where water is inapplicable, and for
facilitating drying.

Ammonia, NH3. (Commercial ammonia, a solution having a sp. gr. of 0.88 to
0.89, and containing about 33 per cent. of ammonia.)—It is used as an alkali
(more commonly than soda or potash), since an excess of it is easily removed by
boiling. The salts of ammonium formed by it may be removed by igniting, or by
evaporating in a porcelain dish with an excess of nitric acid. It differs in a
marked way from soda or potash in its solvent action on the oxides or hydrates
of the metals. Salts of the following metals are soluble in an ammoniacal
solution in the presence of ammonic chloride:—copper, cadmium, silver, nickel,
cobalt, manganese, zinc, magnesium, sodium, potassium, and the alkaline earths.
Dilute Ammonia is made by diluting 1 vol. of commercial ammonia with 2 of
water. The dilute ammonia is always used; but in assays for copper a stronger
solution (1 of strong ammonia to 1 of water) is required.
Ammonic Carbonate (Am2CO3) is prepared by dissolving one part of the
commercial sesquicarbonate of ammonia in four parts of water, and adding one
part of strong ammonia.
Ammonic Bicarbonate (HAmCO3) is prepared by saturating a solution of the
sesquicarbonate of ammonia with carbon dioxide.
Ammonic Chloride, AmCl.—Use the commercial salt in a 20 per cent. solution
in water. The salt should leave no residue on ignition.
Ammonic Molybdate.—The solution is prepared as follows:—Dissolve 100
grams of the powdered commercial salt in 200 c.c. of dilute ammonia, and pour
the solution in a slow stream into 750 c.c. of dilute nitric acid; make up to 1 litre,
and allow the mixture to settle before using. It is used for the purpose of
separating phosphoric oxide from bases and from other acids, and also as a test
for phosphates and arsenates. In using this solution the substance must be
dissolved in nitric acid, and a considerable excess of the reagent added (50 c.c.
is sufficient to precipitate 0.1 gram P2O5); when the phosphate is in excess no
precipitate will be got. The precipitate is phospho-molybdate of ammonia.
Ammonic Nitrate (AmNO3) is used in the separation of phosphoric oxide by
the molybdate method, and occasionally for destroying organic matter. It is
soluble in less than its own weight of water. The solution is made when wanted.
Ammonic Oxalate (Am2C2O4.2H2O) is used chiefly for the separation of lime.
The solution is made by dissolving 15 grams of the salt in 100 c.c. of water.

Ammonic Sulphide may be purchased in the state of a strong solution. It is
yellow, and contains the disulphide, S2Am2. It serves the same purpose as is
obtained by passing a current of sulphuretted hydrogen through an ammoniacal
solution; but has the disadvantage of loading the solution with sulphur, which is
precipitated when the solution is subsequently acidified. It is useful for
dissolving the lower sulphide of tin (SnS).
Baric Carbonate (BaCO3) is sometimes used for precipitating the weaker
bases. It should be prepared when wanted by precipitating a solution of baric
chloride with ammonic carbonate and washing. The moist precipitate is used
without drying.
Baric Chloride, BaCl2.2H2O.—A crystallised salt, soluble in 2-1/2 parts of
water. It is used for the detection and separation of sulphates. Make a 10 per
cent. solution.
"Black Flux."—A mixture of finely divided carbon with carbonate of potash or
with carbonates of potash and soda. It is prepared by heating tartar or "rochelle
salt" until no more combustible gas is given off. One gram will reduce about 2
grams of lead from litharge.
Borax, Na2B4O7.10H2O.—It is chiefly used as a flux in dry assaying, as already
described. It is also used in testing before the blowpipe; many metallic oxides
impart a characteristic colour to a bead of borax in which they have been fused.
Calcium Chloride.—The crystallised salt is CaCl2.6H2O; dried at 200° C. it
becomes CaCl2.2H2O, and when fused it becomes dehydrated. The fused salt,
broken into small lumps, is used for drying gases. It combines with water, giving
off much heat; and dissolves in a little more than its own weight of water. Strong
solutions may be used in baths in which temperatures above the boiling-point of
water are required. One part of the salt and 2 of water give a solution boiling at
112°, and a solution of 2 parts of the salt in 1 of water boils at 158°. The salt is
very little used as a reagent.
Calcium Fluoride or "Fluor Spar," CaF2.—The mineral is used as a flux in dry
assaying; it renders slags which are thick from the presence of phosphates, &c.,
very fluid. Mixed with hydrochloric acid it may sometimes be used instead of
hydrofluoric acid.

Calcium Carbonate, CaCO3.—It is precipitated in a pure state by ammonic
carbonate from a solution of calcium chloride. It is used for standardising. In the
impure state, as marble or limestone, it is used in the preparation of carbonic
acid.
Calcium Hydrate or "Lime Water."—This is used in testing for carbon
dioxide and in estimating the amount of that gas present in air. It may be made
by slaking quicklime and digesting the slaked lime with water. One hundred c.c.
of water at 15° C. dissolves 0.1368 grams of the hydrate (CaH2O2), and hot
water dissolves still less. "Milk of lime" is slaked lime suspended in water.
Cobalt Nitrate (Co(NO3)2.6H2O) is used in a 10 per cent. solution for the
detection of oxides of zinc, aluminium, &c.; on ignition with which it forms
characteristically coloured compounds.
Copper, Cu.—Pure copper, as obtained by electrolysis, can be purchased. This
only should be used.
Copper Oxide, CuO.—It occurs as a black, heavy, and gritty power, and is used
for the oxidation of carbon and hydrogen in organic substances. It should be
ignited and cooled out of contact with air just before using, since it is
hygroscopic. Oxide of copper which has been used may be again utilised after
calcination.
Copper Sulphate (CuSO4.5H2O) contains 25.4 per cent. of copper. It is used in
the outer cell of a Daniell-battery. The commercial salt is used for this purpose.
The re-crystallised and pure salt is used for preparing the anhydrous sulphate,
which is used for detecting moisture in gases. For this purpose it is dried at 200°
C. till no trace of green or blue colour remains. It must be prepared when
wanted. It may be conveniently used in the form of pumice-stone, saturated with
a solution of the salt and dried. Traces of moisture develop a green colour.
Ferric Chloride, Fe2Cl6. (When crystallised, Fe2Cl6.6H2O.)—The solution is
prepared as described under iron. The commercial salt contains arsenic, and,
since the chief use of ferric chloride is for the determination of this substance, it
must be purified (see under Arsenic).
Ferric Sulphate (Fe2(SO4)3) is a yellowish white deliquescent salt. It is used as
an indicator in volumetric silver assaying, and for the separation of iodine from
bromine. It may be purchased as iron alum, Am2Fe2(SO4)4.24H2O. But it is best

prepared by adding strong sulphuric acid to ferric hydrate in equivalent
proportions. Use it as a solution containing 2 or 3 per cent. of iron.
Ferrous Sulphate, FeSO4.7H2O.—The granulated form is best, and can be
purchased pure. It is used for standardising. It keeps better in crystals than in
solution. It is readily soluble in water, but the solution is best made with the help
of a little free acid. As a re-agent use a 10 per cent. solution. The crystals should
be clear bluish-green; if their colour is dark green, brown, or blue, they should
be rejected.
Ferrous Sulphide (FeS) is used for the preparation of sulphuretted hydrogen. It
may be purchased and broken in small lumps, nut-size, for use.
"Fusion Mixture" (K2CO3.Na2CO3) is a mixture of potassic and sodic
carbonates in the proportions of 13 of the former to 10 of the latter, by weight. It
is hygroscopic. A mixture of the bicarbonates is better, being purer and less apt
to get damp.
Gallic Acid (C7H6O5.H2O) is an organic acid, occurring as a pale fawn-coloured
crystalline powder, soluble in 100 parts of cold water, or in 3 parts of boiling
water. It is used for the determination of antimony. A 10 per cent. solution in
warm water is made when required.
Hydrogen (H) is a gas. It is obtained by acting on zinc with dilute hydrochloric
or sulphuric acid. It is used as a reducing agent, and for providing an atmosphere
free from oxygen. It reduces metallic oxides at a high temperature. It must be
freed from water; and special precautions should be taken to prevent an
admixture with air. It is generally required in a current which can be continued
for an hour or more without interruption. The preparation can be conveniently
carried out in the apparatus shown (fig. 33). A quart bottle is half filled with
sheet zinc, and connected with bulbs filled with sulphuric acid, and with a
calcium chloride tube. The last is connected with the apparatus through which
the gas has to be passed. Dilute hydrochloric acid mixed with a few cubic
centimetres (20 c.c. to 1 pint) of stannous chloride sol. to fix any dissolved
oxygen, is placed in the funnel, and let into the bottle by opening the stopcock
when required. Care must be taken to let the hydrogen escape for some time
before starting the reduction.

Gold, Au.—Gold, obtained by cupelling and "parting," is for most purposes
sufficiently pure. It is best kept in the shape of foil. When the purer metal is
required, gold should be dissolved in aqua regia, the solution evaporated to a
paste, diluted, allowed to stand, and filtered. The filtered solution is acidified
with hydrochloric acid, warmed, and precipitated with sodium sulphite. The
precipitate is collected, washed, and fused on charcoal.
Iron, Fe.—The soft wire (thin) is used for standardising. Rods are used in dry
assays as a desulphurising agent. Steel must not be used, since it is not pure, and
contains a variable amount of iron.
Lead, Pb.—Granulated lead or lead-foil is used in the dry assay for silver and
gold, and in the preparation of lead salts. It can be obtained very pure, but
always contains more or less silver, 1 or 2 milligrams in 100 grams. The amount
of silver it contains must be determined and recorded.
Lead Acetate (Pb[=A=c]2.3H2O, or Pb(C2H3O2)2.3H2O) is used as a test,
specially for the detection and estimation of sulphuretted hydrogen. Prepare a 10
per cent. solution for use.
Lead Nitrate (Pb(NO3)2) can be purchased pure. It is used for standardising.

Lead Dioxide (PbO2) occurs as a dark-brown powder. It is used as an oxidizing
agent and for absorbing sulphurous oxide. It can be prepared by digesting red
lead with warm dilute nitric acid; washing and drying the residue.

"Litharge," PbO.—It can be purchased as a yellow heavy powder. It is used in
dry assaying as a flux, as a desulphurising agent, and also as a source of lead. It
always contains some silver, the amount of which must be determined.
Litmus.—This is an organic colouring matter which is turned red by acids and
blue by alkalies. For ordinary purposes it is best used as litmus paper, which
may be purchased in small books. A solution is prepared by digesting 15 or 20
grams of the commercial litmus in 100 c.c. of water on the water bath. After
being allowed to settle, it is filtered and made just faintly red with acetic acid.
Then there is added a drop or two of a solution of soda and 10 c.c. of alcohol. It
should be kept in a loosely-covered bottle.
Magnesia, MgO.—It may be purchased as "calcined magnesia." It is used for
making "magnesia mixture," and should be kept in a corked wide-mouthed
bottle.
"Magnesia Mixture."—Dissolve 22 grams of magnesia in about a quarter of a
litre of dilute hydrochloric acid, avoiding excess. Add 5 grams of magnesia, boil,
and filter. Add 300 grams of ammonic chloride, and 250 c.c. of strong ammonia;
and dilute with water to 2 litres. It should be kept in a stoppered winchester.
Magnesium Sulphate, MgSO4.7H2O.—It can be purchased very pure, and is
occasionally used as a standard salt.
Manganese Dioxide, MnO2.—It is used in the preparation of chlorine. The
commercial article is not pure, but is sufficiently so for this purpose.
Marble, CaCO3.—Fragments of the white crystalline variety only should be
used. It is used as a source of lime and of carbon dioxide.
Mercury, Hg.—This can be purchased pure. It should have a bright surface,
flow without a tail, and leave no residue on ignition. It is used as a standard; for
amalgamation; and as a confining liquid in gas analysis.
Mercuric Chloride (HgCl2) may be purchased pure. Make a 5 per cent. solution
in water. It is used for destroying an excess of stannous chloride; for removing
sulphuretted hydrogen from solution; and as a test for stannous salts.
Microcosmic Salt, HAmNaPO4.8H2O.—When fused NaPO3 is formed. It is
used in testing for metallic oxides and silica before the blowpipe. The crystals
are sometimes used as a standard for phosphoric acid.

"Nessler's Solution."—Mode of preparation: Dissolve 35 grams of potassium
iodide in 100 c.c. of water; dissolve 17 grams of mercuric chloride in 300 c.c. of
water, and pour this solution into that of the iodide till a permanent precipitate is
produced; make up to 1 litre with a 20 per cent. solution of potash; add mercuric
chloride till a precipitate is again formed; allow to settle and decant. It is used
for detecting ammonia.
Nitre.—This is potassium nitrate.
Platinum Chloride, 2HCl.PtCl4. (In the crystallised form it has 6H2O).—It may
be made as follows:—Take 5 grams of clean platinum scrap and dissolve in a
flask at a gentle heat in 50 c.c. of hydrochloric acid with the occasional addition
of some nitric acid; evaporate to a paste; and then dissolve in 100 c.c. of water.
It is used for separating and determining potassium.
Phenolphthalein is an organic compound used as an indicator; more especially
in determining the weaker acids, it cannot be used in the presence of ammonia.
Dissolve half a gram in 100 c.c. of dilute alcohol.
Potassium Bicarbonate, KHCO3.—It may be purchased pure; on ignition it
leaves the carbonate, K2CO3, which may be used as a standard.

Potassium Cyanide, KCN.—It is used in the dry assay as a reducing agent. The
commercial salt is very impure. Purchase that sold as potassic cyanide (gold)
which contains about 95 per cent. of KCN. It is used for copper assaying and
occasionally in separation. Make a 10 per cent. solution when wanted.
Potassium Bichromate, K2Cr2O7. It may be purchased nearly pure. It is used as
an oxidising agent, for determining iron; and as a test solution. For this last
purpose a 10 per cent. solution is prepared.
Potassium Chlorate (KClO3) can be purchased pure. It is used with
hydrochloric acid as a substitute for aqua regia.
Potassium Ferrocyanide (K4Fe(CN)6.3H2O), or "yellow prussiate of potash,"
is used as a test; as an indicator; and for the determination of zinc. Make a 5 per
cent. solution.
Potassium Ferricyanide (K6Fe2(CN)12), or "red prussiate of potash," is used
for testing; and as an indicator. Make a 5 per cent. solution when wanted, as it
decomposes on keeping.

Potassium Hydrate, KHO. Purchase that purified with alcohol. It is an alkali,
and is used for absorbing carbonic acid, &c.
Potassium Iodide, KI. It may be purchased nearly pure. It is used as a test and
for dissolving iodine. It should be used in a 10 per cent. solution freshly made.
The solution decomposes on exposure to light, with separation of iodine.
Potassium Nitrate (KNO3) can be purchased pure. It is used in the dry way as
an oxidizing agent. It is very fusible. It decomposes at a low temperature into
potassium nitrite (KNO2) and free oxygen; and at a higher temperature leaves
potash (K2O). It oxidizes sulphur and carbon with explosive violence. This
action may be moderated by mixing the nitre with carbonate of soda, common
salt, or some other inert body.
Potassium Nitrite, KNO2.—The commercial article is not pure, but is
sufficiently so for the purpose required. A saturated solution is used in the
separation of cobalt; the solution is made when wanted.
Potassium Permanganate, KMnO4.—This salt can be purchased sufficiently
pure. It is much used as an oxidizing agent.
Potassium Bisulphate (KHSO4) is used as a dry reagent for opening up
minerals. It fuses; and at a much higher temperature is converted into potassium
sulphate with loss of sulphuric acid.
Potassium Sulphocyanate (KCNS) is used for the detection and determination
of traces of ferric iron; as also in the separation of silver and copper from some
of the other metals. Make a 10 per cent. solution. It should show no colour on
the addition of hydrochloric acid.
"Red Lead" (Pb3O4) is used in the dry assay as a flux instead of litharge, from
which it differs in containing a little more oxygen. When acted on by nitric acid
a brown residue of lead dioxide is left, nitrate of lead going into solution. Like
litharge it always carries silver; about 2 milligrams in 100 grams.
Silver, Ag.—Pure silver in foil is required as a standard. It may be prepared as
follows:—Dissolve scrap silver in dilute nitric acid and decant off from any
residue; dilute the solution with hot water and add hydrochloric acid until there
is no further precipitate, stir; allow the precipitate to settle; decant and wash; dry
the precipitate, mix it with twice its bulk of carbonate of soda and fuse the

mixture in a crucible until tranquil; clean the button and roll or hammer it into
foil.
Sodium Acetate, NaC2H3O2.3H2O.—The crystals may be purchased
sufficiently pure. Make a 20 per cent. solution in water. It is used for replacing
mineral acids by acetic acid.[7]
Sodium Acetate and Acetic Acid.—A solution is used in the determination of
phosphates and arsenates; 100 grams of the salt is dissolved in 500 c.c. of acetic
acid, and diluted with water to one litre.
Sodium Bicarbonate (NaHCO3)is used as a flux in dry methods. On ignition it
leaves the carbonate (Na2CO3), which is used as a standard reagent. Make a 20
per cent. solution of the carbonate for use. It should be free from chlorides or
sulphates, or if impure the amount of impurities must be determined.
Sodium Hydrate, NaHO. It may be purchased in sticks, which should be kept in
a well-corked bottle. It is sometimes called "caustic soda." It is a strong alkali. It
is used for neutralizing acid solutions and for separations where ammonia is
unsuitable. Make a 5 per cent. solution for use.
Sodium Hyposulphite, Na2S2O8.5H2O.—It may be purchased pure. It is
generally known as "hypo." It is used as a standard.
Sodium Sulphite (Na2SO3.7H2O) is used as a reducing agent.

Sodium Phosphate, Na2HPO4.12H2O. The crystals may be purchased pure, but
they effloresce in dry air with loss of water. It is used as a standard and for
precipitating magnesia, &c. Make a 10 per cent. solution.
Stannous Chloride, SnCl2.2H2O.—The crystals are best purchased. If kept dry
and free from air they are fairly permanent. A solution is made by dissolving 20
grams in 10 c.c. of hydrochloric acid and diluting to 1 litre. The solution is not
permanent. It is a strong reducing agent, and is chiefly used in solution for this
purpose.
Tin, Sn.—Grain tin should be purchased. It is not pure, but contains 99.5 per
cent. of the metal. The chief impurity is copper. It can be used as a standard.
When acted on with hot hydrochloric acid it slowly dissolves (more rapidly in
contact with platinum) and forms stannous chloride.

Uranium Acetate, UO2(C2H3O2)2.H2O.—It is best purchased in crystals. The
solution is used for the determination of phosphates and arsenates. A solution of
3 per cent. strength is occasionally used as an indicator.
Uranium Nitrate, UO2(NO3)2.6H2O.—This salt is very soluble in water and is
sometimes used instead of the acetate, which is somewhat difficult to dissolve.
"Water," H2O.—Spring or well water is sufficiently pure for most purposes,
100 c.c. will leave a residue of from 10 to 30 milligrams, so that where a salt has
to be dissolved out, evaporated, and weighed it should be replaced by distilled
water. Rain water, melted snow, &c., always leave less residue than spring
water; but in other respects they are often dirtier. Distilled water is best prepared
in the office, a glass or tin condenser being used.
Zinc, Zn.—It is sold in a granulated form or in sticks. It generally contains over
1 per cent. of lead, with a little iron and arsenic. It is used for separating metals
from their solutions, and generally as a reducing agent. For the preparation of
hydrogen, and in most other cases, scrap sheet zinc may be used.
Zinc Oxide, ZnO.—The commercial oxide sometimes contains carbonate.
Zinc Sulphate, ZnSO4.7H2O.—It is occasionally used as a standard, and can be
purchased nearly pure.

FOOTNOTES:
[6] 3HCl + HNO3 = Cl2 + NOCl + 2H2O.

[7] NaC2H3O2 + HCl = H4C2O2 + NaCl.

CHAPTER VII.
FORMULÆ, EQUATIONS, ETC.

Formulæ and equations are a kind of short hand for expressing briefly and in the
language of the atomic theory the facts of chemical composition and reaction.
The convenience of this method of expressing the facts justifies a short
description of it here.
On comparing the percentage composition of a series of compounds the
proportions in which the elements combine appears to be regulated by no simple
law. For example:

Realgar. Orpiment. Mispickel. Pyrites.
Arsenic 71.4 60.9 46.0 —
Sulphur 28.6 39.1 19.6 53.3
Iron — — 34.4 46.7
——— ——— ——— ———
100.0 100.0 100.0 100.0

But if in these examples the composition is calculated, not on 100 parts, but on
107, 246, 163, and 120 parts respectively, evidence of a simple law becomes
apparent.

Realgar. Orpiment. Mispickel. Pyrites.
Arsenic 75.0 150.0 75.0 —
Sulphur 32.0 96.0 32.0 64.0
Iron — — 56.0 56.0
——— ——— ——— ———
107.0 246.0 163.0 120.0

It will be seen that the proportion of arsenic is 75 or twice 75, that of iron is 56,
and that of sulphur 32 or some simple multiple of 32. The series of examples
might be extended indefinitely, and it would still be found that the "combining
proportions" held good. The number 75 is spoken of as the "combining weight,"
or, more frequently, as the "atomic weight" of arsenic. Similarly 56 is the atomic

weight of iron, and 32 the atomic weight of sulphur. The importance of this law
of chemical combination is altogether independent of the atomic theory; but this
theory furnishes the simplest explanation of the facts. According to it a chemical
compound is made up of exactly similar groups of particles. The particles of
each elementary substance are all alike, but differ from those of other elements
in weight. Ultimate particles are called atoms, and the groups of atoms are called
molecules. The atomic weight of any particular element is the weight of its atom
compared with the weight of an atom of hydrogen. The atom of sulphur, for
instance, is 32 times as heavy as the atom of hydrogen, and the atomic weight of
sulphur is 32. The molecular weight is the sum of the atomic weights of the
group. The molecule of pyrites contains two atoms of sulphur and one of iron:
on referring to the table of atomic weights it will be seen that the atomic weights
are—sulphur 32, and iron 56. The molecular weight, therefore, is 32 + 32 + 56
—that is, 120. The meaning of this is, 120 parts by weight of iron pyrites contain
64 parts of sulphur and 56 parts of iron; and this is true whether the "parts by
weight" be grains or tons.
The symbol or formula of an atom is generally the initial letter or letters of the
Latin or English name of the substance. The atom of hydrogen is written H, that
of oxygen O, of sulphur S, of iron (ferrum) Fe, and so on. A list of these symbols
is given in the table of atomic weights.
The formula of a molecule is obtained by placing together the symbols of the
contained atoms. Thus, Fe represents an atom of iron, S an atom of sulphur,
while FeS represents the molecule of sulphide of iron as containing one atom of
each element.
When more than one atom of an element is present this is shown by writing a
figure under and after the symbol; thus, FeS2 represents a molecule with one
atom of iron and two atoms of sulphur, Fe2S3 similarly shows one with two
atoms of iron and three of sulphur. When a group of atoms is enclosed in
brackets, a figure after and under the bracket multiplies all within it; for
example, Pb(NO3)2 is another way of writing PbN2O6. Sometimes it is
convenient to represent the atoms of a molecule as divided into two or more
groups; this may be done by writing the formulæ of the groups, and separating
each simple formula by a full stop. Slaked lime, for instance, has the formula
CaH2O2; or, as already explained, we may write it Ca(HO)2; or, if for purposes
of explanation we wished to look on it as lime (CaO) and water (H2O), we could
write it CaO.H2O. A plus sign (+) has a different meaning; CaO + H2O indicates

quantities of two substances, water and lime, which are separate from each
other. The sign of equality (=) is generally used to separate a statement of the
reagents used from another statement of the products of the reaction; it may be
translated into the word "yields" or "becomes." The two statements form an
equation.
Ignoring the quantitative relation, the meaning of the equation CaO + H2O =
CaO.H2O is: "lime and water yield slaked lime." By referring to a table of
atomic weights we can elicit the quantitative relations thus:—

CaO + H2O = CaH2O2
↓ ↓ ↓
Ca = 40 H2 = 2 = 1×2 Ca = 40
O = 16 O = 16 H2 = 2 = 1×2
—— —— O2 = 32 = 16×2
56 18 ——
74

Or, putting it in words, 56 parts of lime combine with 18 parts of water to form
74 parts of slaked lime. This equation enables one to answer such a question as
this:—How much lime must be used to produce 1 cwt. of slaked lime? for, if 74
lbs. of slaked lime require 56 lbs. of lime, 112 lbs. will require (56 × 112)/74, or
about 84-3/4 lbs.
As another example having a closer bearing on assaying take the following
question:—"In order to assay 5 grams of 'black tin' (SnO2) by the cyanide
process, how much potassic cyanide (KCN) will be required?" The reaction is

SnO2 + 2KCN = Sn + 2KCNO
↓ ↓
Sn = 118 K = 39
O2 = 32 C = 12
—— N = 14
150 ——
65×2 = 130

What is sought for here is the relation between the quantities of SnO2 and KCN.
Note that a figure before a formula multiplies all that follows up to the next stop

or plus or equality sign. The question is now resolved to this: if 150 grams of
oxide of tin require 130 grams of cyanide, how much will 5 grams require?
150 : 130 :: 5 : x
x = 4.33 grams.
A problem of frequent occurrence is to find the percentage composition of a
substance when its formula has been given. For example: "What percentage of
iron is contained in a mineral having the formula 2Fe2O3.3H2O?" Bringing this
formula together we have Fe4H6O9. Find the molecular weight.

Fe4 = 224 = 56×4
H6 = 6 = 1×6
O9 = 144 = 16×9
——
374

Then we get: 374 parts of the mineral contain 224 of iron. How much will 100
contain?
374 : 224 :: 100 : x
x = 59.89.
And the answer to the question is 59.89 per cent.
Again, suppose the question is of this kind:—"How much crystallised copper
sulphate (CuSO4.5H2O) will be required to make 2 litres of a solution, 1 c.c. of
which shall contain 0.0010 gram of copper?"
A litre is 1000 c.c., so, therefore, 2 litres of the solution must contain 0.001 gram
× 2000, or 2 grams. How much crystallised copper sulphate will contain this
amount of metal?

Cu = 63.3
S = 32.0
O4 = 64.0 = 16×4
5H2O = 90.0 = 18×5
————
249.3

If 63.3 grams of copper are contained in 249.3 grams of sulphate, in how much
is 2 grams contained.
63.3 : 249.3 :: 2 grams : x
x = 7.8769 grams.
The answer is, 7.8769 grams must be taken.
As a sample of another class of problem similar in nature to the last (but a little
more complicated) take the following:—"What weight of permanganate of
potash must be taken to make 2 litres of a solution, 100 c.c. of which shall be
equivalent to 1 gram of iron?" In the first place the 2 litres must be equivalent to
20 grams of iron, for there are 20 × 100 c.c. in two litres. In the titration of iron
by permanganate solution there are two reactions. First in dissolving the iron
Fe + H2SO4 = FeSO4 + H2
↓
56
and second, in the actual titration,
10FeSO4 + 2KMnO4 + 9H2SO4= 2MnSO4 + 5Fe2(SO4)3 + 2KHSO4 + 8H2O
↓
K = 39
Mn = 55
O4= 64
——
158 × 2 = 316
As before, attention is confined to the two substances under consideration—viz.,
Fe and KMnO4. In the second equation, we find 316 parts of the permanganate
are required for 10 molecules of FeSO4; and in the first equation 56 parts of iron
are equivalent to one molecule of FeSO4, therefore 560 of iron are equivalent to
316 of permanganate; and the question is, How much of the permanganate will
be equivalent to 20 grams of iron?
560 : 316 :: 20 grams : x.
x = 11.286 grams.
The answer is 11.286 grams.

Very similar to this last problem is the question suggested under the head
"Indirect Titration" (p. 43). "If 100 c.c. of the standard permanganate solution
are equivalent to 1 gram of iron, how much peroxide of manganese will they be
equivalent to?" The equation for dissolving the iron is already given; the second
equation is
2FeSO4 + MnO2 + 2H2SO4 = Fe2(SO4)2 + MnSO4 + 2H2O
↓
Mn = 55
O2 = 32
——
87
It will be seen that 87 grams of peroxide of manganese are equivalent to 112
grams of iron. How much then is equivalent to 1 gram of iron?
112 : 87 :: 1 gram : x
x = 0.7767 gram.
It is sometimes convenient to calculate the formula of a substance from its
analysis. The method of calculating is shown by the following example.
Required the formula of a mineral which gave the following figures on analysis:
—

Cupric oxide (CuO) 10.58
Ferrous oxide (FeO) 15.69
Zinc oxide (ZnO) 0.35
Sulphuric oxide (SO2) 28.82
Water (H2O) 44.71
——————
100.15

First find the molecular weights of CuO, FeO, &c., and divide the corresponding
percentages by these figures. Thus, CuO = 63.3+16 = 79.3 and 10.58 divided by
79.3 gives 0.1334. Similarly FeO = 56+16 = 72 and 15.69 divided by 72 gives
0.2179. Treated in the same way the oxide of zinc, sulphuric oxide and water
give as results 0.0043, 0.3602 and 2.484.
Classify the results as follows:—

Bases. Acids. Water.
CuO 0.1334 SO3 0.3602 H2O 2.484
FeO 0.2179
ZnO 0.0043
—————————— —————————— ——————————
RO 0.3556 RO3 0.3602 R2O 2.484

The figures 0.3556, 0.3602 and 2.484 should be then divided by the lowest of
them—i.e., 0.3556; or where, as in this case, two of the figures are very near
each other the mean of these may be taken—i.e., 0.3579. Whichever is taken the
figures got will be approximately 1, 1 and 7. The formula is then RO.SO3.7H2O
in which R is nearly 2/5ths copper, 3/5ths iron and a little zinc.
This formula requires the following percentage composition, which for the sake
of comparison is placed side by side with the actual results.

Calculated. Found.
Cupric oxide 11.29 10.58
Ferrous oxide 15.37 15.69
Zinc oxide nil 0.35
Sulphuric oxide 28.47 28.82
Water 44.84 44.71
——————
99.97 100.15

Trimming the results of an analysis to make them fit in more closely with the
calculations from the formula would be foolish as well as dishonest. There can
be no doubt that the actual analytical results represent the composition of the
specimen much more closely than the formula does; although perhaps other
specimens of the same mineral would yield results which would group
themselves better around the calculated results than around those of the first
specimen analysed. It must be remembered that substances are rarely found pure
either in nature or in the arts; so that in most cases the formula only gives an
approximation to the truth. In the case of hydrated salts there is generally a
difficulty in getting the salt with exactly the right proportion of water.

PRACTICAL EXERCISES.

The following calculations may be made:—
1. Calculate standards in the following cases—
(a) Silver taken, 1.003 gram. Standard salt used, 100.15 c.c.
(b) Iron taken, 0.7 gram. Bichromate used, 69.6 c.c.

2. Calculate percentages:—
(a) Ore taken, 1 gram. Solution used, 65.2 c.c. Standard, 0.987 gram.

(b) Ore taken, 1 gram. Barium sulphate got, 1.432 gram. Barium sulphate
contains 13.73 per cent. of sulphur, and the percentage of sulphur in the ore is
wanted.

(c) Barium sulphate is BaSO4. Calculate the percentage of sulphur it contains,
for use in the preceding question.

3. A method of estimating the quantity of peroxide in a manganese ore is based
on the following reactions:—

(1) MnO2 + 4HCl = MnCl2 + Cl2 + 2H2O.

(2) Cl + KI = KCl + I.

To how much MnO2 is 1 gram of Iodine (I) equivalent?

4. A mineral has the following composition:—

Carbonic acid (CO2) 19.09
Copper oxide (CuO) 71.46
Water (H2O) 9.02

What is its formula?

5. How much copper is contained in 1.5 gram of crystallized copper sulphate
(CuSO4.5H2O)? How much of these crystals must be taken to give 0.4 gram of
copper?

6. How much ferrous sulphate crystals (FeSO4.7H2O) must be taken to yield 2

litres of a solution, 100 c.c. of which shall contain 0.56 gram of iron?

7. Galena is PbS, and hæmatite Fe2O3. What percentages of metal do these
minerals contain?

CHAPTER VIII
SPECIFIC GRAVITY.

The relation of the weight of a substance to its volume should be kept in mind in
all cases where both weight and volume are dealt with. Students are apt to
imagine that on mixing equal volumes of, say, sulphuric acid and water, an acid
of half the strength must be obtained. If the statement of strength is in parts by
weight this will lead to considerable error. For example, 100 c.c. of sulphuric
acid containing 98 per cent. by weight of real acid, will, if diluted with 100 c.c.
of water, yield a solution containing not 49 per cent. by weight, but about 63.5
per cent. of the acid. The reason is this: the 100 c.c. of sulphuric acid weighs 184
grams, and contains 180.32 grams of real acid, while the 100 c.c. of water
weighs only 100 grams; the mixed water and acid weighs 284 grams, and
contains 180.32 of real acid, which is equivalent to nearly 63.5 per cent. by
weight. If, however, the method of statement be volumetric, it would be correct
to say that doubling the volume halves the strength: if 100 c.c. of brine contains
10 grams of salt, and is diluted with water to 200 c.c., it would be of one-half the
former strength, that is, 100 c.c. of the solution would contain 5 grams of salt.
This confusion is avoided by always stating the strengths as so many grams or
"c.c." in 100 c.c. of the liquid. But obviously it would be advantageous to be
able to determine quickly the weight of any particular substance corresponding
to 1 c.c. or some other given volume. Moreover, in descriptions of processes the
strengths of acids and solutions are frequently defined neither by their
gravimetric nor volumetric composition, but by a statement either of specific
gravity or of the degrees registered by Twaddell's or Beaumé's hydrometer.
Thus, in the description of the process of gold parting, one writer gives: "The
acid should be of 1.2 specific gravity"; and another says: "The acid must not be
stronger than 32° Beaumé."
These considerations justify an account of the subject in such a work as this.
And on other grounds the determination of a specific gravity is one of the
operations with which an assayer should be familiar.
The meaning of "specific gravity" is present in the mind of every one who uses
the sentence "lead is heavier than water." This is meaningless except some such

phrase as "bulk for bulk" be added. Make the sentence quantitative by saying:
"bulk for bulk lead is 11.36 times heavier than water," and one has the exact
meaning of: "the specific gravity of lead is 11.36." A table of the specific
gravities of liquids and solids shows how many times heavier the substances are
than water.
It is better, however, to look upon the specific gravity (written shortly, sp. g.) as
the weight of a substance divided by its volume. In the metric system, 1 c.c. of
water at 4° C. weighs with sufficient exactness 1 gram; consequently, the sp. g.,
which states how many times heavier than water the substance is, also expresses
the weight in grams of one c.c. of it. So that if a 100 c.c. flask of nitric acid
weighs, after the weight of the flask has been deducted, 120 grams, 1 c.c. of the
acid weighs 1.2 gram, and the sp. g. is 1.2. The specific gravity, then, may be
determined by dividing the weight of a substance in grams by its volume in c.c.;
but it is more convenient in practice to determine it by dividing the weight of the
substance by the weight of an equal volume of water. And since the volumes of
all substances, water included, vary with the temperature, the temperature at
which the sp. g. is determined should be recorded. Even then there is room for
ambiguity to the extent that such a statement as the following, "the specific
gravity of the substance at 50° C. is 0.9010," may mean when compared with
water at 50° C. or 4° C., or even 15.5° C. For practical purposes it should mean
the first of these, for in the actual experiments the water and the substance are
compared at the same temperature, and it is well to give the statement of results
without any superfluous calculation. In the metric system the standard
temperature is 4° C., for it is at this point that 1 c.c. of water weighs exactly 1
gram. In England, the standard temperature is 60° F. (15.5° C.), which is
supposed to be an average temperature of the balance-room. The convenience of
the English standard, however, is merely apparent; it demands warming
sometimes and sometimes cooling. For most purposes it is more convenient to
select a temperature sufficiently high to avoid the necessity of cooling at any
time. Warming to the required temperature gives very little trouble.
Determination of Specific Gravity.—There is a quick and easy method of
determining the density or sp. g. of a liquid, based upon the fact that a floating
body is buoyed up more by a heavy liquid than by a light one. The method is
more remarkable for speed than accuracy, but still is sufficiently exact. The
piece of apparatus used for the purpose is endowed with a variety of names—sp.
g. spindle, hydrometer, areometer, salimeter, alcoholimeter, lactometer, and so
on, according to the special liquid upon which it is intended to be used. It
consists of a float with a sinker at one end and a graduated tube or rod at the

other. It is made of metal or glass. Generally two are required, one for liquids
ranging in sp. g. from 1.000 to 2.000, and another, which will indicate a sp. g.
between 0.700 and 1.000. The range depends on the size of the instrument. For
special work, in which variations within narrow limits are to be determined,
more delicate instruments with a narrower range are made.

In using a hydrometer, the liquid to be tested is placed in a cylinder (fig. 34)
tall enough to allow the instrument to float, and not too narrow. The
temperature is taken, and the hydrometer is immersed in the fluid. The mark
on the hydrometer stem, level with the surface of the liquid, is read off.
With transparent liquids it is best to read the mark under and over the water
surface and take the mean.
The graduation of hydrometers is not made to any uniform system. Those
marked in degrees Baumé or Twaddell, or according to specific gravity, are
most commonly used. The degrees on Baumé's hydrometer agree among
themselves in being at equal distances along the stem; but they are
proportional neither to the specific gravity, nor to the percentage of salt in
the solution. They may be converted into an ordinary statement of specific
gravity by the following formulæ:—
Sp. g. = 144.3/(144.3-degrees Baumé.)

or putting the rule in words, subtract the degrees Baumé from 144.3, and
divide 144.3 with the number thus obtained. For example: 32° Baumé
equals a sp. g. of 1.285.
144.3/(144.3-32) = 144.3/(112.3) = 1.285
This rule is for liquids heavier than water; for the lighter liquids the rule is
as follows:—
Sp. g. = 146/(136 + degrees Baumé.)
or in words divide 146 by the number of degrees Baumé added to 136. For
example: ammonia of 30° Beaumé has a sp. g. of 0.880 (nearly).
146/(136+30) = 146/166 = 0.8795
A simple series of calculations enables one to convert a Beaumé hydrometer
into one showing the actual sp. g. Graduation, according to sp. g. is the
most convenient for general purposes. In these instruments the distances
between the divisions become less as the densities increase.
Twaddell's hydrometer is graduated in this way: Each degree Twaddell is
0.005 in excess of unity. To convert into sp. g. multiply the degrees
Twaddell by 0.005, and add 1. For example: 25° Twaddell equals a sp. g. of
1.125.
25×.005 = 0.125; + 1.000 = 1.125.
There is a practice which ignores the decimal point and speaks of a sp. g. of
1125 instead of 1.125. In some cases it is convenient, and inasmuch as no
substance has a real sp. g. of much over 20, it can lead to no confusion. The
figures expressed in this way represent the weight of a litre in grams.
Some hydrometers are graduated so as to show at a glance the percentage
composition of the liquid they are intended to be used with. Gay-Lussac
designed one to show the alcoholic strength of mixtures of alcohol and
water; the construction of others upon the same principle is easy and
perhaps useful. But when the principle is applied to complex liquids and
mixed solutions, it is misleading.

The various methods of graduation ought all to give place to one showing a
simple statement of the sp. g.
The method of determining sp. g. with the hydrometer is obviously
inapplicable to the case of solids, and in the case of liquids it should not be
used where exact figures are required. There are several other methods
which may be used, but on the whole those with the specific gravity bottle
are most convenient.

The specific gravity bottle (fig. 35) is a light flask of about 25 c.c.
capacity, provided with a well-fitting perforated stopper. It is essentially a
graduated flask, which measures a constant volume, but it does not much
matter what the volume is.
In taking the sp. g. of a liquid (or, what is the same thing, a fused solid)
there is wanted the weights (1) of the flaskful of water and (2) of the
flaskful of the liquid. Dividing the second by the first gives the required sp.
g. The actual weighings required are—
(1) of the dry and empty flask,

(2) of the flask filled with water, and

(3) of the flask filled with the liquid.

The weighing of the flask once made need not be often repeated. It is well
to do so now and then for safety's sake; but one weighing will serve for a
large number of determinations. The same remarks apply to the weighing of
the bottle filled with water. The bottle is dried by rinsing out first with
alcohol and afterwards with ether; ether is very volatile, and a short
exposure in a warm place will soon drive off the little remaining about the
sides. The ether vapour should be sucked out through a glass tube. See that
the bore of the stopper is dry as well as the bottle. Let the dry bottle stand in
the box of the balance for a minute or two before weighing. The weight is,
strictly speaking, not that of the empty bottle, but of the bottle filled with
air. The empty bottle would weigh from 20 to 30 milligrams less.
Correcting for this would, in most cases, only make a difference in the
fourth place of decimals,[8] so that it is better to ignore the error.
The weight of the flask filled with water is got by filling it with distilled
water, and inserting the stopper. The excess of water will overflow at the
margin and through the bore. The bottle is wiped with a soft, dry cloth,
taking care not to squeeze or warm the bottle. The bottle will remain filled
to the top of the stopper. It is allowed to stand in the balance box for a
minute or two, and then weighed.
Distilled water, as stated, should be used; the use of ordinary water may
increase the weight by 5 or 6 milligrams. Many waters, if they have not
previously been boiled, give off bubbles of air which render the weighing
worthless.
The temperature of the water is of greater importance; lowering the
temperature 2° will increase the weight by 10 or 12 milligrams. A beaker of
water may be warmed or cooled to the required temperature; then the bottle
is filled from it, and quickly weighed. If the balance-room is cooler than the
water, the latter will draw back into the bottle, and a few small bubbles of
air will enter; but even in extreme cases this will only increase the weight
by a very small fraction of a milligram. There is more trouble caused when
the room is warmer, for the liquid then expands and protrudes as a drop
resting on the top of the stopper. There will in this case be loss by
evaporation, which in the case of the more volatile liquids, such as alcohol,
is serious. To prevent this loss, as well as any that may arise by overflow,
the stopper should be dilated above into a small cup, A (fig. 36), which may

itself be stoppered. In a bottle of this kind the neck of the stopper is
graduated, and the bottle is considered full when the liquid stands at the
level of the mark in the neck. On inserting the stopper, the liquid rises into
the cup, and is reduced to the level of the mark by absorption with pieces of
filter-paper.

For most purposes, however, there is no need for cooling and allowing
room for subsequent expansion. The assayer, as a rule, can select his own
standard temperature, and may choose one which will always necessitate
warming. It will be handier in this case to have a bottle with a thermometer
stopper. Of the two types shown in fig. 37, that with the external
thermometer tube (A) is more generally useful.

The bottle is filled at a lower temperature, and is then gently warmed so as
to slowly raise the temperature to the required degree. The superfluous
liquid is then at once wiped off, and the bottle cooled and weighed.
The weight of the flask filled with the liquid whose sp. g. has to be
determined is ascertained in a similar way. Of course the temperature must
be the same. If the liquid does not mix with water, the bottle should be dried
before filling, but otherwise the flask need only be rinsed out two or three
times with the liquid.
Having obtained the three weighings, deduct the weight of the bottle from
each of the others to get the weights of the water and liquid respectively.
Divide the latter by the former, the result shows the sp. g. As an example,
take the following, in which a rather large sp. g. bottle was used:—

1. Weight of bottle 39.299gram
2. Weight of bottle and water 81.884 "
3. Weight of bottle and paraffin 73.146 "

By subtracting 1 from 2 and 3 the result is as follows:—

81.884 grams 73.146 grams
39.299 " 39.299 "
—————— ——————

42.585 of water. 33.847 of paraffin.

Divide the weight of the paraffin by that of the water—
42.585)33.8470(0.7948
29.8095
——————
.......
The sp. g. of the paraffin is 0.7948.
The sp. g. of a fusible solid may be obtained in the same way at a
temperature some degrees above its fusing point.
The sp. g. of a solid in powder or gravel sufficiently fine to pass through the
neck of the bottle is easily determined. If the bottle filled with water weighs
50 grams, and there is placed on the pan alongside of it 20 grams of a sand,
the weight of the two together will of course be 70 grams. But if the sand is
put in the bottle, it evidently displaces its own bulk of water; and if, on
again weighing, the weight is found to be 62 instead of 70 grams, it is
because the 20 grams of sand has displaced 8 grams of water. Bulk for bulk,
the sand is 2-1/2 times as heavy.
In practice, the weight of the bottle filled with water will probably be
already known; if not, it must be determined. A certain quantity, say 20
grams, of the powdered substance is then transferred carefully to the bottle.
The bottle need not be dry inside, but its neck and outside must be. In
making this transference a careful worker will make no loss, and the mode
of working saves a little time. But it is better to weigh the dry flask; put into
it 10 to 20 grams of the powder, and weigh again. The increase in weight
gives accurately the weight of powder in the bottle. About two-thirds fill the
bottle with distilled water, and mix with the powder by gentle shaking. Air
bubbles will disentangle themselves, and rise to the surface of the water.
Wash back anything adhering to the stopper with a jet of water, and fill the
bottle almost to overflowing. Allow it to stand for a minute or so; replace
the stopper; warm to the required temperature; take off the superfluous
moisture; wipe and weigh. As an example, take the following:—

1. Weightof bottle 12.681 grams

2. " " bottle filled with water 37.708 "
3. " " bottle with wolfram 40.821 "
4. " " bottle with wolfram and water 61.199 "

Subtract (1) from (3) to get the weight of wolfram taken:
40.821 grams
12.681 "
——————
28.140 "
add the weight of the wolfram to the weight of the bottle filled with water:
28.140 grams
37.708 "
——————
65.848 "
subtract (4) from this to get the weight of water displaced:
65.848 grams
61.199 "
——————
4.649 "
Divide the weight of the wolfram by the weight of the water displaced to
get sp. g.:
4.649)28.140(6.053
27.894
———————
......
If the solid is soluble in water, or has a tendency to float, some liquid other
than water is used. Paraffin oil or oil of turpentine will do. The process is as
follows:—The weight of the dry and empty bottle having been determined,
add a sufficiency of the substance and weigh again to find how much has
been added. Fill up with paraffin oil and weigh again. Clean out the
substance by rinsing with paraffin; fill up and weigh. Calculate the sp. g. as
if water had been used, and multiply by the sp. g. of the paraffin.

For example:

1.Weightof bottle 39.299grams
2." " bottle and nitre 57.830"
3." " bottle and paraffin 73.146"
4." " bottle and paraffin and nitre 84.665"
5." " bottle and water 81.884"

First from (1),(3), and (5), calculate the sp. g. of the paraffin as already
shown. It will be 0.7948. Deduct (1) from (2) to get the weight of the nitre:
57.830 grams
39.299 "
——————
18.531 "
add this to (3):
18.531 grams
73.146 "
——————
91.677 "
and deduct (4) to find the weight of the equal bulk of paraffin.
91.677 grams
84.665 "
——————
7.012 "
divide the weight of the nitre by the weight of the paraffin:
7.012)18.531(2.6427
——————
......
The sp. g., taking paraffin as the standard instead of water, is 2.6427.
Multiply this by the sp. g. of paraffin, 0.7948, and the result is 2.1004 as the
sp. g. of nitre compared with water.

Similarly, a sp. g. compared with water at say 50° C. can be converted into
one compared with water at standard temperature, by multiplying by the sp.
g. of water at 50° C. The following table gives the sp. g. of water at various
temperatures:—

Degrees Degrees Degrees
Sp. G. Sp. G. Sp. G.
Centigrade. Centigrade. Centigrade.
4° 1.0000 20° 0.9982 40° 0.9923
10° 0.9997 25° 0.9971 50° 0.9881
15° 0.9991 30° 0.9957 100° 0.9586

If, for example, a substance at 50° C. has a sp. g. of 0.9010 as compared
with water at 50° C., it will have (compared with water at 4° C.) a sp. g. of
0.9010 × 0.9881; or 0.8903. The figures 0.8903 represent the sp. g. of the
substance at 50° C. compared with water at 4° C. Except in comparing the
sp. gravities of the same substance at different temperatures, a calculation
of this kind serves no useful purpose.
In taking the specific gravity of a solid not in powder, a lump of it is freed
from loose particles and its exact weight determined. By means of a horse
hair with a slip knot it is suspended to the balance, and beneath it is placed,
out of contact with the balance pan, a beaker of distilled water. The horse
hair must be long enough to keep the mineral well beneath the surface of
the water so as to allow the balance to vibrate. Air bubbles are removed by
touching with a camel-hair pencil. Whilst the mineral is suspended in water
the weight is again taken. It will weigh less than before, and the difference
between the two weighings gives the weight of water (and consequently the
volume) displaced by the mineral. The weight in air divided by the
difference is the specific gravity. Thus
Weight in air 3.2170 grams
Weight in water 2.7050 "
———
Difference 0.5120 gram

3.2170/0.5120 equals 6.28, the sp. g.

The sp. g. of a substance depends mainly on its composition, but is affected
by certain conditions. The effect of temperature has been already
considered. Air holes and empty spaces lessen the specific gravity of
otherwise solid bodies; and metals, which after fusion become imperfect
solids, have their density increased by hammering or rolling. But metals
when free from pores have their density diminished when rolled, without
annealing. The effects of these conditions are slight when compared with
those due to the presence of impurities.
For simple substances, or mixtures of only two substances, a determination
of sp. g. is a sufficient check on the composition for many practical
purposes; and with more complex mixtures, such as slags and some of the
products of dressing operations in which the material does not differ much
in its nature from time to time, such a determination will yield information
of considerable value, and afford a check upon the proper working of a
process.
When the mixing of two substances is accompanied by a change in volume,
the sp. g. of the mixture can only be learnt by experiment. But when the
substances have no such action on each other the resulting sp. g. can be
calculated. Some of these calculations have a practical interest as well as an
educational value. Students should practise them so as to become familiar
with the relations between weight and volume.
When substances are mixed by volume, the sp. g. of the mixture is the mean
of those of its constituents, and may be calculated in the usual way for
obtaining averages. 1 c.c. of a substance having a sp. g. of 1.4 mixed with 1
c.c. of another having a sp. g. of 1.0 will yield 2 c.c. of a substance having a
sp. g. of 1.2. If, however, we write gram instead of c.c. in the above
statement, the resulting sp. g. will be 1.16. The simplest plan is to remember
that the sp. g. is the weight divided by the volume (sp. g. = w/v) and the sp.
g. of a mixture is the sum of the weights divided by the sum of the volumes
(sp. g. = (w + w' + w", &c.)/(v + v' + v", &c.)). In the above example the
sum of the volumes is 2 c.c.; the weights (got by multiplying each volume
by its corresponding sp. g.) are 1.4 gram and 1 gram. The sum of the
weights divided by the sum of the volumes is 2.4/2 or 1.2.

The sp. g. of a mixture of 10 c.c. of a substance having a sp. g. of 1.2, with
15 c.c. of another having a sp. g. of 1.5 may be thus found:—
sp. g. = (12+22.5)/(10+15) = 1.38
multiply each volume by its sp. g. to get its weight:
10×1.2 = 12 15×1.5 = 22.5
add these together (12+22.5 = 34.5) and divide by the sum of the volumes
(10+15 = 25):
25)34.5(1.38
25
—
95, &c.
The sp. g. will be 1.38, provided the mixture is not accompanied by any
change of volume.
The same formula will serve when the proportion of the ingredients is given
by weight. A mixture of 4 parts by weight of galena (sp. g. 7.5) with 5 parts
of blende (sp. g. 4) will have a sp. g. of 5.06:
sp. g. = (4+5)/(0.53+1.25) = 9/1.78 = 5.06
It is necessary in this case to calculate the volumes of the galena and of the
blende, which is done by dividing the weights by the sp. gravities: thus, 4
divided by 7.5 gives 0.53 and 5 divided by 4 gives 1.25.
The converse problem is a little more difficult. Given the sp. g. of a mixture
and of each of the two ingredients, the percentage by weight of the heavier
ingredient may be ascertained by the following rule, which is best
expressed as a formula. There are three sp. gravities given; if the highest be
written H, the lowest L and that of the mixture M, then:
Percentage of heavier mineral = (100×H×(M-L))/(M×(H-L))
Suppose a sample of tailings has a sp. g. of 3.0, and is made up of quartz
(sp. g. 2.6) and pyrites (sp. g. 5.1): then the percentage of pyrites is 27:
(100×5.1×(3-2.6))/(3×(5.1-2.6)) = (510×0.4)/(3×2.5) = 204/7.5 = 27.2

The same problem could be solved with the help of a little algebra by the
rule already given, as thus: the sp. g. of a mixture equals the sum of the
weights of the constituents divided by the sum of the volumes. Then 100
grams of the tailings with x per cent. of pyrites contain 100-x per cent. of
quartz. The sum of the weights is 100. The volume of the pyrites is x/5.1
and of the quartz (100-x)/2.6.
Then we have by the rule
3 = 100/((x/5.1)+(100-x)/2.6)
3 = 1326/(510-2.5x)
204 = 7.5x
and x = 27.2
If the percentage (P) and sp. g. (H) of one constituent and the sp. g. (M) of
the mixture are known, the sp. g. of the other constituent may be calculated
by the following formula, in which x is the required sp. g.:
x = ((100-P)×M×H)/((100×H)-(P×M))
For example, "tailings" (sp. g. 3.0) containing 27.2 per cent. of pyrites (sp.
g. 5.1) will contain (100-27.2), 72.8 per cent. of earthy matter having a
mean sp. g. of x:
x = ((100-27.2)×3×5.1)/((100×5.1)-(27.2×3))
= 1113.84/428.4 = 2.6
The differences in sp. g. corresponding to differences in strength have been
carefully determined and tabulated in the case of the stronger acids and of
many other liquids. Such tables are given at the end of this book.
To Calculate the Weight of a Measured Volume of Mineral or Rock.—
Multiply the cubic feet by 62.4 and then multiply by the sp. g. of the stuff,
the answer gives the weight in pounds. For example, 100 cubic feet of
quartz weighs 100×62.4×2.6 = 16,224 lbs. The weight of any mass of
mineral of known extent and sp. g. is ascertained in this way.
The following table gives the specific gravities of some of the commoner
minerals.

Barytes 4.5
Blende 4.0
Calcite 2.6
Cassiterite 6.9
Chalybite 3.8
Copper pyrites 4.2
Fluor 3.1
Galena 7.5
Hæmatite 5.0
Mispickel 6.2
Pyrites 5.0
Quartz 2.6

FOOTNOTES:
[8] The difference of 20 or 30 milligrams is disregarded here because it detracts
equally from the actual weight of the water and liquid to be determined. If the
liquid is a heavy one the difference shows itself in the third or second place of
decimals. The correction may be made by deducting from the weight of the flask
0.0012 grams for each gram of water it holds.

PART II.—THE METALS.

CHAPTER IX.
SILVER, GOLD, CYANIDES, PLATINUM, MERCURY.

SILVER.

Silver is widely diffused, and has been found in most mining districts. It
occurs native in sufficient quantity to constitute one of the chief ores of the
metal. It also occurs combined with sulphur (as in argentite), with sulphur
and antimony (as in stephanite or brittle silver ore, and in pyrargyrite or
ruby silver), and with copper, sulphur, antimony, and arsenic, as in
polybasite. Chloride of silver occurs native as horn silver or kerargyrite.
Silver is found in the ores of other metals, such as fahlerz, which sometimes
contains from two to ten per cent. of the metal, and galena, which is an
important source of it; in fact, galena is never found entirely free from
silver. It is present also in greater or less quantity in the ores of copper and
zinc.
Silver dissolves readily in nitric acid, forming silver nitrate. It only forms
one family of salts, and of these the chloride and nitrate are of chief
importance to the assayer. The formation of the chloride of silver on the
addition of hydrochloric acid or a soluble chloride to the nitric acid
solution, serves for the recognition and separation of silver. The precipitated
chloride is white (becoming violet on exposure to light), insoluble in nitric
acid, soluble in ammonia, hyposulphite of soda, or concentrated solutions of
chlorides. The best confirmatory test is made by wrapping the precipitate in
a little sheet lead, and cupelling, when the silver will be left in the metallic
state, and is easily recognized.
Dry Assay.—This assay is made up of two parts: (1) the concentration of
the silver in a button of lead; and (2) the cupellation of the resulting alloy.
The concentration of the button of lead may be effected either by
scorification or by fusion in a crucible.

The scorification assay is performed in a scorifier, which is a shallow open-
mouthed dish about 2-1/2 inches across, with a very thick bottom to enable
it to withstand the corrosive action of the slag. A charge of more than 3 or 5
grams of the ore cannot be worked in one, and with such small charges the
unavoidable variations have a serious effect on the figures reported. A
difference of one milligram on the weight of the button of silver got
represents a difference of 6 or 10 ounces per ton. With rich ores such
variation is unavoidable under any conditions, and the only safe plan is to
take the mean of several assays. But with poorer ores the accuracy of the
assay, as well as convenience in working, is much increased by working in
a crucible with larger charges.
In scorification the proportion of lead required for scorifying 1 gram of ore
is in average cases from 10 to 15 grams, sinking in the case of galena to 2
grams, and rising with earthy and refractory substances to from 30 to 40
grams. But by fusing in a crucible with well-selected fluxes, a proportion of
4 of flux to 1 of ore is generally sufficient; and not only is the proportion of
added matter less, but it is also easier to manipulate large quantities in
crucibles, so that, although in some cases the crucible assay is more
troublesome and less satisfactory, yet with poor and earthy ores it is the best
method of dealing with them; while when properly worked it yields results
as accurate as scorification does. As a general rule, if more than 5 grams of
ore must be taken, the crucible assay should be adopted.

Scorification Assay.—The charge of ore is usually 3 grams, sometimes 5;
the lead varies from 30 to 70 grams, and the quantity of soda, borax, or
powdered glass added varies from 0.3 to 3 or 4 grams. It is generally
recommended to have the lead granulated,[9] and to mix the ore with about
half of it in the scorifier; then to put on the rest of the lead; and finally to

sprinkle the borax or glass on the top. It answers just as well, however, to
use the lead in the shape of foil, and wrap the ore up in it; and if the ore
contains much sulphur, the borax may with advantage be added (wrapped in
a little tissue paper) some five or ten minutes after the operation has started.

The process of scorification is as follows:—A scorifier (fig. 38) of
convenient size having been selected (one 2-1/2 inches across is most
generally useful), it is dried at a gentle heat for about ten minutes. The
charge is then put into it, and it is introduced, with the help of a scorifier
tongs (fig. 39), into a muffle heated considerably above redness. The muffle
is then closed, and when the metal has melted down, it is opened, but the
temperature is kept up. A ring of slag will, after a time, form around the
metal, and when this appearance (known as the eye) presents itself, the
temperature may be lowered. When the eye has disappeared—that is, when
the layer of slag has quite closed in—a pinch of powdered culm wrapped in
tissue paper is added. As soon as the slag has again become tranquil, the
scorifier is taken out, and its contents are poured into a mould (fig. 40), the
slag is detached, and saved. If the button of metal weighs more than 30
grams, its size is reduced by another scorification in the same scorifier,
which should have been replaced in the muffle immediately after the
contents had been poured out. If the ore is not a very rich one, the button of
lead will carry practically all the silver; but with rich ores it is more
satisfactory to save the slag, and subsequently to melt it down with the
cupel on which the lead has been treated, so as to recover the silver lost in
the slag, together with that absorbed in the cupel, at one operation. Or, if the
cupellation loss is neglected or calculated in some other manner, the slag or
slags from the scorifier may be powdered and mixed with 20 grams of
oxide of lead, 5 grams of borax, and 1 gram of charcoal. This should be
melted down in a small crucible, and the resulting button of lead cupelled.

If the scorification has been unsatisfactory, the quantity of silver obtained
from the slag will be by no means inconsiderable. The usual explanation is
that with sulphury ores compounds of metallic oxides and sulphides
(oxysulphides) are formed, which remain in the slag, retaining considerable
quantities of the precious metal. It is said that under certain conditions such
a slag may contain as much as 10 per cent. of silver. An excess of lead and a
high temperature prevents the formation of these oxysulphides. But if much
silver is present in the ore, the slag cannot be safely thrown away, even if
sulphur is absent, and the process has been satisfactorily performed.
If the crust which appears on the surface of the lead does not clear, add a
small lump of borax and 20 grams more lead; then close the muffle, and
keep the temperature as high as possible. If the slag forms properly, but
shows unfused or only half-fused lumps, even when the scorification has
proceeded for some time, add more borax, and stir with an iron rod. The
slag adhering to the rod must be detached by hammering, and replaced in
the scorifier.
If the ore consists largely of quartz, soda should be added instead of borax;
or, if it contains much copper, powdered quartz may be used. If the scorifier
at the end of an operation is more than usually corroded, the borax should
be replaced in subsequent assays on similar ores by powdered glass or
quartz.

If a fairly fluid slag is formed which does not clear from the metal and show
the eye, more lead and a higher temperature is wanted.
As a general rule, it may be stated that when a scorification is
unsatisfactory, what is wanted is more heat, more lead, or more borax.
It is a safe plan when work has to be done on a strange ore, to make three or
four assays with varying quantities of lead. The proportion of lead is right
when a further addition does not yield a higher result. The proper
proportion having been found, a note of it should be made for future use.

POT ASSAYS.

The object of the fusion in a crucible, like that of scorification, is to
concentrate the silver in a button of lead which is to be subsequently
cupelled; and to retain the earthy and waste matters in the slag. It is
necessary to consider the quality of the slag and the weight and quality of
the lead. The slag when fused should be liquid and homogeneous, and not
too corrosive on the crucible. The button of lead should be soft, malleable,
and free from a coating of regulus.[10] In weight it should not differ much
from the ore taken. With 20 grams of ore, for example, a button of lead
weighing from 18 to 25 grams will be satisfactory: less than this would
leave an undue proportion of silver in the slag; and more would be
unnecessarily large for cupelling, and would increase the loss in that
operation.
With average ores, take 20 grams of the powdered ore and mix with 30
grams of "soda," 40 grams of red-lead or litharge, 5 grams of borax, and
from 2 to 2.5 grams of flour, and place in an E crucible (Battersea round).
Put these in the furnace at a red heat, cover the crucible, and gradually raise
the temperature until the whole charge has melted down and is in a state of
tranquil fusion. Pour into a mould, and replace the crucible in the furnace.
As soon as the lead is solid, detach the slag and put it back into the crucible;
and when it is again fluid, charge on to it with a copper scoop a mixture of
20 grams of oxide of lead, and 1 gram of charcoal: when fusion has again
become tranquil, pour and detach the button of lead. The lead buttons
should be hammered into discs with rounded edges, and be freed from slag;

if too big for a cupel they may be scorified together in a small scorifier, but
it is better to cupel them separately.
Ores containing Metallic Oxides.—Peroxides of iron, manganese, and
copper interfere by counteracting the effect of the charcoal or flour, and
thus reducing the size of the lead button. Peroxide of iron will reduce the
weight of lead by a little more than its own weight; and peroxide of
manganese has about twice this effect. When these oxides are present an
additional quantity of flour must be used, and precautions must be taken to
prevent reoxidation of the slag by the furnace gases. This may best be
prevented by using a layer of common salt as a cover to the charge. When
the ores contain a good deal of quartz or stony matter, the fluxes just given
(for average ores) will do; but the proportion of soda should be diminished,
and that of the borax, oxide of lead, and flour increased as the quantity of
metallic oxides become greater. If the ore contains practically no quartz, the
soda may be altogether omitted, and some glass or powdered quartz added.
The following charge may be taken as an example: weigh up 20 grams of
the powdered ore, 15 grams each of "soda" and borax, 60 grams of oxide of
lead, and 5 grams of flour. Mix and place them in an E crucible, and cover
with a layer of from a quarter to half an inch of common salt. Place in the
furnace as before. The salt will give off a considerable amount of fume,
which will, to a certain extent, conceal the state of the charge: when the
crucible has been in the furnace for about 25 minutes remove it and pour
out the contents immediately. With ores that produce a thick slag the
addition of 5 grams of fluor spar will be an advantage. It may happen that
with an unknown ore the first assay will be more or less unsatisfactory: but
from it the necessity for adding more or less flour will be learnt, and a
second assay, with the necessary modification of the charge, should give a
good result.
Ores containing much Sulphides.—Ores of this class may be easily
recognized, either by the appearance of the minerals they contain or by the
odour of sulphurous oxide (SO2) which they evolve when roasted on a
spatula. The sulphides most commonly present, in addition to the
sulphurized minerals of silver, are pyrites, galena, blende, and mispickel.
When they are present in only a moderate amount, their effect is simply to
increase the weight of the button of lead; and this is easily counteracted by
reducing the amount of flour, or by omitting it. When in larger amounts,

they not only yield large buttons, but also render the metal sulphury,
sometimes even giving a button of regulus instead of lead. This last evil
may be remedied (1) by putting in a rod of iron as soon as the charge has
fused, or (2) it may be counteracted by a proper addition of nitre, or (3)
when the sulphides present are only those of iron or copper the sulphur may
be removed by calcining, and the ore converted into one of the class
containing metallic oxides. The calcination is effected as follows:—Weigh
up 20 grams of the powdered ore and place it in a wide-mouthed crucible
sufficiently large to perform the subsequent melting down in. The roasting
must be done at a gentle heat at first, so as to avoid clotting: the mouth of
the crucible should project considerably above the coke, and should slope
forward towards the worker. The charge must be occasionally stirred with
the stirrer (fig. 10) so as to expose fresh surfaces to the action of the air, and
to prevent adhesion to the sides of the crucible. The stirrer should not be
removed till the calcination is finished. The temperature should be raised at
the end to a good red heat; and (to ensure the decomposition of any sulphate
that may be formed) the roasted ore should be rubbed up in a mortar with a
pinch of anthracite, and again calcined. It is then mixed with fluxes as
described, and fused in the same crucible.
The calcination of an ore is a work occupying a good deal of time, and, in
most cases, it is better to take advantage of the desulphurizing power of red
lead or nitre. Red lead by itself will do, but a large quantity of it will be
required; 1 part of a metallic sulphide needs from 20 to 50 parts of red lead
to yield a button free from sulphur; whereas at most from 2 to 2-1/2 parts of
nitre are sufficient. There is sometimes an advantage in having a
considerable excess of oxide of lead in the slag, but where there is no such
reason, 2 parts of red lead to 1 of ore is enough. A charge which will do for
most sulphides is the following: 20 grams of ore, 40 to 100 grams of red
lead, 20 grams of "soda," 5 of borax, and sufficient nitre (or perhaps flour)
to give a button of about 25 grams of lead. How much this must be (if not
already known) may be approximately determined by fusing 3 grams of the
ore and 3 grams of "soda" in a small crucible (C) with 50 grams of litharge
(not red lead) under a cover of salt, and weighing the resulting button of
lead. Subtract 3 from the weight of lead obtained, and the difference
multiplied by 1.3 will give the quantity in grams of nitre required. If the
button of lead weighs less than 3 grams flour must be added. If this is not

satisfactory repeat the assay, adding an extra gram of nitre for each 4 grams
of lead in excess of that required, or 1 gram of flour for a 12-gram
deficiency.
In the method in which iron is used as a de-sulphurising agent, only as
much oxide of lead should be added as will give a button of lead of the
required size. Rather a large button of lead should be got, and the slag
should be strongly alkaline; if the ore does not already carry a large amount
of sulphur some should be added. The fusion should be performed at a low
temperature (similar to that for a galena assay), and should be continued for
some time after it has become tranquil. Take 20 grams of the ore, 40 grams
of "soda," 40 grams of oxide of lead, and 5 or 10 grams of borax; place this
mixture in a crucible (with a rod of iron, as in the galena assay), cover, and
fuse for about half an hour. Take out the rod, washing it in the slag, and, in a
minute or two, pour. Clean and cupel the button of lead.
General Remarks on the Fusion.—Other things being equal, the smaller
the quantity of the slag the better, provided there is sufficient to cover the
metal. The presence of peroxides of the heavy metals is prejudicial, since
they tend to increase the quantity of silver retained in the slag. It may be
given as a general rule that when iron, copper, manganese, &c., are present,
there is a more than ordinary need for cleaning the slags, and care must be
taken to keep these metals in the state of lower oxide.
In selecting the fluxes, it should be remembered that soda is the best for
quartz, and borax for lime and metallic oxides. And that with ores almost
free from gangue some quartz or glass should be added to protect the
crucible. Two parts of soda are enough to flux 1 part of quartz; whilst of
borax, or oxide of lead, 4 parts are barely sufficient. Oxide of lead has the
advantage of being heavy and so does not occupy much space in the
crucible; on the other hand, if the melting down be performed too quickly,
or if oxide of lead only is used, this high specific gravity is a disadvantage,
for the lighter earthy matter floats as a pasty mass on the more fluid oxide
of lead, and thus escapes its action.
When metallic sulphides are present in the ore, an excess of oxide of lead
helps to keep the sulphur out of the button of metal. In addition to the oxide
of lead required as a flux, some will be required to provide the lead in

which the silver is to be collected. Oxide of lead, mixed with charcoal or
flour, yields, when heated, a multitude of minute buttons of metal uniformly
distributed through the mass of the charge; as the charge melts down these
run together and fall to the bottom; this shower of lead collects the silver
more easily than a single button at the bottom of the crucible could do. Only
that portion of the oxide of lead which remains in the slag can be
considered as a flux; very often the first indication of an excessive
reduction of lead is the pastiness of the slag rendered thick by the
withdrawal of the oxide of lead which would have kept it fluid. If, in an
assay, it is found that 5 parts of flux are not sufficient for 1 part of ore, the
remedy lies in using a different flux rather than in taking a larger quantity.
On the Reducing Effect of Charcoal, Flour, and Tartar.—The weight to be
got from a given charge will depend (provided sufficient oxide of lead is
present) upon the proportion of the reducing agents in it. We have thought it
well to illustrate this part of the subject by a series of experiments which the
learner will do well to practise for himself before proceeding to the assay of
actual ores. Take 80 grams of litharge and 20 grams of a mixture of borax
and soda. Fuse three lots (1) with 1.5 gram of charcoal, (2) with 3 grams of
flour, and (3) with 7.5 grams of tartar. Weigh the buttons of lead obtained,
and divide each by the weight of reducing agent used. The results will differ
somewhat with the dryness and quality of the flour, etc., used; in one series
of experiments they were as follows:—

Gram. Grams. Gram. Grams.
1.5 charcoal gave 34.0 lead ∴ 1 charcoal = 22.6 lead.
3.0 flour " 33.5 " ∴ 1 flour = 11.2 "
7.5 tartar " 38.0 " ∴ 1 tartar = 5.0 "

The use of flour as a reducing agent has many advantages, and it is well to
remember that 1 gram of flour reduces about 11 grams of lead; and that
charcoal has twice, and tartar one-half, this reducing effect.
On the Reducing Effect of Charcoal, &c., on Red Lead.—It is often easier
to obtain red lead of good quality than it is litharge, and by a large number
of assayers red lead is the form of oxide of lead always used. Red lead,
however, contains an excess of oxygen which will use up some of the
reducing agent before lead separates out. On making a series of experiments

(similar to the last, but using 80 grams of red lead instead of the litharge)
the results were, with the same quantities of the reducing agents:—

With charcoal, 18grams of lead.
" flour, 18 " "
" tartar, 22 " "

Comparing these with the results with litharge, in the previous table it will
be seen that the same quantity of reducing agent has in each case brought
down 16 grams less of lead, so that a larger amount of the reducing agent
must be added to get a button of the same weight as that obtained with
litharge. To get a button of a desired weight, say 22 grams, we must add
reducing agent sufficient to throw down 22 + 16 or 38 grams of lead, which
would require 3.4 grams of flour. If this amount of flour is fused with 80
grams of red lead, a button of lead weighing 22 grams will be formed, the
other 16 grams being kept up by the oxygen of the red lead.
If the quantity of red lead differs from 80 grams, this rule must be modified.
With 40 grams of red lead, for example, we should add an excess of
reducing agent sufficient to throw down 8 grams of lead instead of 16.
Similarly, with 160 grams of red lead, we should add enough to throw down
32 grams.
The following rule will enable one to calculate the weight of flour required
to produce a button of lead of any desired weight from any given quantity
of red lead. Each 5 grams of red lead present diminishes the weight of the
lead by 1 gram. If then we divide the weight of red lead in a charge by 5,
and add this to the weight of lead required, the sum divided by 11 will give
the weight of flour which must be added. Using 80 grams of red lead and
wanting a button of 20 grams, we should add 3.3 grams of flour.
80/5 = 16; 16+20 = 36; 36/11 = 3.3 nearly.
The following are some results obtained which will illustrate the rule:—

Red Lead used. Flour used. Lead got.
40 grams 3grams 25.0grams
100 " 3 " 13.5 "

80 " 4 " 30.0 "
80 " 5 " 40.0 "

On the Reducing Effect of Metallic Sulphides, and the Counteracting Effect
of Nitre.—The sulphides found in ores will reduce a button of lead from
oxide of lead just as flour does; and, as charcoal, flour and tartar differ in
their reducing power, so equal weights of the different mineral sulphides
throw down different weights of lead.
One gram of iron pyrites yields about 11 grams of lead. One gram of copper
pyrites, blende, fahlerz, or mispickel, yields 7 or 8 grams of lead, whilst 1
gram of antimonite will give 6, and 1 gram of galena only a little over 3
grams. It is evident that if an ore carries much of these sulphides, the
quantity of lead reduced will be very much larger than that required for an
assay. To counteract this effect nitre is added; 1 gram is added for each 4
grams of lead in excess of that required. For example: with a 20-gram
charge of an ore containing 50 per cent. of pyrites, if no nitre were added,
110 grams of lead would be got; or, if there was not sufficient oxide of lead
to yield this quantity of metal, the button would be sulphury. To reduce the
weight of the button by 80 grammes, we should add 20 grams of nitre, if
litharge were used; or if red lead were used, we should add 16 grams of
nitre, since the oxidizing effect of 20 grams of red lead is equivalent to that
of 1 of nitre, and since 80 grams of red lead are generally used in a charge.
Two assays of an ore of this kind with these quantities of nitre gave 26.0
grams of lead with litharge, and 22.5 grams with red lead.
It is best to use in these assays 80 grams of red lead, 20 of soda, and 5 of
borax, with 20 grams of the ore. If the lead got by the preliminary fusion in
a small crucible with litharge (described under "ores containing much
sulphides") is known, the following table will indicate the quantity of nitre,
or flour, to be added with this charge:—

Lead got in Preliminary Fusion Flour to be added Nitre to be added to
with 3 grams of Ore. to the Assay. the Assay.
0.0 gram 3.3 grams none
3.0 grams 1.3 gram —
6.0 " none 4.0 grams
9.0 " — 9.0 "

12.0 " — 14.0 "
15.0 " — 19.0 "
18.0 " — 24.0 "
21.0 " — 29.0 "

If litharge is used in the assay instead of red lead 4 grams more nitre, or 1.5
gram less flour must be used. When more than a few grams of nitre are
added to a charge the proportion of "soda" and borax should be increased,
because one of the products of the reaction of nitre upon sulphides in the
presence of soda is sulphate of soda, and because the "soda" thus used up
no longer serves as a flux; more borax should be added, as it is the best flux
for the metallic oxides which are formed in the process. If in an assay too
large a button of lead is got, even after this calculation has been made, and
the assay is repeated, add 1 gram more nitre for each 4 grams of lead in
excess. Sometimes the assay appears tranquil before the nitre has produced
its full effect; in such cases it is well to seize the crucible with the tongs and
mix its fused contents by rotating them; if this causes an effervescence, the
crucible should be replaced in the fire and the fusion continued. The
following experiments will illustrate the extent to which the above rules
may be relied on. In all of them the standard flux was used, viz.:—80 grams
of red lead, 20 of soda, and 5 of borax.

Pyrites 5 5 5 5 2.5 5 10 15 20
Quartz — 20 — 20 17.515 10 5
Nitre — — 5 5 — 4 16 28.5 41
Lead got 42.5 36.0 16.0 19.0 11.5 22.522.5 26.5 27.5
Copper Pyrites 8 8 8 8
Quartz — 12 — 12
Nitre — — 4 4
Lead got 47.5 34.033.026.0
Antimonite 8 8 8 8
Quartz — 12 — 12
Nitre — — 4 4
Lead got 29.0 26.013.013.0
Galena 10 10 10 10 15 20

Quartz. — 15 — 15 5 —
Nitre — — 3 3 3.5 7
Lead got 17.019.08.08.018.518.5

A similar set of experiments, with 80 grams of litharge instead of 80 grams
of red lead, gave:—

Pyrites 4 4 4 4 7 10
Quartz — 15 — 15 13 10
Nitre — — 5 5 12.520
Lead got 46.5 40.5 25.524.527.026.5
Copper Pyrites 5 5 5 5
Quartz — 15 — 15
Nitre — — 5 5
Lead got 44.5 32.5 23.025.0
Blende 5 5 5 5 10
Quartz — 15 — 15 10
Nitre — — 5 5 15
Lead got 41.5 38.5 21.522.521.6
Antimonite 5 5 5 5 10
Quartz — 15 — 15 10
Nitre — — 5 5 10
Lead got 31.0 32.5 11.5 12.518.7
Galena 10 10 10 10 15 20
Quartz — 15 — 15 5 —
Nitre — — 5 5 7.5 11
Lead got 33.5 33.5 13.014.019.522.7

The variation in some of these experiments, in which we might have
expected similar results, is due to the fact that the sulphur, and in some
cases the metals, are capable of two degrees of oxidation. For example:
theoretically 1 gram of iron pyrites (FeS2) would yield 8.6 grams of lead if
the sulphur were oxidised to sulphurous oxide (SO2), and the iron to ferrous
oxide (FeO); whilst if the sulphur were oxidised to sulphate (SO3), and the
iron to ferric oxide, 12.9 grams of lead will be thrown down. Similarly the

yield with copper pyrites would be 7.5 or 11.6; with blende, 6.4 or 8.5; with
antimonite, 5.5 or 8; and with galena, 2.6 or 3.4. As regards the metals, the
lower oxide will always be formed if the assay is carried out properly (fused
under a cover, and with a sufficiency of reducing agent). But the proportion
of sulphur oxidised completely will vary with the conditions of the assay.
With a slag containing much soda the tendency will be to form sulphate,
and, in consequence, a big reduction of lead; whilst with an acid slag
containing much quartz the tendency will be for the sulphur to go off as
sulphurous oxide (SO2). In a fusion with litharge alone all the sulphur will
be liberated as the lower oxide, whilst with much soda it will be wholly
converted into sulphate. For example: 3 grams of an ore containing a good
deal of pyrites and a little galena, gave, when fused with litharge, 16.5
grams of lead. A similar charge, containing in addition 20.0 grams of soda,
gave 22.5 grams of lead.
It will be noted from the experiments that 1 gram of nitre kept up on the
average 4 grams of lead; the range being from 3.2 with acid slags to 5.3
with very basic ones. These facts serve to explain some apparently irregular
results got in practice.

CUPELLATION.

The process is as follows:—The cupels, which should have been made
some time before and stored in a dry place, are first cleaned by gentle
rubbing with the finger and blowing off the loose dust; and then placed in a
hot muffle and heated to redness for from 5 to 10 minutes before the alloy
to be cupelled is placed on them. The reasons for this are sufficiently
obvious: the sudden evolution of much steam will blow a cupel to pieces;
and, if the whole of the water has not been removed before the cupel is
filled with molten lead, the escaping steam will bubble through, and scatter
about particles of the metal. If some particles of unburnt carbon remain in
the bone ash, a similar result will be produced by the escape of bubbles of
carbonic acid as soon as the fused litharge comes in contact with them. The
cupels having been prepared are arranged in a definite order in the muffle,
and the assay buttons are arranged in a corresponding order on some
suitable tray (cupel tray, fig. 41); the heat of the muffle being at bright
redness. Then with the help of the tongs (fig. 42) the assay buttons should

be placed each in its proper cupel; a note having been previously made of
the position it is to occupy, and the door of the muffle closed.

This part of the work should be done promptly, so as not to unduly cool the
muffle: the start requires a fairly high temperature, and is a critical part of
the process. A black crust forms at once on the surface of the lead; but this
ought soon to fuse and flow in greasy drops from off the face of the metal,
so as to leave the latter fluid with a well-defined outline, and much brighter
than the cupel. If this clearing does not take place, the buttons are said to be
frozen; in which case the temperature must be raised, some pieces of
charcoal put in the muffle, and the door closed. If they still do not clear, the
heat must have been much too low, and it is best to reject them and repeat
the assays.

When the buttons have cleared it is well to check the draught of the furnace,
and to partly open the door of the muffle, so as to work at as low a
temperature as is compatible with the continuation of the process.[11] Too
low a temperature is indicated by the freezing of the buttons and the
consequent spoiling of the assays. Experience soon enables one to judge
when the heat is getting too low. A commoner error is to have the heat too
high: it should be remembered that that which was high enough to clear the
buttons at starting is more than sufficient to keep the process going. At the
finish a higher temperature is again required: therefore the door of the
muffle should be closed and the furnace urged. The finish is easily
recognised. The drops of litharge which in the earlier stages flow steadily

from the surface of the alloy, thin off later to a luminous film. At the end
this film appears in commotion, then presents a brilliant play of colours,
and, with a sudden extinction, the operation is finished. The metal again
glows for an instant whilst becoming solid.
If the button is a small one the cupel is withdrawn at once and placed on
that square of the cupel tray which corresponds to the position it occupied
in the muffle. If, however, it is fairly large precautions must be taken to
prevent spirting.
Molten silver dissolves oxygen from the air and gives it off on solidifying;
the escape of the gas on sudden cooling is violent and, by throwing off
particles of the metal, may cause loss. This is called "vegetation" or
"spirting." The silver is apparently solid when spirting takes place; the crust
breaks suddenly and some of the metal is forced out. The evil is best
guarded against by slow cooling and avoiding draughts. With large buttons
of silver precautions should never be omitted. One plan is to allow the
cupels to cool in the muffle itself, the mouth being closed with hot charcoal.
Another is to cover the cupel with another cupel previously heated to
redness; in this case the silver cools between two hot cupels, and, of course,
cools slowly. A third plan is to withdraw the cupel to the door of the muffle,
holding it until it begins to get solid and then immediately to put it back into
the hotter part of the muffle.
Silver remains after cupellation in flattened elliptical buttons, adhering but
only slightly to the cupel. Its upper surface should show faint markings as if
it were crystalline. The presence of platinum renders it still more
crystalline, but removes the characteristic lustre and renders the metal dull
and grey. Copper, if not completely removed, has a very marked effect on
the appearance of the button: the metal is spread out, damping, as it were,
and firmly adhering to the cupel, which latter in the neighbourhood of the
metal is almost black with oxide of copper. Sometimes the silver button is
globular, or even more sharply rounded on its under than on its upper
surface; it is said that this is due to the presence of lead. Gold may be
present even to the extent of 50 per cent. without showing any yellow
colour.

The appearance of the cupel affords some useful information. The presence
of cracks evidently due to shrinkage indicates a badly made cupel. If,
however, they are accompanied by a peculiar unfolding of the cupel, the
margin losing its distinctness, it is because of the presence of antimony.
When lead is the only easily oxidisable metal present, the stained portion of
cupel is yellow when cold. A greenish tint may be due to small quantities of
copper or, perhaps, nickel, cobalt, or platinum. Larger quantities of copper
give a greenish grey or almost black colour. A dark green and corroded
cupel may be due to iron. Rings of pale-coloured scoria may be due to tin,
zinc, antimony, or arsenic. When the cupel shows signs of the presence of
these metals in objectionable quantity, it is well to repeat the assay and
scorify so as to remove them before cupellation.
The button should be detached from the cold cupel by seizing with a pair of
pliers: the under surface should be distorted by squeezing or hammering the
button so as to loosen the adhering bone ash. The cleaning is easily
completed by rubbing with a clean hard brush. After cleaning the buttons
are best put on a tray of marked watch-glasses, and then taken to the
balance and weighed. The weight of silver got needs a small correction; (1)
by deducting for the amount of silver introduced by the lead or oxide of
lead used in the assay;[12] and (2) by adding for the cupellation loss.
Loss in Cupellation.—During the whole process of cupelling a silver lead
alloy a more or less abundant fume may be observed rising from the cupel.
This furnishes an evident loss of lead and a possible loss of silver; for
although silver at the temperature of cupellation gives off no appreciable
vapour, it is known that such fume formed on a large scale contains silver. It
is, however, difficult to believe that the small amount of lead vapourised
carries with it a weighable amount of silver. That it does not do so in the
ordinary way of working is shown by the fact that a button of silver equal in
weight to the silver lost in cupelling may be got by smelting the cupel and
cupelling the resulting button of lead. The loss of silver by volatilisation is
altogether inconsiderable, unless the temperature at which the operation is
performed is much too high.
Another possible source of loss is the infiltration of small particles of alloy
into the cupel. The cupel is necessarily porous, and particles of metal may
perhaps drain into it, more especially if the bone ash is not in fine powder;

but if this is the main source of loss it is hard to see why, in cupelling equal
weights of silver and gold, the loss is not equal in each case. It is not easy to
believe that the mere filtration of the fused alloy will effect such a change in
the proportion of the metals as that which actually occurs. For example: a
cupel on which an alloy consisting of 0.80 gram of silver, 0.47 gram of
gold, and 25 grams of lead had been cupelled, was found to contain 7-1/2
milligrams of silver, and rather less than half a milligram of gold.
Assuming, for the sake of argument, that the gold present had filtered into
the cupel in the form of small drops of alloy, it would have been
accompanied by less than a milligram of silver, and the presence of the
extra 6 or 7 milligrams of silver must have been due to a different cause.
There can, thus, be little doubt that the cause of the greater part of the
"cupellation loss" is a chemical one and cannot be counteracted by a
mechanical contrivance.[13] In cupellation, then, there is a loss, apart from
imperfect working, inherent in the process itself; and as the amount of this
loss varies under different conditions, it is necessary to study it somewhat in
detail.
The following experiments are taken without selection from the work of
one student. Three experiments were made for each determination, and the
mean result is given. By "range" is meant the difference between the
highest and lowest result and the percentage loss is calculated on the silver
present. The silver added in the lead used has been deducted.
Effect of Varying Lead.—In each experiment 0.4 gram of silver was taken
and cupelled with the lead. The silver loss and "range" are expressed in
milligrams.

Lead Used. Silver Lost. Range.Percentage Loss.
Grams.
10 6.5 1.0 1.62
20 7.0 1.0 1.75
40 12.0 1.5 3.00
60 12.7 0.5 3.17

The loss increases with the lead used.

Effect of Varying Temperature.—0.4 gram of silver was cupelled with 20
grams of lead.

Temperature. Silver Lost. Range.Percentage Loss.
Bright red 7.0 1.0 1.75
Clear yellow 17.3 1.7 4.32

The difference in temperature in these experiments was much greater than
would occur even with careless work.
Effect of Varying Silver.—20 grams of lead were used in each cupellation.

Silver Taken. Silver Lost. Range.Percentage Loss.
Milligrams.
12.50.7 0.2 5.6
25.01.4 0.1 5.6
50.01.6 0.4 3.2
100.02.9 0.3 2.9
200.05.6 0.5 2.8
400.07.0 1.0 1.7
800.09.7 1.0 1.2

It will be seen that, although the quantity of silver lost increases with the
silver present, the percentage loss is greater on the smaller buttons.
The following results are often quoted:—Cupelling 1 grain of silver with 10
grains of lead, the loss was 1.22 per cent.; 10 grains of silver with 100
grains of lead, loss 1.13 per cent.; 25 grains of silver cupelled with 250
grains of lead, lost 1.07 per cent. The proportion of silver to lead was the
same in the three experiments, and the largest button gave the best result.
Evidently, if the quantities of lead had been the same in the three
experiments (say, 250 grains in each case), the loss on the smaller quantities
of silver would appear worse in the comparison.
In judging these results, it must be borne in mind that it is difficult to
regulate the temperature, &c., in consecutive experiments so as to get
exactly similar results, so that the range in consecutive cupellations is
greater than that in a batch cupelled side by side.

Effect of Copper and Antimony.—0.1 gram of silver was cupelled with 20
grams of lead, and to one batch 0.5 gram of antimony, and to another 0.5
gram of copper was added.

Silver Lost. Range.Loss in Percentage.
Without addition 2.9 0.3 2.9
With antimony 3.2 0.2 3.2
With copper 4.9 1.7 4.9

Perhaps the antimony has so small an effect because it is eliminated in the
earlier part of the process, while the silver is still alloyed with, and
protected by, a large proportion of lead; whilst the copper on the other hand
makes its fiercest attack towards the close, when the silver is least capable
of resisting it. The ill effects of copper are most strongly felt when the
quantity of lead present is not sufficient to remove it: the coppery button of
silver got under these conditions is very considerably less than the weight
of silver originally taken.
Although the above is a fair statement of the loss attending average work, it
will not do in very important and exact work to place too much reliance on
the figures given, or, indeed, on any other set of figures, with the object of
correcting the result of an assay. Each man must rely on his own work.
It is easy to determine what allowance must be made for the loss in
cupellation by cupelling side by side with the assay piece an alloy of similar
and known composition. For, if the two pieces are very nearly alike, we may
justly conclude that the loss on each will be the same; and if, further, we
take the average of three or four such determinations we shall get results
accurate within 0.1 per cent. The method of getting such results may be best
explained by one or two illustrations. This method of working is termed
"assaying by checks."
Suppose we have an alloy of silver and lead in unknown proportions and
that by cupelling two lots of 10 grams each there is got from I. 0.1226 gram
of silver, and from II. 0.1229 gram. We should know from general
experience that the actual quantity of silver present was from 2 to 4
milligrams more than this. To determine more exactly what the loss is, the
following plan is recommended:—The two silver buttons are wrapped up

each in 10 grams of lead, and cupelled side by side with two other lots of 10
grams of the original alloy. If now the two buttons I. and II. weigh 0.1202
and 0.1203, they will have suffered in this second cupellation an average
loss of 2.5 milligrams. Suppose the two fresh lots of alloy gave 0.1233 and
0.1235 of silver, the average loss on these would also be 2.5 milligrams.
Add this loss to each result, and take the mean; which is in this case 0.1259.
If copper is present in the alloy as well as silver, it is necessary to add about
the same quantity of copper to the checks as is supposed, or known, to be
present in the assays. If the substance to be assayed is an alloy of silver and
copper, first cupel 0.5 gram of it, with, say, 10 grams of lead, and weigh the
resulting button of silver, in order to get an approximate knowledge of its
composition. Suppose the button weighs 0.3935 gram. We know that this is
below the truth: for the sake of round numbers take it as 0.4, and assume
that the rest of the alloy (0.1 gram) was copper. Two check pieces are then
weighed out, each containing 0.4 gram silver and 0.1 gram of copper
wrapped in 5 grams of lead. Of course the silver must be pure. And there is
also weighed out two (or better, four) assay pieces each containing half a
gram of the alloy wrapped in 5 grams of lead. The whole lot are then
cupelled as nearly as possible under the same conditions. With four assay
pieces, the cupels should be placed close together in two rows of three
across the muffle; the two check pieces are put in the middle cupels.
Suppose the buttons of silver got weighed as follows:—

Check pieces I. 0.3940II. 0.3945
Assay pieces I. 0.3905II. 0.3912
III. 0.3910IV. 0.3909

The average loss on the two check pieces is 5.7 milligrams, and the average
result of the four assay pieces is 0.3909. Add the average loss to the average
result, and there is got the corrected result, 0.3966. And if 0.5 gram of alloy
contain 0.3966 of silver, 1000 will contain 793.2 of silver, and this is the
degree of fineness.
A correction for the loss in cupellation is always made in this way when
rich alloys are being assayed; and in the case of rich ores it may be done
after the manner of the first of the above illustrations. There is another
method of working which relies more on experiment. This is to smelt the

cupel as described further on (p. 114), and to again cupel the resulting
button of lead. The button of silver got in this second cupellation is added to
that first obtained. It will sometimes, but not often, happen that the two
buttons together will slightly exceed in weight the silver which was actually
present. This is because of the retention in the buttons of a small quantity of
lead. It has been stated that the proportion of lead thus retained may be as
much as 1% of the silver present; this, however, can only be under
exceptional conditions. A determination of the actual silver in the buttons
got in the series of cupellations quoted on pages 102, 103, gave an average
percentage of 99.85, so that even with the larger buttons the effect of the
retained lead would be only to increase the weight by about 1 milligram. In
the method of working with checks, the retained lead has no disturbing
influence.
The proportion of lead required for the cupellation of any particular alloy
requires consideration. With too much lead the time occupied in the process
is increased, and so is the loss of silver; on the other hand, too little lead is
of greater disadvantage than too much. From 8 to 16 parts of lead are
required for each part of silver alloy, or, if gold is present, about twice as
much as this must be used. For the cupellation of 1 gram of a silver copper
alloy containing different percentages of copper, the following quantities of
lead should be used:—

Percentage of Copper in Alloy. Lead Required.
5 6grams
10 8 "
20 10 "
30 12 "
40 14 "
50-100 16-18 "

The alloy, in not too large pieces, is wrapped in the required weight of lead
foil and charged into the cupel at once; or the lead may be put in first, and,
when the cupellation has fairly started, the alloy may be added wrapped in
tissue paper; or a portion of the lead may be first started and the alloy
wrapped in the remaining lead and subsequently added. The cupellation of
large quantities of alloy or of alloys which contain tin, antimony, iron, or

any substance which produces a scoria, or corrodes the cupel, must be
preceded by a scorification. The advantages of this are that the slag is
poorer in precious metal than that found on a cupel and is more easily
collected and cleaned; that larger quantities of metal can be treated, and
that, even if the substance is in part infusible, or produces at the start a
clinkery mass or scoria, the oxide of lead gradually accumulates, fluxes the
solid matters, and produces a good final result; but if the oxide of lead by
itself is not sufficient for the purpose, borax or some other flux can be
easily added.
If the button of silver got is very small its weight may be estimated from its
size; but it must be remembered that the weight varies as the cube of the
diameter. If one button has twice the diameter of another it is eight times as
heavy and so on. Scales specially constructed for measuring silver and gold
buttons may be purchased; but it is much better to make the measurement
with the help of a microscope provided with an eyepiece micrometer.
If the length of the long diameter of a silver button be taken the following
table will give the corresponding weight in milligrams:—

Diameter. Weight.Diameter. Weight.
0.04 inch 3.6 0.015 inch 0.19
0.035 " 2.4 0.014 " 0.15
0.03 " 1.5 0.013 " 0.12
0.025 " 0.9 0.012 " 0.097
0.02 " 0.45 0.011 " 0.075
0.019 " 0.4 0.010 " 0.056
0.018 " 0.33 0.008 " 0.028
0.017 " 0.27 0.006 " 0.012
0.016 " 0.23 0.004 " 0.004

The weight of a corresponding button of gold is got by multiplying by 2.25.
These figures are based on those given by Plattner, and apply only to
buttons of such shape as those left after cupellation. A sphere of silver 0.01
inch in diameter would weigh 0.09 milligram, and a similar sphere of gold
weighs 0.167 milligram.

It is safer, however, to compare with a micrometer the diameter of the
button whose weight has to be determined with that of a standard button of
nearly equal size whose weight is known. The weights of the two buttons
are proportional to the cubes of their diameters. This plan of working is
described more fully in Appendix B., page 440.
Calculation of the Results.—After deducting for the silver added, and
correcting for the cupellation loss, the calculation is made in the usual way;
reporting as so many parts per thousand in the case of rich alloys and as so
many ounces and pennyweights, or better as ounces and decimals of an
ounce, in the case of poor alloys and ores.
In this last case, however, it is less fatiguing to refer to a set of tables which
give, either directly or by means of simple addition, the produce
corresponding to any weight obtained from certain given weights of the
substance. The following table gives the produce in ounces and decimals of
an ounce per ton of 2240 pounds:—

Weight of Metal Weight of Ore
got. taken.
5 20 50 100
3 grams.
grams. grams. grams. grams.
0.0001 1.09 0.65 0.16 0.06 0.03
0.0002 2.18 1.31 0.33 0.13 0.06
0.0003 3.27 1.96 0.49 0.20 0.10
0.0004 4.36 2.61 0.65 0.26 0.13
0.0005 5.44 3.27 0.82 0.33 0.16
0.0006 6.53 3.92 0.98 0.39 0.19
0.0007 7.62 4.57 1.14 0.46 0.23
0.0008 8.71 5.23 1.31 0.52 0.26
0.0009 9.80 5.88 1.47 0.59 0.29
0.001 10.89 6.53 1.63 0.65 0.33
0.002 21.78 13.07 3.27 1.31 0.65
0.003 32.67 19.60 4.90 1.96 0.98
0.004 43.56 26.13 6.53 2.61 1.31
0.005 54.44 32.67 8.17 3.27 1.63

0.006 65.33 39.20 9.80 3.92 1.96
0.007 76.22 45.73 11.43 4.57 2.29
0.008 87.11 52.27 13.07 5.23 2.61
0.009 98.00 58.80 14.70 5.88 2.94
0.01 108.89 65.33 16.33 6.53 3.27
0.02 217.78 130.67 32.67 13.07 6.53
0.03 326.67 196.00 49.00 19.60 9.80
0.04 435.56 261.33 65.33 26.13 13.07
0.05 544.44 326.67 81.67 32.67 16.33
0.06 653.33 392.00 98.00 39.20 19.60
0.07 762.22 457.33 114.33 45.73 22.87
0.08 871.11 522.67 130.67 52.27 26.13
0.09 980.00 588.00 147.00 58.80 29.40
0.1 1088.89 653.33 163.33 65.33 32.67
0.2 2177.78 1306.67 326.67 130.67 65.33
0.3 3266.67 1960.00 490.00 196.00 98.00
0.4 4355.56 2613.33 653.33 261.33 130.67
0.5 5444.44 3266.67 816.67 326.67 163.33
0.6 6533.33 3920.00 980.00 392.00 196.00
0.7 7622.22 4573.33 1143.33 457.33 228.67
0.8 8711.11 5226.67 1306.67 522.67 261.33
0.9 9800.00 5880.00 1470.00 588.00 294.00
1.0 10888.89 6533.33 1633.33 653.33 326.67

When, as in this table, the fraction of an ounce is expressed by two places
of decimals, it may be reduced to pennyweights (dwts.) by dividing by 5.
For example, 0.40 of an ounce is 8 dwts. The fraction of a dwt. similarly
expressed may be converted into grains with sufficient exactness by
dividing by 4. Thus, 1.63 ozs. equal 1 oz. 12.60 dwts., or 1 oz. 12 dwts. 15
grains. In England it is usual to report in ounces and decimals of an ounce.
The way to use the table is best shown by an example. Suppose a button of
silver weighing 0.0435 gram was obtained from 20 grams of ore. Look
down the 20-gram column of the table, and select the values corresponding
to each figure of the weight, thus:—

0.04 = 65.33 ozs. to the ton
0.003 = 4.90 "
0.0005 = 0.82 "
——————
0.0435 = 71.05 "
Add these together. The produce is 71.05 ozs., or 71 ozs. 1 dwt. to the ton.
Or, suppose an ore is known to contain 1.24 per cent. of silver. Look down
the 100-gram column, select the values, and add them together as before.
1.0 = 326.67 ozs. per ton
0.2 = 65.33 "
0.04 = 13.07 "
——————
1.24 = 405.07 "
This gives 405 ozs. 1 dwt. 10 grains to the ton.
The calculation becomes more complicated when, as is frequently the case,
the ore contains metallic particles. These show themselves by refusing to
pass through the sieve when the ore is powdered. When they are present, a
large portion, or if feasible the whole, of the sample is powdered and sifted.
The weights of the sifted portion and of the "metallics," or prills, are taken;
the sum of these weights gives that of the whole of the sample taken. It is
very important that nothing be lost during the operation of powdering.
Each portion has to be assayed separately. It is usual to assay a portion of
the sifted sample, say, 20 or 50 grams, and to add to the produce of this its
share of the "metallics." This way of calculating, which is more convenient
than correct, is illustrated by the following example:—

Weight of whole sample 400grams
Made up of sifted portions 399 "
" "Metallics" 1 "
————
400 "

Twenty grams of the sifted portion, when assayed, gave 0.1050 gram of
silver. The whole of the "metallics" scorified and cupelled gave 0.842 gram
of silver. Since the 20 grams assayed was 1-20th of the whole, 1-20th part
of the 0.842 gram of silver (from the metallics) must be added to its
produce. We thus get 0.1471 gram (0.1050 + 0.0421).
Referring to the 20 gram column, we get—
0.1 = 163.33
0.04 = 65.33
0.007 = 11.43
0.0001 = 0.16
—————————
0.1471 = 240.25 ounces per ton.
A more legitimate method of calculation is as follows:—Calculate
separately the produce of each fraction as if they were from different ores.
Multiply each produce (best stated in per cents.) by the weight of the
corresponding fraction. Add together the products, and divide by the weight
of the whole sample. Taking the same example for illustration, we have:—
Metallics.—Weight 1 gram.
1 gram of it yielded 0.842 grams of silver.
∴ Produce = 84.2 per cent.
Produce multiplied by the weight is still 84.2.
Sifted Portion.—Weight 399 grams.
20 grams of it yielded 0.105 gram of silver.
∴ Produce = 0.525 per cent.
Produce multiplied by weight (0.525 × 399) is 209.475.
Add together; and divide by 400, the weight of the whole sample—
84.2
209.475
———-
400) 293.675 (0.7342
0.7342 is the total produce of the ore in per cents.

Referring to the 100-gram column in the table we find 239.84 ounces to the
ton as the produce.
0.7 = 228.67
0.03 = 9.80
0.004 = 1.31
0.0002 = 0.06
———
239.84
Comparing this with the result calculated by the first method—viz., 240.26,
we see that that was 0.38 oz., or between 7 and 8 dwts. too high.
With ores containing "metallics" it is of great importance to powder the
whole of the selected sample without loss during the process; and of even
greater importance to well mix the sifted portion, of which the last portions
to come through the sieve are apt to be more than ordinarily rich through
the grinding down of some portions of the metallic prills.
Remarks on Cupellation.—Cupellation is at once the neatest and the most
important of the dry methods of assaying. Its purpose is to remove easily
oxidisable metals, such as lead and copper, from silver and gold, which are
oxidisable with difficulty. Metals of the first class are often spoken of as
base, and gold and silver as noble metals.
When lead is exposed to the action of air at a temperature a little above
redness, it combines with the oxygen of the air to form litharge, an oxide of
lead, which at the temperature of its formation is a liquid. Consequently, if
the lead rests on a porous support, which allows the fused litharge to drain
away as fast as it is formed, a fresh surface of the lead will be continually
exposed to the action of the air, and the operation goes on until the whole of
the lead has been removed. Silver or gold exposed to similar treatment does
not oxidise, but retains its metallic condition; so that an alloy of lead and
silver similarly treated would yield its lead as oxide, which would sink into
the support, while the silver would remain as a button of metal.
The porous support, which is called a cupel(fig. 5), should absorb the slag
(oxide of lead, etc.) just as a sponge absorbs water, but must be sufficiently
fine-grained to be impervious to the molten metal. At first sight it appears

difficult to filter, as it were, a fluid slag from a fluid metal; but an ordinary
filter-paper damped with oil will allow oils to run through and yet retain the
water; but damped with water it will allow water to run through and retain
oils. Similarly, fused slags damp and filter through a cupel, but the molten
metal not damping it withdraws itself into a button, which is retained.
Although, of course, if the cupel is very coarse-grained the metal may sink
into the hollows.
Copper, antimony, tin, and most other metals, form powdery oxides, which
are not of themselves easily fusible, and it is necessary when these are
present to add some solvent or flux to render the oxide sufficiently fluid.
Fortunately, oxide of lead is sufficient for the purpose; hence, mixed oxides
of copper and lead, provided the lead is present in proper proportion, form a
fluid slag. In separating copper from silver or gold, advantage is taken of
this fact; for, although we cannot cupel an alloy of copper and silver, it is
easy to cupel an alloy of copper, silver and lead. If, however, the lead is not
present in sufficient quantity, the whole of the copper will not be removed,
and the button of silver, still retaining copper, will be found embedded in a
coating of black oxide of copper. Copper oxidises less easily than lead does;
and, consequently, the alloy which is being cupelled becomes relatively
richer in copper as the operation proceeds. It is on this account that the ill-
effects of the copper make themselves felt at the close of the operation, and
that the oxide of copper is found accumulated around the button of silver.
Tin and antimony, on the other hand, are more easily oxidised; and the
tendency of their oxides to thicken the slag makes itself felt at the
commencement: if the button of alloy once frees itself from the ring or crust
of unfused oxide first formed, the cupellation proceeds quietly, and leaves a
clean button of silver in the centre. But in either case the cupellation is
imperfect, and should be repeated with a larger proportion of lead. An
unfused and, consequently, unabsorbed slag tends to retain small buttons of
alloy or metal, and thus cause serious loss.
There is a principle underlying many of the phenomena of dry silver
assaying which the student should endeavour to understand; and which
serves to emphasise and explain some facts which without an explanation
may present difficulties. If a button of melted lead be covered with a layer
of slag rich in oxide of lead, and a second metal be added, this other metal
distributes itself between the metal and slag in proportions which depend

mainly upon the ease with which it is oxidised, and to a large extent upon
the relative quantities of material present. Easily oxidisable metals such as
zinc, iron, antimony and tin, will go mainly into the slag, and, if the
proportion of the slag is large, very little will go into the metal. On the other
hand, with metals oxidisable with difficulty, such as silver, gold, and
platinum, the reverse holds true; nearly the whole of the metals will go into
the lead, and very little into the slag. If, however, the slag be very rich, say
in antimony, the lead will contain antimony; and, on the other hand, if the
lead be very rich in silver, the slag will contain silver in appreciable
quantity. Copper, which is near lead in the facility with which it is oxidised,
will serve for the purpose of a detailed example. The results of actual
analyses of metal and slag formed in contact with each other are shown in
the following table:—
Percentage Composition of the Metal.

Lead. Copper.
6.8 93.2
20.0 80.0
28.0 72.0
32.0 68.0
85.0 15.0

Percentage Composition of the Slag.

Lead. Copper.
71.4 21.4
78.0 17.0
80.0 12.5
86.0 6.7
90.0 3.6

It will be seen from this table that the slag is always much richer in lead and
poorer in copper than the metal with which it is in contact. The ratio of lead
to copper in these five samples is:—

In the Metal. In the Slag.

1 : 14 1 : 0.3
1:4 1 : 0.2
1 : 2.5 1 : 0.16
1:2 1 : 0.08
1 : 0.16 1 : 0.04

Assuming these figures to be correct, the following statement is
approximately true. On oxidising an alloy of 10 grams of copper and 10
grams of lead, and pouring off the slag when 3 grams of lead have gone into
it, there will be a loss of (owing to the slag carrying it off) about 0.2 gram
of copper. On repeating the operation, the next 3 grams of lead will carry
with them about 0.5 gram of copper; and on again repeating, 3 grams of
lead will remove 0.8 gram of copper. Finally, the last gram of lead will
carry with it 0.3 gram of copper, and there will be left a button of copper
weighing 8.3 grams. The slag will have carried off altogether 1.7 gram of
copper, which is 17 per cent. of the metal originally present.
With the more perfect exposure to the air, and quicker removal of the slag,
which results from heating on a cupel, the loss would be heavier. Karsten
got by actual experiment on cupelling copper and lead in equal proportions,
a loss of 21.25 per cent.
Going back to the example: if the slag were collected and fused with a
suitable reducing agent so as to convert, say, half of it into metal, that half
would contain nearly the whole of the copper (such a reduction is called
"cleaning the slag"). On reoxidising this metal, another button of copper is
formed which, added to the first, would reduce the loss from 17 per cent. to,
say, 7 or 8 per cent. And it is conceivable that by a series of similar
operations, almost the whole of the 10 grams of copper originally taken
might be recovered. In practice the problem is (as far as the copper is
concerned) not how to save, but how most easily to remove it; and since the
removal of this metal is quicker from an alloy containing not too much lead,
it is evident that two or three operations with small quantities of lead will be
more effectual than a single treatment with a larger quantity. With those
metals (tin, antimony, &c.) which pass quickly into the slag, the contrary is
true; hence with these it is necessary to have enough lead present, so that
the slag formed at the outset shall contain enough oxide of lead to make it
fluid. As silver is so much less easily oxidised than copper, we should

reasonably expect that the proportion of silver carried off in the oxide of
lead would be considerably less than that of the copper indicated in the
above example. Indeed, there are one or two facts which tend to encourage
the hope that the operation may be conducted without any loss. If a piece of
pure silver foil is exposed on a cupel to air at the usual temperature of
cupellation, it undergoes very little change; it does not even fuse; it loses
nothing in weight, and does not oxidise. In fact, even if oxide of silver were
formed under these conditions, it could not continue to exist, for it is
decomposed into silver and oxygen at a temperature considerably below
redness. On the other hand, oxide of silver is not reduced to metal by heat
alone, when mixed with an excess of oxide of lead; while metallic silver is
converted into oxide when heated with the higher oxides of lead, copper,
and some other metals. That silver, and even gold (which is more difficult
to oxidise than silver), may be carried off in the slag in this way, is in
agreement with general experience. If 10 grams of silver are cupelled with
10 grams of lead, there will be a loss of about 50 milligrams of silver,
which is in round numbers 1-30th of the corresponding copper loss; with 10
grams of gold and 10 grams of lead, the loss will be 4 or 5 milligrams,
which is about 1-12th of the corresponding silver loss.
Determination of Silver in Assay Lead.—Scorify 50 grams of the lead
with 0.5 gram of powdered quartz or glass at not too high a temperature.
When the eye has "closed in," pour; reject the slag, and cupel the button of
lead. Remove the cupel from the muffle immediately the operation is
finished. Weigh, and make a prominent note of the result in the assay book,
as so many milligrams of silver contained in 100 grams of lead.
Determination of Silver in Red Lead or Litharge.—Fuse 100 grams of
the oxide with from 10 to 20 grams of borax; and in the case of litharge
with 2 grams or with red lead 4 grams of flour. Cupel the lead, and weigh
the button of silver. Note the result as in the last case.
Determination of Silver in Argentiferous Lead.—Be careful in taking the
sample, since with rich silver lead alloys the error from bad sampling may
amount to several parts per cent. Cupel two lots of 20 grams each, and
weigh the buttons of silver. Add to these the estimated cupel loss, and
calculate the result. Or wrap each button of silver in 20 grams of assay lead,
and re-cupel side by side with two fresh lots of 20 grams each of the alloy.

Calculate the loss incurred, and add on to the weight of the two fresh
buttons got.
Determination of Silver in Bullion.—The remarks made under the last
heading as to the importance of correct sampling apply with equal force
here. Make a preliminary assay by cupelling 0.1 gram of the alloy with 1
gram of assay lead; calculate the percentage composition. Refer to the table
on page 105 to find what weight of lead is required for cupelling 1 gram of
alloy.
Weigh out four lots of 1 gram each, and wrap them in the required quantity
of lead. Make two check pieces by weighing up two lots of fine silver equal
to that which you believe to be present in the assay pieces; add copper to
make up the weight to 1 gram, and wrap in the same quantity of lead as was
used for the assays.

Prepare six cupels and charge them in the annexed order (fig. 43), and
cupel. Guard against spirting. Clean and weigh the buttons of silver. Add
the mean loss on the two check pieces to the mean weight of the four assay
pieces; this multiplied by 1000 will give the degree of fineness.
Determination of Silver in Copper.—The silver is best separated in the
wet way before cupelling, but if the proportion is not too small, it can be

found by cupellation. Weigh up 3 grams of the metal, wrap in 30 grams of
sheet lead, and cupel; when the cupellation has proceeded for fifteen
minutes, add 20 grams more lead, and continue till finished. Weigh the
button of silver.
The cupellation loss will be five or six per cent. of the silver present.
Determine it by powdering the saturated portion of the cupel and fusing in a
large Cornish crucible with 30 grams each of soda and borax, 10 grams of
fluor spar, and 1-1/2 gram of charcoal. Cupel the resulting button of lead,
and add 10 grams more of lead towards the close of the operation. Deduct
the weight of silver contained in the lead used from the weight of the two
buttons, and calculate to ounces to the ton.
In an experiment in which 0.1975 gram of silver was present, the weight of
the button from the first cupellation was 0.1867, and that of the button from
the second, after correcting for the lead added, was 0.0110 gram.
Determination of Silver in Galena. By Pot Assay.—Mix 20 grams of the
powdered ore with 30 grams of red lead, 20 grams of soda, and 5 grams of
borax, as also with from 7 to 10 grams of nitre. Fuse and pour. Clean the
slag if the ore is rich. Cupel the buttons of lead. Make the usual corrections
and calculate in ounces to the ton.
By Scorification.—Take 10 grams of the ore, 30 grams of lead, and 0.5
gram of borax. Scorify, clean the slag by adding anthracite after the "eye"
has closed in: cupel the button of lead. Weigh the button of silver, make the
necessary corrections, and calculate to ounces to the ton.
The determination may also be made by cupelling the button of lead got in
the dry lead assay.
A sample of galena determined by the three methods gave the following
results:—

By pot assay 7.18 ozs. per ton.
" scorification 7.02 "
" lead assay 6.72 "

Determination of Silver in an Ore. By Pot Assay.—Take 20 grams of the
powdered ore and mix with 30 grams of soda, 40 grams of red lead, and 5
grams of borax, as also with from 2 to 3 grams of flour. Fuse: pour. Clean
the slag by fusing with 20 grams of red lead and two grams of flour. Cupel
the buttons of lead; weigh; make the necessary corrections, and calculate to
ounces to the ton.
By Scorification.—Take 5 grams of the powdered ore, 50 grams of lead, and
0.5 gram of "soda" or borax. Scorify. Clean the slag by fusing in a crucible
as in the pot assay. Cupel, &c.
Examples.—By Pot Assay.—Ore taken 20 grams.

Silver got 0.2893 gram
Silver from slag 0.0060 "
Silver lost in cupellation 0.0100 "
———
0.3053 "
Deduct silver in red lead 0.0017 "
———
Silver in ore 0.3036 " = 495.9 ozs. per ton.

By Scorification.—Ore taken, 3 grams.

Silver got. 0.0425 gram
Silver from slag 0.0022 "
Silver lost in cupellation 0.0020 "
———
0.0467 "
Deduct silver in lead 0.0015 "
———
Silver in ore 0.0452 " = 492.2 ozs. per ton.

Determination of Silver in Silver Precipitate.—This substance contains,
in addition to metallic silver and gold, sulphates of lead and lime; oxides of
zinc, copper, and iron; and more or less organic matter. The sample as

received is generally free from "water at 100° C."; and, since it rapidly
absorbs water, care should be taken in weighing it.
Since it contains combined water it is not suited for scorifying; therefore the
determination of silver and gold (fine metal) is made by pot assay. Weigh
up 5 grams of the precipitate, mix with 100 grams of litharge and 1 gram of
charcoal. Melt in a crucible at a moderate heat and pour. Detach the slag,
replace in the crucible, and, when fused, add a mixture of 20 grams of
litharge and 1 gram of charcoal. When the fusion is again tranquil, pour;
and cupel the two buttons of lead.
In a sample worked in this manner the mean of four determinations gave
0.6819 gram of "fine metal"; deducting 1 milligram for the silver contained
in the oxide of lead, and adding 8 milligrams for the cupellation loss, there
is got 0.6889 gram or 13.778 per cent. of silver (and gold) in the sample.
Determination of Silver in Burnt Ores. By Pot Assay.—Roasted
cupriferous pyrites containing small quantities of gold and silver comes
under this heading. The following mixture will give a fluid slag which is
heavy and tough when cold:—

Ore. Borax. Sand.Litharge. Charcoal.
100 50 50 100 7

Mix; place in a large crucible; cover with salt; and melt down under cover.
When fused drop in an iron rod for a few minutes, and about a couple of
minutes after its withdrawal, pour the charge quickly into a large conical
mould. The button of lead should weigh about 50 grams. Cupel and weigh
the silver. The litharge may be replaced by red lead, in which case another
gram of charcoal powder must be added.
In our experience the results obtained by this method are about 20 per cent.
less than the actual content of the ore. The results of two assays, after
deducting for the silver in the litharge used, were 3.9 and 4.1 milligrams;
and a third assay, in which 5.4 milligrams of silver had been added, gave
9.2, which, after deducting the added silver, leaves 3.8 milligrams. The
average of the three results is 3.9 milligrams from the 100 grams of ore.

Two lots of 100 grams of the same ore treated in the wet way gave 5.2 and
5.0 milligrams of silver. Burnt ores from Spanish pyrites carry about 0.005
per cent. of silver.

WET METHODS.

Silver is got into solution from its ores by attacking with nitric acid, but it is
best, after dissolving, to cautiously add dilute hydrochloric acid, and to
carefully avoid excess. If the quantity of silver is very small the solution is
allowed to stand twenty-four hours, but, otherwise, it is warmed and filtered
as soon as it clears. Dry the residue and concentrate the silver in a button of
lead by pot method or scorification, according to the amount of stony matter
present. Cupel the lead, and the resulting button will be free from all metals,
except perhaps gold. It may be weighed; or dissolved in nitric acid, and the
silver determined gravimetrically in the diluted and filtered solution. It is
better to weigh the metal and afterwards to determine the gold in it,
estimating the silver by difference. Silver alloys are dissolved in dilute
nitric acid (free from chlorides), diluted, and filtered. The solution is then
ready for gravimetric determination.
Sulphuretted hydrogen precipitates silver (like copper), completely, even
from fairly acid solutions.

GRAVIMETRIC DETERMINATION.

Add dilute hydrochloric acid in small excess to the hot dilute solution,
which must contain free nitric acid. Heat and stir until the solution clears.
Decant through a small filter, and wash with hot water, acidulated at first
with a little nitric acid if bismuth is suspected to be present. Dry quickly,
transfer as much as possible of the precipitate to a watch-glass; burn and
ignite the filter paper, treating the ash first with two drops of nitric acid and
then with one of hydrochloric, and again dry. Add the rest of the silver
chloride and heat slowly over a Bunsen burner until it begins to fuse. Cool
and weigh.
The precipitate is silver chloride (AgCl) and contains 75.27 per cent. of
silver. The moist precipitate is heavy and curdy; it is decomposed by direct

sunlight, becoming violet under its influence. When heated it is yellowish;
and, since it is volatile at a high temperature, it must not, in drying, be
heated above its fusing point. The fused chloride can be removed from the
crucible (to which it adheres strongly) by digesting with dilute acid and
zinc.
For the determination of silver in nearly pure bullion the following process
is used:—Weigh up 1.5054 gram of the alloy. With this amount of alloy
each 2 milligrams of silver chloride formed is equivalent to 1 degree of
fineness, so that the weight of the silver chloride obtained (stated in
milligrams and divided by 2) will give the degree of fineness. Transfer to a
bottle (known as "bottles for the Indian mint assay") and dissolve in 10 c.c.
of dilute nitric acid, then make up with water to 200 c.c. and add 3 c.c. of
dilute hydrochloric acid. Allow to stand a few minutes and then shake. Fill
the bottle completely with water, allow to settle, and syphon off the clear
liquid; pour on more water, shake gently to break up the lumps, and again
fill the bottle with water. Invert over the mouth of the bottle a porous
Wedgwood crucible, somewhat similar to those used in gold parting. Take
firm hold of the crucible and bottle, and invert promptly so that the silver
chloride may be collected in the crucible. Allow to stand a little while for
the precipitate to settle, and then carefully remove the crucible under water.
[14]
Drain off most of the water and break up the silver chloride with the
help of a well-rounded glass rod. This greatly facilitates the subsequent
drying. Dry first on the water bath and then on the iron plate. Remove the
dried silver chloride, by inverting the crucible, and weigh it.
As an example, 3 determinations of silver in a coin carried out in this way
gave:—

(1) 1.8500gram AgCl = 925.0 fineness.
(2) 1.8498 " = 924.9 "
(3) 1.8502 " = 925.1 "

Determination of Silver in Burnt Ores.—Take 100 grams of the ore and
place in a large beaker of 2-1/2 litres capacity, and cover with 375 c.c. of
hydrochloric acid. Boil for half an hour until the oxides are dissolved and
the residue looks like sand and pyrites; then add 20 c.c. of nitric acid, and
boil till free from nitrous fumes. Dilute to 2 litres with water, and pass a
current of sulphuretted hydrogen till the iron is reduced, the copper and
silver precipitated, and the liquor smells of the gas. This takes about one
hour and a half.
Filter off the precipitate (rejecting the solution) and wash with warm water.
Dry and transfer to an evaporating dish, adding the ashes of the filter paper.
Heat gently with a Bunsen burner until the sulphur burns, and then calcine
until no more sulphurous oxide comes off. When cold add 30 c.c. of nitric
acid, boil and dilute to 100 c.c. Add 1 c.c. of very dilute hydrochloric acid
(1 to 100),[15] stir well, and allow to stand overnight. Decant on to a
Swedish filter paper, dry and calcine.
Mix the ashes with 100 grams of litharge and 1 gram of charcoal, and fuse
in a small crucible. Detach the button of lead and cupel. Weigh and make
the usual corrections. As an example, 100 grams of ore treated in this way
gave 5.8 milligrams of silver; deducting 0.8 for the silver added in the oxide
of lead leaves 5 milligrams obtained from the ore. Another experiment on
100 grams of the same ore to which 5 milligrams of silver had been added
gave 11.0 milligrams. Deduct 5.8 for the silver added; this leaves 5.2
milligrams as the silver obtained from the ore. These give, as a mean result,
0.0051 per cent., or 1.66 ounce per ton.
Determination of Silver in Commercial Copper.—For the method of
doing this, with an example and experiment, see under the heading of
Examination of Commercial Copper.

VOLUMETRIC METHODS.

There are two of these, one adapted for the determination of silver in alloys
of approximately known composition, and the other of more general
application. The first of these, generally known as "Gay-Lussac's" method
is, as regards its working, perfect in principle; but it requires a practically
constant quantity of silver, that is, one which varies by a few milligrams
only in each determination. It is a confirmatory method rather than a
determinative one. The other is known as "Volhard's," and resembles in
principle and method an ordinary volumetric process.
Gay-Lussac's method is based on the precipitation of silver from a nitric
acid solution by a solution of sodium chloride. The point at which the whole
of the silver is precipitated being recognised by the standard solution
ceasing to give a precipitate. The process depends for its success upon, (1)
the ease which silver chloride separates out from the solution leaving it
clear after shaking, and, (2), the cloudiness produced by the reaction of very
small quantities of silver nitrate and sodium chloride. In working, a quantity
of the sodium chloride solution equal to 1 gram of silver is added at once to
the assay; and, when the solution has been rendered clear by shaking, the
residual silver (which should not exceed a few milligrams) is estimated with
the help of a weaker solution of sodium chloride. The success in working
evidently depends upon the accuracy with which the first addition of the
salt solution is made. On this account the standard solution is run in from a
special pipette capable of delivering a practically invariable volume of
solution. It is not so important that this shall deliver exactly 100 c.c. as that
in two consecutive deliveries the volume shall not differ by more than 0.05
c.c. The dilute salt solution is one-tenth of the strength of that first run in,
and 1 c.c. of it is equivalent to 1 milligram of silver. Ordinarily it is run in 1
c.c. at a time (and an ordinary burette may be used for this purpose),
shaking between each addition until it ceases to give a precipitate. If many
such additions have to be made the operation not only becomes tedious, but
the solution also ceases to clear after shaking, so that it becomes impossible
to determine the finishing point.
If the assay contains less than one gram of silver the first addition of the
dilute salt solution of course produces no precipitate. Five milligrams of
silver in solution (5 c.c.) is then added, and the assay proceeded with in the
usual way; 5 milligrams of silver being deducted from the amount found.

There is required for the assay a standard solution of sodium chloride,
which is prepared by dissolving 5.4162 grams of the salt (made by
neutralizing carbonate of soda with hydrochloric acid) in water and diluting
to one litre. 100 c.c. of this is equivalent to 1 gram of silver.
The weaker solution of salt is made by diluting 100 c.c. of the stronger one
to one litre. One c.c. of this will equal 1 milligram of silver, or 0.1 c.c. of
the stronger solution.
A standard solution of silver equivalent to the dilute salt solution is made
by dissolving 1 gram of fine silver in 10 c.c. of dilute nitric acid, and
diluting with water to one litre.

The solution of salt is standardised as follows:—Weigh up 1.003 gram of
fine silver and dissolve in 25 c.c. of dilute nitric acid in a bottle provided
with a well-fitting flat-headed stopper. Heat on the water bath to assist
solution, resting the bottle in an inclined position. When dissolved blow out
the nitrous fumes with the help of a glass tube bent at right angles. Run in

from a stoppered pipette (as shown in fig. 44) 100 c.c. of the standard salt
solution, and shake vigorously until the solution clears. Fill an ordinary
burette with the weaker standard salt solution, and run 1 c.c. into the assay
bottle, letting it run down the side so that it forms a layer resting on the
assay solution. If any silver remains in solution a cloudy layer will be
formed at the junction where the two liquids meet. This is best observed
against a black background If a cloudiness is seen, shake, to clear the liquid,
and run in another c.c. of salt, and continue this until a cloudiness is no
longer visible. Deduct 1.5 c.c. from the amount of the weaker sodium
chloride solution run in. Divide the corrected reading by 10, and add to the
100 c.c. This will give the volume of strong salt solution equivalent to the
silver taken.
If the first addition of the weaker salt solution causes no cloudiness add 5
c.c. of the silver solution from an ordinary pipette, shake, and then run in
the weaker salt solution, working as before. These 5 milligrams of silver
added must be allowed for before calculating. As an example:—1.0100
gram of fine silver was taken for standardising a solution and 4 c.c. of the
weaker salt solution were run in. Deducting 1.5 and dividing by 10 gives
0.25 c.c. to be added to the 100 c.c.
100.25 : 1.0100 :: 100 : x
x = 1.0075
which is the standard of the salt solution.
The method of working an assay may be gathered from the following
example:—In the determination of silver in some buttons left after
cupellation, it was assumed that these would contain 99.5 per cent. of silver.
For the assay it was necessary to take a quantity that should contain a little
more than 1.0075 grams of silver; then
99.5 : 100 :: 1.0075 : x
x = 1.0125
To ensure a slight excess, there was taken 1.0150 gram of the buttons,
which was treated in exactly the same way as for the standardising. The
quantity of the weaker salt solution required was 7 c.c.; deducting 1.5 c.c.,
and dividing by 10, gives 100.55 c.c. of strong salt solution, which is

equivalent to 1.0130 gram of silver. This being obtained from 1.015 gram of
alloy, is equal to 99.8 per cent., or 998.0 fine.
The Effect of Temperature.—The standardising and the assay must be
done at the same time, since a difference of 5° C. makes a difference of 0.1
c.c. in measuring the 100 c.c. of strong solution of salt. It is always best to
prepare a standard with each batch of assays.
SULPHOCYANATE METHOD.—Volhard's process is based upon the
precipitation of silver in nitric acid solutions with potassium sulphocyanate,
the finishing point being the development of a reddish-brown colour,
produced by the action of the excess of sulphocyanate upon ferric sulphate.
The white sulphocyanate settles readily, leaving the liquor clear; and a
persistent brown coloration in the liquid indicates the finish. The assay must
be carried out in the cold; and water free from chlorides[16] must be used.
The standard sulphocyanate of potassium solution is made by dissolving 4-
1/2 or 5 grams of the salt (KCyS) in water, and diluting to 1 litre. 100 c.c.
are about equivalent to 0.5 gram of silver.
The standard silver nitrate solution is made by dissolving 5 grams of fine
silver in 50 c.c. of dilute nitric acid, boiling off nitrous fumes, and diluting
to 1 litre.
The indicator is a saturated solution of iron alum, or a solution of ferric
sulphate of equivalent strength made by titrating acid ferrous sulphate with
potassium permanganate. Use 2 c.c. for each assay.
The sulphocyanate solution is standardised by placing 50 c.c. of the silver
nitrate solution in a flask with 20 c.c. of dilute nitric acid, diluting to 100
c.c. with water, and running in the sulphocyanate until the greater part of
the silver is precipitated; then adding 2 c.c. of the ferric indicator, and
continuing the titration until a reddish-brown colour is developed, and
remains permanent after shaking continuously. The assay is similarly
performed, the silver being used in the state of a nitric acid solution.
The effect of variations in the conditions of the assay may be seen from the
following experiments, in which 20 c.c. of standard silver nitrate were used:
—

Effect of Varying Temperature:—

Temperature 10° C. 30° C. 70° C. 100° C.
Sulphocyanate reqd. 19.6 c.c. 19.3 c.c. 19.0 c.c. 18.6 c.c.

Effect of Varying Nitric Acid:—Varying nitric acid has no effect, except
that with a fairly acid solution the finishing point is somewhat sharper.

Nitric acid added 5 c.c. 10 c.c. 20 c.c. 50 c.c.
Sulphocyanate reqd. 19.6 c.c. 19.5 c.c. 19.6 c.c. 19.6 c.c.

Effect of Varying Bulk:—

Bulk 50 c.c. 100 c.c. 200 c.c. 300 c.c.
Sulphocyanate reqd. 19.5 c.c. 19.6 c.c. 19.6 c.c. 19.7 c.c.

Effect of Varying Ammonic Nitrate:—

Ammonic nitrate 0 gram 1 gram 5 grams 10 grams
Sulphocyanate reqd. 19.6 c.c. 19.6 c.c. 19.7 c.c. 19.9 c.c.

Effect of Varying Silver:—

Silver added 1 c.c. 10 c.c. 20 c.c. 50 c.c. 100 c.c.
Sulphocyanate reqd.1.0 c.c. 9.70 c.c. 19.6 c.c. 49.4 c.c. 99.0 c.c.

This method is valuable for determining silver in salts, alloys, and
solutions, where no more than an ordinary degree of accuracy is demanded.
It is easy, and applicable under most of the usual conditions. Its greatest
disadvantage is the brown coloration produced by the sulphocyanate when
the assay is nearly, but not quite, finished; and the slowness with which this
is removed on shaking up with the precipitate. This is worse with large
quantities of precipitate, and if about 1 gram of silver is present, it gives an
indefiniteness to the finish which lowers the precision of the process to
about 1 in 500; this is useless for the assays of bullion. One writer states
that this inconvenience is due to portions of liquid being entangled in the
precipitate, but it appears much more likely to be due to the action of the
precipitate itself. In attempting to apply the process to the assay of bullion

by working it on the principle of a Gay-Lussac assay, it was found that a
very considerable excess of silver was required to complete the reaction. In
these experiments 100 c.c. of "sulphocyanate" (very accurately measured)
was run into the solution containing the weighed portion of bullion (fine
silver) and, after shaking the solution, was filtered. In the filtrate the
remaining silver, if there should be any, was determined by the ordinary
titration, but with "sulphocyanate" of one-tenth the strength. This final
titration was quite satisfactory. The amount of silver precipitated by the first
100 c.c., however, varied with the quantity of silver present as in the
following series.[17]

Silver present. Silver precipitated.
1.1342gram. 1.1322gram.
1.1375" 1.1335"
1.1405" 1.1351"
1.1484" 1.1379"

These, of course, preclude a method of the kind aimed at, and at the same
time emphasise the importance of uniformity of work in the ordinary
process. In the determination of chlorides in sea-water, Dittmar used a
combined method: precipitating the bulk of the silver as chloride, and after
filtering, determining the small excess of silver by sulphocyanate. This
modification answers admirably when applied to the assay of bullion. In the
ordinary Gay-Lussac method, the precipitation of the bulk of the silver by
the 100 c.c. of salt solution leaves nothing to be desired, either as to ease in
working or accuracy of result; the silver precipitate settles quickly, and
leaves a clear liquor admirably fitted for the determination of the few
milligrams of silver remaining in solution. But the method of determining
this residual silver by adding successive small quantities of salt so long as
they continue to give a precipitate is unsatisfactory, and, judged on its own
merits apart from the rest of the process, could hardly escape condemnation.
It is clumsy in practice, for the continued adding of small portions of salt
solution is laborious and becomes impossible with more than a few
milligrams of silver in solution. The proposed modification is simple;
having precipitated the silver with the 100 c.c. of salt solution, as described
under Gay-Lussac's method (page 120), shake till the liquor clears, and
filter into a flask, washing with a little distilled water. Add 2 c.c. of "ferric

indicator" to the filtrate and titrate with a standard "sulphocyanate solution"
made by diluting the ordinary standard solution to such an extent that 100
c.c. after diluting shall be equivalent to 0.1 gram of silver.[18] Calculate the
weight of silver found by "sulphocyanate" and add it to the weight which
100 c.c. of the salt solution will precipitate.
An advantage of this modification is that an excess of 15 milligrams may be
determined as easily and exactly as 5. In standardising the salt solution,
then, weigh up, say 1.0150 gram of pure silver, dissolve and titrate.
Suppose 13.5 c.c. of "sulphocyanate" required; then these are equivalent to
.0135 gram of silver, (100 c.c. = .1); the silver precipitated by the salt is
1.0150-.0135—i.e., 1.0015 gram, which is the standard.
Application of the Method to Assays for Arsenic.—If silver nitrate be
added to a neutral solution of an arsenate of one of the alkali metals, silver
arsenate (Ag3AsO4), is thrown down as a dark-red precipitate. If, after
adding excess of silver nitrate to insure a complete precipitation, the
arsenate of silver be filtered off, the weight of the arsenic could be
estimated from the weight of silver arsenate formed. But this may be done
much more conveniently by dissolving the precipitate in nitric acid, and
titrating with sulphocyanate; the silver found will be to the arsenic present
as 324 (108×3) is to 75.
The mineral is best treated by the method given in the third paragraph on
page 382; but the solution, after being acidified with nitric acid, should be
made exactly neutral with ammonia. A small excess of silver nitrate should
then be added, and since acid is liberated in the reaction, the liquor must
again be neutralised.[19] The precipitate must then be filtered off, and
washed with distilled water. Then dissolve it in the paper by slowly running
over it 20 c.c. of dilute nitric acid. Wash the filter with distilled water,
collecting with the filtrate in a small flask. Add 2 c.c. of "ferric indicator"
and titrate.
If the sulphocyanate solution be made up with 11 or 12 grams of the
potassium salt to the litre, and be then standardised and diluted, so that for
100 c.c. it shall equal 1.08 gram of silver, (see p. 38), then it will also equal
.25 gram of arsenic (As). Except for ores rich in arsenic, it will be better to
work with a solution one half this strength. The standard as calculated from

an experiment with pure silver should be checked by another using pure
resublimed white arsenic, As2O3, which contains 75.75 % of the metal. The
quantity of white arsenic taken, .1 or .2 gram, should contain about as much
arsenic as will be present in the assays. It is converted into sodium arsenate
by evaporating to a small bulk with nitric acid and neutralising with soda.
The precipitation and titration of the silver arsenate should be exactly as in
the assays.
The difficulty of the method is in the neutralising; which has to be very
carefully done since silver arsenate is soluble in even faintly acid solutions;
one drop of nitric acid in 100 c.c. of water is enough to produce an
absolutely worthless result; and an excess of acid much less than this is still
very prejudicial. The addition of a little sodium acetate to the solution after
the final neutralising has a good effect.
Arsenic in Mispickel.—Weigh up .250 gram of the finely-powdered ore,
and place in a Berlin crucible about 1-1/4 or 1-1/2 inch in diameter. Treat
with 10 or 12 drops, one drop at a time, of strong nitric acid, warm very
gently, but avoid much heating. Put on a thin layer of nitre, and rather more
than half fill the crucible with a mixture of equal parts of soda and nitre.
Heat quickly in the blow-pipe flame, and when the mass is fused and
effervescing, withdraw and allow to cool. Boil out with water, filter and
wash. Insert a piece of litmus paper and cautiously neutralise with nitric
acid, using ammonia to neutralise any accidental excess of the acid. Add a
gram or so of ammonium nitrate and silver nitrate in excess, neutralise
again with ammonia and add two or three grams of sodium acetate. Filter
off the precipitate, wash and titrate. In the fusion care should be taken to
avoid much effervescence (an excess of the soda mitigates this) and the
operation should be stopped as soon as the whole has entered into fusion.

COLORIMETRIC DETERMINATION.

There is, properly speaking, no colorimetric method, but the following,
which is sometimes used, is based on similar principles. It is useful for the
determination of small quantities of silver in substances which yield clear
solutions with nitric acid.

Dissolve a weighed quantity of the substance in nitric acid, and dilute to a
definite bulk. Divide into two equal parts. To one, add a drop or two of
dilute hydrochloric acid, stir and filter. To the other, add a similar amount of
dilute acid, and then to the filtered portion run in from a burette standard
silver nitrate (1 c.c. = 0.5 milligram silver) until the solutions are equally
turbid. Calculate in the usual way.

GOLD.

Gold occurs in nature chiefly as metal. It always contains more or less
silver, and, in alluvial sands, &c., may be associated with platinum and
iridium.
Gold is insoluble in hydrochloric or nitric acid, but is dissolved by aqua
regia or by solutions of iodine, bromine, or chlorine. It is taken up by
mercury, forming an amalgam, from which the mercury may be driven off
by heat.
When gold occurs in particles of any size, it is readily detected by its
appearance, but when finely disseminated through a large quantity of rock,
it is separated and detected by the amalgamation assay—described below—
or by a process of washing somewhat similar to vanning, or by the
following test:—Powder and, if necessary, roast 50 to 100 grams of the ore,
put on it three or four crystals of iodine and enough alcohol to cover it;
allow to stand for half an hour; a piece of filter paper moistened with the
liquid and burnt leaves an ash with a distinctly purple tint if any gold is
present. It is better, however, to filter off the solution, evaporate, and ignite.
Then, either take up with mercury, and ignite the amalgam so as to get a
speck of the metallic gold; or treat with a few drops of aqua regia, and test
the solution with stannous chloride: a purple coloration indicates gold.
AMALGAMATION ASSAY.—This does not attempt to give the total
produce of gold, but rather the quantity which can be extracted on a large
scale; therefore it should imitate as closely as possible the process adopted
in the mine or district for extracting the metal.
Take 2 lbs of the ore in powder and roast; make into a stiff paste with hot
water and rub up for an hour or so with a little mercury. Wash off the sand

carefully, and collect the amalgam. Drive off the mercury by heat, and
weigh the residual gold. It is best to cupel it with lead before weighing.
In an experiment on a lot of ore which contained 0.189 gram of gold, 0.179
gram was obtained by the above process, equal to about 94-1/2 per cent.
recovered. With ores generally, the yield may be from 80 to 90 per cent. of
the actual gold present.

DRY ASSAY.

The dry assay of gold ores resembles in its main particulars the dry assay
for silver by the crucible method; and for much that is of importance in its
discussion the student is referred to what is written under Silver on pp. 90-
113.
Size of Assay Charges.—Gold ores rarely contain more than a few ounces,
often only a few pennyweights of gold to the ton; consequently, the button
of gold obtainable from such quantities of ore as may be conveniently
worked by assaying methods is often so small as to require more than
ordinary care in its manipulation. One milligram of gold forms a button of
about the size of one of the full-stops on this page, and compared with a
million similar particles of quartz (about four ounces), represents a produce
of a quarter of an ounce to the ton: a proportion such as the assayer is
frequently called on to determine. It is evident, therefore, that a charge of
half an ounce or less of the ore, such as is usual with silver ores, would
demand of the worker both skill and care in the handling of the minute
quantity of gold to be obtained from it. Fortunately the work is simple and
precise, so that in practised hands and with only a 5-gram charge the assay
of a 5-dwt. ore is practicable; with so small a charge, however, the result is
barely perceptible on a sensitive balance: the button of gold should be
measured under a microscope. It follows, therefore, that larger charges of
say 50, 100, or even 200 grams, have an advantage in that they lessen the
strain on the worker's attention, and, except in the case of the poorest
mineral, bring the button of gold within the scope of the balance. On the
other hand, the inconvenience of the larger charges lies in the amount of
fluxes and consequent size of the crucibles required to flux them.

Sampling.—A further consideration in favour of the larger charges is the
matter of sampling. In preparing his ore, the student should ask himself
what reasonable expectation he has that the portion he puts in the furnace
will be of average richness. The larger charges are likely to be nearer than
the smaller ones to the average of the parcel of ore from which they are
taken. In explanation of this, let us suppose a large heap of 5-dwt. ore, in
sand of the coarseness of full-stops, and containing all its gold in particles
of 1 milligram, as uniformly distributed as care and labour in the mixing
can accomplish. Such a heap could not possibly occur in practice, but it will
serve for purposes of illustration. Now, one ton of the sand, however taken,
would contain appreciably the same quantity of gold as any other ton. For a
ton would contain about 8000 particles of gold; and even if two separate
tons differed by as much as 100 particles (which they are just likely to do),
this would mean only a difference of 1 or 2 grains to the ton. On the other
hand, two portions of 14 lbs., which should contain on the average 50
particles of gold, are likely enough to differ by 10 particles, and this,
calculated on a ton, means a difference of 1 dwt. It is easy to see that
something like this should be true; for on calculating the 14-lb. lot up to a
ton, the deviation from the average, whatever it may be, is multiplied by
160; whereas, if the ton were made up by adding 14-lb. lot to 14-lb. lot, up
to the full tale, then a large proportion of the errors (some being in excess
and some in defect) would neutralise each other. An average which is
practically true when dealing with thousands, and perhaps sufficiently exact
with hundreds, would be merely misleading when applied to tens and units.
Reasonable safety in sampling, then, is dependent largely on the number of
particles of gold in the charge taken, and the risk of an abnormal result is
less, the larger the charge taken.
By doubling the charge, however, we merely double the number of
particles. Powdering finely is much more effective; for, since the weight of
a particle varies as the cube of the diameter, halving the diameter of the
particles increases their number eight-fold. If, now, we modify our
illustration by assuming the particles to have only one-sixth the diameter of
a full-stop (which would represent a powder of a fineness not unusual in
ores prepared for assaying), we should multiply the number of particles by
200 (6 × 6 × 6 = 216). We should then reasonably expect a 14-lb. parcel of
the powder to give as safe a sample as a ton of the sand would give; and

portions of a size fit for crucible work, say 50 or 100 grams, would be as
safe as 10 or 20-lb. samples of the coarser stuff. For example, 60 grams of
such powder would contain, for a 5-dwt. ore, about 100 particles; and in the
majority of cases the error due to sampling would be less than 10 or 12
grains to the ton, and would only occasionally exceed a pennyweight. With
richer ores the actual deviation stated as so much to the ton of ore might be
greater, but it would represent a smaller proportion, stated in percentage of
the gold actually present, and would ultimately fall within the limits of
unavoidable error.
It will be seen that the size of the quartz particles has no direct bearing on
the argument; and, in fact, the coarseness of the quartz only interferes by
preventing the uniform mixing of the sand and by binding together several
particles of gold; in this last case, particles so united must, of course, count
as one larger particle. Now, there are some natural ores in which the gold
particles are all very small; with these fine powdering and mixing yields a
product from which a sample may be safely taken. Then, again, in
"tailings," before or after treatment with cyanide, we have a similar
material, inasmuch as the coarser gold has been removed by previous
amalgamation. With these, it is not unusual to take the portion for assay
without any further powdering, since they are poor in gold, and have
already been stamped and passed through a sieve of say thirty holes to the
inch (linear).
But there are other ores, in lump showing no visible gold, which contain the
gold in all possible degrees of fineness, from say prills of a milligram or so
down to a most impalpable powder. The treatment of these cannot be so
simple and straightforward. Suppose a parcel of 1000 grams (say 2 lbs.) of
such ore in fine powder, containing on an average 1 particle of 1 milligram
(the presence or absence of which makes a difference of .6 dwt. on the ton),
10 others of about .5 milligram (each representing .3 dwt.), and 100 others,
which are too coarse to pass through an 80 sieve, and having an average
weight of .1 milligram (each .06 dwt.), and that the rest of the gold,
equivalent altogether to 2 ounces to the ton, is so finely divided that a
charge of 50 grams may be taken without any considerable risk of its
interfering with the sampling. Then in a 50-gram charge there would be one
chance in twenty of getting the milligram particle, in which case the result
would be 12.35 dwts. too high; on the other hand, if it were not present the

result would on this account be .65 dwt. too low. Of the ten .5-milligram
particles, it is as likely as not that one will be present, and its presence or
absence would cause an error of 3.3 dwts., more or less. Of the 100
particles of .1 milligram, there would probably be from 3 to 7, instead of 5,
the proper number; this would mean a variation of 2.6 dwts. from the true
proportion. So that the probable result would range about 5 dwts. more or
less than the 2-1/2 ozs., which is the true produce, and there are possibilities
of astounding results. It is true that the majority of the results would be well
within these limits, and now and again the heart of the student would be
gladdened by a beautiful concordance in duplicate assays; nevertheless,
there can be no reasonable expectation of a good assay, and to work in this
way, on a 50-gram charge, would be to court failure. The coarse gold must
ruin the assay.
The difficulty may be met by concentrating the whole of the coarse gold in
a small fraction of the ore, by sifting and making a separate assay of this
fraction. A portion of the ore, of about 1000 grams, is ground to a very fine
powder and passed through an 80 sieve, re-grinding when necessary, until
only 20 or 30 grams is left of the coarser powder. This is mixed with fluxes
and carried through as a separate assay. The sifted portion is thoroughly
mixed, and a portion of it, say 30 or 50 grams, taken for assay. The weights
of the two portions must be known, and care must be taken that nothing is
lost in the powdering. The method of calculating the mean result from the
two assays is shown on page 109. In this way of working there is no
advantage in continuing the grinding until the coarser fraction is reduced to
a gram or so—rather the contrary; and rubbing on until all the gold is sent
through the sieve is to be distinctly avoided. The student must bear in mind
that what he is aiming at is the exclusion of all coarse gold from the portion
of ore of which he is going to take only a fraction.
The question of the smaller sampling of gold ores has been dwelt on at
considerable length, as befits its importance, in order that the student may
be impressed with a sense of its true meaning. Sampling is not a mystery,
nor does the art lie in any subtle manner of division. It is, of course,
absolutely necessary that the stuff to be sampled shall be well mixed, and
the fractions taken, so that each part of the little heap shall contribute its
share to the sample. Moreover, it must be remembered that tossing about is
a poor sort of mixing, and that everything tending to separate the large from

the small, the light from the heavy, or the soft from the hard (as happens in
sifting), must be avoided, or, if unavoidable, must be remedied by
subsequent mixing.
With a well-taken sample, we may rely on a great majority of our results
falling within normal limits of error; but nothing can be more certain than
that, in a moderately large experience we shall get, now and again,
deviations much more considerable. These erratic assays can only be met by
the method of working duplicates, which call attention to the fault by
discordant results. Such faulty assays should be repeated in duplicate, so
that we may rest the decision on three out of four determinations.
The likelihood of two very faulty assays being concordant is remote; but
with very important work, as in selling parcels of ore, even this risk should
be avoided, as concordance in these cases is demanded in the reports of two
or more assayers. The following actual reports on a disputed assay will
illustrate this: (a) 5 ozs. 1 dwt.; (b) 5 ozs. 10 dwts. 12 grains; (c) 5 ozs. 11
dwts.; (d) 5 ozs. 11 dwts. 12 grs. The mean result of several assays, unless
there be some fault in the method, will be very fairly exact; and individual
assays, with an uncertainty of 1 in 20, may, by repetition, have this reduced
to 1 in 100 or less.
Assay Tons, etc.—Having decided on taking a larger or smaller portion, the
exact quantity to be used will be either some round number of grams, such
as 50 or 100, easily calculable into percentage; or it will be that known as
the "Assay Ton" (see page 13) or some simple multiple or fraction of it,
which is easily calculable into ounces. The reports, too, are at least as often
made as ounces in the short ton of 2000 lbs., as on the more orthodox ton of
2240 lbs. Now the short ton is equal to 29,166.6 troy ounces; and the
corresponding "assay ton" is got from it by replacing ounces by milligrams.
The advantage of its use is that if one assay ton of ore has been taken, the
number of milligrams of gold obtained is also the number of ounces of gold
in a ton of the ore, and there is absolutely no calculation. Even if half an
assay ton has been taken the only calculation needed is multiplying the
milligrams by two. On the other hand with a charge of two assay tons the
milligrams need halving. Where weights of this kind (i.e., assay tons) are
not at hand they may be easily extemporised out of buttons of tin or some
suitable metal, and it is better to do this than to array out the grams and its

fractions at each weighing. The sets of "assay tons," however, are easily
purchased. As stated on page 13, the assay ton for 2240 lbs. is 32.6667
grams; and for the short ton, 29.1667 grams. If, however, the round number
of grams be used and the result brought by calculation to the produce on
100 grams, the conversion to ounces to the ton may be quickly effected by
the help of the table on page 107. As this table only deals with the ton of
2240 lbs., it is supplemented here by a shortened one dealing only with the
produce of 100 grams and stating the result in ounces troy to the short ton
of 2000 lbs.
Estimation of Small Quantities of Gold.—By the Balance. In estimating
minute quantities of gold there are one or two points, of importance to an
assayer only in this assay, where they will often allow one to avoid the
working of inconveniently large charges. One of these is known as
"weighing by the method of
TABLE FOR CALCULATING OUNCES TO THE SHORT TON FROM
THE YIELD OF GOLD FROM 100 GRAMS OF ORE.

Ounces to the Ounces to the Ounces to the
Milligram. Milligram. Milligram.
Ton. Ton. Ton.
0.01 0.003 0.4 0.117 7.0 2.042
0.02 0.006 0.5 0.145 8.0 2.333
0.03 0.009 0.6 0.175 9.0 2.625
0.04 0.012 0.7 0.204 10.0 2.916
0.05 0.014 0.8 0.233 20.0 5.833
0.06 0.017 0.9 0.262 30.0 8.750
0.07 0.020 1.0 0.292 40.0 11.666
0.08 0.023 2.0 0.583 50.0 14.583
0.09 0.026 3.0 0.875 60.0 17.500
0.10 0.029 4.0 1.167 70.0 20.416
0.20 0.058 5.0 1.458 80.0 23.333
0.30 0.087 6.0 1.750 90.0 26.250

vibrations." Suppose a balance at rest in perfect equilibrium, with the
pointer exactly over the middle point of the scale. Let the scale be a series
of points at equal distances along a horizontal line; then, if a small weight

be placed on one pan, the pointer will deviate from its vertical position and
come to rest opposite some definite part of the scale, which will depend
upon the magnitude of the weight added. The law determining this position
is a very simple one; the deviation as measured along the points of the scale
varies directly as the weight added. For example, with an ordinarily
sensitive balance, such as is used for general purposes, one milligram will
move the pointer along, say, three divisions of the scale; then two
milligrams will move it six divisions; half a milligram, one and a half
divisions; and so on. Of course, with a more sensitive balance the
deviations will be greater. Now the point at which the needle comes to rest
is also the middle point about which it vibrates when swinging. For
example, if the needle swings from the third to the seventh division on the
right then [(7+3)/2] it will come to rest on the fifth. In working by this
method the following conventions are useful: Always place the button to be
weighed on the left pan of the balance, the weights on the right; count the
divisions of the scale from the centre to right and left, marking the former +
and the latter -; thus -5 is the fifth division to the left. Then the position of
rest is half the algebraic sum of two readings. For example, let the readings
be 7 to the right and 3 to the left, then (+7-3)/2 = +2. The mean division is
the second division to the right. If the student will place himself in front of
a balance and repeat the following observations and replace the figures here
given by his own, he will have no difficulty in grasping the method. First
determine the bias of the balance; suppose the unloaded balance swings
+1.25 and -1; the bias then is (1.25-1)/2 = +.125 or one-eighth of a division
to the right. Now having put on the button to be weighed let the readings be
+7.5 and +9.25, and (7.5+9.25)/2 = +8.375. Then the effect of the button
has been to move the pointer from +.125 to +8.375, or 8.25 divisions to the
right; we should, therefore, add the weight equivalent of 8.25 divisions to
the weights, whatever they may be on the right hand pan of the balance; if
the divisions were to the left (- divisions) we should subtract. The value of 1
division is easily determined. Suppose the button in the example were a 1
milligram weight, then we should have found that 1 milligram = 8.25
divisions ∴ 1 division = .121 milligram. This method of working adds very
considerably to the power of a balance in distinguishing small quantities.

By the Microscope.—The use of the microscope also is a real advantage in
estimating the weights of minute buttons of gold where there is no undue
risk in sampling, and where an error of say 1 in 20 on the quantity of gold is
tolerable. For ores with copper, lead, zinc, &c., as well as for tailings rather
poor in gold, this leaves a wide field of usefulness. The method is described
on page 440, but the description needs supplementing for those who are not
accustomed to the use of a microscope. The eye-piece of a microscope (fig.
44a, A) unscrews at a, showing a diaphragm at b, which will serve as a
support for an eye-piece micrometer. This last, B, is a scale engraved on
glass, and may be purchased of any optical instrument maker, though it may
be necessary to send the eye-piece to have it properly fitted. When resting
on the diaphragm it is in focus for the upper lens, so that on looking through
the microscope, the scale is clearly seen in whatever position the instrument
may be as regards the object being looked at. Suppose this to be a small
button of gold on a shallow, flat watch-glass, on the stage of the
microscope. Bring the button under the "objective" (i.e., the nose of the
microscope), which should be about a quarter of an inch above the watch-
glass; then looking through the instrument, raise the tube until the button of
gold, or at least some dust on the glass, comes into focus. If the button is
not in the field, rest the thumbs and index fingers, using both hands, on the

edge of the watch-glass, pressing lightly but steadily, and give the glass a
slow, short, sweeping motion; the button will perhaps appear as an ill-
defined blackness, because not quite in focus. Bring this into the centre of
the field. Raise or lower the microscope until the button appears with sharp
outlines. If the scale does not cover the button, rotate the eye-piece; this will
bring the scale into a new position. Since the divisions over the button are
less distinct than the others, it is best to read the latter. Thus, in fig. 44b,
there are 36 divisions on one side of the button, and 35 on the other, making
altogether 71. The whole scale is 80, therefore the diameter of the button is
9 divisions. The value of each division obviously varies with the
magnifying power employed. With most microscopes there is a telescopic
arrangement whereby the tube may be lengthened; if this be done and the
button again brought in focus, it will be seen that, as measured on the scale,
the button is much larger than before. It is evident, therefore, the
micrometer must always be used in the same way. The method given in the
appendix (page 440), for finding the value of the scale when gold buttons
are to be measured is easy and satisfactory. When the button of gold is so
small that there is considerable risk of losing it in transferring to a watch-
glass, it may be measured on the cupel, but for this purpose it must be well
illuminated; this is best done by concentrating light on it with a lens, or with
what comes to the same thing, a clean flask filled with water.

Most assayers, however, using a micrometer in this way, would like to
know its absolute value. To do this, a stage micrometer must be purchased.
This is like an ordinary microscope slide (fig. 44a, C), and when looked at
through a microscope it shows (fig. 44c) lines ruled on the glass at distances
of tenths and hundredths of a millimetre, ten of each, so that the full scale is
1.1 mm. In the case illustrated, 60 divisions of the scale in the eye-piece are
just equal to the 1.1 mm., therefore 1 division equals .0183 mm. A cube of
this diameter would contain (.0183×.0183×.0183) .0000061285 cubic mm.
The corresponding sphere is got by multiplying by .5236; this gives
.000003209 cb. mm. The weight of 1 cb. mm. of water is 1 milligram; and,
since gold is 19.2 times as heavy as water (sp. g. = 19.2), the contents in cb.
mm. must be multiplied by 19.2. This gives .0000616 milligram as the
weight of a sphere of gold measuring 1 division.

If every result had to be calculated in this way the method would be very
laborious; but, having the figures for the first division, those of the others
may be calculated by multiplying by the cube of the corresponding number.
Thus, for the third division (3×3×3 = 27), the content of the cube
(.0000061285×27) is .0001655 cb. mm.; the content of the sphere
(.000003209×27) is .0000866 cb. mm.; and the corresponding sphere of
gold (.0000616×27) is .00166 milligram. With the help of a table of cubes

the whole calculation for 25 or 30 divisions may be made in half an hour,
and the results preserved in the form of a table will simplify all future work.
Assay Operations.—The actual work of the assay resolves itself into three
operations:—(1) The fusion of the ore and concentration of the "fine metal"
(i.e., gold and silver) in a button of lead; (2) The cupellation of the lead,
whereby a button of fine metal is obtained; and (3) the "parting" of the gold
which separates it from the accompanying silver. The following description
takes the order as here given, but the student, in learning the method, should
first practise cupellation if he has not already done so; next he should
practise the separation of gold from silver, taking known weights of fine
gold (p. 63), varying from .5 or .3 gram down to quite minute quantities,
and not resting satisfied until a sensitive balance can barely distinguish
between the weights of gold taken and found. It may be noted here that if he
has not a flatting mill at his disposal, then for large buttons it is better to
make an alloy with eight or nine parts of silver to one of gold, and attack it
with acid without previous flattening, rather than accept the risk and labour
of beating out a less easily attacked alloy to the necessary thinness with a
hammer. It is only after a sense of security in gold parting has been
acquired, that the attack of an ore can be profitably accomplished, and even
then simple and easy ores should be first taken, passing on to others more
difficult, either because of a more complex mineral composition or a
difficulty in sampling.
Concentration of the fine Metal in Lead.—The best flux for quartz,
which makes up the earthy matter of most gold ores, is soda, and this is best
added as carbonate or bicarbonate. By theory,[20] 50 grams of quartz will
require 88.5 grams of the carbonate, or 140 grams of the bicarbonate, to
form sodium silicate, which is a glassy, easily-fusible substance, making a
good slag. If the bicarbonate is used, and heat is applied gradually, steam
and carbonic acid are given off at a comparatively low temperature, and the
carbonate is left; at a higher temperature (about 800° C., or a cherry-red
heat) the carbonate fuses attacking the quartz, and giving off more carbonic
acid; as the heat increases, and the attack on the quartz (which of itself is
infusible) becomes complete, the whole mass settles down to a liquid
sodium silicate, which is sufficiently fluid to allow the gold and lead to
settle to the bottom. The fluid slag does to a certain extent dissolve some of
the crucible, but not seriously. In a perfect working of this experiment, the

first evolution of gases (steam and carbonic acid) should be gentle, so as to
run no risk of its blowing the fine powder out of the crucible; and the heat
at which the second evolution of carbonic acid is produced should be
maintained until the reaction is completed, so that there may be little or no
formation of gas in the fused mass to cause an effervescence which may
force some of the charge over the edges of the crucible. Of course, in
practice the ideal fusion is not attained, but there is no difficulty in
approaching it closely enough to prevent the charge at any time rising above
the level it reached at first in the crucible, and this should be accomplished.
It is usual with quartzose ores to rely mainly on the action of carbonate of
soda, but not entirely. Litharge is also used; it forms, on fusion with quartz,
a silicate of lead, which is a yellow glass, easily fusible, and more fluid in
the furnace than silicate of soda is. By theory, 50 grams of quartz would
require 186 grams of litharge.[21] The reaction takes place without evolution
of gas, and in its working the only point is to so regulate the heat that the
litharge shall not fuse and drain under the unattacked quartz, leaving it as a
pasty mass on the surface. Now, if in making up a charge for 50 grams of
ore, we took 100 grams of bicarbonate of soda (equivalent to about 63
grams of the carbonate), this being five-sevenths of 140 grams (which by
itself would be sufficient), leaves two-sevenths of the quartz to be fluxed by
other reagents: two-sevenths of 186 grams (say 52 grams) of litharge would
serve for this purpose. But if we used 10 grams of borax, which has a
fluxing action about equal to that of the litharge, then 40 grams of the latter,
or (making an allowance for the quartz being not quite pure) say 35 grams,
will suffice. The fluxes, then, for the 50 grams of ore would be: bicarbonate
of soda 100 grams, litharge 35 grams, and borax 10 grams; we could
decrease any of these, and proportionately increase either or both of the
others, and still rely on getting a fusible slag, which is the whole of the
function of a flux, considered simply as a flux. It should be remembered,
however, that the slag is a bi-silicate or acid slag, and that its acid character
is increased by increasing the proportion of borax.
But in addition to the fluxes there is required about 30 or 40 grams of lead
to collect the silver and gold. This is best added as litharge (say 40 grams)
and flour (4 grams), or charcoal powder (2 grams). See pages 93 and 94.
The full charge, then, would be:

Ore 50 grams.
Bicarbonate of soda 100 "
Litharge 75 "
Borax 10 "
Flour 4 "

These should be mixed, placed in a suitable crucible (a G Battersea, round,
will do), and heated, at first at a red heat, but finally much hotter, so as to
get a fluid and clean slag. When the charge has been in tranquil fusion for
some little time, take it out and pour it into an iron mould. When cold,
detach the button of lead. The slag should be glassy, all through alike, and
easily separable from the metal. With ordinary ores, this slag may be
considered as free from gold. In an experiment in which 90 milligrams of
gold were added, the full amount was obtained from the lead produced by
the first fusion. But in certain cases, more especially where large amounts
of metallic oxides are present, the slag is not so clean, and with these the
slag should be powdered, mixed with 40 grams of litharge and 4 of flour,
and melted again; it is an advantage to add a small prill of say 2 or 3
milligrams of silver to the charge, as it insures a visible product in the
cupellation. Indeed, this last precaution is a good one to be taken wherever
there is reason to expect very small buttons. It has the further advantage,
that, if the quantity of silver necessary for inquartation is known, the right
quantity may be added here, so as to save a subsequent operation.
Ores containing Oxides of Iron.—Of the metallic oxides likely to be
present in a slag, oxide of iron is the most important. Gold is occasionally
found in a matrix of this substance, and in the assay of "concentrates"
largely made up of pyrites, this oxide will be formed in the preliminary
calcination. Now, the lower oxide of iron (ferrous oxide, FeO) is easy to
deal with; fused borax will dissolve about its own weight of it, and a silicate
of soda (such as makes up the bulk of a slag in a gold assay) will take up at
least half as much. But the higher oxide (ferric oxide, Fe2O3) is more
refractory; even 6 parts of borax yields a poor product, and slags with any
considerable percentage of it are not satisfactory. A student attempting to
recover gold from some hæmatite (in which there was about half an ounce
of the metal), found in the slag nearly a gram of gold, although in the first
fusion the slag appeared perfectly fluid. There is, however, no difficulty in

getting good slags, even with large quantities of iron. For example, with 50
grams of ferric oxide, 10 of quartz, 30 of borax, 30 of soda,[22] 50 of
litharge, and 7 of flour, the result was quite satisfactory. So, too, was 25 of
quartz, 50 of soda, 50 of litharge, and 7 of flour. It is well, however, in such
cases to have an ample proportion of flux and to aim at a larger button of
lead than usual by increasing the proportion of flour or charcoal (see also
page 91). A charge used on the Randt for roasted "concentrates" (which we
may roughly speak of as quartz and ferric oxide), is one assay ton (about 30
grams) each of ore, soda, and borax, and one and a half assay ton of litharge
and 2 grams of charcoal. Whilst, for the same material, from which most of
the gold has been extracted by "chloridising," 2.5 tons each of ore, borax,
and soda, 4 of litharge, and 4 grams of charcoal are needed. This quantity
requires a large crucible (I Battersea, round). In this the proportion of
silicate of soda and borax counted together is to the oxide of iron as 4 to 1,
on the supposition that the quartz and oxide of iron of the ore are in about
equal quantities; but, in the larger charge especially, much oxide of lead
would also remain as a flux.
Ores containing Sulphides.—In assaying ores containing a large
proportion of pyrites or mispickel, or both, the best plan is to take a portion
and calcine so as to convert it into a product of the kind just considered. The
weighed portion of ore should be placed in a clean crucible and be heated to
incipient redness: with pyrites the first effect is to drive off about half the
sulphur as vapour which burns as flame over the ore. At this stage care
should be taken that there is no great increase of temperature, otherwise
there may be more or less fusion, which would spoil the operation. When
the sulphur flame ceases the solid sulphide of iron burns with visible
incandescence and the charge should now be stirred with a flattened iron
rod so as to expose fresh portions to the air. The top of the furnace must be
open, so that air may have free access to the crucible. When stirring is no
longer followed by visible burning the heat may be raised to full redness.
The crucible is then lifted out (the stirrer still resting in it) and if the charge
gives off no odour of burning sulphur it is shaken out into an iron mortar
and mixed with the fluxes, taking care to clean the stirrer in the mixture.
The charge is then replaced in the crucible in which the roasting was done
and fused in the furnace. The resulting button of lead is cupelled for fine
metal. Ores rich in sulphides requiring this treatment are frequently

"concentrates." For their assay take 1 assay ton (30 grams), calcine and mix
with an equal weight of soda and of borax (30 grams each), and half as
much again of litharge (1.5 tons or 45 grams), and with 2 grams of charcoal
or 5 grams of flour.
Where the sulphides are present in smaller proportion (10 per cent. or less),
they may be taken as serving the purpose of flour or charcoal (see page 95);
the sulphur and iron are oxidised at the expense of the litharge with a
consequent separation of lead as metal. If the proportion of sulphides is not
sufficient to give a large enough button of lead, some charcoal or flour
should be added. On the other hand, if they are in small excess and give a
button of lead somewhat sulphury, i.e., hard and brittle, it may be remedied
by the judicious addition of nitre; this last reagent, however, should not be
used in large quantity. A plan much used to prevent sulphury buttons is to
insert an iron rod or a nail in the charge in the crucible; the iron takes the
sulphur forming sulphide of iron which in moderate quantity does not form
a separate layer of matte but dissolves in the slag. A slag formed of 50
grams of quartz, 100 soda, and some borax, may take up in this way some
10 or 12 grams of sulphide of iron. If, however, an ore gives a layer of
matte or speise, it is best to repeat the assay by the method of calcining
before fusion.
Cyanide Charges, etc.—In assaying the "tailings" which are to be treated
in a cyaniding plant the following charge is used:

Tailings 3 assay tons or 100grams.
Litharge 4.5 " 150 "
Soda 4.5 " 150 "
Borax .75 " 25 "

The sand is assayed without any further crushing and the assay is made in
duplicate.
The residues after treatment with cyanide, differing from the tailings merely
in being poorer in gold because of the extraction by the solution of cyanide,
are run down with the same fluxes in the same relative proportions. But
four charges of 2.5 assay tons (say 75 grams) are worked, and two of the
resulting buttons are scorified together and then cupelled, etc., so as to give

duplicate assays on charges of 5 assay tons. This is one of the cases in
which it is desirable to add a small portion of silver before cupelling.
In assaying the "cyanide liquors" for gold, 2 assay tons of the liquor are
measured out (58.3 c.c. for the ton of 2000 lbs., 65.3 c.c. for the other) and
are evaporated to dryness in a lead dish weighing about 35 grams. Such a
dish is easily extemporised out of a piece of lead foil, if the ordinary vessel
is not at hand; but care must be taken that the lead is free from gold. The
dish with the dried residue is then scorified and the resulting button of lead
is cupelled.

In some cases the fusion of the ore may be replaced by a treatment with
solution of cyanide of potassium and the gold recovered from the solution
in the way just described. For this purpose the ore should be in not too fine
powder, otherwise there will be great difficulty in filtering; a sand which
will pass a 30 sieve and having no large proportion of very fine stuff will
do. Not less than 200 grams should be taken; and as an extraction apparatus

a bell jar capable of holding half as much again may be used. Such a jar
may be extemporised by cutting off the bottom of a bottle by leading a
crack around it with a red hot poker; or a lamp chimney will serve the
purpose. The smaller mouth of the jar is closed by a perforated cork
provided with a clipped tube after the manner of a burette (see fig. 44d). In
the jar, just over the cork, put a plug of loose asbestos or glass wool, or a
piece of sponge to act as a filter; a layer of broken glass, coarse at the
bottom and fine at the top, will serve the same purpose. On this, place the
charge of ore to be extracted. Prepare a solution of cyanide of potassium in
water, with 5 or 10 grams of the salt to the litre. It may be that the whole
point of the assay depends on the solution being of a definite strength; as,
for example, where the relative efficiency of solutions of different strengths
is being determined, when it will be best to estimate the quantity of cyanide
of potassium in the dilute solution by the method given at the end of this
article (page 160). Pour the cyanide solution on to the ore, letting the first
portions to come through run into the beaker, but as soon as the ore is
thoroughly wetted close the clip and allow to stand for several hours. Then,
opening the clip, run through more cyanide solution and then water, so as to
wash the gold-carrying liquor thoroughly into the beaker. It is no matter if
the liquor is a little bit turbid; transfer it to a lead dish, evaporate, scorify,
and cupel in the usual fashion.
The assay of gold-zinc slimes, which is the precipitate formed by zinc
acting on cyanide solutions of gold, may be made by wrapping 2 or 3 grams
in 40 grams of sheet lead and scorifying, cupelling, &c. The amount of
impurity in the stuff varies greatly; it is usually calcined and mixed
thoroughly with soda 40 per cent., borax 30 per cent., and sand 10 per cent.,
and melted in graphite pots. The buttons of bullion obtained are afterwards
remelted with borax and run into bars, the fineness of which varies from
600 to 830 thousandths. The bars are sampled by chipping off diagonally
opposite corners: or better, by drilling, the drillings being freed from pieces
of steel with the help of a magnet.

Cupellation.[23]—The cupellation of lead for gold differs very little from
that of lead carrying silver. When the gold is accompanied by a larger
proportion of silver, and both have to be determined, the cupellation must
be conducted exactly as in a silver assay, the usual precautions being taken
to moderate the temperature so as to lessen the cupellation loss and to

promote a slow and undisturbed solidification in order to avoid spirting. If,
however, the gold predominates the finish should be effected at a higher
heat, as the melting-point of gold is 100° higher than that of silver. The bad
effect of a higher temperature in increasing the cupellation loss need hardly
be considered in the case of such small buttons of gold as are obtained in
assaying gold ores, as any loss there may be is hardly appreciable by the
balance. With larger quantities of gold, however (as in assaying gold
bullion), this loss becomes important; and it is therefore necessary to very
carefully regulate the temperature of the muffle so as to minimise the loss.
The cupels are made of well-burnt bone-ash, of the fineness of coarse
wheat flour, moistened with one-twelfth its weight of water and compressed
into shape in suitable moulds. The moulds sold for this purpose are often of
unsuitable shape. Since lead has a specific gravity of over 11, a cup to hold
from 15 to 25 grams of molten lead need not have a capacity of more than
about 2 c.c. A hollow about 1 inch across and 1/4 inch deep is sufficient;
and the body of the cupel to absorb this weight of lead should itself weigh
from 20 to 25 grams. The button of lead in a gold assay may be twice as
heavy as this. For these larger buttons a hollow 1-1/3 inch across and 1/3
inch deep will be sufficient. If these larger cupels are not at hand the larger
buttons will have to be reduced in size by a scorification before cupelling.
In some cases this preliminary scorification is advantageous or even
necessary: this may be because the lead is hard and impure, or it may be
that a very small button of gold is expected. In the latter case it is best to
scorify the lead down to something less than 1 gram, and to perform the
cupellation on a specially prepared small fine cupel. These small cupels are
best made by grinding the unsaturated portion of a used cupel to a fine
powder, and compressing the dry powder into a small Berlin crucible or
scorifier; the face should be made quite smooth by pressure from a pestle.
On such cupels a small speck of gold (less than .01 milligram) will be left
in a good shape and easily visible; but the cupel must be withdrawn from
the muffle as soon as the cupellation is finished to make sure of always
getting the button in good condition. In places, such as Mints, where large
numbers of bullion assays are regularly made a special form of cupel is
used so that not less than six dozen assays may all be cupelled at the same
time in a muffle of ordinary size. These cupels are square blocks, a little
less than 2 inches across, and a little more than three quarters of an inch

deep. Each block carries four hollows of about .7 inch across and .3 inch
deep. A muffle, on a floor space of 6 inches by 12, would take 3 of these
blocks abreast and 6 deep, and thus provide the means for 72 assays.[24]
Cupels made with wet bone-ash should be slowly dried; and if in the muffle
they can be slowly brought to an orange-red heat it is all the better. Under
no circumstances must the lead be placed on the cupel before the latter has
been so thoroughly heated that it can no longer give off steam or gas of any
kind. For this gas bubbling through the molten metal spatters it, thus
spoiling one assay and throwing doubt on all the rest. Again, the risk of
freezing at the start is much greater with a cupel which has not been
properly heated.
The best plan is to do all the cupellations in batches. After the muffle has
cooled down for the withdrawal of the last batch, and the old cupels have
been taken out, the new cupels for the next batch should be put in their
place. The furnace should then be stoked and made ready for the next
cupellations; by the time the furnace is ready the cupels will be ready also.
There should be no unnecessary handling of the cupels once they have been
placed in the muffle.
The cupellation temperature for gold is an orange-red heat or perhaps a
little hotter. Beginners, who are apt to overheat their furnace, should avoid a
heat which can properly be called yellow. Dr. T.K. Rose[25] has determined
the temperature of a muffle during the cupellation of gold-silver alloys at
the Royal Mint. In one muffle the temperature ranged from 1065° to 1095°
C.; the lower temperature was of course in the front of the muffle. In
another it ranged from 1022° to 1062°, and here the muffle appeared to the
eye "decidedly cooler than usual." The alloy left after cupelling was made
up of 1 part of gold to 2-1/2 parts of silver, and was fused at 952°; hence the
usual temperature of cupellation was, say, 120° or 130° above the melting-
point of the residual metal. To obtain some real knowledge as to the
meaning of these figures, the student should prepare pointed pieces of the
following metals: silver, which melts at 945°; gold, which melts at 1035°;
and an alloy, half silver, half gold, which melts at 990°. These should be
placed on clean cupels in a muffle almost entirely closed; the temperature
should be very slowly raised, and the appearance of the muffle when each
metal begins to melt should be carefully noted. The cupelling temperature

in Dr. Rose's experiment was as much above the melting-point of gold as
this is above that of the silver-gold alloy. The finish of the cupellation of
gold or gold-silver alloys is practically the same as with pure silver; there is
the same thinning out of the litharge into a luminous film which becomes
iridescent before the brightening. But the danger of spirting decreases as the
proportion of gold becomes greater, and disappears when the gold is much
over 30 per cent. Nevertheless it is well to let such buttons become solid
undisturbed and protected from draughts in the body of the muffle. This
means closing the muffle and allowing the furnace to cool down somewhat
before withdrawing the cupels. Buttons solidified in this way are more
malleable than when they are withdrawn promptly on the finish of the
cupellation. This is important with large buttons, as in a bullion assay. On
the other hand, very small buttons, especially such as have to be measured
rather than weighed, should be withdrawn as soon as the luminous film has
disappeared. For when this is done the button can be loosened from the
cupel by merely touching it with the point of a pin, and is then safely and
easily transferred to a watch glass by touching it with the head of a pin
which has been moistened. It adheres to this, and if the pin is not too wet
comes off at once on touching the glass, or in any case will do so on gentle
warming.
Molten gold, with little or no silver, has a peculiar colour which is easy to
recognise; it is more globular than a button of silver of the same size would
be, and it shows less adhesion to the cupel. Just after becoming solid it
glows beautifully, and this is so marked that it is a valuable help in finding
the position of a button when it is more than ordinarily minute.
If the button left from cupellation is yellow it is at least half gold, and a
rough guess as to the proportion of gold may be made from its yellowness;
the rest of the metal is generally silver. The presence of platinum or one of
the platinum group of metals makes the surface of the button dull and
crystalline. The native alloy of osmium and iridium does not alloy with
gold, however, but falls to the bottom of the molten metal. It shows itself in
the subsequent parting as a black spot or streak on the under surface.
The buttons are removed from the cupel with a pair of pliers and then
brushed to remove adherent litharge and bone-ash. Some assayers advise
cleaning by dipping in warm dilute hydrochloric acid followed by washing

in water and drying. The button is next weighed. When the quantity of
silver obtained is not required to be known the weighing may sometimes be
omitted. The next operation in either case is parting either with or without a
previous inquartation.
The loss of gold in cupellation is by no means always inconsiderable. In
three cupellations of 1 gram of gold with 20 grams of lead made purposely
at a very high temperature the cupel absorbed 6.04, 6.20, and 6.45
milligrams of gold. Hence at a high temperature there may easily be a loss
of more than half a per cent. of the gold. In ten cupellations with the same
quantities of gold and lead, but at an ordinary temperature, the gold
recovered from the cupels varied from 1.37 to 1.92 milligrams, and gave an
average of 1.59 milligrams. In round numbers the cupellation loss of pure
gold is .15 per cent.
But if the gold be alloyed with silver the loss is diminished, as is shown by
the following experiments. Gold, .3 gram, was cupelled with 10 grams of
lead and varying amounts of silver, and the cupels were assayed for gold
with the following results:

Silver in the alloy .3 gram .6 gram .9 gram
Gold in the cupel .47 milligram .32 milligram .17 milligram

These, calculated on the .3 gram of gold, give the loss as .157, .107 and
.057 per cent. respectively. The effect of copper, on the other hand, is to
increase the cupellation loss, which, silver being absent, may from this
cause rise to .3 per cent., even when the temperature is not excessive.
In the ordinary assay of gold-copper alloys a constant weight of the alloy is
always taken; hence as the weight of copper in a cupel charge increases, the
weight of gold decreases. The silver, on the other hand, is always very
nearly two and a half times as much as the gold, whatever its quantity may
be. But the cupellation loss is smaller with less gold and greater with more
copper, and it so happens in these assays that these two opposites nearly
neutralise one another. Mr. W.F. Lowe[26] found the gold recoverable from
the cupels on which 20 grains of gold bullion had been treated varied only
between .014 and .015 grain (i.e. from .07 to .075 per cent. of the bullion
treated), although the quality of the bullion varied from 9 to 22 carat.[27]

But in the poorest bullion there was only 7.5 grains of pure gold, while in
the richest there were 18.3 grains; yet each lost on the cupel the same
weight of gold, viz., .014 grain. When reckoned in percentages of the actual
gold present the losses are .187 per cent. and .076 per cent. respectively.
The heavier percentage loss is mainly due to the increased quantity of
copper.
As with silver so with gold the predominant cause of the cupellation loss is
the solution of the metal in the molten litharge which passes into the cupel.
Three lots of 1 gram of gold cupelled each with 20 grams of lead
repeatedly, so as to make 13 cupellations in all, lost in actual weight 35.72
milligrams. The gold recovered from the cupels amounted altogether to
34.56 milligrams. This shows that, compared with the absorption by the
cupel, the other causes of loss are inconsiderable.
The loss of gold by volatilisation is, however, a real one. The dust from the
flues of assay furnaces has been tested on several occasions and found to
contain gold, though in small quantity. Thus Mr. Lowe found .073 per cent.
of silver and .00033 per cent. of gold in such a material. The lead volatilised
from a gold bullion assay would need to be ten times as rich as this to
account for a loss of gold equal to the hundredth part of a milligram. Dr.
Rose, in the paper already quoted, believes that on a .5 gram charge of
standard bullion the loss from volatilisation is not less than .025 nor more
than .05 milligram of gold.
By way of conclusion it may be said that the cupellation loss of gold is
about .07 per cent., and that it is largely met or even over corrected by a
compensating error due to silver retained in the gold after parting.
Inquartation.—The method of separating the gold from the silver in gold-
silver alloys by boiling with nitric acid does not act equally well in all
cases. An alloy half silver half gold, rolled to thin sheet and boiled for half
an hour with nitric acid, may still retain more than two-thirds of its silver.
An alloy of 1 part gold and 1.7 parts of silver gives up practically the whole
of its silver under similar treatment. The gold is left in a coherent, though
easily broken, sheet retaining the shape of the original alloy. The gold thus
left is quite spongy and porous, so that the acid can penetrate into its
innermost portions. But if the silver is in large excess in the alloy, the

removal of the silver is less complete, and the residual gold, instead of
holding together in a form easy to manipulate, falls to a powder which
requires care and time in its treatment. The older assayers, therefore, added
silver to their gold in such proportion that the alloy for parting should be
one quarter gold to three quarters silver. This operation they called
inquartation.
The modern practice is to aim at getting an alloy with 2-1/2 parts of silver
and 1 part of gold. In gold bullion assays this proportion should be obtained
with fair exactness. And in the parting of such gold buttons as are obtained
in assaying ores it is well to aim at this proportion, though absolute
precision is not a matter of importance.
If the button left on cupelling the lead from an assay of an ore appears
white, it is best to assume that it already contains at least a sufficiency of
silver, in the absence of any knowledge to the contrary. This will be true in
almost all cases. But if, on parting, it does not lose at least two-thirds of its
weight, this indicates that the assumption was not justified; and also what
quantity of silver must be added to the button before again attempting to
part. Generally the fault will be in the other direction; the silver will be in
excess and the gold will break up and demand very careful treatment.
If, however, such a button is yellow, then, from its weight and depth of
colour, a rough estimate can be made of how much gold is contained in it.
Silver must be added to make the total weight 3-1/2 times as much as that
of the gold supposed to be present. Thus, if the button weighs 10 milligrams
and is supposed to contain 8 milligrams of gold, then 8 multiplied by 3-1/2
is 28; the button must, in such case, be made up to 28 milligrams by adding
18 milligrams of silver. In judging of the quality of the gold button, no
ordinary error will very seriously affect the result. If, in the example just
given, the quantity of gold present was really 7 or even 9 milligrams of
gold, the resulting alloy would still have been suitable for such partings. In
fact, in routine assays, where the quantity as well as the quality of the gold
is known within fair limits, it is often the custom to add the silver for
inquartation to the lead during the first cupellation.
But in the assay of rich gold alloys such approximate work will not do. If
the composition is not already known with a fair degree of accuracy

preliminary assays must be made. Weigh up two lots of 100 milligrams of
the alloy and wrap each in 3 grams of lead. To one add 300 milligrams of
silver. Cupel both. The button containing the added silver must be flattened
and boiled with 15 c.c. of nitric acid; and the resulting gold must be
washed, dried, ignited and weighed. This, in milligrams, gives directly the
percentage of gold. The weight of the other button gives the percentage of
gold and silver; the difference between the two gives the percentage of
silver. The rest will, perhaps, be copper.
The composition of the alloy being known, or having been determined as
just described, the calculation of how much silver must be added is fairly
simple. The following is an example. Suppose the bullion contains 92 per
cent. of gold, 1 per cent. of silver and 7 per cent. of copper, and that .5 gram
of it is to be taken for an assay. The .5 gram, then, will contain

Gold .460gram
Silver .005"
Copper .035"

But the total silver required is .46 gram × 2.5. This equals 1.15. Allowing
for the .005 gram of silver already present, 1.145 gram of silver must be
added.
The silver is incorporated with the gold, and at the same time the copper is
eliminated, by cupelling with sheet lead. How much sheet lead must be
used will depend partly on how much bullion is taken, partly on how much
copper it contains. Four grams of lead will do for a .5 gram charge; and for
a .3 gram charge, 3 grams may be used. But with 20 per cent. of copper
these amounts should be doubled; with 40 per cent. of copper they should
be trebled; and with over 60 per cent. of copper four times as much lead
should be used. For small buttons of gold as little lead as may be relied on
to start cupelling may be taken; the lead may conveniently be in the form of
little cups made by folding lead foil on a piece of glass rod. With a large
number of bullion assays systematically worked and checked a simple plan
would be to always use the quantity of lead required by the alloy containing
most copper which turns up for assay. This weight, cut out of lead foil,
would be kept in stock folded into little bags ready to receive the bullion
and silver.

The silver used for inquartation must, of course, be free from gold and is
best prepared by the assayer who is to use it (see p. 66). It should not be in
long strips or angular pieces likely to perforate the lead in which it is
folded. When wrapped in the lead it should be in the middle and should
make as compact a parcel as possible.
Each little parcel, as completed, should be placed on a tray in its properly
numbered compartment. Its position here should correspond to that it will
occupy in the muffle and eventually in the cupel tray. The cupellation must
be made with all the requisite precautions. A good smooth malleable button
is needed for the next operation, which is known as flatting.

Flatting.—Small buttons, such as are got in assaying most gold ores, are
placed on a polished steel anvil and flattened by one or two blows with a
hammer. The flattened discs are heated to dull redness on a clean cupel and
are then ready for parting. Somewhat larger buttons may be similarly
treated, but they should be annealed (i.e. heated to redness and allowed to
cool) during the flattening. The silver-gold alloy left from the cupellation is
soft and bends like lead; but after hammering or rolling it becomes harder,
gets a spring in it like a piece of mainspring and cracks or splits somewhat
easily. There should be no cracks or stripping or even roughness on the
flattened metal, since such defects may cause the loss of small particles
either during the flattening or in the subsequent treatment with acid. The
softness of the metal is restored by heating. In bullion assays the flatting of

the buttons requires care and practice for its skilful working. The strips of
alloy for parting should be of uniform thickness and condition so that the
action of the acid shall be equal in all cases. The button is taken from the
cupel, cleaned and placed on the anvil: it is then struck a heavy blow which
widens it to about 3/4 inch in diameter; this blow is followed by two others,
one a little in front, the other behind, which lengthen the disc and give a
very blunt roof-like slope to its upper face. It should then be annealed. This
may be done by putting it in a just red-hot scorifier heated in a muffle: it
very soon attains the right heat and may then be transferred to a cold
scorifier; the hot scorifier should be put back into the muffle. The softened
disc is then taken to the rolls (Fig. 45). The rolls are loosened until the disc
can be pressed between them. Looking through the interval between them
the rolls should appear exactly parallel; if they are not, one adjusting screw
should be loosened and the other tightened until parallelism is obtained. The
rolls are now turned and the disc should be drawn through without any great
effort. Beginners are apt to err by trying to do too much with one turn of the
handle. It is easy to stop whilst the rolls are only just gripping the metal and
then to bring the disc back by reversing the action. If the disc was originally
level and the rolls are parallel, the metal will appear as a strip which has
been merely lengthened. If the rolls are tighter on one side the strip will be
bowed; the tighter side will correspond with the outer curve of the crescent.
A mistake of this kind may be amended by passing the strip through the
rolls the other way, so as to reverse the irregularity and so straighten the
strip. The screw on the looser side should then be tightened until parallelism
is obtained; after which more care should be taken to tighten the two screws
equally. The rolling should be stopped when the strip is 3 or 4 inches long
and of the thickness of an ordinary visiting card. The strip should be
annealed during the rolling and again at the finish.
Parting.—The thin sheet of metal is dropped into hot dilute nitric acid and
boiled for five or six minutes after the brisk action of the acid on the metal
has ceased. At this stage nearly all the silver has gone into solution as
nitrate of silver and the acid is charged with this salt. This acid is poured off
and the residual metal is again boiled for from 20 to 30 minutes with a
second lot of stronger acid. This leaves the gold almost pure, though it may
still retain from .05 to .1 per cent. of silver. Treatment with the first acid
only would probably leave three or four times as much.

The nitric acid used should be free from hydrochloric, sulphuric, iodic and
telluric acids. In testing it for the first of these add nitrate of silver and
dilute with distilled water; there should be no turbidity. In testing for the
others evaporate three lots in dishes over a water-bath. Test one for
sulphates by adding water and barium chloride. Test another for iodates by
taking up with a little water, adding a few drops of starch paste and then
dilute sulphurous acid solution a little at a time; there should be no blue
colour. Test the third for tellurium by heating with 1 c.c. of strong sulphuric
acid until dense fumes come off; allow to cool considerably; a piece of tin
foil added to the warm acid develops a fine purple colour if only a trace of
tellurium is present.
The presence of lower oxides of nitrogen, which impart a brown colour to
the acid, is objectionable; they, however, are removed by boiling the diluted
acid before using it for parting. It is usual to keep a stock of the acid
suitably diluted to the two strengths required for the parting. These are
known as the parting acids. The first parting acid is the weaker and is used
in the first attack on the metal. The specific gravity generally recommended
for it is about 1.2. It may be prepared either by diluting the strong acid with
about its own volume of distilled water, or by suitably diluting the second
parting acid which has been already used in an assay; the small proportion
of silver this contains is not harmful for this purpose. The second parting
acid has a specific gravity of about 1.3, and may be made by diluting the
strong acid with half its volume of distilled water.
Parting in Flasks.—Flasks are most convenient for the larger partings, as in
bullion assays; and should always be used for this purpose unless some of
the special parting apparatus, like that used in Mints, is available. Many
assayers use flasks, though of a smaller size, for the ordinary partings in
assaying gold ores. The flasks are either bulbs with long necks (Fig. 46)
which ought to be heated on rose burners of special construction; or they
are small flat-bottomed conical flasks which may be conveniently heated on
a hot-plate and are, in this respect, much easier to deal with in general work.
The following instructions apply to the parting of an alloy containing a few
decigrams of gold together with the proper proportion of silver.

The strip from the rolls, after being softened by annealing, is folded on
itself on a glass rod into a roll or cornet. It should be so plastic that it will
retain the shape thus given it and not spring open on removing the pressure
of the fingers. About 50 c.c. of the first parting acid are placed in a 6-ounce
conical flask and heated to boiling; the flask is then withdrawn, and tilted a
little to one side, whilst the cornet is cautiously dropped into it; there will be
a sudden issue of hot vapours and a prompt withdrawal of the hand is
advisable. The flask is replaced on the hot plate and the acid is kept boiling
for 10 or 15 minutes. The flask is then withdrawn and the acid diluted with
about an equal volume of distilled water. If the flask has a thick glass band
around its neck, a little way down,[28] care must be taken to use hot water,
for any sudden chill will certainly crack the flask where it is thus thickened.
The liquor is carefully decanted into a clean beaker and is then thrown into
a jar marked "waste silver." About 40 c.c. of the second parting acid, heated
to boiling, is then poured into the flask, which is then replaced on the hot
plate. The boiling is continued for 15 or 20 minutes or even longer. At this
stage bumping has to be specially guarded against; after a little experience
it is easy to see when this is imminent and the flask should be withdrawn to
a cooler part of the plate; it is better to prolong the heating at a temperature

below boiling than to run the risk of disaster. Some of the older writers,
however, are rather insistent on vigorous boiling with large bubbles. The
addition of a small ball of well-burnt clay of about the size of a pea has
been recommended, as it lessens the tendency to irregular and dangerous
boiling. At the end of the treatment with the second acid the flask is
withdrawn from the plate and the acid is diluted with an equal volume of
distilled water. The liquor is carefully decanted into a beaker, and then
poured into a jar or Winchester marked "acid waste"; it serves for making
the first parting acid. The flask is then washed twice with hot distilled
water; the washings must be carefully decanted from the gold. The flask is
then filled with water. A parting cup (size B) is then placed over its mouth,
like a thimble on the tip of a finger. This cup is of unglazed porous
earthenware of such texture that it absorbs the last few drops of water left
on drying; and with a surface to which the gold does not adhere even on
ignition. The gold should fall out cleanly and completely on merely
inverting the cup over the pan of the balance. The flask and cup are then
inverted so that the flask stands mouth down in the cup; a little of the water
from the flask flows into the cup, but only a little. The gold falls steadily
through the water into the cup. When time has been allowed for even the
finest of the gold to have settled into the cup, the flask is removed. This is
easiest done under water. The cup, with the flask still resting in it, is dipped
under water in a basin; as soon as the neck of the flask is immersed the
crucible can safely be drawn away from under it and then lifted out of the
water. The flask should not be taken away first, for the rush of water from it
may easily sweep the gold out of the cup. The water in the cup is then
drained off and the cup is dried at not too high a temperature; for if the last
drop or two of water should boil there is danger of spattering the gold out of
the crucible. When it is dry, the cup is heated on a pipe-clay triangle over a
Bunsen burner, or on a slab of asbestos in a muffle, to a dull-red heat. This
brings the gold to "colour"; that is, the loose tender dark coloured gold
becomes bright yellow and coherent; and is in a state fit to be transferred to
the balance and weighed. All unnecessary transferences must be avoided.
As soon as the cup is cool it may be inverted over the pan of the balance,
when the gold will fall out cleanly or, at the worst, a gentle tap with the
finger will be sufficient to detach it.

Parting in test-tubes, or in the smaller conical flasks, is used in the assay of
gold ores of ordinary richness. The work is exactly like that just described
in all its main features. Generally speaking much less acid will be used; for
example, in test-tubes and for small buttons, 3 or 4 c.c. of each acid is quite
enough. Again, the action need not be so prolonged; 10 or 15 minutes in
each acid is sufficient. So, too, the heating may be less; it is very convenient
to support the test-tubes in a water-bath, or merely to rest them in a beaker
of boiling water; and there is no serious objection to doing this. A smaller
parting cup should be used; the A size is suitable. The button, on the other
hand, should be beaten thinner than is needed for the larger partings. If the
silver should be in excess and the gold becomes much broken up, ample
time should be given for subsidence from the test-tube or flask into the
parting cup.
Parting in glazed crucibles or dishes.—This method of working has the
advantage that there is no transference of the gold until it is placed on the
pan of the balance. On the other hand, in the boiling more care is required
in adjusting the temperature. The following instructions apply to the
treatment of very small buttons, to which the method is more particularly
applicable; but very little modification is needed for the treatment of larger
buttons. The smallest sized Berlin crucibles answer admirably. They should
be cleaned by treatment with hot and strong sulphuric acid, followed by
washing in distilled water; the comfort and ease of working mainly depends
on the thoroughness of this cleaning. The crucible, one-third full with the
first parting acid, is heated on the hot plate until the acid is almost boiling.
The flattened and annealed button is dropped into it and the heating
continued with, at most, gentle boiling for a few minutes. The crucible is
then filled with distilled water, which cools it enough for easy handling; and
when the gold has settled the liquor is poured off along a glass rod into a
clean beaker. Any greasiness of the crucible makes itself felt here and is
very objectionable. The crucible is then one-third filled with the second
parting acid and the heating resumed, care being taken not to raise the
temperature too high; this should be continued much longer than before, say
for five or ten minutes or even longer according to the size of the button.
Distilled water is again added and, when it is drained off, the washing with
distilled water is twice repeated. It will not be possible to drain off the last
drop of water; but if the gold is coherent, the crucible can be so inclined

that this drop drains away from the gold, in which case the drying can be
done rapidly; the boiling of the water will do no harm. But when the gold is
much broken up, it will collect in the middle of this drop and the drying
must be done gently; best by putting the crucible in a warm place. When
dry, the crucible is heated till the gold changes colour, but the heat must be
kept well below redness. When cold, the gold is transferred directly to the
pan of the balance. With minute specks of gold which will require
measuring, it is best to put a small piece of lead foil (say .1 gram) in the
crucible over the gold, and then heat the crucible to above redness over a
blowpipe. Whilst the lead is oxidising it is easily swept round in a bath of
molten litharge by merely tilting the crucible. In this way any separated
specks of gold can be taken up with certainty. When the worker is satisfied
that the lead has had ample opportunity for taking up the gold, the lead must
be kept in one place and the heat slowly lowered. By this means the button
becomes supported in comparatively pure litharge and when solid can be
picked out quite easily with a pair of pliers and in a very clean condition.
The lead button is then cupelled on a very fine cupel, as already described.
The method of working last described destroys the crucible. If the gold is
not quite so small this may be avoided. A small piece of lead foil should be
hammered out until it is perfectly flexible. It is then shaped into a tray and
the gold is transferred to it. The lead is then folded over, with the help of
two pins; and cupelled.
If the crucible shows a black stain on heating it is because some silver
remains through bad washing. It shows poor work and the assay should be
repeated.
The silver retained in the gold after parting is, in bullion assays, an
important matter; it is roughly equal to the loss of gold due to absorption by
the cupel. Mr. Lowe working on .5 oz. of gold, obtained by parting in
assaying bullion, found it to contain .123 per cent. of silver. Dr. Rose in
some special assay pieces found by a less direct method of assaying, from
.06 to .09 per cent. of silver. The proportion of silver retained varies in a
marked way with the proportion of gold to silver in the alloy before parting.
It is generally stated that the retained silver is least when this proportion is 1
to 2-1/2, and more or less silver than this leads to a less pure gold after
parting.

Platinum in an alloy being parted is dissolved along with the silver either
altogether or in part. It imparts a straw yellow colour to the parting acid.
Palladium gives an orange colour to the acid.
The loss of gold by solution in the acid during parting is small, but easily
demonstrable. On a 500-milligram charge of bullion it may amount to from
.05 to .15 milligram; i.e. from .01 to .03 per cent. It is due to gold actually
dissolved and not merely held in suspension.
Assaying with checks. Surcharge.—It will be seen from what has been
stated that the errors in gold parting are of two kinds: viz. (1) a loss of gold
on the cupel and to a less extent by solution in the acid, and (2) an apparent
gain of gold due to the retention of silver in the parted material. Both errors
are small, and as they are of an opposite character they tend to neutralise
each other. Hence they are altogether without effect on the accuracy of the
assays of ores when the total gold is reckoned in milligrams. And even with
the larger amounts present in bullion assays their influence is so small that
an uncorrected result is still fairly accurate; the resultant error would not be
more than one part in two or three thousand.
It is customary to report the purity of bullion, or its fineness as it is called,
in parts per thousand of bullion. The sum of the errors of an assay, which is
called the surcharge, is reported in the same way. Thus a surcharge of + .3
means that the gold as weighed was .3 part per 1000 more than the gold
actually present. But a surcharge - .3 means that on the whole there was a
loss of .3 part per 1000 in the assay.
Speaking roughly the retained silver will vary with the weight of gold
present; if one alloy contains twice as much gold as another the retained
silver will be about twice as much also. On the other hand, as already
explained, the cupellation loss on the poorer alloy is as much as, or even
more than, with the richer one, because of the copper, &c. present. With
rich gold alloys the silver more than compensates for the loss and the
surcharge is positive; but with poorer alloys the loss is greater and the
surcharge is negative.
In Mints and places where bullion assays must be made with the highest
attainable accuracy, the surcharge is determined by experiment, and the
proper correction is made in the reports on the bullion. This is done by

making assays of gold of the highest degree of purity alongside of those of
the bullion whose quality has to be determined. These "checks" are so made
that they do not differ from the actual assays in any material point. Thus,
being of the same quality and weight and undergoing exactly the same
treatment, they may reasonably be expected to have the same surcharge as
the assays they imitate. Suppose the bullion being assayed varies only a
little, up or down, from 900 gold and 100 copper in the thousand, and that
.5 gram of it is used in each assay. A quantity of gold differing only a little
from .450 gram would be very exactly weighed and placed with .050 gram
of copper in the same weight of lead as is being used in the other assays. It
would be cupelled, parted, &c., as nearly as possible under the same
conditions as the actual assays. Suppose the pure gold weighed .45016 gram
and the parted gold weighed .45025 gram, the gain in weight, .00009 gram,
would be deducted from the actual assays. A surcharge correction is never
applied except to bullion of the same quality as that represented by the
"check assay" it was calculated from.
It is evident that unless the gold is of the highest degree of purity these
check assays will introduce an error almost equal to that which it is
designed to remedy. Moreover, to work the checks to the greatest
advantage, a very systematic and uniform method of working must be
adopted.
Parting in special apparatus.—One plan for obtaining greater uniformity
is to stamp each cornet with a number for purposes of identification, and to
treat several, including one or more check assays in the same acid contained
in a beaker; all the assays under these conditions evidently receive precisely
the same acid treatment. Such a plan can of course only be adopted where
there is no risk of the gold breaking up during the parting. An improvement
on this is to have a porcelain basin[29] about 8-1/2 inches in diameter and
with a capacity of about 1-1/2 litres. It is provided with a porcelain cover
with 30 numbered holes through which tubes dip into the acid. The cover is
removable. The tubes are like test-tubes and are supported by the cover;
their bottoms are perforated with holes or slits. The acid is placed in the
basin and boiled over a flat burner; it enters the tubes through the slits. The
cornets are placed each in its proper tube. When the boiling is finished, the
cover with the tubes is lifted and at the same time the acid drains back into
the basin. A dip into a basin of distilled water washes at one operation all 30

assays. The cover is then put on a basin containing the stronger parting acid
which is already boiling. This boiling is continued for half an hour. The
cover with the 30 cornets is then lifted out from the acid and dipped two or
three times in distilled water to wash off the last traces of acid. To transfer
the cornets from the tubes to the porous cups the whole of the tube must be
dipped under the water; otherwise the operation is exactly as when working
with test-tubes.

A still simpler method of working is to use small platinum cups[30] provided
with fine slits which admit the acid but retain the gold. A number of these,
say 60, are supported on a platinum tray. The parting acids are boiled in
platinum dishes under a hood; and the 60 cornets (each in its proper cup)
are placed in the acid all at once: the tray carrying the cups is provided with
a handle suitable for this purpose. After a proper boiling the tray is lifted out
of the weaker acid into the stronger one, where it undergoes the second
boiling. It is next dipped several times in distilled water and lastly, after a
gentle drying, it is raised to an annealing temperature which must not be too
high for fear of the gold sticking to the platinum. After cooling, the cornets
are transferred from the platinum cups directly to the pan of the balance.
Here all 60 cornets have exactly the same treatment and the "checks" may
be compared with great exactness with the other assays accompanying
them. There is, too, a great saving of labour.[31]
Silver, &c., in gold bullion.—The base metals are generally determined by
cupelling .5 gram of the alloy with 5 grams of lead. The loss in cupellation
having been allowed for by any of the usual methods (see p. 104) the gold
and silver contents are given. By deducting the gold the proportion of silver
is obtained. The silver is generally determined by difference in this way. If
it is desired to dissolve out the copper, silver, &c., and to determine them in
the wet way, the gold must first be alloyed with a sufficiency of some other
metal to render it amenable to the attack by acid. Cadmium is the metal
generally recommended, and the alloy is made by melting together a
weighed portion of the gold with five or six times its weight of cadmium in
a Berlin crucible and under a thin layer of potassium cyanide.
Lead with gold or silver.—Large quantities of lead carrying gold and silver
are sold to refiners in bars weighing about 100 lbs. each. The assay of these

alloys presents no special difficulties, but the sampling of them is a question
which may be profitably discussed.[32]
A molten metal may be conceived to have all the physical states observed in
ordinary liquids, although these cannot be actually seen owing to its
opaqueness. There is no doubt that pure lead at a temperature only a little
above its melting-point can contain a large proportion of gold in such a
manner that it may in a figurative way be spoken of as a clear solution. Any
small portion withdrawn from the molten metal would afford a perfect
sample. The same would be true of any pure alloy of lead and silver in
which the silver does not exceed the proportion of 2-1/2 per cent.[33] On the
other hand, if the molten metal contains much more than .5 per cent. of
zinc, more than .1 per cent. of copper, or a larger quantity of silver, it may
be likened to a turbid liquor. The resemblance holds good so far that if the
molten lead be further heated, whereby its solvent power on the added
metal is increased, the turbidity will disappear, or at least be considerably
diminished. A portion taken at random from such a molten metal may, or
may not, give a good sample. The suspended insoluble matter will tend to
concentrate itself in the upper or lower parts of the liquid according to
whether it is heavier or lighter than it; and this separation may occur with
extreme slowness or with fair rapidity. However, it is generally agreed that
in the case of such alloys as occur in practice, samples taken in this way are
quite satisfactory and are the best obtainable. The precautions insisted on
are that the lead shall be made as hot as practicable; that it shall be stirred
up at the time of taking the sample; and that the portion withdrawn shall be
taken out with a ladle at least as hot as the molten metal. The further
precaution that if any dross be on the surface of the metal it shall be
skimmed off and separately sampled and assayed is almost too obvious to
require mention. An alternative and, perhaps, better way of taking the
sample is to withdraw portions at equal intervals from the stream of metal
whilst the pot is being emptied; equal weights taken from these portions and
mixed (by melting or in some other way) give a fair sample of the whole. In
addition, separate assays of each portion will show to what extent the metal
lacks uniformity in composition For example, samples taken at the
beginning, middle, and end of a run gave the following results in ozs. of
silver per ton: 475, 472, 466, showing an average result of 471 ozs. Fifteen
fractions taken at regular intervals during the same pouring ranged from

475 ozs. to 464 ozs.: the average result was 469.8 ozs. The same lead cast
into bars and sampled by sawing gave an average of 470 ozs.[34] In another
case[35] samples drawn at the beginning, middle, and end of a run gave
1345 ozs., 1335 ozs. and 1331 ozs. The mean result in such cases is always
a reasonably safe one, but evidently where the metal varies a good deal it is
safer to take more than three dips.
Imagine such lead run into moulds and allowed to become solid as bars; the
difference between bar and bar would not be greater than that between
corresponding dip samples. But in each bar the distribution of the silver and
gold is very seriously affected during solidification. Chips taken from the
same bar of auriferous lead may show in one place 23 ozs. of gold to the
ton, in another 39 ozs.; similarly with silver they may vary as much as from
900 ozs. to 1500 ozs. to the ton.
This rearrangement of the constituents of a bar takes place whilst the lead is
partly solid, partly liquid. The most useful conception of such half-
solidified metal is that of a felted spongy mass of skeleton crystals of
comparatively pure lead saturated with a still fluid enriched alloy. If the
solidification of an ingot of impure tin be watched it will be evident that the
frosted appearance of the surface is due to the withdrawal of the fluid
portion from a mat of crystals of purer tin which have been for some time
solid and a contraction of the mass. The shrinking of the last part to become
solid is further shown by the collapse of the surface of the ingot where
weakest; that is, a furrow is formed on the flat surface. In other cases of
fused metal there is expansion instead of contraction in this final stage of
the solidification, and the enriched alloy then causes the upper face of the
ingot to bulge outwards. There are other causes effecting the redistribution
of the metals through the ingot. There can be no general rule of wide
application showing which part of a bar is richest and which poorest in the
precious metals. This will depend on the quantities of gold or silver, on the
quantities and kinds of other metals present and on the manner of casting.
The student is advised to consult Mr. Claudet's paper which has been
already referred to.
The best method of sampling such bars is to melt them all down and to take
a dip sample of the molten metal in one or other of the methods already
described. According to Mr. Claudet this should be done in all cases where

the gold exceeds one or two ounces or where the silver exceeds 200 ozs. to
the ton. If during the melting down some dross has formed this must be
skimmed off, weighed and separately sampled and assayed. The clean lead
also must be weighed, sampled and assayed. The mean result must be
calculated. Thus 14 tons 5 cwts. of clean lead assaying 32 ozs. to the ton
will contain 456 ozs. of silver; 15 cwt. dross assaying 20 ozs. to the ton will
contain 15 ozs. of silver. The 15 tons of lead and dross will contain 471 ozs.
of silver or 31.4 ozs. per ton.
Of the methods of sampling which avoid melting the bars, that known as
sawing is the only one which is thoroughly satisfactory. In it the bars are
brought to a circular saw having fine teeth and are sawn across either
completely or halfway through; in this way a quantity of lead sawdust is
obtained (say 1 lb. or so from a bar) which represents exactly the average of
the bar along the particular cross section taken and approximately that of
the whole bar. A bar of lead, which by dip assay gave 334 ozs. to the ton,
gave on three transverse sections 333 ozs., 335 ozs. and 331 ozs. The
variation may be greater than this, but with a large number of bars, where
each bar is cut across in as far as possible a different place, these variations
tend to neutralise each other and a good sample is obtained. Two or three
cwt. of sawdust may be obtained in this way; this is thoroughly mixed and
reduced by quartering in the usual way or by a mechanical sampler. A
sample of 2 or 3 lbs. is sent to the assayer. This being contaminated with the
oil used in lubricating the saw is freed from it by washing with carbon
bisulphide, ether or benzene and dried. Then, after mixing, 100 to 200
grams of it are carefully weighed and placed in a hot crucible, the heat of
which should be sufficient to melt all the lead. The molten lead should not
be overheated and should show no loss due to the melting. The removal of
the oil may have decreased the weight by perhaps one half per cent. If the
lead gives dross on heating it may be melted under 10 or 20 grams of
potassium cyanide, which prevents the formation of dross. Samples are
sometimes taken with a drill, gouge or chisel, though no method of this kind
is quite satisfactory. One plan adopted is to use a punch which, when driven
into the bar, gives a core or rod of metal about half as long as the bar is
thick and about one-eighth of an inch across. With five bars side by side it
is customary to drive in the punch at one end on the first bar, and at the
opposite end on the last one, and on the others in intermediate positions in

such a manner that all the holes will be along a diagonal of the rectangle
enclosing the bars. The bars are then turned over and similar portions
punched out through the bottoms of the bars and along the other diagonal.
Or one set of five may be sampled along the top and the next set along the
bottom of the bars.
Silver and gold present in bars of copper are subject to the same irregularity
of distribution as in lead. The sampling of such bars is guided by the same
principles.[36]

CYANIDES.

The cyanides ought perhaps to be considered along with chlorides,
bromides and iodides in Chapter XV. But they are treated here because they
owe their importance to their use in the extraction of gold and because their
determination has become a part of the ordinary work of an assayer of gold
ores.
Formerly, the cyanide most easily obtained in commerce was potassium
cyanide; and it was generally sold in cakes which might contain as little as
40 per cent. or as much as 95 per cent. of the pure salt. It became customary
to express the quality of a sample of commercial cyanide by saying it
contained so much per cent. of potassium cyanide. The commercial product
now made by improved methods of manufacture is actually sodium
cyanide, but is called "potassium cyanide" (probably with the words
"double salt" on the label); it contains cyanide equivalent to something over
100 per cent. of potassium cyanide in addition to a large proportion of
sodium carbonate and other impurities. What is wanted in most cases is
merely a soluble cyanide, and it is a matter of indifference whether the base
be sodium or potassium. But since 49 parts of sodium cyanide (NaCN = 49)
are equivalent to 65 parts of potassium cyanide (KCN = 65) it is evident
that a pure sample of sodium cyanide would contain cyanide equivalent to
little less than 133 per cent. of potassium cyanide. Therefore a sample of
cyanide reported on in this way may be rich in cyanide, and yet have much
impurity.
The commonest impurity in commercial cyanide is carbonate of sodium or
potassium. This may be tested for by dissolving, say, 2 grams in a little

water and adding barium chloride. There may be formed a white precipitate
of barium carbonate, which if filtered off, washed and treated with acid,
will dissolve with effervescence. Cyanate may be tested for in the solution
from which the barium carbonate has been filtered by adding a little soda
and boiling; if cyanates are present they decompose, giving off ammonia
(which may be tested for in the steam) and yielding a further precipitate of
barium carbonate.[37] If the soda alone gave a further precipitate of barium
carbonate, this may, perhaps, be due to the presence of bicarbonates.
Alkaline sulphides may be present in small quantity in commercial cyanide.
Their presence is shown at once when the sample is being tested for its
strength in cyanide, inasmuch as the first few drops of silver nitrate solution
produce at once a darkening of the liquor. A special test for sulphide may be
made by adding a drop or two of solution of acetate of lead to four or five
c.c. of soda solution and adding this to a clear solution of the suspected
cyanide. This will cause a black precipitate or colour, if any sulphide is
present.
The cyanides of the heavier metals combine with the alkaline cyanides to
form double cyanides. Some of these, ferrocyanide and ferricyanide of
potassium for example, have such characteristic properties that the fact that
they are cyanides may be overlooked. Others, such as potassium zinc
cyanide (K2ZnCy4), have much less distinctiveness: they behave more or
less as a mixture of two cyanides and are, moreover, so easily decomposed
that it may be doubted if they can exist in dilute alkaline solutions. In
reporting the cyanide strength of a cyanide liquor as equivalent to so much
per cent. of potassium cyanide, there is a question as to whether the cyanide
present in the form of any of these double cyanides should be taken into
account. It must be remembered that the object of the assay is not to learn
how much of the cyanide exists in the solution as actual potassium cyanide;
reporting the strength in terms of this salt is a mere matter of convenience;
what is really desired is to know how much of the cyanide present in the
liquor is "free" or "available" for the purposes of dissolving gold. Every one
is agreed as to the exclusion of such cyanides as the following: potassium
ferrocyanide (K4FeCy6), potassium ferricyanide (K3FeCy6), potassium
silver cyanide (KAgCy2), and potassium aurocyanide (KAuCy2); and the
double cyanides with copper or nickel. But with cyanide liquors containing
zinc the position is less satisfactory. One method of assay gives a lower

proportion of cyanide when this metal is present; and the loss of available
cyanide thus reported depends, though in a fitful and uncertain way, upon
the quantity of zinc present. The other method of assay reports as full a
strength in cyanide as if no zinc were present. Unfortunately, using both
methods and accepting the difference in the results as a measure of the
quantity of zinc present, or at any rate of the zinc present as cyanide, is not
satisfactory. It appears best to use the method which ignores the zinc; and to
determine the amount of zinc by a special assay of the liquor for this metal.
The cyanide present as hydrogen cyanide or prussic acid (HCy) is
practically useless as a gold solvent. Hence any report on the strength of a
cyanide liquor which assigned to this the same value as its equivalent of
alkaline cyanide would be misleading. On the other hand, it is "available
cyanide" inasmuch as a proper addition of sodium hydrate[38] would restore
its value. The question of the presence or absence of free prussic acid is
involved in the larger one as to whether the cyanide solution has the right
degree of alkalinity. The assay for "cyanide" should include the hydrogen
cyanide with the rest.
A rough test of the power of a cyanide liquor for dissolving gold may be
made by floating a gold leaf on its surface and noting the time required for
its solution. This test might, perhaps, be improved by taking, say, 20 c.c. of
the liquor and adding three or four gold leaves so that the gold shall always
be in considerable excess. The liquor should not be diluted as this will
affect the result. It should be allowed to stand for a definite time, say at
least two or three hours, or better, that corresponding to the time the liquor
is left in contact with the ore in actual practice. The liquor should then be
filtered off and, with the washings, be evaporated in a lead dish as in the
assay of cyanide liquors for gold (p. 141). The gold obtained on cupelling,
less any gold and silver originally present in the liquor, would be the
measure of the gold dissolving power.

THE ASSAY FOR CYANIDE BY TITRATION WITH SILVER
NITRATE.

The determination of the quantity of a cyanide is made by finding how
much silver nitrate is required to convert the whole of the cyanide into

potassium silver cyanide[39] or one of the allied compounds. It will be seen
from the equation that 170 parts by weight of silver nitrate are required for
130 parts by weight of potassium cyanide. As already explained it is
customary to report the cyanide-strength in terms of potassium cyanide,
even when only the sodium salt is present. One gram of potassium cyanide
will require 1.3076 gram of silver nitrate. The standard solution of silver
nitrate is made by dissolving 13.076 grams of silver nitrate in distilled
water and diluting to 1 litre; 100 c.c. of such a solution are equivalent to 1
gram of potassium cyanide.[40]
The titration is performed in the usual way, running the standard solution of
silver nitrate into a solution containing a known weight or volume of the
material containing the cyanide. The finishing point is determined in one of
two ways, both of which are largely used. In the first place, as long as there
remains any free cyanide in the solution the silver nitrate will combine with
it forming the double cyanide and yielding a clear solution; but as soon as
all the free cyanide is used up the silver nitrate will react with the double
cyanide[41] forming silver cyanide, which separates as a white precipitate
and renders the solution turbid. But, in the second place, if potassium iodide
is present in the solution the excess of silver nitrate will react with it,[42]
rather than with the double cyanide; and silver iodide will separate as a
yellowish turbidity which is easily recognised.
In working with pure solutions, the two finishing points give the same
results; and this is true even when there is much difference in the degree of
dilution. The finishing point with the iodide, however, has an advantage in
precision. Moreover, it is but little affected by variations in alkalinity, which
render the other finishing point quite useless. The great difference between
the two is shown when zinc is present in the solution. In this case, when
working without the iodide, the first appearance of a turbidity is less
distinct; the turbidity increases on standing and as a finishing point is
unsatisfactory. It can be determined with precision only by very systematic
working and after some experience. The turbidity is due to the separation of
an insoluble zinc compound. A most important point (to which reference
has already been made) is that less silver nitrate is required to give this
turbidity and, consequently, a lower strength in cyanide is reported. On the

other hand, as much silver nitrate is required to give the yellow turbidity
due to silver iodide as would be required if no zinc were present.
Unfortunately the difference in the two titrations does not depend merely on
the quantity of zinc present; as it is also influenced by the extent of dilution,
the degree of alkalinity of the solution, and the quantity of cyanide present.
In an experiment with .055 gram of zinc sulphate and .1 gram of potassium
cyanide the difference in the two finishing points was only .1 c.c.; whereas
with .4 gram of potassium cyanide, the other conditions being the same, the
difference was 1.5 c.c. of standard silver nitrate. On the assumption that all
the zinc was present as potassium zinc cyanide (K2ZnCy4) the difference
should have been 5 c.c. in each case. Again, repeating the experiment with
.4 gram of potassium cyanide, but with .11 gram of crystallised zinc
sulphate, the difference was 6.5 c.c.: that is, merely doubling the quantity of
zinc increased the difference by more than four times. Hence it would
appear better to use the method with the iodide and make a separate assay
for the zinc. But since the student may be called on to use the other method,
he is advised to practice it also.
The assay without iodide.—The standard solution of silver nitrate is
placed in a small burette divided into tenths of a c.c. Ten c.c. of the cyanide
solution to be assayed is transferred to a small flask and diluted with water
to about 70 c.c. The silver solution is then run in from the burette (with
constant shaking of the flask), a little at a time but somewhat rapidly, until a
permanent turbidity appears. Since 1 c.c. of the silver nitrate solution
corresponds to .01 gram of potassium cyanide, it also corresponds to .1 per
cent. of this salt counted on the 10 c.c. of cyanide solution taken. The
titration should be performed in a fairly good uniform light. The learner
should practice on a fairly pure solution of potassium cyanide at first, and
this may conveniently have a strength of about 1 per cent. For practice with
solutions containing zinc make a solution containing 1.1 gram of
crystallised zinc sulphate in 100 c.c. and slowly add measured quantities of
from 1 to 5 c.c. of this to the 10 c.c. of cyanide liquor before diluting for the
titration.
If a cyanide solution blackens on the addition of the silver nitrate it contains
sulphide. In this case, shake up a considerable bulk of the liquor with a few

grams of lead carbonate, allow to settle and make the assay on 10 c.c. of the
clear liquor.
If the cyanide liquor be suspected to contain free prussic acid, take 10 c.c.
for the assay as usual; but, before titrating, add .1 or .2 gram of sodium
carbonate. On no condition must caustic soda or ammonia be added. The
difference between the results, with and without the addition of carbonate of
soda, is supposed to measure the quantity of free prussic acid. If this has to
be reported it is best done as "prussic acid equivalent to ... per cent. of
potassium cyanide." Suppose, for example, the difference in the two
titrations equals 1 c.c. of standard silver nitrate; the prussic acid found
would be equivalent to .1 per cent. of potassium cyanide.
The assay with iodide.—The standard solution of silver nitrate is placed in
a burette divided into tenths of a c.c. Take 10 c.c. of the cyanide liquor,
which should previously have been treated with white lead for the removal
of sulphides if these happened to be present. Transfer to a small flask, add 3
or 4 drops of a solution of potassium iodide and 2 or 3 c.c. of a solution of
sodium hydrate; dilute to 60 or 70 c.c. with water. If much zinc is present
the soda may be increased to 20 or 30 c.c. with advantage. The standard
solution should be run in somewhat rapidly, but a little at a time, so that the
precipitate at first formed shall be small and have only a momentary
existence. The titration is continued until there is a permanent yellowish
turbidity. The most satisfactory and exact finish is got by ignoring any faint
suspicion of a turbidity and accepting the unmistakable turbidity which the
next drop of silver nitrate is sure to produce. This finishing point gives
results which are exactly proportional to the quantity of cyanide present;
and it can be recognised with more than ordinary precision even in
solutions which are not otherwise perfectly clear.
Each c.c. of the standard silver nitrate solution corresponds to .01 gram of
potassium cyanide; and if 10 c.c. of the liquor are taken for assay this
corresponds to .1 per cent. or 2 lbs. to the short ton or 2.24 lbs. to the long
ton. As already explained the result should be reported as "cyanide
equivalent to so much per cent. of potassium cyanide."
The following experimental results were obtained with a solution of
potassium cyanide made up to contain about 1.2 per cent. of the salt.

Effect of varying cyanide.—The bulk before titration was in each case 60
c.c.; 2 c.c. of soda and 3 drops of potassium iodide were used in each case.

Cyanide added 40 c.c. 30 c.c. 20 c.c. 10 c.c. 5 c.c. 1 c.c.
Silver required 47.0 c.c. 35.25 c.c. 23.5 c.c. 11.7 c.c. 5.8 c.c. 1.15 c.c.

Accepting the result for 40 c.c. as correct, the others are in very satisfactory
agreement.
Effect of varying dilution.—The conditions were those of the 40 c.c.
experiment in the last series; but varying amounts of water were used in
diluting.

Water added none 100 c.c. 200 c.c. 400 c.c.
Silver required 47.0 c.c. 47.0 c.c. 47.0 c.c. 47.05 c.c.

Very considerable dilution therefore has no effect.
Effect of varying soda.—The conditions were those of the 40 c.c.
experiment in the first series, except that varying amounts of soda solution
were used.

Soda added none 10 c.c. 30 c.c.
Silver required 46.95 c.c. 47.0 c.c. 47.0 c.c.

This alkali therefore has no prejudicial effect.
Effect of ammonia.—Soda causes turbidity in some cyanide liquors; with
these it should be replaced by 2 or 3 c.c. of dilute ammonia with a gram or
so of ammonium chloride. The following experiments with dilute ammonia
show that larger quantities of this reagent must be avoided.

Ammonia added none 10 c.c. 30 c.c. 60 c.c.
Silver required 46.95 c.c. 47.15 c.c. 47.7 c.c. 49.5 c.c.

Effect of sodium bicarbonate.—In this experiment 1 gram of bicarbonate
of soda was used instead of the soda or ammonia of the other experiments.
The silver nitrate required was only 46.45 c.c. instead of the 47.0 c.c. which

is the normal result. This is probably due to the liberation of prussic acid
and shows the importance of having the solution alkaline.
Effect of zinc.—In each experiment 40 c.c. of the cyanide solution and .5
gram of zinc sulphate crystals were used and the bulk was made up to 100
c.c. before titrating.

Soda added 1 c.c. 5 c.c. 10 c.c. 25 c.c.
Silver required 47.1 c.c. 47.0 c.c. 46.9 c.c. 46.9 c.c.

The work was easier with the more alkaline solutions. The titration in the
presence of zinc is comparatively easy, but, in learning it, it is well to have
a burette with cyanide so that if a titration be overdone it can be brought
back by the addition of 1 or 2 c.c. more cyanide and the finish repeated; a
quarter of an hour's work in this way will ensure confidence in the method.
Effect of other substances.—It was found that an alkaline cyanate,
sulphocyanate, ferrocyanide, nitrite, borate, silicate or carbonate has no
effect. The ferricyanide had a small influence and, as might be expected,
hyposulphite is fatal to the assay. The addition of salts of lead and cadmium
was without effect. On the other hand, nickel produces its full effect; and
the quantity of nickel added can be calculated with accuracy from the extent
of its interference with the titration.
Assay of commercial cyanide of potassium.—Break off 20 or 30 grams of
the cyanide in clean fresh pieces, weigh accurately to the nearest centigram.
Dissolve in water containing a little sodium hydroxide; transfer to a 2-litre
flask: dilute to 2 litres; add a few grams of white lead; shake up and allow
to settle. Run 50 c.c. of the clear liquor from a burette into an 8 oz. flask;
add 2 or 3 c.c. of soda solution and 3 drops of potassium iodide. Titrate with
the standard solution of silver nitrate. The percentage may be calculated by
multiplying the number of c.c. used by 40 (50 c.c. is one fortieth of the 2
litres) and dividing by the weight of commercial cyanide originally taken.
Alkalinity of commercial potassium cyanide and of cyanide solutions.—
Hydrocyanic acid like carbonic acid has no action on methyl-orange;[43]
hence the alkaline cyanides may be titrated with "normal acid" as easily as
the carbonates or hydrates. 100 c.c. of normal acid will neutralise 6.5 grams
of pure potassium cyanide.[44] A solution of commercial cyanide prepared
as for the assay last described, but best without the addition of white lead,
may be used for the test. Take 50 c.c. of it; tint faintly yellow with methyl-
orange and titrate with normal acid till the liquor acquires a permanent
reddish tint. In the case of the purer samples of cyanide the quantity of acid
used will correspond exactly with that required to neutralise the actual
quantity of cyanide present as determined by the assay with nitrate of silver.

The less pure samples will show an excess of alkalinity because of the
presence of sodium carbonate or of potassium carbonate.
In comparing the alkalinity and cyanide strength of a solution the simplest
plan is to take 65 c.c. of the solution and titrate with normal acid; for in this
case each c.c. of normal acid corresponds to .1 per cent. of potassium
cyanide. In systematic assays of this kind, the alkalinity would no doubt be
generally in excess of that required by the cyanide present: there would be
no inconvenience in recording such excess in terms of potassium cyanide.
Determination of the acidity of an ore.—Most ores have the power of
destroying more or less of the alkalinity of a cyanide solution and in a
proportionate degree of damaging its efficiency. An assay is needed to
determine how much lime or soda must be added for each ton of ore in
order to counteract this. Whether this acidity should be reported in terms of
the lime or of the soda required to neutralise it will depend on which of
these reagents is to be used in the actual practice. Again, if the ore is
washed with water before treating with cyanide on the large scale, then the
assay should be made of the acidity of the ore after a similar washing.
The standard solutions of acid and alkali used for this determination may
be one-fifth normal. 200 c.c. of the normal solution should be diluted to 1
litre in each case, 1 c.c. of the resulting solutions would be equivalent to 8
milligrams of soda (NaHO) or 5.6 milligrams of lime, CaO. It must be
remembered this refers to the pure bases in each case. Suppose it is desired
to report as so many lbs. of lime to the short ton (2000 lbs.) of ore. Since 1
c.c. of the standard solution is equivalent to 5.6 milligrams of lime, if we
take 2000 times this weight of ore (i.e. 11,200 milligrams or 11.2 grams) for
the assay, each c.c. of standard solution will be equivalent to 1 lb. of lime to
the short ton.[45]
Total acidity.—Weigh out 11.2 grams of the ore, place them in a four-inch
evaporating dish and measure on to it from a burette 10 or 20 c.c. of the
standard solution of soda. Stir the soda solution into the ore and allow to
stand for 15 or 20 minutes with occasional stirring. Stir up with 30 or 40
c.c. of water, float a piece of litmus paper on the liquid and titrate with the
standard solution of acid. If the ore is strictly neutral the quantity of "acid"
required to redden the litmus will be the same as the quantity of "soda"

originally used. If the ore is acid, less acid will be used. For example, if 10
c.c. of soda were used and only 7 c.c. of acid were required, the ore will
have done the work of the remaining 3 c.c. of acid. And the ton of ore will
require 3 lbs. of lime to neutralise its acidity.
Acidity after washing.—Take 11.2 grams of the ore; wash thoroughly with
water and immediately treat the residue, without drying, exactly as just
described.
Examination of cyanide solutions for metals, &c.—Take a measured
quantity of the solution, say 20 c.c.[46] and evaporate in a small dish with,
say, half a c.c. of strong sulphuric acid. Evaporate at first, on a water-bath in
a well ventilated place, but finish off with a naked Bunsen flame, using a
high temperature at the end in order to completely decompose the more
refractory double cyanides. Allow to cool; moisten with strong hydrochloric
acid; warm with a little water and test for the metals in the solution by the
ordinary methods. Since the quantities of the metals likely to be present
may be given in milligrams the work must be carefully performed. It may
be worth while to determine the proportions of lime and magnesia as well
as those of the metals proper.
Or the 20 c.c. of cyanide liquor may be evaporated with 5 c.c. of strong
nitric acid to dryness and gently ignited and the residue taken up with 2 or 3
c.c. of strong hydrochloric acid.
Copper, iron, and zinc can be rapidly determined in such a solution, as
follows. Dilute with water to 10 or 15 c.c., add an excess of ammonia, and
filter. The precipitate will contain the iron as ferric hydrate; dissolve it in a
little hot dilute sulphuric acid: reduce with sulphuretted hydrogen; boil off
the excess of gas, cool and titrate with standard potassium permanganate (p.
236). Determine the copper in the filtrate colorimetrically (p. 203); but
avoid further dilution. Then add dilute hydrochloric acid, so as to have an
excess of 4 or 5 c.c. after neutralising the ammonia; add some clean strips
of lead foil, and boil until the solution has for some time become colourless.
Titrate with standard potassium ferrocyanide (p. 263) without further
dilution, and bearing in mind that at most only one or two c.c. will be
required.

Examination of an ore for "cyanicides."—Place 100 grams of the ore
with 200 c.c. of a cyanide solution of known strength (say .1 or .2 per cent.)
in a bottle and agitate for a definite time, such as one or two days. Filter off
some of the liquor and assay for cyanide, using say 20 c.c. Calculate how
much cyanide has been destroyed in the operation. Evaporate 20 c.c. with
sulphuric or nitric acid and examine for metal. Test another portion for
sulphides, &c.
The student who has mastered the methods of assaying can greatly improve
himself by working out such problems as the above.

PLATINUM.

Platinum occurs in nature in alluvial deposits associated with gold and some
rare metals, generally in fine metallic grains, and, occasionally, in nuggets.
It is a grey metal with a high specific gravity, 21.5 when pure and about
18.0 in native specimens. It is fusible only at the highest temperature, and is
not acted on by acids.
It is dissolved by warm aqua regia, forming a solution of "platinic chloride,"
H2PtCl6. This substance on evaporation remains as a brownish red
deliquescent mass; on drying at 300° C. it is converted into platinous
chloride, PtCl2, and becomes insoluble, and at a higher temperature it is
converted into platinum. All platinum compounds yield the metal in this
way. Platinic chloride combines with other chlorides to form double salts,
of which the ammonic and potassic platino-chlorides are the most
important.
Platinum alone is not soluble in nitric acid; but when alloyed with other
metals which dissolve in this acid it too is dissolved; so that in gold parting,
for example, if platinum was present, some, or perhaps the whole of it
would go into solution with the silver. Such alloys, however, when treated
with hot sulphuric acid leave the platinum in the residue with the gold.
Platinum is detected when in the metallic state by its physical characters
and insolubility in acids. In alloys it may be found by dissolving them in
nitric acid or in aqua regia, evaporating with hydrochloric acid, and treating

the filtrate with ammonic chloride and alcohol. A heavy yellow precipitate
marks its presence.
The assay of bullion, or of an alloy containing platinum, may be made as
follows: Take 0.2 gram of the alloy and an equal weight of fine silver, cupel
with sheet lead, and weigh. The loss in weight, after deducting that of the
silver added, gives the weight of the base metals, copper, lead, &c. Flatten
the button and part by boiling with strong sulphuric acid for several
minutes. When cold, wash, anneal, and weigh. The weight is that of the
platinum and gold. The silver may be got by difference. Re-cupel the metal
thus got with 12 or 15 times its weight of silver, flatten and part the gold
with nitric acid in the usual way (see under Gold), and the platinum will
dissolve. The gold may contain an alloy of osmium and iridium; if so, it
should be weighed and treated with aqua regia. The osmiridium will remain
as an insoluble residue, which can be separated and weighed. Its weight
deducted from that previously ascertained will give the weight of the gold.
When the platinum only is required, the alloy must be dissolved by
prolonged treatment with aqua regia, the solution evaporated to dryness,
and the residue extracted with water. The solution thus obtained is treated
with ammonic chloride in large excess and with some alcohol. A sparingly
soluble[47] yellow ammonic platinum chloride is thrown down, mixed,
perhaps, with the corresponding salts of other metals of the platinum group.
Gold will be in solution. The solution is allowed to stand for some time, and
then the precipitate is filtered off, washed with alcohol, dried, and
transferred (wrapped in the filter paper) to a weighed crucible. It is ignited,
gently at first, as there is danger of volatilising some of the platinum
chloride, and afterwards intensely. With large quantities of platinum the
ignition should be performed in an atmosphere of hydrogen. Cool and
weigh as metallic platinum.

IRIDIUM

Occurs in nature alloyed with osmium as osmiridium or iridosmine, which
is "rather abundant in the auriferous beach sands of Northern California"
(Dana). It occurs in bright metallic scales, which do not alloy with lead, and
are insoluble in aqua regia. Iridium also occurs in most platinum ores, and

forms as much as two per cent. of some commercial platinum. In chemical
properties it resembles platinum, but the ammonic irido-chloride has a dark
red colour, and on ignition leaves metallic iridium, which does not dissolve
in aqua regia diluted with four or five times its volume of water and heated
to a temperature of 40° or 50° C.
The other metals of the platinum group are Palladium, Rhodium, Osmium,
and Ruthenium. They differ from gold, platinum, and iridium by the
insolubility of their sulphides in a solution of sodium sulphide. Palladium is
distinguished by the insolubility of its iodide; and Osmium by the volatility
of its oxide on boiling with nitric acid.

MERCURY.

Mercury occurs native and, occasionally, alloyed with gold or silver in
natural amalgams; but its chief ore is the sulphide, cinnabar. It is
comparatively rare, being mined for only in a few districts. It is chiefly used
in the extraction of gold and silver from their ores (amalgamation); for
silvering mirrors, &c.
Mercury forms two series of salts, mercurous and mercuric, but for the
purposes of the assayer the most important property is the ease with which
it can be reduced to the metallic state from either of these. Mercury itself is
soluble in nitric acid, forming, when the acid is hot and strong, mercuric
nitrate. Cinnabar is soluble only in aqua regia. Mercurous salts are generally
insoluble, and may be converted into mercuric salts by prolonged boiling
with oxidising agents (nitric acid or aqua regia). The salts of mercury are
volatile, and, if heated with a reducing agent or some body capable of fixing
the acid, metallic mercury is given off, which may be condensed and
collected.
Mercury is separated from its solutions by zinc or copper, or it may be
thrown down by stannous chloride, which, when in excess, gives a grey
powder of metallic mercury, or, if dilute, a white crystalline precipitate of
mercurous chloride. Nitric acid solutions of mercury yield the metal on
electrolysis; and, if the pole on which the metal comes down be made of
gold or copper, or is coated with these, the separated mercury will adhere
thereto. It may then be washed and weighed.

The best tests for mercury next to obtaining globules of the metal are: (1) a
black precipitate with sulphuretted hydrogen from acid solutions, which is
insoluble in nitric acid; and (2) a white precipitate with stannous chloride.

DRY METHOD.

Weigh up 5 grams, if the ore is rich, or 10 grams, if a poorer mineral. Take a
piece of combustion tube from 18 inches to 2 feet long, closed at one end,
and place in it some powdered magnesite, so as to fill it to a depth of 2 or 3
inches, and on that a layer of an equal quantity of powdered lime (not
slaked). Mix the weighed sample of ore in a mortar with 10 grams of finely
powdered lime and transfer to the tube; rinse out the mortar with a little
more lime, and add the rinsings. Cover with a layer of six or seven inches
more lime and a loosely fitting plug of asbestos. Draw out the tube before
the blowpipe to the shape shown in fig. 47, avoiding the formation of a
ridge or hollow at the bend which might collect the mercury. Tap gently,
holding the tube nearly horizontal, so as to allow sufficient space above the
mixture for the passage of the gases and vapours which are formed. Place
the tube in a "tube furnace," and, when in position, place a small beaker of
water so that it shall just close the opening of the tube. The point of the tube
should not more than touch the surface of the water. Bring the tube
gradually to a red heat, commencing by heating the lime just behind the
asbestos plug, and travelling slowly backwards. When the portion of the
tube containing the ore has been heated to redness for some time the heat is
carried back to the end of the tube. The magnesite readily gives up carbonic
acid, which fills the tube and sweeps the mercury vapour before it. Some of
the mercury will have dropped into the beaker, and some will remain as
drops adhering to the upper part of the neck. Whilst the tube is still hot cut
off the neck of the tube just in front of the asbestos plug (a drop of water
from the wash bottle will do this), and wash the mercury from the neck into

the beaker. The mercury easily collects into a globule, which must be
transferred, after decanting off the bulk of the water, to a weighed Berlin
crucible. The water is removed from the crucible, first by the help of filter
paper, and then by exposing in a desiccator over sulphuric acid, where it
should be left until its weight remains constant. It should not be warmed.
Example:—5 grams of an ore treated in this way gave 4.265 grams of
mercury, equivalent to 85.3 per cent. Pure cinnabar contains 86.2 per cent.

WET METHODS.

Solution.—Since solutions of chloride of mercury cannot be boiled without
risk of loss,[48] nitric acid solutions should be used wherever possible. No
mercury-containing minerals are insoluble in acids; but cinnabar requires
aqua regia for solution. In dissolving this mineral nitric acid should be used,
with just as much hydrochloric acid as will suffice to take it up.
To separate the mercury, pass sulphuretted hydrogen in considerable excess
through the somewhat dilute solution. The precipitate should be black,
although it comes down at first very light coloured. It is filtered, washed,
and transferred back to the beaker, and then digested with warm ammonic
sulphide. The residue, filtered, washed, and boiled with dilute nitric acid,
will, in the absence of much lead, be pure mercuric sulphide. If much lead
is present, a portion may be precipitated as sulphate, but can be removed by
washing with ammonic acetate. To get the mercury into solution, cover with
nitric acid and a few drops of hydrochloric, and warm till solution is
effected. Dilute with water to 50 or 100 c.c.

GRAVIMETRIC DETERMINATION.

This may be made by electrolysis. The same apparatus as is used for the
electrolytic copper assay may be employed, but instead of a cylinder of
platinum one cut out of sheet copper should be taken, or the platinum one
may be coated with an evenly deposited layer of copper. Fix the spiral and
weighed copper cylinder in position, couple up the battery, and when this
has been done put the nitric acid solution of the mercury in its place.[49] The
student had better refer to the description of the Electrolytic Copper Assay.

The mercury comes down readily, and the precipitation is complete in a few
hours: it is better to leave it overnight to make sure of complete reduction.
Disconnect the apparatus, and wash the cylinder, first with cold water, then
with alcohol. Dry by placing in the water oven for two or three minutes.
Cool and weigh: the increase in weight gives the amount of metallic
mercury.
It must be remembered that copper will precipitate mercury without the aid
of the battery; but in this case copper will go into solution with a
consequent loss in the weight of the cylinder: this must be avoided by
connecting the battery before immersing the electrodes in the assay
solution. The electrolysed solution should be treated with an excess of
ammonia, when a blue coloration will indicate copper, in which case the
electrolysis is unsatisfactory. With a little care this need not happen. Gold
cylinders may preferably be used instead of copper; but on platinum the
deposit of mercury is grey and non-adherent, so that it cannot be washed
and weighed.

VOLUMETRIC METHODS.

Several methods have been devised: for the details of these the student is
referred to Sutton's "Handbook of Volumetric Analysis."

QUESTIONS.

1. The specific gravity of mercury is 13.596. What volume would 8 grams
occupy?
2. If 3.169 grams of cinnabar gave 2.718 grams of mercury, what would be
the percentage of the metal in the ore?
3. Pour solution of mercuric chloride on mercury and explain what happens.
4. On dissolving 0.3 gram of mercury in hot nitric acid, and passing
sulphuretted hydrogen in excess through the diluted solution, what weight
of precipitate will be got?

FOOTNOTES:
[9] Lead may be granulated by heating it to a little above the melting point,
pouring it into a closed wooden box, and rapidly agitating it as it solidifies.
[10] A rod of iron placed in the crucible with the assays will decompose any
regulus that may be formed.
[11] With buttons poor in silver the lowering of the temperature at this stage is
not a matter of importance.
[12] 100 grams of the lead, or of its oxide, will contain from 1.5 to 2.5
milligrams.
[13] Still the precautions of having cupels well made from bone ash in fine
powder, and of working the cupellation at as low a temperature as possible are
very proper ones, provided they are not carried to an absurd excess.
[14] Be careful to remove the crucible before taking the bottle out of the basin of
water; if this is not done the chloride may be washed out of it.
[15] 1 c.c. of this dilute acid will precipitate 8 or 9 milligrams of silver.
[16] Chlorides interfere not merely by removing silver as insoluble silver
chloride, but also by making it difficult to get a good finishing point, owing to
the silver chloride removing the colour from the reddened solution.
[17] These results were obtained when using ammonium sulphocyanate, and
cannot be explained by the presence of such impurities as chlorides, &c.
[18] Multiply the standard by 1000, and dilute 100 c.c. of the standard solution
to the resulting number of c.c. Thus, with a solution of a standard .495, dilute
100 c.c. to 495 c.c., using, of course, distilled water.
[19] HNa2AsO4 + 3AgNO3 = Ag3AsO4 + HNO3 + 2NaNO3.

[20]
SiO2 + Na2CO3 = CO2 + Na2SiO3
SiO2 + 2NaHCO3 = 2CO2 + Na2SiO3 + H2O.

[21] PbO + SiO2 = PbSiO3

[22] Here and elsewhere in this article when a flux is spoken of as soda the
bicarbonate is meant.
[23] See the description of the process commencing on p. 98 and the explanatory
remarks on p. 110.
[24] Percy, Metallurgy of Silver and Gold, p. 258.
[25] "Limits of Accuracy attained in Gold-bullion Assay," Trans. Chem. Soc.,
1893.

[26] "Assaying and Hall-marking at the Chester Assay Office." W.F. Lowe.
Journ. Soc. Chem. Industry, Sept. 1889.
[27] Fine or pure gold is 24 carat. Nine carat gold therefore contains 9 parts of
gold in 24 of the alloy; eighteen carat gold contains 18 parts of gold in 24; and
so on.
[28] The mouth of the flask must not have a rim around it.
[29] See "Assaying and Hall-marking at the Chester Assay Office," by W.F.
Lowe. Journ. Soc. Chem. Industry, Sept. 1889.
[30] Percy, Metallurgy of Silver and Gold, p. 263.
[31] See also "The Assaying of Gold Bullion," by C. Whitehead and T. Ulke.
Eng. and Mining Journal, New York, Feb. 12, 1898.
[32] Consult Percy's Metallurgy of Silver and Gold, p. 172; A.C. Claudet, Trans.
Inst. Mining and Metallurgy, vol. vi. p. 29; G.M. Roberts Trans. Amer. Inst.
Mining Engineers, Buffalo Meeting, 1898; J. and H.S. Pattinson, Journ. Soc.
Chem. Industry, vol. xi. p. 321.
[33] Heycock and Neville, Journ. Chem. Soc., 1892, p. 907.
[34] G.M. Roberts.
[35] A.C. Claudet.
[36] "The Sampling of Argentiferous and Auriferous Copper," by A.R. Ledoux.
Journ. Canadian Mining Institute, 1899.
[37] NaCNO + BaCl2 + NaHO + H2O = NH3 + BaCO3 + 2 NaCl.

[38] HCy + NaHO = NaCy + H2O.

[39] 2KCN + AgNO3 = KAg(CN)2 + KNO3.

[40] If it be desired to make a solution so that 100 c.c. shall be equivalent to 1
gram of sodium cyanide, then 18.085 grams of silver nitrate should be taken for
each litre.
[41] AgNO3 + KAgCy2 = 2 AgCy + KNO3.

[42] AgNO3 + KI = AgI + KNO3.

[43] See pp. 322, 323, and 324 for a description of the methods for measuring
the quantity of acid or alkali.
[44] KCN + HCl = KCl + HCN
[45] Taking 16.0 grams of ore, each c.c. = 1 lb. of soda to the short ton. The
corresponding figures for the long ton are 12.544 grams for lime and 17.92
grams for soda.
[46] In which case each .01 gram of metal found equals 1 lb to the short ton of
solution.

[47] 100 c.c. of water dissolves 0.66 gram of the salt; it is almost insoluble in
alcohol or in solutions of ammonic chloride.
[48] According to Personne mercuric chloride is not volatilised from boiling
solutions when alkaline chlorides are present.
[49] The solution should contain about 0.25 gram of mercury, and a large excess
of nitric acid must be avoided.

CHAPTER X.
COPPER—LEAD—THALLIUM—BISMUTH—ANTIMONY.

COPPER.

Copper occurs native in large quantities, especially in the Lake Superior
district; in this state it is generally pure. More frequently it is found in
combination. The ores of copper may be classed as oxides and sulphides.
The most abundant oxidised ores are the carbonates, malachite and
chessylite; the silicates, as also the red and black oxides, occur less
abundantly. All these yield their copper in solution on boiling with
hydrochloric acid.
The sulphides are more abundant. Copper pyrites (or yellow ore), erubescite
(or purple ore), and chalcocite (or grey ore) are the most important. Iron
pyrites generally carries copper and is frequently associated with the above-
mentioned minerals. These are all attacked by nitric acid. They nearly all
contain a small quantity of organic matter, and frequently considerable
quantities of lead, zinc, silver, gold, arsenic, bismuth, &c.
The copper ores are often concentrated on the mine before being sent into
the market, either by smelting, when the product is a regulus or matte, or by
a wet method of extraction, yielding cement copper or precipitate. A regulus
is a sulphide of copper and iron, carrying from 30 to 40 per cent. of copper.
A precipitate, which is generally in the form of powder, consists mainly of
metallic copper. Either regulus or precipitate may be readily dissolved in
nitric acid.
Copper forms two classes of salts, cuprous and cupric. The former are pale
coloured and of little importance to the assayer. They are easily and
completely converted into cupric by oxidising agents. Cupric compounds
are generally green or blue, and are soluble in ammonia, forming deep blue
solutions.

DRY ASSAY.

That, for copper, next after those for gold and silver, holds a more important
position than any other dry assay. The sale of copper ores has been
regulated almost solely in the past by assays made on the Cornish method.
It is not pretended that this method gives the actual content of copper, but it
gives the purchaser an idea of the quantity and quality of the metal that can
be got by smelting. The process is itself one of smelting on a small scale. As
might be expected, however, the assay produce and the smelting produce
are not the same, there being a smaller loss of copper in the smelting. The
method has worked very well, but when applied to the purchase of low
class ores (from which the whole of the copper is extracted by wet methods)
it is unsatisfactory. The following table, which embodies the results of
several years' experience with copper assays, shows the loss of copper on
ores of varying produce. The figures in the fourth column show how rapidly
the proportion of copper lost increases as the percentage of copper in the
ore falls below 30 per cent. For material with more than 30 per cent. the
proportion lost is in inverse proportion to the copper present.

LOSS OF COPPER.

Copper present. Dry Assay. Margin. Loss on 100 Parts of Copper.
Per cent. Per cent. Per cent.
100 98 2.0 2.0
95 92-1/2 2.5 2.6
90 87-3/8 2.6 2.9
85 82-3/8 2.6 3.0
80 77-3/8 2.6 3.2
75 72-3/8 2.6 3.5
70 67-1/2 2.5 3.6
65 62-1/2 2.5 3.8
60 57-5/8 2.4 4.0
55 52-3/4 2.3 4.2
50 47-3/4 2.2 4.4
45 43 2.0 4.5

40 38-1/8 1.8 4.6
35 33-1/4 1.7 4.8
30 28-1/2 1.50 5.0
25 23-1/2 1.50 6.0
20 18-1/2 1.56 7.8
18 16-1/2 1.53 8.5
16 14-1/2 1.48 9.3
14 12-5/8 1.40 10.0
12 10-5/8 1.37 11.4
10 8-3/4 1.28 12.8
8 6-7/8 1.14 14.3
6 5 1.05 17.5
5 4 1.00 20.0
4 3 1.00 25.0
3.75 2-3/4 0.97 26.0
3.50 2-9/16 0.94 27.0
3.25 2-5/16 0.91 28.0
3.00 2-1/8 0.87 29.0
2.75 1-15/16 0.82 30.0
2.50 1-3/4 0.77 31.0
2.25 1-1/2 0.72 32.0
2.00 1-5/16 0.66 33.0

The wet assay being known, the dry assay can be calculated with the help of
the above table by deducting the amount in the column headed "margin"
opposite the corresponding percentage. For example, if the wet assay gives
a produce of 17.12 per cent., there should be deducted 1.5; the dry assay
would then be 15.62, or, since the fractions are always expressed in eighths,
15-5/8. With impure ores, containing from 25 to 50 per cent. of copper, the
differences may be perhaps 1/4 greater.
Wet methods are gradually replacing the dry assay, and it is probable that in
the future they will supersede it; for stock-taking, and the various
determinations required in smelting works and on mines, they are generally
adopted, because they give the actual copper contents, and since it is
obvious that a knowledge of this is more valuable to the miner and smelter.

Moreover, the working of the dry method has been monopolised by a small
ring of assayers, with the double result of exciting outside jealousy and,
worse still, of retarding the development and improvement of the process.
The principal stages of the dry assay are: (1) the concentration of the copper
in a regulus; (2) the separation of the sulphur by calcining; (3) the reduction
of the copper by fusion; and (4) the refining of the metal obtained.
The whole of these operations are not necessary with all copper material.
Ores are worked through all the stages; with mattes, the preliminary fusion
for regulus is omitted; precipitates are simply fused for coarse copper, and
refined; and blister or bar coppers are refined, or, if very pure, subjected
merely to washing.
The quantity of ore generally taken is 400 grains, and is known as "a full
trial"; but for rich material, containing more than 50 per cent. of copper, "a
half trial," or 200 grains, is used.
Fusion for Regulus.—The ore (either with or without a previous imperfect
roasting to get rid of any excess of sulphur) is mixed with borax, glass,
lime, and fluor spar; and, in some cases, with nitre, or iron pyrites,
according to the quality of the ore. The mixture is placed in a large Cornish
crucible, and heated as uniformly as possible in the wind furnace, gradually
raising the temperature so as to melt down the charge in from 15 to 20
minutes. The crucible is removed and its contents poured into an iron
mould. When the slag is solid, it is taken up with tweezers and quenched in
water. The regulus is easily detached from the slag. It should be convex
above and easily broken, have a reddish brown colour, and contain from 40
to 60 per cent. of copper. A regulus with more than this is "too fine," and
with less "too coarse." A regulus which is too fine is round, compact, hard,
and of a dark bluish grey on the freshly broken surface. A coarse regulus is
flat and coarse grained, and more nearly resembles sulphide of iron in
fracture and colour.
If an assay yields a regulus "too coarse," a fresh determination is made with
more nitre added, or the roasting is carried further. With low class ores a
somewhat coarse regulus is an advantage. If, on the other hand, the regulus
is too fine, less nitre or less roasting is the remedy. With grey copper ores
and the oxidised ores, iron pyrites is added.

Calcining the Regulus.—It is powdered in an iron mortar and transferred
to a small Cornish crucible, or (if the roasting is to be done in the muffle) to
a roasting dish or scorifier. The calcining is carried out at a dull red heat,
which is gradually increased. The charge requires constant stirring at first to
prevent clotting, but towards the end it becomes sandy and requires less
attention. If the temperature during calcination has been too low sulphates
are formed, which are again reduced to sulphides in the subsequent fusion.
To prevent this the roasted regulus is recalcined at a higher temperature,
after being rubbed up with a little anthracite. The roasted substance must
not smell of burning sulphur when hot. It is practically a mixture of the
oxides of copper and iron.
Fusion for Coarse Copper.—The calcined regulus is mixed with a flux
consisting of borax and carbonate of soda, with more or less tartar
according to its weight. Some "assayers" use both tartar and nitre, the
former of course being in excess. The charge is returned to the crucible in
which it was calcined, and is melted down at a high temperature, and, as
soon as tranquil, poured. When solid it is quenched and the button of metal
separated.
The slag is black and glassy. The small quantity of copper which it retains is
recovered by a subsequent "cleaning," together with the slags from the next
operation.
The button of "coarse copper" obtained must be free from a coating of
regulus. It will vary somewhat in appearance according to the nature and
quantity of the impurities.
Refining the Coarse Copper.—The same crucible is put back in the
furnace, deep down and under the crevice between the two bricks. When it
has attained the temperature of the furnace the coarse copper is dropped
into it and the furnace closed. The copper will melt almost at once with a
dull surface, which after a time clears, showing an "eye." Some refining
flux is then shot in from the scoop (fig. 48), and, when the assay is again
fluid, it is poured. When cold the button of metal is separated.

The button of "fine" copper is flat or pitted on its upper surface, and is
coated with a thin orange film; it must have the appearance of good copper.
If it is covered with a red or purple film, it is overdone or "burnt." If, on the
other hand, it has a rough, dull appearance, it is not sufficiently refined.
Assays that have been "burnt" are rejected. Those not sufficiently fine are
treated as "coarse copper," and again put through the refining operation.
Cleaning the Slags.—These are roughly powdered and re-fused with tartar,
etc., as in the fusion for coarse copper. The button of metal got is separated
(if big enough refined) and weighed.
The details of the process are slightly varied by different assayers: the
following will be good practice for the student.
Determination of Copper in Copper Pyrites.—Powder, dry, and weigh up
20 grams of the ore. Mix with 20 grams each of powdered lime and fluor,
15 grams each of powdered glass and borax, and 5 or 10 grams of nitre.
Transfer to a large Cornish crucible and fuse under a loose cover at a high
temperature for from 15 to 20 minutes. When fluid and tranquil pour into a
mould. When the slag has solidified, but whilst still hot, quench by dipping
two or three times in cold water. Avoid leaving it in the water so long that it
does not dry after removal. When cold separate the button, or perhaps
buttons, of regulus by crumbling the slag between the fingers. See that the
slag is free from regulus. It should be light coloured when cold and very
fluid when hot. Reject the slag.
Powder the regulus in a mortar and transfer to a small crucible. Calcine,
with occasional stirring, until no odour of sulphurous oxide can be detected.

Shake back into the mortar, rub up with about 1 gram of powdered
anthracite, and re-calcine for 10 minutes longer.
Mix the calcined regulus with 10 grams of tartar, 20 grams of soda, and 3
grams of borax; and replace in the crucible used for calcining. Fuse at a
bright red heat for 10 or 15 minutes. Pour, when tranquil.
As soon as solid, quench in water, separate the button of copper, and save
the slag.
To refine the copper a very hot fire is wanted, and the fuel should not be too
low down in the furnace. Place the crucible well down in the fire and in the
middle of the furnace. The same crucible is used, or, if a new one is taken, it
must be glazed with a little borax. When the crucible is at a good red heat,
above the fusing point of copper, drop the button of copper into it, and close
the furnace. Watch through the crevice, and, as soon as the button has
melted and appears clear showing an eye, shoot in 10 grams of refining
flux, close the furnace, and, in a few minutes, pour; then separate the button
of copper. Add the slag to that from the coarse copper fusion, and powder.
Mix with 5 grams of tartar, 0.5 gram of powdered charcoal, and 2 grams of
soda. Fuse in the same crucible, and, when tranquil, pour; quench, and pick
out the prills of metal.
If the copper thus got from the slags is coarse looking and large in amount,
it must be refined; but, if small in quantity, it may be taken as four-fifths
copper. The combined results multiplied by five give the percentage of
copper.
The refining flux is made by mixing 3 parts (by measure) of powdered
nitre, 2-1\2 of tartar, and 1 of salt. Put in a large crucible, and stir with a
red-hot iron until action has ceased. This operation should be carried out in
a well-ventilated spot.
For pure ores in which the copper is present, either as metal or oxide, and
free from sulphur, arsenic, &c., the concentration of the copper in a regulus
may be omitted, and the metal obtained in a pure state by a single fusion.[50]
It is necessary to get a fluid neutral slag with the addition of as small an
amount of flux as possible. The fusion should be made at a high
temperature, so as not to occupy more than from 20 to 25 minutes. Thirty

grams of ore is taken for a charge, mixed with 20 grams of cream of tartar,
and 10 grams each of dried borax and soda. If the gangue of the ore is basic,
carrying much oxide of iron or lime, silica is added, in quantity not
exceeding 10 grams. If, on the other hand, the gangue is mainly quartz,
oxide of iron up to 7 grams must be added.
Example.—Twenty grams of copper pyrites, known to contain 27.6 per
cent. of copper, gave by the method first described 5.22 grams of copper,
equalling 26-1/8 per cent. Another sample of 20 grams of the same ore,
calcined, fused with 40 grams of nitre, and washed to ensure the removal of
arsenic and sulphur, and treated according to the second method, gave a
button weighing 5.27 grams, equalling 26-3/8 per cent. The ore contained a
considerable quantity of lead. Lead renders the assay more difficult, since
after calcination it remains as lead sulphate, and in the fusion for coarse
copper reappears as a regulus on the button.
The Estimation of Moisture.—The Cornish dry assayer very seldom
makes a moisture determination. He dries the samples by placing the papers
containing them on the iron plate of the furnace.
It is well known that by buying the copper contents of pyrites by Cornish
assay, burning off the sulphur, and converting the copper into precipitate, a
large excess is obtained.

NOTES ON THE VALUATION OF COPPER ORES.

Closely bound up with the practice of dry copper assaying is that of valuing
a parcel of copper ore. The methods by which the valuation is made have
been described by Mr. Westmoreland,[51] and are briefly as follows:—The
produce of the parcel is settled by two assayers, one acting for the buyer,
the other for the seller; with the help, in case of non-agreement, of a third,
or referee, whose decision is final. The dry assayers who do this are in most
cases helped, and sometimes, perhaps, controlled, by wet assays made for
one or both of the parties in the transaction.
In the case of "ticketing," the parcels are purchased by the smelters by
tender, and the value of any particular parcel is calculated from the average
price paid, as follows:—The "standard," or absolute value of each ton of

fine copper in the ore, is the price the smelters have paid for it, plus the
returning charges or cost of smelting the quantity of ore in which it is
contained. The value of any particular parcel of ore is that of the quantity of
fine copper it contains, calculated on this standard, minus the returning
charges. The ton consists of 21 cwts., and it is assumed that the "settled"
produce is the actual yield of the ore.
If at a ticketing in Cornwall 985 tons of ore containing 63.3 tons of fine
copper (by dry assay) brought £2591 12s., the standard would be £83 15s.
This is calculated as follows:—The returning charge is fixed at 55s. per ton
of ore. This on 985 tons will amount to £2708 15s. Add this to the actual
price paid, and there is got £5300 as the value of the fine copper present.
The weight of copper in these 985 tons being 63.3 tons, the standard is
£5300/63.3, or £83 15s. (nearly).
The value of a parcel of 150 tons of a 6 per cent. ore on the same standard
would be arrived at as follows:—The 150 tons at 6 per cent. would contain
9 tons (150×6/100) of fine copper. This, at £83 15s. per ton, would give
£753 15s. From this must be deducted the returning charges on 150 tons of
ore at 55s. per ton, or £412 10s. This leaves £341 5s. as the value of the
parcel.
At Swansea the returning charge is less than in Cornwall, and varies with
the quality of the ore. This appears equitable, since in smelting there are
some costs which are dependent simply on the number of tons treated, and
others which increase with the richness. The returning charge then is made
up of two parts, one fixed at so much (12s. 2d.) per ton of ore treated, and
the other so much (3s. 9d.) per unit of metal in the ore. In this way the
returning charge on a ton of ore of 8-3/4 produce would be 12s. 2d.+(8-
3/4×3s. 9d.), or £2 5s.
If, for example, Chili bars, containing 96 per cent. of copper, bring £50 per
ton, the standard is £71 9s. 4d. It is got at in this way. The returning charge
on a 96 per cent. ore is 12s. 2d.+(96×3s. 9d.), or £18 12s. 2d. This added to
£50 gives £68 12s. 2d., and this multiplied by 100 and divided by 96 (100
tons of the bars will contain 96 tons of fine copper) will give £71 9s. 4d.
The price of 100 tons of pyrites, containing 2-1/4 per cent. of copper by dry
assay, would be got on this standard as follows:—The parcel of ore would

contain 2-1/4 tons of copper. This multiplied by the standard gives £160
16s. 0d. From this must be deducted the returning charge, which for 1 ton of
ore of this produce would be 12s. 2d. + (2-1/4 × 3s. 9d.) or £1 0s. 7d., and
on the 100 tons is £102 18s. 4d. This would leave £57 17s. 10d. as the price
of the parcel, or 11s. 7d. per ton. This would be on the standard returning
charge of 45s. (for 8-3/4 per cent. ore); if a smaller returning charge was
agreed on, say 38s., the difference in this case, 7s., would be added to the
price per ton.

WET METHODS.

The solubility of the ores of copper in acid has already been described, but
certain furnace products, such as slags, are best opened up by fusion with
fusion mixture and a little nitre.
The method of dissolving varies with the nature of the ore. With 5 grams of
pyrites, a single evaporation with 20 c.c. of nitric acid will give a residue
completely soluble in 30 c.c. of hydrochloric acid. If the ore carries oxide of
iron or similar bodies, these are first dissolved up by boiling with 20 c.c. of
hydrochloric acid, and the residue attacked by an addition of 5 c.c. of nitric.
When silicates decomposable by acid are present, the solution is evaporated
to dryness to render the silica insoluble; the residue extracted with 30 c.c. of
hydrochloric acid, and diluted with water to 150 c.c. It is advisable to have
the copper in solution as chloride. To separate the copper, heat the solution
nearly to boiling (best in a pint flask), and pass a rapid current of
sulphuretted hydrogen for four or five minutes until the precipitate settles
readily and the liquid smells of the gas. When iron is present it will be
reduced to the ferrous state before the copper sulphide begins to separate.
The copper appears as a brown coloration or black precipitate according to
the quantity present. Filter through a coarse filter, wash with hot water
containing sulphuretted hydrogen, if necessary. Wash the precipitate back
into the flask, boil with 10 c.c. of nitric acid, add soda till alkaline, and pass
sulphuretted hydrogen again. Warm and filter, wash and redissolve in nitric
acid, neutralise with ammonia, add ammonic carbonate, boil and filter. The
copper freed from impurities will be in the solution. Acidulate and
reprecipitate with sulphuretted hydrogen. When the nature of the impurities
will allow it, this process may be shortened to first filtering off the gangue,

then precipitating with sulphuretted hydrogen and washing the precipitate
on the filter first with water and then with ammonium sulphide.
Having separated the copper as sulphide, its weight is determined as
follows. Dry and transfer to a weighed porcelain crucible, mix with a little
pure sulphur, and ignite at a red heat for 5 or 10 minutes in a current of
hydrogen. Allow to cool while the hydrogen is still passing. Weigh. The
subsulphide of copper thus obtained contains 79.85 per cent. of copper; it is
a greyish-black crystalline mass, which loses no weight on ignition if air is
excluded.
Copper may be separated from its solutions by means of sodium
hyposulphite. The solution is freed from hydrochloric and nitric acids by
evaporation with sulphuric acid; diluted to about a quarter of a litre; heated
nearly to boiling; and treated with a hot solution of sodium hyposulphite
(added a little at a time) until the precipitate settles and leaves the solution
free from colour. The solution contains suspended sulphur. The precipitate
is easily washed, and under the proper conditions the separation is
complete, but the separation with sulphuretted hydrogen is more
satisfactory, since the conditions as to acidity, &c., need not be so exact.
Zinc or iron is sometimes used for separating copper from its solutions, but
they are not to be recommended.

ELECTROLYTIC ASSAY.

The separation of copper by means of a current of electricity is largely
made use of, and forms the basis of the most satisfactory method for the
determination of this metal. If the wire closing an electric circuit be broken,
and the two ends immersed in a beaker of acidulated water or solution of
any salt, the electricity will pass through the liquid, bringing about some
remarkable changes. Hydrogen and the metals will be liberated around that
part of the wire connected with the zinc end of the battery, and oxygen,
chlorine, and the acid radicals will be set free around the other. Different
metals are deposited in this way with varying degrees of ease, and whether
or not any particular metal will be deposited depends—(1) on the conditions
of the solution as regards acid and other substances present, and (2) on the
intensity of the current of electricity used. For analytical purposes the metal

should be deposited not only free from the other metals present, but also as
a firm coherent film, which may afterwards be manipulated without fear of
loss. This is, in the case of copper and many other metals, effected by a
simple control of the conditions. It is necessary that the electrodes, or wires
which bring the electricity into the solution, should be made of a material to
which the deposited metal will adhere, and which will not be attacked by
substances originally present or set free in the solution. They are generally
made of platinum. There are various arrangements of apparatus used for this
purpose, but the following plan and method of working is simple and
effective, and has been in daily use with very satisfactory results for the last
five or six years.
The battery used is made up of two Daniell cells, coupled up for intensity as
shown in fig. 49—that is, with the copper of one connected with the zinc of
the other. For eight or ten assays daily the quart size should be used, but for
four or five two pint cells will be sufficient.

The outer pot of each cell is made of sheet copper, and must be clean and
free from solder on the inside. It is provided near the top with a perforated
copper shelf in the shape of a ring, into which the inner or porous cell
loosely fits. It is charged with a saturated solution of copper sulphate, and
crystals of this salt must be added, and always kept in excess. When the
battery is at work copper is being deposited on the inner surface of this pot.

The inner or porous pot contains the zinc rod, and is charged with a dilute
acid, made by diluting one volume of sulphuric acid up to ten with water.
The object of the porous pot is to prevent the mixing of the acid and copper
sulphate solutions, without interrupting the flow of electricity. The copper
sulphate solution will last for months, but the acid must be emptied out and
recharged daily.
The zinc rods must be well amalgamated by rubbing with mercury under
dilute acid until they show a uniformly bright surface. They should not
produce a brisk effervescence when placed in the acid in the porous pot
before coupling up.
The battery when working is apt to become dirty from the "creeping" of the
copper and zinc sulphate solution. It must be kept away from the working
bench, and is best kept in a box on the floor.

The connection of the battery with, and the fixing of, the electrodes may be
made by any suitable arrangement, but the following is a very convenient
plan. The wire from the zinc is connected by means of a binding screw with
a piece of stout copper wire, which, at a distance sufficiently great to allow
of easy coupling with the battery, is led along the back of a piece of hard
wood. This is fixed horizontally about one foot above the working bench.
The general arrangement is shown in fig. 50, in which, however, for the
sake of economy of space, the battery is placed on the working bench
instead of on the floor. The piece of wood is one inch square and three or
four feet long. It is perforated from front to back at distances of six inches
by a number of small holes, in which are inserted screws like that shown in
fig. 51. These are known as "terminals," and may be obtained of any
electrician. The head of each screw is soldered to the wire mentioned above

as running along the back and as being connected with the zinc end of the
battery. These terminals serve to fix the electrodes on which the copper is to
be deposited. The wire from the copper end of the battery is similarly
connected by a connecting screw (fig. 52) with another wire (H in fig. 53),
which runs along the top of the rod and has soldered to it, at distances of six
inches, cylindrical spirals of copper wire. These should project from the rod
at points about half-way between the terminals already described. They may
be made by wrapping copper wire around a black-lead pencil for a length of
about three inches.

The rod is perforated from top to bottom with a series of small holes, one in
advance of each terminal but as near it as possible. Into these short pieces of
glass tube are inserted to ensure insulation. These receive the other
electrodes, which are connected with the wire leading to the copper end of
the battery, through the spirals, with the help of a binding screw. The figure
will make this clear. (Fig. 53.)

The electrodes consist of a platinum spiral and cylinder. The spiral should
have the shape shown in A, fig. 54. When in work it is passed through one
of the holes fitted with glass tubes and connected with the copper end of the
battery. The thickness of the wire of which it is made is unimportant,
provided it is stout enough to keep its form and does not easily bend. The
spiral will weigh about 8 grams. The cylinder (C, fig. 54) will weigh about
12 grams. It should have the shape shown in the figure. In working it is
clamped to one of the terminals, and on it the copper is deposited. A
cylinder will serve for the deposition of from 1 to 1.5 gram of copper. It is
made by rivetting a square piece of foil on to a stiff piece of wire, and then
bending into shape over a glass tube or piece of rounded wood. Each
cylinder carries a distinctive number, and is marked by impressing Roman
numerals on the foil with the blade of a knife. The weight of each is
carefully taken and recorded. They lose slightly in weight when in use, but
the loss is uniform, and averages half a milligram per month when in daily
use. The cylinders are cleaned from deposited copper by dissolving off with
nitric acid and washing with water; and from grease by igniting.

The beakers, to contain the solution of copper to be electrolysed, are
ordinary tall beakers of about 200 c.c. capacity, and are marked off at 100
c.c. and 150 c.c. They are supported on movable stands, consisting of
wooden blocks about six inches high and three inches across. The bar of
wood which carries the connecting wires and electrodes is permanently
fixed over the working bench, at such a height that, with the beakers resting
on these blocks, the electrodes shall be in position for working.
To fix the electrodes to the rod, remove the stand and beaker and pass the
long limb of the spiral up through one of the glass tubes. Connect it with the
free end of the copper spiral by means of a connecting screw (fig. 52), and
then draw out and bend the copper spiral so that the platinum one may hang
freely. Screw the wire of the cylinder to the terminal, and, if necessary, bend
it so that the cylinder itself may be brought to encircle the rod of the spiral
in the manner shown in fig. 53.
The general method of working is as follows:—The quantity of ore to be
taken for an assay varies with the richness of the ore, as is shown in the
following table:—

Percentage of Copper in the Ore. Quantity of Ore to be taken.
1 to 5 5 grams
5 to 10 3 "
10 to 30 2 "
30 to 50 1.5 "
50 to 100 1 "

The weighed quantity of ore is dissolved by evaporating with nitric acid and
taking up with hydrochloric, as already described. Any coloured residue
which may be left is generally organic matter: it is filtered off, calcined, and
any copper it contains is estimated colorimetrically. Nearly always,
however, the residue is white and sandy. The copper is separated from the
solution as sulphide by means of a rapid current of sulphuretted hydrogen.
The liquid is decanted off through a filter, the precipitate washed once with
hot water and then rinsed back into the flask (the filter paper being opened
out) with a jet of water from a wash bottle. Fifteen c.c. of nitric acid are
added to the contents of the flask, which are then briskly boiled until the
bulk is reduced to less than 10 c.c. The boiling down is carried out in a

cupboard free from cold draughts, so as to prevent the condensation of acid
and steam in the neck of the flask. Twenty c.c. of water are next added, and
the solution is warmed, and filtered into one of the beakers for electrolysis.
The filtrate and washings are diluted with water to the 100 c.c. mark, and
the solution is then ready for the battery. It must not contain more than 10
per cent. by volume of nitric acid.
The number and weight of the platinum cylinder having been recorded,
both electrodes are fixed in position and the wooden block removed from
under them. The beaker containing the copper solution is then brought up
into its place with one hand, and the block replaced with the other so as to
support it. All the assays having been got into position, the connecting wires
are joined to the battery. If everything is right bubbles of oxygen at once
stream off from the spiral, and the cylinder becomes tarnished by a deposit
of copper. If the oxygen comes off but no copper is deposited, it is because
the assay solution contains too much nitric acid. If no action whatever takes
place, it is because the current is not passing. In this case examine the
connections to see that they are clean and secure, and the connecting wires
to see that they are not touching each other.
The action is allowed to go on for sixteen or seventeen hours, so that it is
best to let the current act overnight. In the morning the solutions will appear
colourless, and a slow stream of oxygen will still be coming off from the
spiral.
A wash-bottle with cold distilled water and two beakers, one with distilled
water and the other with alcohol, are got ready. The block is then removed,
the spiral loosened and lowered with the beaker. The cylinder is next
detached and washed with a stream of water from the wash-bottle, the
washings being added to the original solution. The current from the battery
is not stopped until all the cylinders are washed. After being dipped in the
beaker of water and once or twice in that with the alcohol, it is dried in the
water-oven for about three minutes, and then weighed. The increase in
weight is due to deposited copper. This should be salmon-red in colour,
satin-like or crystalline in appearance, and in an even coherent deposit, not
removed by rubbing. It is permanent in air when dry, but sulphuretted
hydrogen quickly tarnishes it, producing coloured films. With ores
containing even very small proportions of bismuth, the deposited copper

has a dark grey colour, and when much of this metal is present the copper is
coated with a grey shaggy deposit.
It still remains to determine any copper left undeposited in the solution.
This does not generally exceed four or five milligrams, and is estimated
colorimetrically. Thirty c.c. of dilute ammonia (one of strong ammonia
mixed with one of water) are added to the electrolysed solution, which is
then diluted up to the 150 c.c. mark with water. It is mixed, using the spiral
as stirrer, and, after standing a few minutes to allow the precipitate to settle,
100 c.c. of it are filtered off through a dry filter for the colorimetric
determination. Since only two-thirds of the solution are taken for this, the
quantity of copper found must be increased by one-half to get the quantity
actually present.

The colorimetric determination may be made in the manner described
under that head, but where a number of assays are being carried out it is
more convenient to have a series of standard phials containing known
amounts of copper in ammoniacal solution. By comparing the measured
volume of the assay solution with these, the amount of copper present is

determined at a glance. These standard bottles, however, can only be
economically used where a large number of assays are being made daily.
A convenient plan is to get a quantity of white glass four-ounce phials, like
that in fig. 55, and to label them so that they shall contain 100 c.c. when
filled up to the bottom of the labels. The labels should be rendered
permanent by coating with wax, and be marked with numbers indicating the
milligrams of copper present. The bottles are stopped with new clean corks,
and contain, in addition to the specified quantity of copper, 6 c.c. of nitric
acid and 10 c.c. of strong ammonia, with sufficient water to make up the
bulk to 100 c.c. The copper is best added by running in the requisite amount
of a standard solution of copper, each c.c. of which contains 0.001 gram of
the metal.
The standard bottles should be refilled once every three or four months,
since their colorimetric value becomes slowly less on keeping. The
following determinations of a set which had been in use for three months
will illustrate this. The figures indicate milligrams of copper in 100 c.c.: the
first row gives the nominal and the second row the actual colorimetric value
of the standards. The difference between the two shows the deterioration.

1 2 3 4 6 8 10 12 14
1 2 3 3.7 5.5 7.5 9 11 13

The amount of copper in the assay is got by increasing that found
colorimetrically by one-half and adding to that found on the platinum
cylinder. The percentage is calculated in the usual way. The following
examples will illustrate this, as well as the method of recording the work in
the laboratory book:—

_____________________________________________
Cylinder I. + Cu 9.5410
Cylinder I. 9.5170
——————
0.0240
By colour 100 c.c. = 0.0015}
0.0007} 0.0022
—————— ——————
0.0022 0.0262
IX. Sample. Took 5 grams.

Copper = 0.52%
_____________________________________________
Cylinder VI. + Cu 10.5705
Cylinder VI. 10.0437
——————
0.5268
By colour, 100 c.c. = 0.0070}
0.0035} 0.0105
—————— ——————
0.0105 0.5373
Matte, No. 1070. Took 1.5 gram.
Copper = 35.82%
_____________________________________________
Cylinder XIII. + Cu 12.0352
Cylinder XIII. 11.0405
——————
0.9947
By colour 100 c.c. = 0.0005}
0.0002} 0.0007
—————— ——————
0.0007 0.9954
X. Sample, Cake copper. Took 1.0053 gram.
Copper = 99.00%
____________________________________________

In the electrolytic assay of metals, alloys, precipitates, and other bodies rich
in copper, the preliminary separation of the copper by sulphuretted
hydrogen is unnecessary. It is sufficient to dissolve the weighed sample in
10 c.c. of nitric acid, boil off nitrous fumes, dilute to 100 c.c. with water,
and then electrolyse.
General Considerations.—In the preliminary work with the copper
sulphide there is a small loss owing to its imperfect removal in washing the
filter paper, and another small loss in dissolving in nitric acid owing to the
retention of particles in the fused globules of sulphur. To determine its
amount the filter-papers and sulphur were collected from forty assays, and
the copper in them determined. The average amount of copper in each assay
was 0.175 gram; that left on the filter paper was 0.00067 gram; and that
retained by the sulphur 0.00003 gram; thus showing an average loss from
both sources of 0.00070 gram. The determinations from another lot of forty-
two similar assays gave on an average

Copper left on filter paper 0.00070gram

Copper retained by sulphur. 0.00004"

The loss from these sources is trifling, and need only be considered when
great accuracy is required.
The deposition of the copper under the conditions given is satisfactory, but,
as already stated, if the solution contain more than 10 per cent. of nitric acid
it is not thrown down at all; or if a stronger current is used, say that from
three Bunsen cells, it will be precipitated in an arborescent brittle form, ill
adapted for weighing. It may be noted here that increasing the size of the
cells does not necessarily increase the intensity of the current.
In two determinations on pure electrotype copper the following results were
obtained:—

Copper Taken. Copper Found.
0.8988 gram 0.8985 gram
0.8305 " 0.8303 "

The presence of salts of ammonia, &c., somewhat retards the deposition,
but has no other ill effect.
The organic matter generally present in copper ores interferes more
especially in the colorimetric determination of the residual copper. It can be
detected on dissolving the ore as a light black residue insoluble in nitric
acid. It is filtered off at once, or, if only present in small amount, it is
carried on in the ordinary process of the assay and separated in the last
filtration before electrolysis.
The following experiments were made to test the effect of the presence of
salts of foreign metals in the solution during the precipitation of copper by
electrolysis:—

Copper Taken. Other Metal Added. Copper Found.
0.1000 gram 0.1000 gram of silver 0.1800
0.1050 " 0.1000 " " 0.2000
0.1030 " 0.1000 " mercury 0.2010
0.1037 " 0.1000 " " 0.2015

0.1020 " 0.1000 " lead 0.1020
0.1030 " 0.1000 " " 0.1028
0.1010 " 0.1000 " arsenic 0.1010
0.1007 " 0.1000 " " 0.1022
0.1030 " 0.1000 " antimony 0.1050
0.1034 " 0.1000 " " 0.1057
0.0990 " 0.1200 " tin 0.0990
0.1014 " 0.1000 " " 0.1015
0.1000 " 0.1000 " bismuth 0.1662
0.1040 " 0.1000 " of cadmium 0.1052
0.1009 " 0.1300 " zinc 0.1017
0.1014 " 0.1000 " nickel 0.1007
0.1079 " 0.1200 " iron 0.1089
0.1054 " 0.1000 " chromium (Cr2O3) 0.1035
0.1034 " 0.1000 " " (K2CrO4) 0.1010
0.1075 " 0.1000 " aluminium 0.1078
0.1010 " 0.1000 " manganese 0.0980

It will be seen from these that mercury, silver, and bismuth are the only
metals which are precipitable[52] along with the copper under the conditions
of the assay. Mercury, which if present would interfere, is separated because
of the insolubility of its sulphide in nitric acid.
Bismuth is precipitated only after the main portion of the copper is thrown
down. It renders the copper obviously unsuitable for weighing. It darkens,
or forms a greyish coating on, the copper; and this darkening is a delicate
test for bismuth. In assaying ores containing about three and a half per cent.
of copper, and known to contain bismuth in quantities scarcely detectable in
ordinary analysis, the metal deposited was distinctly greyish in colour, and
would not be mistaken for pure copper. Ten grams of this impure copper
were collected and analysed, with the following results:—

Copper 99.46per cent.
Bismuth 00.30 "
Iron 00.14 "

Arsenic 00.10 "
———
100.00

The quantity of copper got in each assay was 0.175 gram, and consequently
the bismuth averaged 0.00053 gram.
To separate the bismuth in such a case the deposit is dissolved off by
warming it in the original solution. The bismuth is precipitated by the
addition of ammonic carbonate, and the solution, after filtering and
acidifying with nitric acid, is re-electrolysed.
Determination of Copper in Commercial Copper.—Take from 1 to 1.5
gram, weigh carefully, and transfer to a beaker; add 20 c.c. of water and 10
c.c. of nitric acid; cover with a clock glass, and allow to dissolve with
moderate action; boil off nitrous fumes, dilute to 100 c.c., and electrolyse.
The cylinder must be carefully weighed, and the electrolysis allowed to
proceed for 24 hours. The weight found will be that of the copper and silver.
The silver in it must be determined[53] and deducted.
Determination of Copper in Brass, German Silver, or Bronze.—Treat in
the same manner as commercial copper. If nickel is present, the few
milligrams of copper remaining in the electrolysed solution should be
separated with sulphuretted hydrogen, the precipitated sulphide dissolved in
nitric acid, and determined colorimetrically.

VOLUMETRIC PROCESSES.

There are two of these in use, one based on the decolorising effect of
potassic cyanide upon an ammoniacal copper solution, and the other upon
the measurement of the quantity of iodine liberated from potassic iodide by
the copper salt. The cyanide process is the more generally used, and when
carefully worked, "on certain understood and orthodox conditions," yields
good results; but probably there is no method of assaying where a slight
deviation from these conditions so surely leads to error. An operator has no
difficulty in getting concordant results with duplicate assays; yet different
assayers, working, without bias, on the same material, get results uniformly
higher or lower; a difference evidently due to variations in the mode of

working. Where a large number of results are wanted quickly it is a very
convenient method. The iodide process is very satisfactory when worked
under the proper conditions.

CYANIDE METHOD.

The process is based upon the facts—(1) that when ammonia is added in
excess to a solution containing cupric salts, ammoniacal copper compounds
are formed which give to the solution a deep blue colour; and (2) that when
potassic cyanide is added in sufficient quantity to such a solution the colour
is removed, double cyanides of copper and potassium or ammonium being
formed.[54] In the explanation generally given the formation of cuprous
cyanide is supposed[55]; but in practice it is found that one part of copper
requires rather more than four parts of cyanide, which agrees with the
former, rather than the latter, explanation.
Reliance on the accuracy of the process cannot rest upon the supposition
that the cyanide required for decoloration is proportional to the copper
present, for varying quantities of ammonia salts, ammonia and water, and
differences of temperature have an important effect. The results are
concordant and exact only when the cyanide is standardised under the same
conditions as it is used. It is best to have the assay solution and that used for
standardising as nearly as possible alike, and to titrate the two solutions side
by side. This demands an approximate knowledge of the quantity of copper
contained in the ore and a separation of the bulk of the impurities.
For the titration there is required a standard solution of potassium cyanide
made by dissolving 42 grams of the salt, known to dealers as Potassium
Cyanide (Gold), in water and diluting to one litre: 100 c.c. of this will be
about equivalent to one gram of copper. For poor ores the solution may
conveniently be made half this strength.
The solution of the ore and the separation of the copper as sulphide are
effected in the same ways as have been already described for electrolysis.
Similarly, too, the sulphide is attacked with 15 c.c. of nitric acid and the
assay boiled down to 10 c.c. Add 20 c.c. of water and warm, filter into a
pint flask, wash well with water, and dilute to about 150 c.c.; add 30 c.c. of
dilute ammonia, and cool.