Bulletin No. 253 Series E, Chemistry and Physics, 44 

DEPARTMENT OF THE INTERIOR

UNITED STATES GEOLOGICAL SURVEY

CHARLES D. WALCOTT, DIRECTOR

COMPARISON

A WET AND CRUCIB1E-FIRE METHODS

FOB THM

ASSAY OF GOLD TELLURIDE ORES

WITH NOTES ON THE ERRORS OCCURRING IN THE OPERATIONS OF 

FIRE ASSAY AND PARTING

W. F. HILLEBRAND and E. T. ALLEN

WASHINGTON

GOVERNMENT PRINTING OFFICE

1905

CONTENTS.

Pago.

Letter of transmittal. ..................................................... 5

Introduction. ___ .__._..._..._.._._...-......-- _.__.-.-.._...._.._._.-_..... 7

Part I. Assay of Cripple Creek telluride ores -...I........................ 7

Nature of the ores assayed ......-.-..-...- ... ......................... 7

Preparation of the samples. ---...-....---...............--........... 9

Combination wet and fire assay ......-.-....--..-..-...-------..--.. 9

Fire assay ..............................^............................... 12

Conduct of the assay......-.--....---..:..-..........-........... 12

Charges used in assaying slags and cupels ..............^.,....-... 12

Assays for gold and silver together............................... 12

Discussion of results........-...-.....--........-..-----..... 14

Assays for gold alone .........--.....-.--...-.....--.,..-........ 15

Comparison of results .......................... ^...................... 18

Part II. The errors in gold assaying..---..--.-..--.--.--..-.-.--..-_..... 19

Cupellation losses..--...-.......-..---.-...--.-....................... 19

Absorption and volatilization .-....-......:.........-.-.......... 19

Retention of lead by cupelled beads .............................. 23

Losses of gold in slags ............................................... 24

Errors in parting :................................................... 24

Strength of acid ..................--.......--..-.--..----......... 24

Incomplete extraction of silver................................... 25

Impurities in acid ....^................................'.......... 25

Action of nitrous acid ............................................ 26

Action of nitric acid -.......-.-_-...-............-.-............. 27

Summary of results.....--.....--...-.......---.--........-........--.... 28

Index -.-..--...._.......................-.-........-.................... 31

. 3 '

LETTER OF TRANSMITTAL.

DEPARTMENT OF THE INTERIOR, 

UNITED STATES GEOLOGICAL SURVEY,

Washington, D. C. , November 1, 1904.

SIR: I transmit herewith, for publicatioiras a bulletin, a paper by 

Messrs. W. F.. Hillebrand and E. T. Alien, in which the subject of 

inquiry is the accuracy of the crucible assay method for telluride 

gold ores.

The question is a very important one in the very important telluride gold fields, and it is satisfactory to be assured that the doubts 

which have been entertained as to the accuracy of the dry method 

are not well founded. x

While this main point is definitely established, the investigation as 

a whole is less complete than most of those which are issued from the 

Survey laboratories, and several problems suggested by the experiments are not fully worked out. This is due to the unfortunate fact 

that the quarters occupied by the Survey have become so crowded as 

to make it necessary, in the interest of other divisions, to abandon 

assaying altogether and to dismount the assay furnaces before this 

research had been completed.

Very respectfully, GEORGE F. BECKER,

Geologist in Charge Division of

Chemical and Physical Research. 

Hon. CHARLES D. WALCOTT,

Director United States Geological Survey.

5
rL
COMPARISON OF A WET AND CRUCIBLE-FIRE METHODS FOR 

THE ASSAY OF GOLD TELLURIDE ORES, WITH NOTES ON 

THE ERRORS OCCURRING IN THE OPERATIONS OF FIRE 

ASSAY AND PARTING.

By W. F. HILLEBRAND and E. T. ALLEN.

INTRODUCTION.

The SCOrification assay for telluride ores has long been believed, 

and apparently proved, to give low results by reason of volatilization,

and it is now seldom used for ores of that class. The assay by crucible is supposed to be far more reliable, and Furman a maintains that 

it is entirely so when properly carried out. He gives a very few comparative determinations by fire and wet assay in proof of his assertion, but the tests are too few in number and the agreement is not so 

close as might be desired. A more critical investigation of this problem seemed to promise results of some value to those engaged in 

mining and smelting the telluride ores which are so important in 

certain parts of our own and other countries, and the data and conclusions set forth in the following pages are the outcome of experiments on telluride ores from Cripple Creek, Oolo. From them it 

will be seen that there is no occasion to question the substantial 

accuracy of the crucible method, as tested by careful determinations 

in the wet way. Incidentally the question of losses of gold in assaying, particularly during cupellation and parting, has been looked 

into, for concerning these points the most contradictory statements 

are to be found in the literature. Only after most of the work upon 

the ores had been completed was this fully impressed upon us and the 

extent of some of these losses duly appreciated. Had this appreciation come earlier we might have been able to offer still more satisfactory results for our assays.

PART I. ASSAY OF CRIPPLE CREEK TELLURIDE ORES.

NATURE OF THE ORES ASSAYED.

Efforts were made to procure two ores of somewhat widely differing 

gold content and, with a view to the preparation of a homogeneous 

sample, containing as little free gold as possible. As received,

a A Manual of Practical Assaying, 1899, p. 399.
METHODS OF ASSAY OF GOLD TELLURIDE ORES. [BOLL. 253.

through the kindness of Mr. W. M. Bainbridge, of the El Paso Consolidated Gold Mining Company, and of Dr. Waldemar Lindgren, of 

the Geological Survey, the samples were marked 6 and 12 ounces per 

ton respectively, but they were actually much richer and of nearly 

equal value, approximately 15 and 19 ounces to the ton. At least 10 

pounds of each was available. The ores were from a narrow fissure 

in granite and were themselves practically granite, carrying pyrite in 

some quantity, the higher grade sample Z> more than a, some calcite; 

also traces of copper and molybdenum, as shown by determinations 

on 100 grains. Other constituents were not looked for. The tellurium also was quantitatively determined, in 50-gram portions, in 

order to judge of the composition of the telluride mineral. As given 

below, silver is, no doubt, a trifle low, because of the impossibility of 

making exact allowance for loss in assaying.

TABLE I. Amounts of certain constituents of the ores.

Te. ................... ..........

An....... .................... ......

Ag.. ........... ....................

On. .......................... ------

Mo-....--.--..-......-.----..-.---.

a.

Per cent. ' 

0. 0742

.0506

.0075

.0059

.0015

b.

Per cent. 

0.092

.060

.0103

.0070

.0018

Assuming none of the gold and silver to be free, but all in combination with tellurium, the following percentage compositions for the 

telluride have been calculated from the foregoing table:

Calculated composition of the tellurides.

Te. ..........

An... ........

A0.

a

Per cent. 

56. 09

38.24

5.67

100.00

Atomic 

ratio.

1.78

| 1.00

b

Per cent. 

56.68

( 36. 97

1 6.35

100. 00

Atomic 

ratio.

1.80

} 1.00

If the tellurium has been correctly determined, the above results 

indicate the probable presence of free gold, for from what we know 

of the occurrence of gold-silver tellurides at Cripple Creek they probably all conform to the general formula MTe2 . That the telluride 

here present must then be largely if not wholly sylvanite, assuming 

absence of other silver minerals, is indicated by the amount of silver
HILLEBRAND"] 

AND ALLBN.J PREPABATION OF THE SAMPLES. 9

shown by the analyses, which is considerably in excess of that in 

even the richest known calaverite. It is not impossible that both 

calaverite and sylvanite as well as free gold are present. From the 

ratio of gold to silver in a second pair of samples, a± and 61} which, as 

mentioned later, were taken from a lower position in the original 

sample bags, so that a considerable mechanical sorting had ensued 

since the bags were filled, the suspicion may be justified that the 

silver is in part a constituent of some mineral other than a telluride. 

For if the assays are correct there is a diminution in the silver as 

the gold increases, as shown by the following comparison:

J\g.. \& .........

ATL.. .........

a.

Ounces.

2.19

14.53

a,.

Ounces.

2.07

15.60

b.

Ounces,

3.00

17.75

fti.

Ounces.

2.87

19.63

That not all the silver may thus be assigned elsewhere is, however, 

likely from the fact that the ratio of tellurium to gold alone is much 

in excess of 2 to 1. But too much weight must not be attached to 

the above values for silver, and therefore speculations of this sort are 

of little importance.

PREPARATION OF THE SAMPLES.

Considerable portions from the tops of the bags containing the 

coarsely crushed samples received were ground fine enough to. pass a 

sieve of 100 meshes to the linear inch and mixed for what was considered a sufficient time to insure homogeneity. The amounts thus 

prepared proved inadequate for the completion of the work, so other 

portions were ground fine enough to pass a sieve of 150 meshes and 

mixed more intimately even than in the first instance, this being done 

because the first results on sample fr were somewhat less satisfactory 

and less accordant than those on a, a condition which it was hoped finer 

grinding would remedy. These second samples were found to be 

markedly richer in gold than the first, due doubtless to their having 

been taken from a greater depth in the original bags. They represented practically new ores, and, to simplify references, will be designated at and blt

COMBINATION WET AND FIRE ASSAY.

Throughout the literature it is prescribed that for wet assay of 

gold ores they should be subjected to a preliminary roasting for the 

removal of sulphur chiefly. Since, however, it is pretty certain that 

there is loss of gold by volatilization in the roasting of telluride 

ores, it seemed imperative to disregard these directions and to treat 

Bull. 253 05 2
10 METHODS OF -.ASSAY OF GOLD TELLURIDE OKES. [BULL. 253.

chemically the unroasted ore. No attempts were made to test different methods of attack and subsequent treatment because of lack of 

time. The method employed in all cases was the following:

Fifty-gram portions in duplicate were mixed with water in large 

porcelain basins, and strong nitric acid was added by degrees with 

constant stirring till action had nearly ceased. Then bromine water 

was added in excess to oxidize the gold and sulphur and precipitate 

the silver. No artificial heat was employed. The following morning 

the insoluble matter was collected on filters and washed ten times 

with water containing bromine and some hydrochloric acid, then two 

or three times with pure water.

It was found impossible to extract the whole of the gold by wet 

treatment. The residues always yielded by fire assay small amounts 

of gold, usually under 0.1 mg. when properly washed, besides nearly 

all the silver.

The combined filtrates were evaporated in the original porcelain 

basins to near dryness, then treated with hydrochloric acid and covered for a time. When action ceased the covers were removed and 

evaporation continued, hydrochloric acid being added two or three 

times more at proper intervals. When the residues were now digested with dilute hydrochloric acid and a little bromine water, to 

insure solution of the gold, there remained undissolved considerable 

calcium sulphate, which was collected on a suitable-sized filter and 

the filtrate received in a beaker. The first calcium sulphate residues 

thus obtained were found by fire assay to be free from gold, but all 

those obtained in subsequent tests were added to and assayed with 

the main residues.

The filtrates contained all the iron in an oxidized form and a slight 

excess of bromine. Without attempting to reduce the ferric iron to 

ferrous by sulphurous acid, whereby tellurium would have been in 

part precipitated as well as the gold, a large excess of filtered ferrous 

sulphate solution was added, which induced complete precipitation of 

the gold alone, without any tellurium. The completeness of precipitation was proved by the failure to detect any gold in the subsequently precipitated tellurium.

After twenty-four hours the precipitated gold was collected on a 

9 cm. paper filter of 0.00005 gm. ash content, carefully washed with 

cold water, then with warm dilute hydrochloric acid to extract iron 

from the paper, and finally with hot water. Paper and contents were 

burned in a clean porcelain crucible, and the contents most carefully 

transferred to the pan of the assay balance and weighed as nearly as 

might be to hundredths of a milligram. As a check the gold was then 

generally cupelled with 3 or 4 grams of pure lead in order to eliminate 

the error introduced by paper ash and any possible failure to wash 

the filter thoroughly. A companion cupellation with a similar amount 

of proof gold afforded the correction for gold loss during cupellation.
COMBINATION WET AND FIKE ASSAY. 11

A number of the beads thus obtained from the ore were collectively 

tested most carefully for silver, with negative results.

It was said above that it was not possible to extract quite all of the 

gold from the ore, and that the residues insoluble in acid were treated 

by fire assay in order to recover the amount retained. The retention 

was probably owing to particles of telluride inclosed in gangue matter, or of gold in sulphur which had been separated but not fully 

oxidized by the treatment with nitric acid and bromine. These residues were assayed in crucibles with the following charge: Litharge, 2 

A. T.; soda, 1 A. T.; borax, 10 grams; argol, 2 grams; salt, cover. The 

lead button thus obtained weighed about 20 grams and yielded on 

cupellation nearly all the silver of the ore and a small amount of gold, 

which latter was obtained pure by direct parting. It was not deemed 

necessary to examine slags or cupels for their possible gold contents, 

owing to the minute amount in question, nor was any attempt made to 

determine by the wet way the exact amount of silver that the Ore held.

TABLE, II. Combination wet and fire assays for gold in Cripple Creek ores.

[50 grains in each case.]

Sample.

a -.:.--.--......

a^... ............

b ...............

b,.. .............

Residues.

Milligrams.

0.08 

.04 

.08 

.06

f .:«

.05 

I .12

( .«

1 .07 

i .12

f .98 

| .37 

.10 

.11

By FeSO4.

Milligrams.

25.29 

25.22 

25.73 

25.63

26.48 

26.32 

26.69 

26.89

29.56. 

30.84 

30.50

32.54 

33.13 

33.10 

33.85

Total.

Milligrams. 

25.37 

25.26 

25.85 

25.69

26.60

26.48 

26.74 

. 27.01

29.68 

30.91 

30.62

33.52 

33.50 

33.20 

33.96

Oz. per ton. 

14.80 

14.73 

. 15.08 

14.98

15. 52 

15.45 

15.60 

15. 76

17.32 

18.03 

17.86

19.55 

19.54

\ 619.58

After cupellation.

Milligrams.

25.22 

 «25.42 

«25.30

26.51 

26.45 

26. 59 

26.73

29.45 

30.57 

30.17

33.35 

33.32 

f 33. 07 

I 33. 76

Oz. per ton.

14.71 

 14. 82 

14.76

15.46 

15.43 

15. 51 

15.59

17.' 18 

17.83 

17.60

19.45 

19.44

| 619.49

"These beads contained a very little silver, which was lost in process of parting.

& The assays were made in pairs, and by accident a part of the washings of one assay was 

evaporated in the wrong dish. The assays were run through separately, but only the mean is 

taken for final comparison.

There is seen to be a very fair agreement in each of the several 

series except ft, an exception which was apparent also in the fireassay tests of the ore. It was largely for this reason that fresh samples of each ore were prepared (a^ and fy) as before mentioned.
12 METHODS OF ASSAY- OF GOLD TELLUKIDE OKES. [BULL. 253.

FIRE ASSAY. 

CONDUCT OF THE ASSAY.

The assays were all made in Battersea F crucibles, heated in a gasfed melting furnace, as the muffle available was far too small to 

admit crucibles of such size. A very satisfactory charge was found 

to be that given below, and it was adhered to throughout except in a 

few cases where the litharge was reduced to four assay tons without 

any apparent difference in results.

Ore ......_-.-_---....-.--------.-..--_... 1 assay ton (29.166 grams).

Sodium bicarbonate ................................... 1 assay ton.

Litharge -.-.------_------.__....-- ..._....._-.._..----. 6 assay tons.

Borax (fused). ............................................ 10 grams.

Salt cover.

Neither iron nor niter was used. In most cases the requisite amount 

of silver was added to give two and one-half to three times the weight 

of the gold. During the earlier stages of firing the temperature was 

purposely kept low, and it was not made high until escape of gas had 

nearly ceased. The buttons obtained were rather large for cupellation about 25 grams in weight but it was felt that by reducing a 

large amount of lead the collection of all the gold would be more certain. The buttons were in all cases soft and malleable. Not every 

slag was assayed, since it was found that the gold losses here were 

very slight, in direct disagreement with Fulton's results on Cripple 

Creek tellurides."

The larger number of tests were made by one of us (A), but in 

order to have the check of another worker a number were made by 

his colleague (H). The latter's results seem to be in general a little 

below those of A, a fact probably explained by the employment of a 

somewhat higher temperature in cupellation. The powerful influence of slight changes in temperature on the cupellation results will 

be^ emphasized in the second part of this paper.

CHARGES USED IN ASSAYING SLAGS AND CUPELS.

The charge used for the slag assay was: Litharge, 1 A. T.; argol, 2 

grams; salt. For cupels it was: Litharge, 2 A. T.; soda, 1 A. T.; 

borax glass, 1.5 A. T.; argol, 2 grams; salt.

ASSAYS FOR GOLD AND SILVER TOGETHER.

The first assays were made in order to ascertain not only the gold, 

but also the silver content of the ores, though no importance was 

attached to the determination of the latter. Incidentally these tests 

showed the losses of gold in slags and cupels.

a Jour. Am. Chem. Soc.. vol. 20,1898, p. 596; see also later in this bulletin, p. 14.
HILLEBIIAND~| 

AND ALLEN.J FIRE ASSAY. 13

TABLE III. Crucible assays of Grille Creek telluride ores for gold and silver

together.

[Values are the weights obtaiiied in milligrams, and denote ounces per ton of 2,000 pounds.]

Main assay, Au+Ag.

a ...-..._..---------.---- ....

Average ...............

From-slag ....................

Total... ................

Less average gold, from Table 

. VII...... ..................

Silver. .......................

b. ............... ............

Average ...............

From slag .......-..-.....-.-

From cupel ..:-..--.---..-...

Total..................

Silver. . ......................

0! ---...---.-.-...-..-.--.-.-,

Average ...............

"Averages obtained b

16. 54 

16.37 

16.97 

16.50 

16.58 

16. 54

16.58 

.055 

.085

16.72 

14.53

2.19

"20.54 

20.63 

20.58 

20.88 

20.21

20.57 

.08 

.10

20. 75 

17.75

3.00

-17.39 

17.40 

17.43 

17.25

17.37 

7 combin

Slags.

Au+Ag.

0.07 

.04

.055

.08 

.08

.08

.43 

.05 

.07 

.03

.145 

ng all the be

Au.

} 0.03

.015

a. 027 

ads (8) from

Cupels.

Au+Ag.

0.10

.07

.085

( Lost. 

I 0.10

T10

Lost. 

0.17

.15

.17

.16 

a\ and 61.

Au.

1

0.04

.04

«.07
14 METHODS OF ASSAY OF GOLD TELLURIDE ORES. [BULL. 253.

TABLE III. Crucible assays of Cripple Creek telluride ores for gold and silver

together Continued.

Main assay, Au+Ag.

From slag ................... 0. 145

From cupel .................. .16

Total.................. 17.675

Less average gold ............ 15. 60

Silver ........................ 2. 075

22. 33 

, 22. 34 bi..... .......................

22.42

.22. 38

Average--....-........ 22.37

From slag -......-.--........ .05

From cupel ..---...-.-....... .18

Total ................... 22.50

Less average gold ............ 19. 63

Silver........................ 2.87

Slags.

Au+Ag.

0.06 

.05 

.06 

. -04

.05

Au.

« 0.027

Cupels.

Au+Ag.

0.22

.17

.16 

.18

.18

Au.

«0.07

a Averages obtained by combining all the beads (8) from cti and 6^

Discussion of results. The cupellation loss, even without that due 

to volatilization, is here seen to be markedly greater than in the slag. 

This observation is in agreement with the general tenor of statements 

found in the literature, but directly contradicts Fulton's statement,

already referred to, as to telluride ores from Cripple Creek." May not 

his high losses in the slag have resulted from his employment of iron 

nails, with resultant formation of auriferous sulphide in the slag? 

. In order to have a check upon the actual loss, including that due 

to volatilization, proof metal of the composition of that obtained from 

the ore was cupelled with 25 grams of lead. The results follow:

a Jour. Am. Chem. Soc., vol. 20,1898, p. 596.
AND ALLEN. FIRE ASSAY. 

TABLE IV. Cupellation losses on proof alloy.

15

Ag ..............

Au .............

Au+Ag .. 

Ag.. ...I. ......

Au .............

Au+Ag .. 

Ag. ............

Au .............

Au+Ag .. 

Ag. .__ '.___...

Au .............

Au+Ag -.

Taken.

Milligrams. 

2.09 

15.59

17.68

2.26

15.74

18.00

2.28 

19.75

oo f>q ##. Uo

2.58 

19.64

22.22

Pound.

Milligrams.

17.53

17.84

21.51

22.07

Loss.

Milligrams.

0.15

.16

.52

.15

Per cent.

<

0.85

.89

2.36

.68

Recovered from cupels.

Milligrams. 

0.06

.077

.137

.08 

.077

" .'157

.07 

.08

.15

.10

.07

.17

Per cent. 

2.87 

.49

3.54 

.49

3.07 

.40

3.88 

.36

When cupellation takes place at a low temperature, with formation 

of considerable feather litharge, as in the above cases, the loss by 

volatilization is seen to be practically negligible, or if not absolutely 

inconsiderable to be compensated, perhaps by retention of lead, but 

that the case is different when the temperature is higher is plainly 

shown elsewhere in this paper (Tables VIII and IX).

ASSAYS FOR GOLD ALONE.

In all these cases the requisite amount of silver was added at the 

start to give a bead suitable for parting.

c

. TABLE V. Crucible assays of Cripple Creek telluride ores for gold alone. 

[Values are the weights obtained in milligrams and denote ounces per ton of 2,000 pounds.]

Main assay, Au.

14.50 ; 

a 14' 43

i4-. 45 ;

14.47 :

Average ........... 14. 46

From slag and cupel ..... .05

Total.............. 14.51 (Alien).

Slags.

Au+Ag.

0.25 

.36 

.16

.18

.24

Au.

0.02

Cupels.

Au+Ag.

0.72 

' .82

' .77

0, '

Au.

0.03
16 METHODS OF ASSAY OF GOLD TELLUEIDE OEES. [BOLL. 253. 

TABLE V. Crucible assays of Cripple Creek telluride ores for gold alone Cont'd.

Main assay, Au.

14. 63

14.63 

a.-..-.....-------.-.-.....

14.53 

14.21

14.50 

From slag and cupel ..... .05

Total.............. 14.55 (Hbd.).

15. 50 

15. 52 

15.55

d! _..-.......--.-.-.--....

15.61 

15. 57 

.15.53

Average ...._-_-..- 15. 55

From slag and cupel ..... .07

Total .............. 15.62 (Alien).

r!5.50 

at _..--.---------------- J

\15.48

Average ...... .... 15. 49

From slag and cupel ..... .08

Total.............. 15.57 (Hbd.).

- (17. 76 b. _----..--_-------.. _.-J

\17.83

Average ........... 17.79

From slag and cupel . . . . . .05

Total.............. 17.84 (Alien).

17. 69 

b ... 1173

17.47 

17.14& ,

Average ........... 17.63

From slag and cupel ..... . 055

Total. ............. 17.685 (Hbd.).

Slags.

Au+Ag.

0.08 

.09

.09 

(«)

..32

.28

.30

(a) 

(a) 

(a) 

(a)

Au.

0.04 

.03-

.02

.02

.02 

.02 

.03 

.03

.025

Cupels.

Au+Ag.

0.92 

1.00 

1.23 

1.07

1.05

(a) 

(a).

.87 

' .82

.85

Au.

0.02 

.02 

.04 

.03

-

.03

.10 

.03

.06

.03

.03 

.03 

.03 

.03

.03

"Not weighed. & Excluded from average as undoubtedly low.
ASSAY. 17 

TABLE V. Crucible assays of Cripple Creek telluride ores for'gold alone Cont'd.

Main assay, Aia.

19. 39 

19.53 

19. 51 

b,. ........................ 19.57

19. 55 

19.62 

19. 56

Average ----.----.- 19.54 

From slag and cupel ..... . 085

Total .............. 19.625 (Alien).

b /19.79

119. 48

Average ........... 19. 63

Prom slag and cupel ..... .07

Total.............. 19.70& (Hbd.).

Slags.

Au+Ag.

0.14 

.14

.14

(«) 

(«)

All.

0.04

.02

Cupels. ; 

Au+Ag.

1.48 

1.34 

1.28 

1.29

1.35

Au.

0.05' 

.06 

.03 

.04

.045

.05

"Not weighed.

b Proof inetal (19.85 mg. gold and 49 mg. silver) was crucibled, cupelled, and parted at the same 

time as these last tw.o assays. The final gold was 19.84+0.07, or 19.91 in all, showing that in this 

case there must have been a marked retention of silver after parting. If a like correction has 

to be applied to the regular assays, the Average drops to 19.64, almost exactly that of Alien.

In order to check the actual loss, including that by volatilization, 

the following cupellations were made with alloys corresponding to the 

above and about 25 grams in weight:

TABLE VI. Cupellation losses on proof alloy.

Ag. ............

Au .............

Ag ............

Au .............

Acr

Au .............

As?

Au .............

Taken. 

Milligrams.

48.03 

15.79

55.37 

15.82

63.65

19.72

63.81 

19.74

Found.

Millie/rams.

46.90 

15.70

54.77 

15.75

62.27 

19.63

62.46 

19.64

Loss.

Milligrams.

1.13 

.09

.60

.07

1.38 

.09

1.35 

.10

Per cent. 

2.35

.57

1.08 

.44

2.17 

.46

2.11

.51

Recovered from cupels.

Milligrams.

0.82 

.02

1.09 

.02

1.04 

.03

Per cent. 

1.71

 .12

1.71 

.10

1.63 

.15

Bull. 253 05 3
18 METHODS OF ASSAY OF GOLD TELLUKIDE ORES. [BULL. 253.

When the above percentage recoveries are compared with corresponding ones for the alloy far richer in gold (p. 15, Table IV), it is seen, as 

might be expected, not only that there is a selective absorption of the 

metals, but also that the gold loss by cupellation is absolutely, and, 

relatively to the silver, greater in the richer gold alloy; but it is singular that whereas in the latter case practically all the lost gold (and 

silver) seemed to be recovered from the cupel, in the case of the rich 

silver alloy by far the greater part of the lost gold has been volatilized instead of absorbed. That a portion, and relatively a large portion, is always thus volatilized under similar conditions the results 

given in Table IX on page 21 clearly show, but there the volatilization 

loss, no matter at what temperature in the muffle, though very variable, is with one exception considerably below that by absorption. The 

explanation of this disagreement we are, unfortunately, unable to 

offer.

COMPARISON OF RESULTS.

Comparing the average results by combination wet and fire test and 

by fire assay alone, we have the following showing:

TABLE VII. Comparison of results by the tivo methods.

Combination assay.

a ..^.. ...........

A vpTa fyp

b ................

Average ...

a^. ..............

Average ...

b!.. ..............

Average -..

1 H «14.76

I H 17.71.

1 H 15.50

1 H 19.47

Fire atsay.

H 14.55 

A 14.51

& 14. 53

H 17.69 

A 17. 84

& 17.75

H 15.57 

A 15.62

& 15. 61 '

H 19. 64 or c 19. 70 

A 19.62 19.62

H9.63or C19.64

a Probably a little high from retained silver.

t> Averaged from the results of both H. and A. by giving proper credit for the number of tests 

made by each 

o See footnote to Table V, p. 17, for explanation of the alternative value. «

From the above, it would seem to be be clear, as claimed by Fur 

man, that for telluride ores of the composition of those of Cripple Creek 

the crucible fire assay gives results perfectly satisfactory, as compared 

with wet extraction. In three of the four ores assayed by us the
ERRORS IN GOLD ASSAYS. ' 19

advantage would seem to lie very slightly with the fire assay, -but the 

 determinations are too few in number and not sufficiently uniformly 

in one direction to permit us to regard this as demonstrated.

Because possibly some one may raise the objection that the fire 

assay failed to extract from the residues insoluble in the liquid reagents the whole of their gold contents, and that therefore the results 

do not prove the accuracy of the fire assay in comparison with the 

other, it is to be regretted that recourse had to be taken to the former 

to complete the wet assays, but there seemed no help for it. No 

reasonable person will entertain any doubt on the subject.

PART II. THE ERRORS IN GOLD ASSAYING.

CUPELLATION LOSSES. 

ABSORPTION AND VOLATILIZATION.

That the loss of silver in assaying may be yery marked is, of course, 

well known, particularly that loss which is due to too high heat 

during cupellation. That gold suffers similar losses, though to a much 

less degree, has been fairly well established by various workers. 

"That loss of gold takes place in cupellation if this is conducted at 

too high a temperature, or if the alloy is cupelled without quartation 

silver, is an already long-known experimental fact. "a The data relating 

to gold are, however, not so satisfactory as those with respect to silver, 

and they are at times uncertain or even conflicting. It is in general 

said that the cupellation of gold may be carried out at a considerably 

higher temperature than that of silver, and the assayer is commonly 

advised to complete the final stage of the operation farther back in the 

muffle than the earlier stages. Some authorities insist on the absence of 

feather litharge at the end. There seems to be in the literature no adequate appreciation of the very marked effect that slight differences in 

temperature actually do produce on the yield of gold by cupellatiou, 

especially if the ore be rich, nor of the very considerable volatilization loss that accompanies cupel absorption if the heat is not kept at 

a minimum. Doubtless these facts are known to individual assayers, but they are not sufficiently emphasized in chemical literature, 

especially that to which practical assayers in this country usually have 

access. . " Many yet believe that the degree of heat has little or no 

influence on the result." 6

Our own experiments, which follow, were all made with proof gold 

and silver from the mint. The results are not therefore necessarily 

comparable with those obtained by assaying ordinary bullion, where 

the presence of copper is said to demand a higher temperature and 

to influence the gold loss.

a Bodeman-Kerl's Assaying, translation by Goodyear, 1880, p. 194. 

bRoessler in Dinglers Polytech. Jour., vol. 206,1872, p. 186.
20 METHODS OF ASSAY OF GOLD TELLUKIDE OEES. [BULL. 253.

First, gold alone was cupelled in order to ascertain primarily the 

loss with increase of temperature, and, secondarily, how this loss was 

affected by the amount of lead used. Ordinarily six cupels were set 

in a row, from front to back of the 12 by 6 inch muffle, so that the 

front cupel always showed at the end abundant feather litharge, and 

the rear one was at the back end of the muffle. The cupellations 

were made in most cases with an altogether unnecessarily large amount 

of lead (25 grams), in order to duplicate as nearly as might be the 

conditions under which the assays described in Part I were cupelled. 

Table VIII shows these results.

Then quartation mixtures were cupelled and the silver as well as 

the gold losses determined (Table IX).

TABLE VIII. Losses of gold when cupelled without silver as affected by temperature and amount of lead.

[25 grams in Series A and B, 5 grams in Series C.]

o5 .2 '£

CD 

02

A. 

B. 

C.

rl_____.

2......

3......

4.......

5......

.6.....'.

(I .. ..

2

3......

4......

5......

.6....... 

1......

2......

8......

' 4......

5...... 

.6......

Gold 

taken.

Mgs. 

30.58 

30.32 

30. 63 

30.45 

30.16 

30.66

10.34 

10.25

10.29

10.27 

10. 17 

Lost -.

10. 36 

10.31 

10.31 

10.17 

10.29 

Lost ..

Gold 

found.

Mgs. 

30.47 

29. 96 

30.09 

29.30 

28.90 

29.30

10.31

9.77

10.15

9.20 

8.56

10.33 

10.23 

10.16

9.87 

9.84

Loss.

Mgs.

0.11 

.36 

.54 

1.15 

1.26 

' 1.36

.03

.48

.14

1.07 

1.67

.03 

.08 

.15 

.30

.45

Per cent. 

0.36 

1.19 

1.76 

3.78 

4.17 

4.43

.29

4.68

1.36

10.42 

16. 43

.29 

.78 

1.45 

2.95 

4.37

In cupels.

Mgs. 

0.15 

.29 

.57 

.89 

1.17 

1.25

Volatilized.

Mgs.

0.07

.26 

.09 

.11

Percentage of loss.

Absorption.

136

80 

105 

77 

93 

92

Volatilization.

20

23

7 

8

(

Litharge formed faster than it 

could he absorbed during much 

of the time, yet the furnace 

ran at an unusually high heat.

 5 grams lead.
TABLE IX. Losses of gold and silver when cu

[25 grams lead.]

Series.

A. 

B.

O,

rl_--___

3

'

8...:_.

4.--.-.

5......

«-- -:.

rl_.___-

2......

8.._...

4_____.

5......

.6......

M 

rl......

2

3..----

4......

5-.--..

 6......

Gold 

taken.

Mgs. 

30.06 

30. 40 

30. 60 . 

30.07 

30.61 

Lost.

15.56 

15.14 

15.15 

15.44 

15.52 

15.39 

3an ----.

10.67 

10.57 

10.53 

10.63 

10.60 

10.21

Gold 

found.

Mgs. 

29.91 

30.03 

29.89 

28.94 

29.42

15.53 

15.08 

14.92 

15.12 

15.12 

15.02

10.62 

10. 40 

9.99 

9.50 

9.28 

8.93

Gold loss.

V

Mgs. 

0.15" 

.37 

.71 

1.18 

1.19

.03 

.06 

.23 

.32

.40 

.37

.05 

.17 

.54 

1.13 

1.32 

1.28

Per cent. 

0.50 

1.22 

2.32 

3.76 

3.89

.19 

.40 

1.52 

2.07 

2.59 

2.40

.47 

1.61 

5.13 

10.63 

12.46 

12.53

.Silver 

taken.

Mgs. 

90.51 

90.19 

90. 74 

90.67 

90.75

45.06 

45.19 

45.41 

45.30 

45.59 

45.05

30.33 

30.64 

30.42 

30.52 

30.38 

30.44

Silver 

found.

Mgs. 

88.97 

86.82 

'85. 71 

83.73 

83.51

44.20 

43.70 

43.53 

42.68 

42.61 

42.07

29.67 

28.90 

27. 32 ' 

25.64 

24.81 

24.75

Silver loss.

Mgs. 

1.54 

3.37 

5.03 

6.94 

7.24

.86 

1.49 

1.88 

2.62 

2.98 

2.98

.66 

1.74 

3.10 

4.88 

5.57 

5.69

Per cent. 

1.70 

3.73 

5.51 

7.66 

7.98

1.91 

3.30. 

4.14 

5.78 

6.55 

6.61

2.17 

5.68 

10.19 

15. 99. 

18.34 

18.69

A
22 METHODS OF ASSAY OF GOLD TELLUEIDE . OEES.' [BOLL. 253.

The foregoing tables show most striking^ the effect of increasing 

temperature on the loss of gold (and silver) and that this loss with 

pure gold is almost wholly by cupel absorption. A difference of 

position in the muffle amounting to no more than the width of a cupel 

is of great moment. The percentages given in the last two columns of 

Series A, Table VIII, and of Series B, Table IX, are of course subject to considerable probable error because of the difficulty of weighing such small beads to hundredths of a milligram and the probability 

of losses inherent in the operations by which they were obtained. 

Further, because of the-probable retention of traces of lead by the 

large gold beads (see p. 24), the volatilization figures are, if anything, 

too low and those f^r absorption too high.

It is clear that the following statements are far from correct:

"A high temperature has no ill effect, the loss of gold in cupellation 

being so small that-it need riot be considered. It is about balanced 

by the silver left in the gold after parting. The cupellation loss is 

about 0.07 per cent."«

"The loss of gold by cupellation is insignificant (about 0.7 per 

1,000), compensated moreover by the small quantity of silver which 

the gold retains after parting." 6 0 '.:

"All the silver lost is absorbed by the cupel." c .'.

Our experience goes to show that, as a general direction, the following should be modified, for with pure gold and telluride ores no 

higher heat is needed than for silver cupellation.

"A high heat is maintained throughout the operation [cupellation] 

and. especially at the time of brightening, that is to say, a higher 

degree than is admissible in a silver assay . . ." d

The average loss named in the first two of these extracts is from 

three to seven times less than the lowest of those in Tables VIII and 

IX those suffered at the lowest permissible limit of temperature.

Though not connected with the present discussion, it is impossible

to see why the following, credited to Aidarow, should be true:

"The gold, however, only soaks into the cupel when it has been 

scorified with lead beforehand. If gold is added to lead that is already 

fused on the cupel, no loss of gold takes place on cupellation." 6

That a small but appreciable portion of the total loss is due to volatilization the results of Series A, Table VIII, and Series B, Table 

IX, clearly show. That such loss occurs is mentioned by some writers/ but most writers make no mention of it. According to Rose,^ 

10 per cent of the total loss in bullion assay is by volatilization, a

aBeringer, A Textbook of Assaying, 1890, p. 128. 

bCampredon, Guide Pratique-du Chimiste M6tallurgiste, 1898, p. 689. 

eCampredon, ibid., p. 304. 

«*Aaron, Assaying, 1900, p. 36.

eBodeman-Kerl, Assaying, translated by Goodyear, 1880, p. 171.

/Makins, Quart. Jour. Chein. Soc., vol. 13,-1861, p. 97; Roessler in Dinglers Polytech. Jour., vol. 

206,1872, p. 189; Ricketts and Miller, Notes on Assaying, 1897, pp. 128 and 129. 

0 Metallurgy of Gold, p. 479.
EBROBS IN ASSAYING. 23

proportion roughly agreeing with the returns in Table VIII, Series A, 

final columns.

' That gold volatilizes slowly if kept above its melting point Napier 

has shown/4 but this condition does not ordinarily, if at all, obtain 

after the bead has brightened, so that the direction that it is quite 

safe to leave gold beads in the muffle after brightening for almost 

any length of time seemed probably justified. To test the point, however, a number of trials were made by cupelling several beads simultaneously and taking them out at intervals of one-half, five, and 

twenty minutes after pushing them back in the muffle to brighten. 

No constant losses were suffered by those left in for the longer 

periods, but curiously enough there seemed to be a slight tendency 

to increase in weight. To what this could be attributed is not clear.

On their face the results of Series'C, Table VIII, as compared with 

Series B of the same table, seem to indicate that the amount of lead 

exerts a very marked effect on the gold loss. Roessler 6 is fully persuaded from his own table of tests that this is generally true. But 

compare for a moment the three series of Table IX. The same 

amount of lead was used in them all (25 grams) and the absolute 

losses of gold with 30 mgs. and with 10 mgs. were practically identical, while with 15 mgs. (Series B) they dropped to about one-third. 

Here the only seemingly probable explanation is that the temperature throughout the muffle must have been appreciably lower with B 

than with A and C. Hence it is not improbable that a similar explanation may be the proper one to adopt for the low results of Series C 

in Table VIII. The cause is certainly not conclusively shown, and it 

has been made plain that slight differences in temperature produce 

marked effects.

The final columns in Series B, Table IX, are of interest as showing 

that with quartation alloys not only is the volatilization loss of gold 

greater than when pure gold is cupelled, but also that the ratio of 

volatilization loss to absorption loss is about the same for both gold 

and silver, or, roughly, as 30 to 70. We regret that similar comparisons were not made for the other series. It is hoped that at some 

future time opportunity may offer to test this and other points of 

interest more critically. For the present, however, this is impossible owing to the enforced abandonment of the Survey's fire-assay 

laboratory. It is largely due to this latter fact that the work herein 

described is rather fragmentary and not altogether satisfying.

RETENTION OF LEAD BY CUPELLED BEADS.

Inasmuch as in hardly a single case did our corrected assays on 

proof metal show more than was weighed at the start, even at the 

lowest heat, with full feather litharge, it did not seem necessary to

"Quart. Jour. Chern. Soc., vol. 10,1858, p. 229. 

. bDinglers Polytech. Jour., vol. 206,1872, p. 188.
24 METHODS OF ASSAY OF GOLD TELLURIDE ORES. [BULL. 253.

assume the presence of lead in the beads obtained. Nevertheless, in 

one case a number of them from the same series of assays were dissolved in aqua regia together, the gold precipitated, and the solution evaporated to a few drops. By exercise of the utmost care 

it was possible to detect a minute quantity of lead, too small, perhaps, 

to have influenced the results to any appreciable extent.

Later the following experiments were carried out:

Experiment 1. Three beads of proof gold, weighing, respectively, 

30.14, 50.40, and 10.35 mgs., were cupelled with 2 grams of lead each, 

in the ordinary way, and weighed after boiling with dilute hydrochloric acid to remove any film of lead oxide or particles of lithargesoaked bone ash. They were then dissolved together in aqua regia, 

evaporated, diluted with sulphurous acid, the solution filtered and 

evaporated till fumes appeared. The residue, consisting of lead sulphate and a little metallic gold, was transferred to a filter after taking 

up with alcohol, the sulphate extracted by ammonium acetate, the 

lead reprecipitated by hydrogen sulphide, reconverted into sulphate, 

and weighed. The weight was 0.0004 gram, equivalent to 0.3 per 

cent of lead.

Experiment 2. Three beads of very nearly the same weights as 

those first acted on were treated in a similar manner, except that 

they were left in the muffle twenty minutes after brightening. The 

amount of lead recovered from all three together was 0.37 per cent.

This shows clearly, as had our earlier experience, that the error, 

though of some magnitude, is not lessened by leaving the beads in 

the muffle for a considerable time after brightening. . It should be 

said that the sulphate weighed was certainly of lead and not silver. 

The percentages above given are, however, to be regarded as approximate only.

LOSSES OF GOLD IN SLAGS.

From a statement in Fulton's paper, "Assay on telluride ores,"« 

we were led to expect much greater losses of gold in the slag than 

appear from the tables in Part I of this bulletin. Fulton says that 

these are much greater with Cripple Creek telluride ores than with 

ordinary gold ores. They appear, however, so far as our own work 

goes, to be slight. We have suggested on page 14 a possible explanation for his higher losses.

ERRORS IN PARTING. 

STRENGTH OF ACID.

Most diverse is the practice in parting as to the strengths of acid 

employed. Vauquelin seems to have first used acid of 1.16 and 1.26 

specific gravity, and these are the strengths most frequently recommended to-day. But there are not wanting those who prescribe other

«Jour. Am. Chem. Soc., vol. 20,1898, p. 589.
AND ABLRLBND ] ERRORS IN PARTING. 25

strengths, or who reverse the order of application that is, employ 

the stronger acid first or who are content with acid of uniform 

strength. o

INCOMPLETE EXTRACTION OF SILVER.

Not less diverse are the statements as to the completeness of the 

silver extraction. Campredon puts the average silver retention for 

bullion assay, tested upon an aggregate of 100 cornets, at 0.0002 gram. 

According to Beringer, 05 the surcharge may amount to 0.05 per cent. 

In Brown's Assaying (p. 396) it is said that "A small amount of silver 

will always remain in the cornet, no matter how carefully the manipulations may have been conducted." On the other hand, Ricketts and 

Miller 6 say that the inaccuracy due to silver retention in parting can 

be avoided by careful work. Later, however, they say (p. 128) with 

respect to gold bullion, "On the other hand, the gold always contains some silver and occluded gases." That cornets retain occluded

gases was proved by Graham, 0 and Varrentrapp showed that the 

amount varies with the temperature of annealing. a

Our own experience in the present work is that while sometimes by 

dissolving a number of beads together traces of silver were detected, 

quite as often none were found. This was before we learned the 

need for more than two or three boilings with acid. We believe that 

some of the unfavorable results reported by various writers .may have 

been due to insufficient treatment with acid and inadequate washing. 

Not infrequently the direction is given to wash three times with 

water. That this is insufficient for such cornets as are obtained in 

bullion assay our own tests have proved to us. (See further, p. 27.)

There is great lack of agreement, too, as to the best gold-silver 

ratio in the quartation alloy. For instance, Chaudet and Kandelhardt are credited in Bodeinan-Kerl e with the statement that a goldsilver alloy with 2£ parts of silver retains less silver when parted with 

nitric acid than an alloy with 3 parts. The same authority attributes 

to Pettenkofer the statement that the separation can succeed well 

with as little as If parts of silver to 1 of gold. Rammelsberg is 

quoted/ to the effect that pure nitric acid dissolves all the silver out 

of an alloy containing. 80 per cent and more of silver and leaves all 

the gold behind; and, on the contrary, the separation is imperfect 

with a proportion of from 15 to 80 per cent of silver.

IMPURITIES IN ACID.

Of course, if the acid used for parting contains certain impurities, 

losses will occur. The impurity most to be guarded against is hydrochloric acid or chlorine. Bodeman-Kerl? mention also sulphuric and.

«A Text-book of Assaying, 1890, p. 129. e Goodyear's tranlation, 1900, p. 183. 

*>Notes on Assaying, 1897, p. 109. /Ibid., p. 184. 

oPhilos. Trans. Roy. Soc., 1866, p. 433. 0Ibid., p. 187. 

dKose, Metallurgy of Gold, p. 481.
26 , METHODS OF ASSAY OF GOLD TELLURIDE OEES. [BULL. 253.

sulphurous acids. Rose a ascribes, on the basis of his own tests, 8 

per cent of the total loss in a gold assay to the solvent action of the 

acid, without specifying further. Lenher* confirms the statements of 

several authors as to the solubility of gold in a hot mixture of strong 

sulphuric and nitric acids, and that on dilution with water it precipitates, this precipitation being supposedly brought about by nitrous 

acid formed from the nitric acid. He also shows that the sulphuricnitric acid mixture dissolves gold even at 0° C.

ACTION OF NITROUS ACID.

The statement, which, according to Makins, can be traced to Berzelius, is met with in several places that loss of gold ensues in parting 

from the solvent action of nitrous acid; and Makins, c by repeated 

boilings of assay gold with nitric acid of specific'gravity 1.25 and 

1.35, was able to establish a steady loss which he attributed to nitrous 

acid, though the proof is by no means convincing, as he does not 

clearly show the entire absence of traces of hydrochloric acid in his 

nitric acid, nor of silver in his cornets, nor did he evaporate the filtrates and prove that they contained gold. The active agent could 

not be nitrous acid formed from the solution of the 'quartation silver 

for its formation must have practically ceased after the second or 

third boiling. Van Liew^ obtained astonishing results. He condensed nitrous acid, which he prepared by reducing potassium nitrate 

with lead and treating the nitrite with sulphuric acid, in a 30 per cent 

solution of nitric acid in which had been placed leaf gold. At 15° C. 

about 1 to 2 per cent of the gold was found to have dissolved, and 

upward of 38 per cent at 82° C., and 51.9 per cent at an undetermined 

lower temperature.

In order to obtain light on this point one of us prepared nitrous 

acid and tested its effect on spongy gold. Cold aqueous solutions

were obtained by the method of Lunge from starch and nitric acid.

Experiment 1. Twenty-five c. c. of the solution, containing 0.032 

gram HN02 and 0.401 gram HNO3 per cubic centimeter, was placed 

in a pressure bottle with very thoroughly washed spongy gold and 

left in contact for thirty-six hours, the final temperature being about 

30°. Evaporation of the filtrate to dryness showed a small, yellowishwhite residue, which proved to be mostly organic (a blank test gave 

the same result). When this was heated to redness a faint purple 

stain remained. This, when dissolved in a few drops of aqua regia 

and tested with stannous chloride, showed a trace of gold.

a Metallurgy of Gold, p. 479.

6 Jour. Am. Chem. Soc., vol. 26,1904, p. 552. Lenher in this paper gives interesting data on the 

solvent action on gold of a number of bodies other than the halogens, but they are outside the 

present discussion.

o Quart. Jour. Chem. Soc., vol. 13,1861, p. 99.

rfEng. and Min. Jour., vol. 69, p. 498.
HILLBBHAND"] 

AND ALLEN.J EEROES IN PAETING. 27

Experiment 2. The solution was more concentrated, holding 0.0548 

=gram HNO2 and 0.075 gram.HN03 per cubic centimeter. A trace of 

gold was found.

Experiments 3 and 4- In each of these 10 c. c. of solution, containing 0.062 gram of HNO2 and 0.219 gram of HN03 per cubic centimeter, was sealed in a tube with spongy gold and heated for seven 

hours at about 150°. No gold was found in either case.

It is difficult to see how the above conditions can have differed 

materially froin those in Van Liew's experiments; nevertheless, 

nitrous acid was made from pure potassium nitrate arid its action on 

spongy gold observed at a temperature of about 70° to 80° in pressure bottles. No solution of gold could be proved.

In all of the above experiments the acid was filtered through a close 

plug of ignited asbestos before testing for gold. 

 The results of these severe tests show that solvent action by nitrous

acid during an assay need not be considered. The reported precipitating action of nitrous acid in dilute solution (see above, p. 26) 

seems to be in direct opposition to such solvent action. Van Liew's 

abnormal results must have been due to using a nitrate containing 

chloride as the source of his nitrous acid.

ACTION OF NITEIC ACID.

. To test the possible effect of nitric.acid itself the following experiments were made:

Four portions of pure gold were cupelled with three times their 

weight of silver and 5 grams of lead each.' Five boilings with acid, 

with intervening washings, were needed to extract the silver to such 

an extent that an ordinary test failed to show it longer in the extracts. 

Then'No. 1 was boiled with 20 c. c. HN03 for fifteen minutes, washed, 

annealed, and weighed. No. 2 was boiled two periods of fifteen minutes each with acid and properly washed, then annealed and weighed. 

With Nos. 3 and 4 the boiling with acid was repeated two and three 

times, respectively, the duration of acid treatment in case of No. 4 

lasting between one and two hours. The following table shows that 

no systematic losses appear, and that in the most severe test no loss 

whatever occurred:

!_.. ....................

2

3..-..-----....--..---..

4.-.-....-...--.--....-.

Taken.

Mgs. 

250. 27

250. 23

250. 57

250. 38

Found.

Mgs. 

249. 99

 250.29

250. 53

250. 38

Loss.

Mgs. 

-0.28

+ .06

- .04

.00
28 METHODS OF ASSAY OF GOLD TELLDEIDE OEES. [BULL. 253.

The acid thus used was tested for gold as follows: It was evaporated in clean porcelain dishes, the residues were moistened with a few 

drops of water and wiped out clean with small pieces of filter paper, 

these were wrapped in lead cornets to which a tiny piece of silver was 

added, cupelled, and parted.

No. 1 gave no gold. a

No. 2 gave 0.01 mg. gold.

No. 3 gave 0.01 mg. gold.

No. 4 gave no gold.

The acid used for parting was also carefully tested for gold after 

first precipitating the silver. That from the four first partings combined gave no gold. That from the second gave 0.02 mg. gold in all. 

This gold may have been float, gold, though the utmost care was taken 

to guard against this source of error.

From the above it is plain that the losses in parting with pure acid, 

whether traces of gold really dissolve or not, are negligible in an ore 

assay at least.

In conclusion it may be said that the gold beads, after the repeated 

treatments with nitric acid detailed on page 27, were dissolved in 

aqua regia and the solutions tested for silver with the greatest care, 

both before removal of the gold and afterwards. In not a single case 

could the slightest trace of silver be detected,

SUMMARY OF RESULTS.

Not all of the conclusions here summarized are to be regarded as 

fully established, while some are but confirmations of earlier statements by others. More work is needed along certain lines, for there 

is more than one obscure point connected with the fire assay that 

needs greater attention than we were able to give it. To what extent

the results for cupel losses by different workers will be influenced by

a difference in the quality of the cupels used there is, unfortunately, 

no means of determining. The possibility of such a disturbing influence must always be borne in mind.

It is clearly established that the fire assay by crucible for gold 

telluride ores gives results which are quite equal to those obtained 

by the wet way, provided due corrections are made for slag and cupel 

losses.

The gold loss in the slag is very small and generally negligible with 

the charge used by us, 6 but the cupel losses are very appreciable, and 

considering the far greater value of gold than of silver,, weight for

"The absence of gold here shows that the large apparent loss in weight of the cornet, as shown 

by No. 1 of the table above, was not due to solution of gold.

bit is quite conceivable that some ores, differing from ours in the character of their gangue, 

may give slags that have greater solvent action on gold.
SUMMAEY OP RESULTS. 29

weight, it may be doubted whether the statement that "The loss of 

gold in the assay of any ore, except when extremely rich, is too small 

to require correction " a is justified when very exact work is called 

for. Much depends on what is meant by " extremely rich." Doubtless the ores treated of in Part I fall within this category, but the 

losses are there so detectable that correction seems called for with 

ores far less rich. In view of the very appreciable errors that always 

occur and which it is quite impossible to wholly allow for in any ordinary assay, it seems that those who demand an assay balance weighing to less than the hundredth of a milligram are decidedly straining 

at a gnat if they are willing to swallow the camel of an uncorrected 

assay.

The cupellation loss of gold by volatilization is generally small as 

compared with that by absorption, and at a temperature allowing 

the formation of abundant feather litharge is negligible, or perhaps

compensated by retention of lead (Table IV), but the case is otherwise

at high temperatures (Tables VIII and IX), when it may average onehalf of that by absorption in the case of a quartation alloy. The 

results of Table VI seem to contradict the first statement, for they show 

a much greater apparent loss by volatilization than by absorption.

The loss of gold by absorption is a very important one, and is 

influenced, far more than is generally supposed, by slight-changes in 

temperature. It is greater with pure gold and alloys poor in silver 

than with alloys rich in silver. e

From a limited number of tests it appears that the ratio of volatilization loss to absorption loss for quartation alloys at higher than the 

normal temperature of cupellation is about the same for both gold 

and silver, or roughly, 30 to 70. This point needs further testing.

Our experiments failed absolutely to show the need for a higher 

temperature at the end of cupellation with gold beads than with those 

of silver. The most exact results were obtained when feather litharge 

was still abundant at the time of brightening.

Furthermore, it is altogether unnecessary to leave gold beads in the 

muffle for some time after brightening in order to remove the last of 

the lead, for there is no loss in weight from so doing, but if anything 

a very slight tendency to increase.

Our results on absorption as influenced by the amount of lead used 

in cupellation are inconclusive.

The error caused by retention of lead in the beads is one of some 

magnitude if the results of two careful tests are to be depended on, 

which showed 0.30 and 0.37 per cent, respectively, of lead. The 

amount of this retention is not lessened by leaving the beads in the 

muffle for some time after brightening.

Silver can be completely extracted from quartation alloys by nitric 

acid, but more than two repetitions of the acid treatment and subsea Ricketts and Miller, p. 110.
30 METHODS OF ASSAY OF GOLD TELLURIDE OEES. [BOLL. 253.

quent washing are called for if any certainty of complete extraction 

is to be expected.

Tests made with, mixtures of pure nitrous and nitric acids show 

that the solvent action of the former acid is so slight, if indeed there 

is any at all, that it need not be considered as a possible disturbing 

factor in parting.

Similarly it was shown that the losses in parting with pure nitric 

acid, whether traces of gold really dissolve or not, are quite negligible, 

in an ore assay, at least.
INDEX.

Page. 

Aaron, C. H., on effect of temperature on

goldassays ..............1..... 22

Absorption, loss due to, in gold assays... 19-22 

Acid, impurities in, loss of gold due to... 25-26

strength of ............................ 24

Aidarow, on loss of gold in cupellation .. 22 

Bainbridge, W. M., acknowledgments to. 8 

Beringer, C. and J. J., on effect of temperature on gold assays....... 22

on incomplete extraction of silver ... 25 

Bodeman-Kerl, on gold-silver ratio in

quartation alloy............... 25

on loss of gold due to impurities in

acid............................ 25

on loss of gold in assaying............ 19

Brown, W. G., on incomplete extraction

of silver........................ 25

Campredon, L., on incomplete extraction

of silver........................ 25

on loss of gold by cupellation......... 22

Chaudet and Kandelhardt, on gold-silver

ratio in quartation alloy.. _. 25 

Combination wet and flre assay, comparison of results of fire assay and 

of............................... 18-19

 resultsof .............................. 9-11

Crucible assay. See Fire assay. 

Cupellation losses in gold assays, discussion of.......................... 19-24

Cupelled beads, retention of lead by ..... 23-24

Cupels, cliarge used in assaying_...... ( 12

Errors in gold assaying, discussion of._ 19-29 

Extraction of silver, completeness of.... 25

Fire assay, conduct of ......_.......... 12

comparison of results of combination

wet and fire assay and of .. _ 18-19 

results of .............................. 12-15

Fire and wet assay, com parison of results

of fire assay and of............ 18-19 

results of .............................. 9-11

Fulton, C. H., on Cripple Creek telluride. 12,14 

on loss of gold in slags ................ 24

Furman, H. van F., on assay by crucible. 7 

Gold, combination wet and flre assays

for ............................. 9-11

flre assays for.......................... 15-18

loss of, by cupellation................. 19-24

in assays, when cupelled without

silver, table showing ......... 20

inslags ............................ 24

solubility of, in acids.................. 26

Gold and silver, flre assays for ........... 12-15

loss of, when cupelled together....... 21

Gold assaying, errors in .................. 19-29

Graham, Thomas, on occluded gases..... 25

Page. 

Kandelhardt and Chaudet, on gold-silver

ratio in quartation alloy...... 25

Lead, effect of amount of, on gold lost... 23 

retention of, by cupelled beads... 23-24 

Lenher, V., on solubility of gold in acids. 26 

Lindgren, Waldemar, acknowledgments to............-...---.... 8

Makins, G. H., on loss of gold by volatili-

. zation.......................... 22

on loss of gold due to nitrous acid.... 26

Miller, E. H., and Bicketts, P. deP., on inComplete extraction of Silver. 26 

'on loss of gold by volatilization ...... 22

on loss of gold in slag .......'.......... 29

Napier, James, on loss of gold by volatilization............._...... 23

Nitric acid, action of, on gold ............ 27-28

Nitrous acid, action of, on gold........... 26-27

Ores assayed, nature of......_.......... 7-9

preparation of...._.................. 9

Parting, errors in.......................... 24-28

Pettenkofer, Max, on gold-silver ratio in

quartation alloy.-....--------- 25

Preparation of samples, -method of.... _ 9

Rainmelsberg, C. F., on gold-silver in

quartation ratio............... 25

Eicketts, P. deP., and Miller, E! H., on incomplete extraction of silver. 25 

on loss of gold by volatilization....... 22

on loss of gold in slag.................. 29

Roessler, H., on effect of amount of lead

onlossof gold...........:._ 23

on effect of temperature on gold assays ............................ 19

on loss of gold by volatilization ...... 22

Rose, T. K., on loss of gold by volatilization ......_.............._ 22

on loss of gold due to impurities in

acid..:......................... 25

Samples, preparation of ..............._ 9

Silver, extraction of, completeness of, in

parting ..........._.......... 25

Silver and gold, loss of, when cupelled

together....................... 21

Slags, charge used in assaying............ 12

loss of gold in..._.................... 24

Temperature, effect of. on gold assays... 19-23 

Van Liew, W. R., on loss of gold due to

nitrous acid................._ 26

Vauquelin, L. N., strength of acid used by. 24 

Volatilization, loss due to, in gold assays. 22-23 

Varrentrapp, F., on occluded gases...._ 25

Wet and fire assay, comparison of results

'of............................... 18-19

resultso'f.................._......._ 9-11

0

31