CHAPTER I._A text-book of assaying

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A TEXT-BOOK OF ASSAYING:

FOR THE USE OF THOSE CONNECTED WITH MINES.

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.

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

CHAPTER III.

WET GRAVIMETRIC METHODS.

Wet gravimetric methods 27

(a) Solution 29

(b) Precipitation 30

CHAPTER IV.

VOLUMETRIC ASSAYS.

Titrometric assays 35

(a) Standard solutions 36

(b) Standardising 37

Colorimetric assays 44

Gasometric assays 44

CHAPTER V.

WEIGHING AND MEASURING.

Weighing 47

Measuring liquids 49

(a) Graduated flasks 49

(b) Pipettes 50

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.

[Illustration: FIG. 1.

CONE

PARTLY REDUCED CONE

PLAN OF FRUSTRUM DIVIDED.

ELEVATION OF FRUSTRUM DIVIDED.]

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.

[Illustration: FIG. 2.]

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%.~

[Illustration: FIG. 3.]

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.

+----------+----------+--------------------------+----------------+

+----------+----------+--------------------------+----------------+

| Feb. 1 | 482 | Tough Copper | For Arsenic. |

| " 2 | X | Piece of Metal | For Ni and Cu. |

| " | 483 | Tough Copper. | |

| " | 73 | Silver Precipitate, | 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 As_{2}O_{3} = 23.3 c.c. Uranium.

.'. 100 cc. Uranium = 0.5 gram As.

______________________________________________________________

10071 5 grams

Tin Ore Cruc. and SnO_{2} 9.6065 grms.

Feb. 3/89 Cruc. and Ash 9.4235 "

------

SnO_{2} = 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.

Key to following example page of Assay book:

DR = DATE REPORTED.

Not Det. = Not detected

EXAMPLE OF PAGE OF ASSAY BOOK.

-------------------------------------------+----+-------+---------------+----

DESCRIPTION OF SAMPLE. | | Water | Assay on |

------+--------------------+---------------| |Lost at| the Dry |

Date. | Material. | Weight. |No. |100° C.|Material. | DR

------+--------------------+---+---+---+---+----+-------+---------------+----

1889 | |ton|cwt|qrs|lbs| | | |

Feb. 1|Tough cake copper | | | | | 482| |Arsenic, 0.52% | 7

" |Tough cake copper | | | | |2082| |Arsenic, 0.63% | 7

" |Tough cake copper | | | | | 491| |Arsenic, 0.68% | 7

| | | | | | | | |

Feb. 2|Nickel disc for C.R.| | | | | X | |Copper, 73.75 | 7

| | | | | | | | ----- |

| | | | | | | | 100.27 |

| | | | | | | | ------ |

" |Silver precipitate, | | 24| 1| 0| 73| Not | |

| 4 casks | | | | | | det. |Silver, 4.851 | 10

" |Purple ore |200| | | | 494| Not |Copper, 0.13% | 11

| | | | | | | det. |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 Amount of ore, &c. to

to be determined. be weighed.

100-10 1 gram.

10-5 2 grams.

5-1 5 "

1-0.1 10 "

0.1-0.01 20 "

[Illustration: 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.

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.

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.

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.

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.

7. Calculate in grains per gallon:--

(a) 0.51 gram per litre.

(b) 24.6 parts per 100,000.

8. Convert into ozs. (troy) per ton:--

(a) 7 loths per centner.

(b) 30 grams per quintal.

FOOTNOTES:

[1] Ether or carbon bisulphide.

[2] Such substances are best dried by pressing between folds of dry

filter-paper.