Laboratory instructions for fire assays of gold, silver, and lead. Mining 107 a-b-

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UC-NRLF

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GIFT OF

UNIVERSITY OF CALIFORNIA
DEPARTMENT OF MINING AND METALLURGY

ASSAYING LABORATORY

S. B. CHRISTY
Dean

ABORATORY INSTRUCTIONS FOR

FIRE ASSAYS OF GOLD,

SILVER, AND LEAD

MINING 107 a-b

BY

WALTER S. MORLEY
Assistant Professor of Metallurgy

UNIVERSITY OF CALIFORNIA
DEPARTMENT OF MINING AND METALLURGY

ASSAYING LABORATORY

S. B. CHRISTY
Dean

LABORATORY INSTRUCTIONS FOR
! FIRE ASSAYS OF GOLD,
SILVER, AND LEAD

MINING 107 a-b

BY

WALTER S. MORLEY
Assistant Professor of Metallurgy

UNIVERSITY OF CALIFORNIA PRESS

BERKELEY

1913

PREFACE

These instruction sheets have been prepared in this form to
take the place of mimeograph copies which were formerly used
by students electing the course in fire assaying at the University
of California. These notes are concise directions for the carry-
ing out of certain laboratory work, and in no wise cover the
ground of the lectures which supplement the instruction in the
laboratory.

The purpose of the course is not to teach merely practical
fire assaying, but also to familiarize the student with metal-
lurgical principles which underlie smelting operations on the
large scale; and in the lectures as much emphasis is placed on
the underlying metallurgical principles as on the practical
details of fire assaying.

The laboratory work is divided into twenty-five assignments,
thirteen of which are intended to cover the work of the first
half-year, and the remaining twelve, the work of the second half-
year. In addition to the twelve assignments of the second half-
year, there will be given, as time permits, assignments relating
to the fire assays of mercury, tin, cobalt, and antimony, and the
determination of carbon, sulphur, and ash in coal and coke.

323637

TABLE OF CONTENTS

CHAPTER I

PRELIMINARY WORK PAGE

1. Weighing 8

2. Practice in Cupellation 10

3. Parting Gold and Silver 13

4. Fire Assay of Lead Bullion 14

CHAPTER H

SCORIFICATION ASSAYS

5. Scorification in General 15

6. Scorification of Silver Ores 17

7. Quartz-Gold Ores containing less than one-half Pyrite 19

8. Quartz-Gold Ores containing more than one-half Pyrite 20

9. Scorification Assay of Samples High in Copper 21

10. Scorification Assay of Ores and Products Fusible with Difficulty. 21

11. General Practice 22

CHAPTER III

CRUCIBLE ASSAYS

12. Determination of Gold and Silver in Litharge: of the Reducing

Powers of Charcoal, Argol, and Flour, and Assay of Fluxes 23

13. Quartz Ores 25

14. Nitre Assay for Ores containing Moderate Amounts of Pyrite 27

15. Nitre Assay for Ores containing Large Amounts of Pyrite 29

16. Roasting Method for Ores containing Large Amounts of Pyrite .... 30

17. Assay of Sulphide Ores by "Nail Method" 31

18. Assay of Tailings 33

CHAPTER IV

BULLION ASSAYS

19. Assay of Gold Bullion 35

20. Fire Assay of Silver Bullion 39

CHAPTER V

MISCELLANEOUS WORK

21. Furnace Repairs. Making Cupels 40

22. Melting, Refining, and Sampling of Gold and Silver Bullion 41

23. Comparison of Methods for Assay of Copper-bearing Materials .... 42

24. Practice in Sampling, Panning, and Amalgamating 43

25. Fire Assay of Lead Ores 45

[5]

THE NOTE-BOOK

Systematic, neat and accurate note-taking is of just as much, if not
more, importance to the student of fire assaying than the actual details
of the work itself. Careful note-taking involves close observation and
concentration. A mere numerical result, without a statement of the
conditions governing the experiment, is not wanted. With a particular
numerical result should be given all the essential facts concerning the
experiment, as determined by observation, reading and thought.

System and neatness are essential characteristics of a good engineer,
especially a mining engineer, whose duties often take him into places
where hardship, dirt, and disorder are the rule.

You should take your note-book not only into the lecture room, but
into the furnace room as well. Write down your observations while
they are fresh in your memory. Keep your note-book clean, your writing
legible, and your drawings neat.

Properly taken carbon copies are expected, and they should be handed
in not later than one week after the experiment has been performed.

[6]

REGULATIONS COVERING WORK IN THE ASSAYING
LABORATORY

Before commencing work, each student should be supplied
with the folowing articles :

1. Copy of Fulton's Manual of Fire Assaying.

2. Capsule-box containing six porcelain capsules, three
unglazed capsules, three Kennedy parting-flasks, three
test tubes, needle and magnifier.

3. Laboratory folder with centimeter-ruled paper, ar-

ranged for carbon copy.

4. HHH lead pencil.

5. Set of Becker (or equivalent) weights from 1 gram to

%0 milligram.

6. Pair of bone-tipped pincers.

7. Pair of steel pincers.

8. Pair of bead pliers.

9. Blow-pipe.

10. Laboratory suit.

Each student should report promptly for work at the ap-
pointed hour, and should stay the full laboratory period, unless
excused.

All fires should be drawn by 4:15 o'clock.

All apparatus should be left in its proper place and in order.

[7]

CHAPTER I

PRELIMINARY WORK

ASSIGNMENT 1

WEIGHING

In this assignment you are expected to become thoroughly
familiar with the construction, adjustment, use, and care of an
assay balance. Read carefully all the references given at the
end of this assignment.

An assay balance is similar to a chemical balance, except that
the moving parts are made lighter, and instead of limiting the
sensibility to .10 mg. it is made to weigh accurately to .01 mg.
With the balance you use this is done entirely by the method of
deflections. Later on you will be given a balance with a rider.

Calculate the sensibility of the balance assigned to use, as
follows : After a couple of oscillations take readings, first on one
side and then on the other — for example:

PANS EMPTY

Left Right

—2.9 2.6

—2.5 2.2

—2.0 1.8

—1.6

4) 9.0 3) 6.6

—2.25 2.2

—2.25
2.20
2)~705~

— .03 Resting point, left

Repeat the readings until uniform results are obtained. If
the resting points do not agree, increase the number of readings
on a side. After a little practice, two readings on one side and
one on the other will suffice.

In this assignment record all readings in your notes. After
you have become skillful in reading, recording will not be neces-
sary. In taking readings do not let the needle vibrate more than
a total of five divisions.

[8]

Now place your !/<> mg. weight on the left pan, allow needle
to swing as before, and determine the new resting-point — for
example :

14 MG. WEIGHT ON LEFT PAN

Left Right

3.1 7.3

3.3 7.0

3.6

3) 10.0 2) ]4.3

3.33 7.15

7.15
3.33

2) 10.48

5.24 2nd resting point, right

— -03 1st resting point, left

5.27 Difference

The difference between the two resting points is the number
of divisions on the ivory index through which y2 mo- nas caused
the needle to be deflected.

The sensibility of a balance may be defined as the deflection
caused by 1 mg. ; or, expressed in another way, the sensibility
is the weight in hundredths of a milligram which will cause the
needle to be deflected one division on the index.

In the above example, the sensibility is 10.54 divisions, or
.095 mg.

As you can readily read to tenths of a division, the balance
will be sensible to .01 mg. Were the beam perfectly rigid, this
sensibility would hold for much greater loads than are usually
weighed on an assay balance ; but for a load of even !/2 grm.
there is a perceptible change. To test this, place a load of 1 gram
on each pan of the balance and determine the sensibility as you
did when the load was zero. Compare results.

You are now ready to weigh by the deflection method. Obtain
a number of pieces of gold or silver weighing from 1 to 10 mg.
Weigh each separately and then all together. Add up the
separate weights and compare with the total weight. Repeat
until your results are consistent.

[9]

Make a full and complete record of your observations.

Before leaving the balance see that the beam is lowered and
the case closed.

The foregoing method of weighing is specially adapted to
the poorer grades of balances in use. When a rider attachment
is available, the ''rider-deflection" method may be used, the
rider taking the place of the 2 mg. and smaller weights. This
method is the one ordinarily used in assay work. With an
exceedingly sensible and well-constructed balance the method
of ''no deflections" (see Fulton's Manual of Assaying) may be
used.

Make a drawing of your balance shelf, showing the method
used to secure a firm foundation : give dimensions of foundation
supports, width of shelf, etc.

REFERENCES

Fulton: Manual of Assayng, pp. 42-52.
Aaron: Assaying, part I, pp. 27-38 and 127.
Thorpe: Quantitative Chemical Analysis, pp. 3-33.

Catalogues descriptive of the balances made by Becker, Troemner,
Ainsworth, Smith & Thompson, and Keller.

ASSIGNMENT 2

PRACTICE IN CUPELLATION

A. Place half a dozen dry cupels in the muffle and build a
fire in the usual manner. As coke is used for fuel, it is necessary
to have a good coal fire started before adding the coke. Let the
muffle heat up until a bright red is reached, then arrange the
dampers so that the heat remains uniform. Before cupelling,
see that the fire is solid around the muffle.

Now obtain six lead buttons, weighing approximately 15
grams each. Cupel in sets of two each in the following manner :
Bring two of the cupels forward say within two inches of the
front of the muffle. Place a lead button in each, using the cupel

[10]

tongs, then shove the cupels back into the hotter part of the
muffle. Close the muffle door for a few minutes. When the
buttons have melted and cupellation has commenced, as shown
by the fumes of litharge, open the door and arrange the dampers
so that the fumes are drawn slowly off through the rear of the
muffle. Try to keep the temperature constant, increasing it only
when the lead has almost disappeared. The heat is too low
when the fumes become heavy and dark, and a scum forms
around the edge of the molten lead. The heat is too high when
the fumes of litharge rise rapidly to the top of the muffle and
the lead appears to be boiling. The heat is just right when
crystals of litharge form in the cupel. Practice will enable you
to fix upon the proper conditions.

Cupel the remaining four and as many more as you have time
for during the period.

Make a full and complete record of your observations.

Study carefully the following references.

REFERENCES

Brown: Manual of Assaying, pp. 214-223.
Furman: Assaying, pp. 138-142.
Bodeman & Kerl: Assaying, pp. 110-124.
Knead & Sexton: pp. 109-115.

B. Roll out a silver coin to the thickness of an ordinary
visiting card. Anneal the coin repeatedly during the operation.
Clean with sandpaper. Now weigh out on your bead balance six
assays of this coin, three weighing 25-30 mgs. each, and three
weighing 90-100 mgs. each. Weigh each bead accurately. Wrap
each assay in 15 grams of sheet lead, weighed approximately on
the pulp balance. Arrange these six assays in sets of two each,
so that each set will contain one 25-30 mg. assay and one 90-
100 mg. assay. Cupel one set at a time, as in "A," only using
more care on account of the resulting silver bead. Try to
feather the cupels and at the same time brighten the beads.

[11]

Remove the beads from the cupels and clean thoroughly by
brushing. Place them in their respective capsules, and take to'
the balance room. Weigh, and report in this form :

Silver
Date Assay No. Coin Lead Wt. Bead fineness Remarks

Sept. 1 1 27.6 15 24.3 880 Well feathered;

brightened at
proper heat.

The term silver fineness, as used above, refers to the number
of parts of silver found in 1000 parts of the coin.

Compare the results, and note the average percentage silver
losses for the 25-30 mg. assays and for the 90-100 mg. assays,
taking into account, of course, the known fineness of the silver
coin used.

Dissolve a piece of coin silver in nitric acid and test with
ammonium hydroxid for copper; also test a well-brightened bead
in the same manner. What deductions do you make?

C. Obtain from the storeroom, six lead buttons or packets,
having, respectively, the following compositions:

1. Lead and silver.

2. Lead, silver and gold.

3. Lead, silver and antimony.

4. Lead, silver and copper.

5. Lead, silver and tin.

6. Lead, silver and iron.

Cupel in sets of two each. Endeavor, in each case, to make
a perfect cupellation. Observe, carefully, in each case, the heat,
time and manner of brightening; observe the formation of any
scoria, its color, etc. ; note, in each case, the color and general
appearance of the cupel after brightening.

Make a full and complete report.

[12]

ASSIGNMENT 3

PARTING GOLD AND SILVER

To be a successful assayer one must be able to part correctly
and rapidly. Nicety of manipulation is required, and the only
• way to acquire this is to practice conscientiously. Do not take
anything for granted. There are three steps in an assayer 's
work where he must be sure, viz: (1) That the sample is what
it represents; (2) that nothing but gold is weighed as such;
(3) that the weighing is done accurately. With these points
looked to carefully, the rest will be comparatively easy. Perform
carefully the following experiments :

Weight out six pieces of gold foil on your bead balance to
.01 mg. ; each piece should contain about 5 mgs. Alloy the pieces
of gold with 1, 2, 3, 4, 5, and 8 times their weight of silver,
respectively. Weigh the silver also on your bead balance, to
.10 mg. only.

To alloy, wrap the gold and silver in about 1 gram of sheet
lead and cupel in the muffle, taking care to have the cupel
smooth, dry and hot. The muffle should be slightly hotter than
for ordinary cupellation. After alloying remove the bead from
the cupel by means of the bead plyers, brush thoroughly, and
place in the porcelain capsule used for parting. Treat each of
the five beads thus and remove to the parting room. Now flatten
each bead, using the small blowpipe, hammer and anvil. Return
them to their respective capsules, which should then be filled
with hot, weak parting acid; heat slowly on the hot plate, and
when all action has- ceased, replace the spent acid with strong
acid and heat for three minutes at boiling point. Then remove
from the plate, pour off the acid, and wash three times with
distilled water (preferably warm). Dry over the hot plate, then
anneal by heating to redness over the Bunsen flame. Cool and
weigh. Report results in this form :

Wt. Gold taken Silver Ratio Wt. of Parted Gold Remarks

6.21 1 to 1 12.10 Not acted upon by acid.

What do your experiments determine?

[13]

Describe minutely the action of the acid on the several beads.
NOTE. — Flatten all dore beads unless they are known to
contain more than eight parts of silver.

REFERENCES

Brown: Manual of Assaying, pp. 224-239.
Furman: Manual of Assaying, pp. 145-146.
Bodeman & Kerl: Assaying, pp. 198-200.
Fulton: Manual of Assaying, pp. 107-110.

ASSIGNMENT 4

FIRE ASSAY OF LEAD BULLION

Chip or saw off four y2 A.T. samples from the bullion given
you, and assay according to the directions given in Fulton's
Manual of Assaying.

Report both gold and silver values in this form:

Per ton

No. of Gold Silver Gold Silver

Date bullion Dore nigs. ings. ozs. ozs.

Sept. 28 22 60.22 5.10 55.12 10.20 110.24

To get correct results in this assignment the cupellation must
be done perfectly, so in a measure this assignment will test your
ability to cupel.

Explain the term A.T.

REFERENCES
Furman: Manual of Assaying, pp. 14-17 and 272-275.

[14]

CHAPTER II

SCORIFICATION ASSAYS
ASSIGNMENT 5

SCORIFICATION IN GENERAL

In this assignment you are expected to get a general idea of
the scorification process as far as can be done by reading.
A brief outline of the work in scorification is given below :

1. Sorification assay of silver ores.

(a) Silver-lead ores.

(6) Those containing no lead, but quartz, metallic silver,
and silver compounds.

A general charge for ores of the above class is :

Ore %0 A.T.

Granulated lead 30-35 grams
Borax glass % gram

Use 2% in. scorifiers.

This charge, with slight modifications, will treat any silver ore;
if the ore contains more than 50 per cent of pyrite, the lead
may be increased up to 45 grams. And if much lime, zinc or
antimony is present, the borax glass must be increased up to as
much as three grams in exceptional cases. Where the sample
is rich and at the same time homogeneous the weight of the
sample taken may be decreased to %0 A.T.

2. Scorification assay of rich gold ores.

As will be noticed from your reading, the "scorification
assay" is primarily adapted for silver ores and rich gold ores,
for evident reasons. To get the required amount of sample for
a medium or poor gold ore, it becomes necessary to make several
scorifications and then scorify the resulting buttons together,
or cupel separately and part together.

[15]

(a) Quartz gold ores consisting largely of quartz. — The
charge given for silver ores can be used equally well for quartz
gold ores containing less than 50 per cent pyrite, though in most
cases it is better to take more than */w A.T. and consequently
more lead and a larger scorifier if necessary.

When yw A.T. of ore is used 30 grams are sufficient, and
with % A.T. 50 grams of lead had better be used.

(6) Quartz-pyrite gold ores consisting largely of pyrite. —
Owing to the large amount of lead required to scorify ores of
this class, it is impracticable to use more than l/w A.T. of ore
in one scorifier, so to accomplish the same result it becomes
necessary to unite the lead buttons as suggested above.

The usual charge is :

Ore Ko A-T.

Granulated lead 40-45 grams
Borax glass % gram.

3. Scorification assay of samples containing a large percentage
of copper.

In samples of this class it is necessary to use a small amount
of ore and at the same time a large amount of lead ; this is due
to the retention of copper in the lead and the difficulty in
removing it unless a sufficient amount of lead is used. If any
great amount of copper is retained in the lead button, a high
heat will be necessary to remove it during cupellation. This
will produce a high loss in silver value ; so it is almost imperative
to add enough lead during scorification to insure the removal
of the copper, when the silver loss will be small in comparison.

The usual charge is :

Ore i/10 or i/20 A.T.

Granulated lead 45 or 50 grams
Silica and borax.

4. Scorification assay of ores and products fusible with
difficulty.

[16]

Ores containing tin, zinc or antimony will come under this
head. A large amount of borax glass and a high heat are
necessary.

REFERENCES

Rhead & Sexton: Manual of Assaying, pp. 106-121.
Furman: Manual of Assaying, pp. 122-126.
Bodeman & Kerl: Assaying, pp. 107-116.
Brown: Assaying, pp. 195-214.

ASSIGNMENT 6

SCORIFICATION ASSAY OF SILVER ORES
Preparation of the charge :

On all assays the following method of selecting the portion
of ore to be assayed (assuming that the sample is correct and
ground to pass a 100-mesh, or finer, sieve) should be rigidly
followed. If there be any doubt as to the uniformity of the
sample, pour out on a piece of glazed paper and remix, then
replace the sample in the bottle or bag. Assay charges are then
to be taken direct from the bottle or bag by means of a spatula.

Consult your lecture notes for an explanation of this method
of selecting the portion of ore to be assayed.

Copper spoons are provided for measuring the granulated
lead.

Measure one-half the lead required into the scorifier, add the
ore, mix with the spatula, spread the remaining half of the lead
over the charge, and sprinkle with borax glass over all.

Sample A silver-lead ore.

Prepare six charges as follows :

Ore i/10 A.T.

Granulated lead 30-35 grams
Borax glass % gram

Use 2l/2 in. scorifiers.
[17]

Make the six scorifications at one time in the muffle assigned
to you. Have the muffle at a bright red before placing the
charges in it; when the charges are in, close the muffle door. In
a few minutes draw the door aside slightly, and if the lead is
melted, remove the door completely and arrange the drafts so
that there will be a strong current of air through the muffle.
During the scorification the heat should be maintained at a
medium orange, or a little above a good heat for cupelling.
Increase toward the end to a full yellow. As soon as the ' * bull 's
eye" has disappeared, pour into a warmed scorification mould.
When all the charges have been removed from the muffle, place
six cupels in so they may be ignited by the time the buttons are
slagged and ready for cupelling.

Cupel the six buttons at one time, feathering them all. Weigh
each bead separately ; part the beads together in sets of three in
Kennedy parting flasks, and weigh the gold.

Sample A silver ore containing a quartzose gangue.

Sample A quartz-silver ore.

Prepare six charges of this sample, each as follows :

Ore %0 A.T.

Granulated lead 30-35 grams
Borax glass % gram

Use 2y2 in. scorifiers.

Cupel the six buttons at one time, feathering them all. Weigh
each bead separately ; part the beads together in sets of three in
test tubes and weigh the gold. Record the results in this form :

Per ton

Assay Sample Wt. Wt. Wt. Oz. Oz.

Date No. No. Ore Dore Gold Gold Silver

Oct. 1 1 226 3/10A.T. 10.23mgs. .04 mgs. .4 101.9

Average the results of the assays of each sample after calcu-
lating them separately for each assay.

Under remarks give composition of the ore, manner of
scorifying and cupelling.

If there is any reason to doubt the accuracy of the assay the
results should never, under any circumstances, be accepted.

[18]

Describe in your notes the appearance of the scorifiers and the
character of the lead buttons. Weigh out %0 A.T. of the ore
and wash on a batea, making yourself familiar with the con-
stituent minerals and the approximate percentage of each. Use
the blowpipe if necessary. Make a detailed report in your notes.

Follow the above directions in all future assignments.

ASSIGNMENT 7

QUARTZ-GOLD ORES CONTAINING LESS THAN
ONE-HALF PYRITE

Sample

Make three assays of this sample as follows :

Ore y10 A.T.

Granulated lead 30-35 grams
Borax glass % gram

Use 2^/2 in- scorifiers.
and three assays :

Ore Vs A.T.

Granulated lead 45-50 grams
Borax glass % gram

Use 2% in. scorifiers.

Unite the three buttons from the first charge by re-scorifica-
tion before cupelling. Cupel the other three buttons directly
and in separate cupels; weigh separately for dore; part one
alone and two together. If necessary to add silver before part-
ing, alloy by means of blowpipe and dry cupel.

Note the time required by each charge and the size of the
lead button. Reduce by scorifying, the size of any button
weighing over 20 grams. Observe the following periods in
scorification of sulphide ores (consult your lecture notes) :

1. Melting. Have muffle at a low yellow heat. Introduce the
scorifier and close the muffle door.

[19]

2. Roasting. When the charges have thoroughly melted, open
the door carefully and allow as much air as possible to pass
through the muffle ; if they begin to bubble, close the door so as
to decrease the air supply. The heat during the roasting period
should be kept at about a medium orange. A high heat will
cause mechanical loss by too rapid oxidation.

3. Fusion, or formation of the "bull's eye." When the
roasting has ceased, close the door and fuse down the chargn
thoroughly.

4. Scorification. When the charge has thoroughly fused, as
shown by the formation of the "bull's eye," open the door and
allow a good current of air to pass through the muffle. During
this period the lead content is reduced and the slag becomes more
fluid.

5. Final heating. When the lead is covered or almost covered
by the slag, close the door and increase the heat to a full yellow,
and then pour into heated moulds.

ASSIGNMENT 8

QUARTZ-GOLD ORES CONTAINING MORE THAN
ONE-HALF PYRITE

Sample

Prepare six charges of this sample, as follows :

Ore i/10 A.T.

Granulated lead 45-50 grams
Borax glass }£ gram

Use 2% in. scorifiers.

Scorify as directed in previous assignment; cupel separately,
feathering carefully. Clean the beads carefully, combine in sets
of three, weigh and part; weigh again. Report both gold and
silver.

[20]

ASSIGNMENT 9

SCORIFICATION ASSAY OF SAMPLES HIGH IN COPPER

Sample and Sample A copper matte.

Prepare five charges of each sample, as follows :

Ore i/10 A.T.

Granulated lead 50-55 grams
Borax glass 1 gram

Use 3 in. scorifiers.

For copper matte add 1 gram of silica with borax glass.
Control the furnace as in assaying ores rich in pyrite. Buttons
from matte must be rescorified ; use 2% in. scorifiers and have
total lead 40 grams. Cupel separately the five, buttons from
each sample, feathering the cupels at first and raising the heat
at the end so as to ensure the removal of all the copper. Beads
from each sample should be weighed separately, but parted
together.

What are the difficulties encountered in assaying ores rich
in copper?

REFERENCES

Furman: Manual of Assaying, pp. 290-291.
Peters: Modern Copper Smelting, pp. 67-71.

ASSIGNMENT 10

SCORIFICATION 'ASSAY OF ORES AND PRODUCTS
FUSIBLE WITH DIFFICULTY

Sample Contains much antimony.

Prepare three charges as follows:

Ore 1/10 A.T.

Granulated lead 30-35 grams
Borax glass 3-4 grams

Use 2% in. scorifiers.
[21]

Scorify in the usual manner except at the end, when the heat
must be raised to a full yellow. The buttons should be perfectly
malleable; if they are not, rescorify with an addition of test
lead. Cupel, feathering carefully; weigh the beads separately.
Part in one capsule.

ASSIGNMENT 11

GENERAL PRACTICE

Take the sample given you and reduce by methods suggested
until you have at least two pounds passing a 100 mesh sieve ;
bottle this and keep for assay.

In this assignment you will be given three or more ores, in
addition to the one sampled. You are to determine their com-
position and then decide on charge you would use for assay by
scorification method.

Make a detailed report, indicating the composition of each
sample, the charge used, etc. Give both gold and silver values.

[22]

CHAPTER III

CRUCIBLE ASSAYS

ASSIGNMENT 12

DETERMINATION OF GOLD AND SILVER IN LITHARGE :

OF THE REDUCING POWERS OF CHARCOAL,

ARGOL, AND FLOUR, AND ASSAY

OF FLUXES

Before making any assays, the assayer should determine the
precious metal content of the litharge used. Litharge contain-
ing more than a trace of gold should be rejected. The presence
of a small but uniform amount of silver is rather an advantage
than otherwise, as the silver serves to help collect the gold.

The assayer must also determine the reducing powers of the
various reducing agents; and an assay of all the reagents used
should be made in order to detect any "salting" of the fluxes.

First. To determine the precious metal content of the
litharge, prepare two charges as follows :

Litharge 100 grams

Argol 2 grams

Salt cover.

Use a No. 7 crucible.

To one charge, add a small amount of test silver. Fuse
quickly both charges in a hot fire until thoroughly liquid, then
withdraw and pour; cupel the resulting buttons. Determine
gold and silver by weighing the unalloyed bead for the dore,
and parting the other for gold. If more than a trace of gold is
found, the. litharge must be rejected. Note the silver content,
and correct all assays according to the amount of litharge used.

For example :

Silver in 100 grams of litharge weighs .50 mg., litharge used
in assays weighs 60 grams, dore bead weighs 2.24 mg., then
.60 X -50 = .30 mg., and the corrected dore is 2.24 — .30, or

1.94mg.

[23]

Second. To determine the reducing powers of the reducing
agents, argol, flour and charcoal, prepare charges as follows:

1. Argol 1 gram
Silica 30 grams
Litharge 75 grams
Soda 25 grams
Borax cover.

Use a No. F crucible.

2. Substitute for argol,

Flour 1 gram

3. Substitute for argol,

Charcoal 1 gram

Reducing agents, only, should be weighed carefully. Fuse,
at one time, the above charges ; when quiet, pour into clean, warm
slag moulds. Clean and weigh the resulting lead buttons.
Weigh, using pulp scales, to the nearest 10 mg.

Note that the above determinations are made under normal
working conditions.

Third. To make an assay of the fluxes used, prepare one
charge, as follows:

Silica 30 grams

Litharge 75 grams

Soda 25 grams

Borax glass 5 grams

Reducer for 18-gram button.
Salt and Borax cover.

Use a No. F crucible.

Add a weighed amount of test silver to the charge, and fuse
in the usual way Determine gold and silver, if any.

[24]

ASSIGNMENT 13

QUARTZ ORES

In ores of this class are included only those whose principal
constituent is quartz, with the addition of 0—15 per cent of iron
pyrites. Any exception will be noted below. The following
charge will treat any ore of this class :

Ore 1 A.T.

Litharge 75 grams

Soda 25 grams

Borax glass 5 grams

Salt, borax glass, or ground borax cover.

Reducer, preferably flour, to reduce 18-22 gram button.

Observe first, that 1 A.T. of ore is used. This is the standard
amount for ordinary crucible assays. When the ore is rich in
gold it is advisable to take less, say % or !/4 A.T. ; when the ore
is poor in gold, as in stamp mill tailings, it may be advisable to
take as much as 4 A.T.

The amount of litharge in a charge for ore of this character
should be such that every particle of the ore is oxidized. For
an ore containing minimum amount of pyrites, 60 grams of
litharge will suffice. Any great excess is to be guarded against,
as the crucible is apt to be corroded and the assay lost.

The third element of the charge is soda (sodium carbonate) ;
this flux is used in every crucible assay. To some extent it may
replace the litharge; so that if the soda in the above charge is
increased, the litharge can be decreased, thereby making a more
economical charge, although the slag would not be so clear.

Borax as borax glass is a most useful flux. If the ore to be
treated contains, besides the quartz, any great amount of infusible
oxides, the borax may be increased up to 20 grams.

A cover of salt or borax (hydrous) should be used on all
assays ; it acts as a wash and prevents an overflow.

[25]

Sufficient reducer should be used to produce an 18-25 gram
lead button. If the ore is oxidizing, several grams of reducer
may be necessary. If the ore contains more than about 10 per
cent pyrites and no oxidizing agent, reducer will be at a mini-
mum and a small amount of oxidizer as nitre may be necessary.
An ore containing more than about 15 per cent pyrites cannot
be properly treated by this charge.

Other reducing agents which may be used instead of flour
are argol, charcoal or sugar.

Prepare three charges of Sample , using the charge

given before and an F crucible. This sample consists mainly of
quartz with less than 5 per cent of sulphides of iron, lead and
copper, and reduces grams of lead per one assay ton of ore.

Unless the proportion of silver to gold in an ore is known to
be at least five to one, test silver should be added to all charges
except one. Weigh the test silver added to one of the charges.

This illustrates three methods of working:, and we have two
checks on the gold and two on the silver. The amount of silver
in the litharge will of course be deducted from the weight of
the dore.

On one charge use a salt cover, on the others a borax cover.

Heat the charge until in quiet fusion, then increase the heat
to low yellow for about five minutes, or until the charge becomes
smooth on top. Pour into clean, warm slag moulds, and when
cool break away the lead button; clean, hammer into cubes, and
then cupel in the usual manner.

Report results in the following form :

Per ton

A

Gold

Assay Sample Gold Silver Gold Silver @ $20.67

No. No. Ore Dore mg. mg. oz. oz. per oz.

1 483 1A.T. 2.22 1.00 1.22 1.00 1.22 $20.67

Under remarks indicate everything that has a bearing upon
the results. Describe the appearance of slag, buttons, etc. Which
cover do you prefer, and why ?

[26]

Make a vanning test of the ore, using about %0 A.T. of ore
and a batea. Eeport this test also.

If necessary to use % A.T. of ore or to make the fusions in a
muffle, the following charge will be found suitable :

Ore y2 A.T.

Litharge 60 grams

Soda 20 grams

Borax glass 5 grams

Eeducer to give a 20-gram button.
Ground borax cover.

Use a No. F crucible.

Prepare two charges of Sample , using the above

charge. Add a weighed amount of test silver to one charge, and
an unweighed amount of test silver to the other. Make the usual
tests, and fuse as before.

To mix any charge, put the soda into the crucible first, then
the litharge and other fluxes, lastly the ore, and mix all together
with a fork; sprinkle salt or borax over the top. Make certain
the ore and fluxes are thoroughly mixed.

ASSIGNMENT 14

NITRE ASSAY FOR ORES CONTAINING MODERATE
AMOUNTS OF PYRITE

Ores of this class are usually richer in gold and silver than
those of the previous class, and consequently less material may
be taken in assaying them.

The following charge will be found suitable for ores of this
class :

Ore 1/2 A.T.

Litharge 90 grams

Soda 20 grams

Silica 10 grams

Borax glass 5 grams

Nitre according to preliminary.

Cover.

Use a No. 7 fluxing pot.
[27]

Fusion of the charges should be carried on as before.

Calculate the amount of nitre (see below) to add so that a
20-gram button will be produced from % A.T. of ore.

Make three asays of Sample , using the charge given

above. Sample contains about sulphides.

Add test silver to all charges but one. Before commencing
work on this assignment, be sure that the previous one is
thoroughly understood.

The reducing agents which are commonly present in ores are
antimony, zinc, arsenic and sulphur. Iron pyrites, the sulphide
commonly met with, reduces as follows:

Fifteen per cent of pyrite will reduce about nineteen grams
of lead, and there will be no excess. More than 15 per cent of
pyrite, using % A.T. of ore, will reduce too large a lead button.
Nitre, an oxidizer, is used to control the size of the lead button,
and is usually stated in terms of lead ; that is, one gram of nitre
oxidizes four grams of lead, more or less, according to conditions.

In order to know how much nitre to add, so as to reduce,
within a few grams, a 20-gram lead button, we must determine
or guess correctly the amount of lead which will be reduced
before making an assay.

The reducing power of a sample may be determined by making
a preliminary, thus:

Sample i£0 A.T.

Litharge 75 grams

Salt cover.

Use a No. 7 fluxing pot.

Have the fire hot when the charge is put in, and fuse quickly ;
when in quiet fusion, remove, and pour. Clean and weigh the
button.

Calculate the amount of nitre which must be used on a basis
of one gram of nitre to four grams of lead.

In preparing charge of Sample , add to one charge

i/2 gram more nitre and to the other y2 gram less nitre than the
calculated amount. Weigh the resulting lead buttons, and
calculate the oxidizing power of nitre under these conditions.
Compare this result with the amount used above.

[28]

ASSIGNMENT 15

NITRE ASSAY FOR ORES CONTAINING LARGE
AMOUNTS OF PYRITE

Material of composition as indicated above, is in general,
assayed in the same way as that of the previous assignment.
The increase in the amount of iron pyrites calls for an increase
of litharge to complete the oxidation ; and as the amount of
silica in the ore is decreased and the charge becomes more basic,
more acid in the form of silica or window-glass must be added.

The following charge will suffice :

Ore

1/2 A.T.

Litharge

120 grams

Soda

20 grams

Silica

15 grams

Borax glass

10 grams

Nitre according to preliminary.
Salt or Borax cover.

Use a No. F or 30-gram crucible.

Calculate for 20 gram button.

When the material to be assayed is a pure concentrate or
nearly so, the amount of nitre to be added can usually be placed
at about 25 grams for y2 A.T. of ore. For material of unknown
percentage of pyrites, a preliminary assay must be made.

In general, ores of this type are richer than the type treated
in Assignment 14, and a smaller amount of ore can be assayed,
thus effecting a saving in time and fluxes; when a sensitive
balance is available and the values are uniformly distributed,
the results obtained by using a smaller amount, 14 A.T., will
be even more accurate than those from % A.T.

[29]

The following charge will do when 14= A.T. is used :

Ore

%A.T.

Litharge

75 grams

Soda

15 grams

Silica

5 grams

Borax glass

5 grams

Nitre according to preliminary.
Salt or Borax cover.

Use a 20-gram crucible.

Calculate for 16-20 gram button.

The ore to be assayed in this assignment is a concentrate
obtained from one of our mill runs, and is a typical concentrate.

Prepare three charges of Sample , using y2 A.T., and

three using 14 A.T.

Time required for fusion about thirty minutes, ten minutes
yellow heat at the end. Avoid heating too rapidly, as a sudden,
uneven heating may cause the assay to boil over.

Report the mean of the several assays as the correct value,
first discarding any assay that appears to have been abnormal
in any manner.

ASSIGNMENT 16

ROASTING METHOD FOR ORES CONTAINING LARGE
AMOUNTS OF PYRITE

Obtain two 4-inch roasting dishes with covers. Line each
dish thoroughly by rubbing a blackboard crayon over its interior.

Weigh out % A.T. of Sample into each dish. Place

the dishes, covered, in the muffle, which should be heated to low
redness only around the edges and corners. The material to be
roasted should be spread out in the roasting dishes and ridged
across, so as to expose as much surface as possible to the air.
After one-half hour, take out the roasts, letting them cool a
little before removing the covers; then turn the ore over with
a wire provided for the purpose, leaving it spread about and

[30]

ridged, as before. Return the dishes, without covers, to the
muffle, if the heat is still low. After one-half hour take out and
stir again. Continue this treatment, gradually increasing the
heat, till no odor of sulphur dioxide is perceived on stirring.
Finally, increase the heat to a bright red for about ten minutes,
to decompose any iron sulphate which may have formed.
Smelt the roasted concentrate with this charge :

Concentrates roasted

Litharge 60 grams

Soda 15 grams
Flour 4 grams

Silica 15 grams

Borax glass 10 grams
Borax cover.

Use a No. F crucible.

After the concentrates have been transferred to the crucible,
the roasting dish should be ''dry-washed" out with a little soda,
which is added to the charge.

Compare results with those obtained from the nitre assay.

ASSIGNMENT 17

ASSAY OF SULPHIDE ORES BY THE "NAIL METHOD"

The "nail method" is adapted to gold ores containing vary-
ing amounts of sulphides, although it may with certain modifi-
cations be applied to all gold ores. Ores containing appreciable
amounts of arsenic and antimony cannot be assayed by this
method, with this exception : arsenic frequently occurs in vanner
concentrates and may then be removed in the first stages of the
assay by oxidation with nitre, part of the sulphur being
oxidized at the same time.

In general, gold values will be higher by this method and
silver values lower.

[31]

Prepare two charges of Sample as follows :

Ore i/2 A.T.

Litharge 30 grams

Flour 2.5 grams

Soda 45 grams

Salt or borax cover.
Three twenty-penny nails.

Use a No. F crucible.

Add a weighed amount of test silver to each charge.

Stick the nails into the charge after the borax cover has been
added, points down. Should the nails first added be eaten up
by the charge before fusion is finished, take out what remains
of the old ones and add two or three fresh ones.

First fuse the charges in the usual way; when foaming has
ceased, give a very bright yellow heat for ten or fifteen minutes,
continuing the heating until, on removing the nails and rinsing
them in the slag, they are seen to be free from adhering globules
of lead.

Prepare one charge of Sample as directed for Sample

, except add 5 grams S1O2, and add no test silver.

Prepare two charges of Sample as follows:

Ore

1/2 A.T.

Litharge^

25 grams

Soda

45 grams

Borax glass

10 grams

Silica

15 grams

Five twenty-penny nails.
Salt or borax cover.

Use a No. F crucible.

Make the fusion as instructed before.

When arsenic is present it is well to omit the nails, add 10
to 20 grams of nitre, and * ' boil down ' ' the charge ; then add the
nails and finish at a high heat as usual.

[32]

In each of the above cases compare results with those obtained
from the litharge assay.

What do your results indicate?

In what way is the nail assay convenient ?

ASSIGNMENT 18

ASSAY OF TAILINGS

Mine samples usually consist of one or all of the following:
quartz, quartz and varying amounts of sulphides, concentrates
and tailings. You have had practice in assaying all the above
except the last. Tailings are composed (i.e., stamp-mill tailings)
mainly of quartz. No sulphide should be present.

It is often necessary to determine values in tailings as close
as five cents per ton. This would require 4-5 A.T. to be taken
at one time, or a number of smaller charges, and the resulting
buttons combined. In the latter case 5-8 gram buttons could be
reduced and cupellation effected without previous scorification.

The following charges can be used to advantage:

Ore

Litharge

Sodium carbonate
Flour

Borax glass
Borax cover.
Crucible

2 A.T.

3 A.T.

4 A.T.

120 grams

180 grams

240 grams

60 grams

90 grams

100-120 grams

2 grams

2 grams

2 grams

10 grams

10 grams

10 grams

Sample Prepare three charges, using 2 A.T., 3 A.T.

and 4 A.T. Add one or two mgs. of silver to each charge.
Report in the usual way.

[33]

CHAPTER IV

BULLION ASSAYS

Bullion, in general, is classified* as follows:

1. Lead bullion, usually the product of the lead blast-furnace ;
95 per cent and more lead, containing some copper, antimony,
etc., silver and gold.

2. Base bullion, containing from 100 to 925 parts of silver
per 1000, gold in varying amounts, and a large percentage of
base metals, chiefly copper, zinc, lead, etc. Produced most
frequently by cyanide mills.

3. Dore bullion, containing 925 to 990 parts of silver per
1000, some gold, and base metals, mostly copper, but also lead,
antimony, zinc, etc.

4. Fine silver bullion, free from gold, containing 990 to 1000
parts silver per 1000, but some base metals, usually copper.

5. Silver bullion, containing little base metal and less than
half its weight in gold.

6. Gold bullion, containing little base metal and more than
half its weight in gold.

7. Fine gold bullion, free from silver, containing from 990 to
1000 parts gold per 1000.

You have had practice in assaying lead bullion (see Assign-
ment 4). Gold bullion and fine gold bullion may be considered
under a general case; likewise base bullion, dore bullion, fine
silver bullion, and silver bullion.

* Fulton: Manual of Assaying.

[34]

ASSIGNMENT 19

ASSAY OF GOLD BULLION

The bullion that you are to assay in this assignment is made
to represent as closely as possible that which comes from Cali-
fornia mines.

The Preliminary. — In this class of bullion an expert can tell
with sufficient exactness the fineness of a particular sample and
so can alloy in the proportion of 2% to 1, without making a
preliminary survey. Ordinarily the base metal fineness is not
taken into account, a small amount of copper being added to the
proof center to prevent sprouting, and to toughen the bead.
But, for the general case, and for completeness, a preliminary
assay must be made in order to determine the approximate fine-
ness of gold, silver and base.

Weigh out 90-100 mgs. of the bullion given you ; also 300 mgs.
of test silver; wrap silver and bullion together in 3 grams of
sheet lead, and cupel at a little above the feathering point.
Brighten well; clean, and part the bead.

Report the results according to the following example :

Bullion taken 100.0 mgs.

Test silver 300.0 mgs.

Bullion and test silver 400.0 mgs.

Cupelled bead 385.0 mgs.

Fineness

Base metal 15.0 mgs. 150

Gold (by parting) 80.0 mgs. 800

Silver (by difference) 5.0 mgs. 50

1000

These results should be corrected by adding 15-20 points to
the silver fineness and substracting the same amount from the
base metal fineness. This is to allow for silver lost during
cupellation.

[35]

Having determined the approximate fineness of the bullion
given you, weigh out two samples of 500 mgs. each (within
.05mgs.); also weigh out test silver (within .10 mgs.) so that
the ratio of silver to gold will be 2y2 to 3.

Make a proof center of pure gold, silver and copper to
correspond with the regular samples.

Wrap each sample in 6 grams of sheet lead, or as many as
the following table calls for. For bullion containing over 900
points of precious metal 5 grams of lead are sufficient to remove
all the base metal. The following table* is valuable for determin-
ing the least amount of lead to use :

Precious Metal in Alloy Lead required
.900 10 parts

.800 16 parts

.700 22 parts

.600 24 parts

.500-.050 34 parts

Cupellation. — Before introducing the samples into the muffle,
have the temperature at a bright red (higher than for ordinary
cupellation), and the cupels, which should be smooth, hot.

Place the lead packets in their respective cupels, the proof
in the center, and then close the muffle door. As soon as they
uncover, open the door and allow the cupellation to proceed in
the usual manner.

Take care that no metal is sticking to the sides of the cupels.
Brighten the cupels at a bright red heat, cover with hot cupels
and remove gradually from the muffle. Clean carefully and
weigh.

Now hammer the beads into the shape of a rectangular bar,
then roll out to the thickness of an ordinary visiting card by
means of the rolls ; or in the absence of rolls, hammering will do
In rolling, anneal the alloy whenever it becomes hard and elastic

* Mitchell: Manual of Assaying, p. 610.

[36]

and before it begins to get rough on the edges. The three beads
should be of the same thickness after rolling. Great care should
be taken to prevent mechanical loss.

Parting. — Clean thoroughly three matrass flasks. Place 2 ozs.
of 1.16 sp. gr. nitric acid in each; now introduce the flattened
and annealed beads, first rolling them into a spiral coil, and
heat to boiling.

Boil gently until action ceases (ten to fifteen minutes), then
pour off the spent acid and add 2 ozs. of strong acid (1.26 sp.
gr.). Boil gently with this acid for fifteen to twenty minutes.
Place a piece of charcoal or burnt brick in each flask to prevent
bumping. Pour off this acid and wash the gold three times in
hot water. Transfer to annealing cups and anneal.

Weigh the gold carefully; and record the results according
to the following example:

Bullion

Pure gold

Test silver

Copper

Sheet lead

Total gold and silver

Cupelled bead

Cupellation loss-
Proof center loss

Base metal
Parted gold
Pure gold

Surcharge

Corrected gold

Base metal

Silver (by difference)

Silver fineness
Base metal fineness
Gold fineness

Sample A

Proof center

Sample B

500.0 mg.

500.0 mg.

400.0 mg.

975.0 mg.

1000.5 mg.

975.6 mg.

75 mg.

6 grams

6 grams

6 grams

1475.0 mg.

1400.5 mg.

1475.6 mg.

1395.0 mg.

1395.2 mg.

1396.3 mg.

80.0 mg.

4.3 mg.

79.3 mg.

4.3 mg.

4.3 mg.

75.7 mg.

75.0 mg.

402.1 mg.

400.5 mg.

402.0 mg.

400.0 mg.

.5mg.

.5mg.

.5mg.

401.6 mg.
75.7 mg.
22.7 mg.

45.4
151.4

803.2

401.5 mg.
75.0 mg.
23.5 mg.

47.0
150.0
803.0

[37]

Remarks. — Gold results should agree within 14 point. Silver
results will depend on the care used in cupellation, as will the
base metal fineness. Surcharge is the difference between gold
lost during cupellation and parting (gold may be lost by
volatilization, cupel absorption, or solution in the acid) and the
amount of silver retained in the parted. It is generally positive.
A negative surcharge is to be regarded with suspicion.

In order to get precise results it is necessary to have pure
gold for the proof ; parted gold from previous assays will do, but
then an arbitrary surcharge will have to be used, say i/4 to 1
point.

When rolls cannot be obtained, the gold bullion assay may
be made as follows :

1. Make a preliminary assay as usual.

2. Weigh out 250 mgs. of bullion, instead of 500 mgs., and
add sufficient test silver to make the proportion 3 to 1. Make
up the proof in the usual way.

After weighing the cupelled beads, hammer until flat ; anneal
and part, using acid of 1.10 sp. gr. Boil for five minutes, decant,
wash three times, anneal and weigh.

This method has the advantage of being much more rapid
than the former and at the same time, if carefully done, is
sufficiently accurate.

EEFEEENCES

Eose: Metallurgy of Gold, pp. 462-492.

Aaron: Assaying, part 2, pp. 30-47.

Pack: "Assaying of Gold and Silver in the U. S. Mint," Min. and

Sci. Press, Nov. 14, 1903.
Whitehead & Ulke : « ' The Sampling and Assaying of Gold Bullion, ' ' The

Engineering and Mining Journal, Feb. 12, 1898.

[38]

ASSIGNMENT 20

FIRE ASSAY OF SILVER BULLION

Make an assay of the sample of silver bullion obtained in
Assignment 22, following the directions given in Fulton 's Manual
of Assaying.

Record results according to the following example:

PRELIMINARY ASSAY
Bullion 500 mg.

Lead 10 grm.

Cupelled bead 440.4 mg.

Parted gold none

Silver to be used in proof 445-450 mg.

Gold to be used in proof none

Copper to be used in proof 50-55 mg.

FINAL ASSAY

Bullion

Silver

Gold in proof

Copper in proof

Lead

Gold and silver in proof

Cupelled bead

Cupellation loss
Proof center loss

Base metal
Parted gold

J3ase metal fineness
Gold fineness
Silver fineness

Sample A
500 mg.

Proof
447.8 mg.

Sample B
500 mg.

none

3.5 grm.
442.5 mg.

52 mg.
3.5 grm.
447.8 mg.
444.8 mg.

3.5 grm.
443.0 mg.

57.5 mg.
3.0 mg.

3.0 mg.

57.0 mg.
3.0 mg.

54.5 mg.

54.0 mg.

none

none

none

109

108

000

000

891

892

1000

1000

Use the utmost care in cupelling. Do not neglect to cover
the cupels with hot cupels when the beads have brightened; a
bead that has sprouted must be discarded and the entire assay
repeated. EEFERENCES

Fulton: Manual of Assaying, pp. 175-177.
Furman: Manual of Assaying, pp. 276-280.

[39]

CHAPTER V

MISCELLANEOUS WORK

ASSIGNMENT 21

FURNACE REPAIRS. MAKING CUPELS

1. You are assigned to a particular furnace, and you are
expected to keep it in repair. Examine it carefully ; especially see
that the muffle is in good condition. If it is not, remove it and
put in a new one. The following points should be considered in
placing a new muffle : Have the muffle well supported in the back
and front, give it a slight inclination downwards toward the
front, and have the cementing material of the right consistency.
Fire-clay with a certain proportion of sand or broken fire-brick
is the cementing material used.

Make up the cement in the following manner : Take fire-clay
and fire-brick in the proportion of one to three. Crush the fire-
brick to pass an eight or ten mesh screen. Mix the two dry and
then add water until the whole is in a plastic condition. Moisten
thoroughly the brick-work and those portions of the muffle that
come in contact with the clay, then press the clay in firmly and
allow to dry at least twenty- four hours.

Make a drawing (plan and sections) of your furnace, to scale.

2. Make 200 good cupels, as follows :

Take the box used for the purpose and fill with bone-ash,
add water and mix. Use as many ounces of water as pounds of
bone-ash. The right amount of water has been added when the
bone-ash loses its harsh, gritty feeling, and at the same time is
not pasty. A small amount of potassium carbonate (2 per cent
solution) may be added to assist in cementing the bone-ash, but
this is not necessary. When the bone-ash is thoroughly mixed
and is of the right consistency, sift through a twenty mesh sieve
and then place a sufficient amount in the mould, and hammer
down firmly; two hard blows are enough; do not use anything
but the wooden mallet in striking the mould. Place your cupels
on a board and set away to dry.

[40]

ASSIGNMENT 22

MELTING, REFINING AND SAMPLING OF GOLD AND
SILVER BULLION

Melting. — The object of this experiment is to make you
familiar with some of the conditions under which a gold bar
of moderate size is melted and cast.

Build a solid fire in one of the wind furnaces. Anneal a
No. 12 graphite crucible, if one has not already been annealed,
and place in the fire. Then place the copper bar given you in
the crucible, and when the copper is melted cover with powdered
charcoal to prevent oxidation. When the metal is perfectly fluid,
pour into a large mould that has been first heated and oiled.
When the metal is solified, turn the mould over, and the bar
can then be readily removed. Immerse the bar in a dilute
solution of sulphuric acid, and scour with sand. Weigh in
avoirdupois pounds and ounces. Convert into troy ounces.

Calculate the value of this bar on the supposition that it is a
gold bar of the fineness of the bullion assayed in Assignment 19.
Give the value of the silver in troy ounces only, of the gold in
dollars as well as ounces.

Refining and Sampling. — Weigh the silver bar given you,
recording the result in troy ounces.

Anneal (if necessary) a graphite crucible of proper size, and
place in the furnace. Then place the silver bar in the crucible.
When the silver is thoroughly melted, refine, following instruc-
tions as given in references, and lecture notes.

W7hen sufficiently refined, pour or cast the bar as directed for
the copper bar. Clean, dry and weigh, as before.

Take samples by cutting off from diagonally opposite corners,
two or three grams of metal. Keep these samples separate.
Assay as directed in Assignment 20. Reweigh the bar. Make
detailed report.

EEFERENCES

Aaron: Assaying, part 2, pp. 21-46.
Kose: Metallurgy of Gold, p. 387.

[41]

ASSIGNMENT 23

COMPARISON OF METHODS FOR ASSAY OF COPPER
ORES AND COPPER-BEARING MATERIALS

Scarification. — The standard charge for the assay of copper
ores and copper bearing material by the scorification method is
as follows :

Ore Ho A.T. to ifa A.T.

Granulated lead 50 grams
Borax glass 1 gram

Silica 1 gram

Use 3 in. scorifiers.

Re-scorify lead buttons, using 2% inch scorifiers and enough
test lead to make total weight of lead not less than 35 grams.
When the ore or copper-bearing material is poor in copper the
silica and re-scorification may be omitted. As many assays may
be run as are necessary to give accurate results.

Cupel and slag corrections, either or both, are sometimes
required and may be made as follows:

The cupels are taken in lots of two each, the unused portion
discarded, the remainder crushed to pass 100 mesh, and fused
with 100 grams of litharge, 20 grams sodium carbonate, 40 grams
borax glass, and 3 grams of argol.

The slag is merely ground to pass 100 mesh, and fused with
the addition of, say, 40 grams of litharge, 20 grams of sodium
carbonate and 3 grams of argol.

In both cases the resulting lead buttons are cupelled and
the silver and gold obtained added to the first weights.

Crucible. — A standard method for the assay of the above by
the crucible method is as follows:

Ore Ho to %0 A-T.

Litharge 80 grams

Silica 12 grams

Sod. Garb. 16 grams

Borax glass 12 grams

Eeducer or oxidizer for a 20 gram button.
Use a No. F crucible.
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For ores or other material containing less than 40 per cent
copper, the amount used for assay may be increased up to % A.T.
Some or all of the silica may be omitted.

Prepare eight charges of Sample , a copper matte

rich in copper, according to the methods already outlined, four
by scorification and four by crucible fusion. Use proper pre-
cautions in melting and cupellation.

Make a tabular report, showing comparative losses, etc.

ASSIGNMENT 24

PRACTICE IN SAMPLING, PANNING AND
AMALGAMATING

Sampling. — Take the sample of ore given you, weigh, and
then reduce in size (weight and mesh) by approved methods.
Your assay sample should weigh at least % Ib. and should pass
at least a 100-mesh screen. Take care that all scales pass the
screens used.

Reserve all discarded ore passing the 40-mesh screen and keep
for the panning and amalgamating experiments.

Amalgamating :* — Weigh out five charges (100 grams each)
of the 40-mesh lot, and place in bottles provided for the purpose
with mercury (10 grams), and 150 c.c. of water; and agitate for
thirty minutes. Separate the mercury by panning (see below)
and transfer to a porcelain dish, dry with filter or blotting paper.
Place the thoroughly dry mercury in a 20 gram or F crucible
in which 20 grams of lead have been placed. Add to the crucible
a charge consisting of 30 grams of litharge, 10 grams of soda,
5 grams of borax glass, and 0.5 grams of argol, and silver enough
to part the gold.

Fuse the charge very slowly, starting with a slow fire in the
wind furnace or if using a muffle, one that has not yet become

* A modification of Fulton 's method.

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red and has a good draught through it to prevent the escape
of fumes into the room. When fusion is quiet, pour and cupel
the resulting lead button. Part and weigh the gold.

Panning. — The contents of the bottles are to be panned
separately and then together. Pour the contents of one bottle
into a gold pan, add water if necessary, and wash by giving the
pan a vibratory motion as in vanning, washing off the light
particles of gangue and leaving only the mercury and the heavy
portion of the ore, as black sand, sulphides, etc. The light
portion of the ore should be washed into a pan or vessel of
somewhat larger size than the gold pan. Repeat the panning of
the mercury and concentrates in order to separate the mercury.
If the mercury is floured, add a small globule of liquid sodium
amalgam.

Treat the contents of the other bottles in the same way. The
light portion or tailings of the several bottles may be poured
into the same vessel. Re-pan this material to make sure that
no mercury or sulphides have escaped. If any is recovered, add
to the amount recovered before.

Treat the collected mercury as directed above (see amalga-
mating).

Allow the tailings to settle, pour off the surplus water, and
dry by any convenient means. When dry, weigh, pass through
a 100-mesh screen, mix well and assay.

Wash the heavy material or concentrates into a small por-
celain pan, or if more convenient on to a batea. Examine under
a magnifying glass or low power microscope, and note the
different kinds of sulphides or other constituent, and the approxi-
mate amount of each.

Dry, weigh, crush to pass a 100-mesh screen, and assay.

Prom your results calculate the following:

1. Assay value of ore in ounces, and dollars and cents per ton.

2. Gold amalgamated in ounces, and dollars and cents per ton.

3. Assay value of concentrates in ounces, and dollars and
cents per ton.

[44]

4. Assay value of tailings in ounces, and dollars and cents per
ton.

5. Percentage of gold amalgamated.

6. Total gold contents in comparison with assay value.

7. Percentage and different kinds of sulphides or other heavy
material.

REFERENCES

Fulton: Manual of Assaying, pp. 36-4 land 154-156.

Furman: Manual of Assaying, chap. 2.

Brown: Manual of Assaying, pp. 185-194.

Hersam: "Principles in the Practice of Sampling," Min. and Eng.

World, Nov. 25, 1911.
Brunton: "Modern Practice in Ore Sampling, Trans. Am. lust. Min.

Eng., vol. 40, p. 567.
Hofman: The Metallurgy of Lead, chapter V.

ASSIGNMENT 25

FIRE ASSAY OF LEAD ORES

Lead assays are usually made in the muffle ; but, in the absence
of such, a wind furnace may be used. The muffle is to be
preferred because the heat can be more uniformly controlled,
and there is less danger of loss by volatilization.

All our lead assays will be made in the muffle furnace.

The fire assays of lead ores may be placed under three heads,
according to the kind and proportion of flux used :

1. Fusion with sodium bicarbonate, borax, argol, and iron.

2. Fusion with mixture of sodium bicarbonate and potassium
carbonate, borax, argol, and iron.

3. Fusion with potassium cyanide.
Sample Pure assayers' litharge.

[45]

Prepare three charges as follows:

Ore 10 grams

Lead flux 35 grains

Salt cover.

Use Battersea P crucibles.

Manage heats as directed below j pour into scorification
moulds.

Clean buttons by brushing and hammering. Weigh on pulp
balance to .01 gram. Report to .10 per cent.

Sample Pure Galena.

Prepare three charges as before, with the addition of four
nails to each crucible, and five grams of borax glass on top of
each charge before salt cover.

Assay Sample in the same manner.

Your results should check to within .5 per cent. Write your
notes carefully.

The lead flux is a mixture of sodium bicarbonate (12 parts)
potassium carbonate (12 parts), argol (3 parts).

The first method should be used only when potassium car-
bonate cannot be procured. The third should be used only on
pure ores. The second is preferable in all cases.

Heats. — The fusions are best managed by giving a low heat
for about twenty minutes and then a high heat to finish. This
is necessary for two reasons : ( 1 ) Lead and its compounds are
volatile; and if the heat is so high that boiling takes place
violently, some lead is almost certain to be lost. (2) Lead forms
a double sulphide with sodium ; and to reduce this by means of
iron, a high heat is necessary.

The nails should not be removed until entirely free from lead.

Fusion with cyanide requires a very low heat.

EEFEEENCES

Hofman: The Metallurgy of Lead, chapter V.
Fulton: Manual of Assaying, pp. 199-201.

[46]

— -saws."""

OVERDUE.

lord Bros.

Makers

Syracuse, N. Y.
W. JAN. 21, 1908

YC 18949

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