(c) THE ESTIMATION OF GOLD IN GOLD BULLION.

Various grades of bullion are met with-rich in gold and poor in gold; gold alloyed with silver, or silver and copper, or silver, copper, and zinc; or gold with silver, copper, lead, and zinc. For full information on this subject the student is referred to Rose, Beringer, Furman, and Ricketts and Miller.

Here the student is asked to determine the gold in standard gold coinage which contains 916.6 parts gold per 1000, that is, its 'fineness' is 916-66. When the student attains to the necessary degree of accuracy in this assay, he may then proceed with the estimation of gold in base bullion as described in the works mentioned, provided he has the spare time, which is not likely unless he extend his three years' course to four as advised. The composition of the English half-sovereign is as follows:

Gold, 916-6 parts; silver, nil; copper, 83.3 parts.

Method.-A certain weight of the coin* is taken, and sufficient silver is added to bring the proportion of the total silver present to the gold as 2.5 to 1. The gold and silver are wrapped in sheet lead, cupelled, the button flattened, rolled up, and parted in nitric acid, and the resulting gold weighed. Any losses are estimated by running along with the assay a 'check' made up of pure gold, silver, and copper. The error so found is

allowed for.

Apparatus.—The assay balance, pulp scales, weights, a fine file or scissors, the muffle furnace and accessories, a thermometer and beakers (or the special parting bulbs), bullion rolls.

Reagents. Pure gold, pure silver, pure copper, test lead, and the two parting acids.

Details of the Assay. As a high degree of accuracy is required, the balance must be in perfect adjustment, and the bullion weights used must be checked, as was done with the weights used in gravimetric analysis. Scrupulous care must be exercised in keeping all apparatus, including the operator's hands, clean. It is only by scrupulous care in every step, combined with intelligent work, that a high degree of accuracy can be obtained. Take a half-sovereign' piece and pass it through the bullion rolls till of the thickness of an ordinary visiting card.

Place in the right-hand pan of the balance the half-g f-gram weight. This weight in a bullion set is marked 1000, and from it the weights range down to 100 Thoroughly clean the rolled coin by brushing. Cut off with a

pair of scissors (holding the clean gold in a pair of forceps) about one-sixth of the coin, letting the piece fall on a clean sheet of glazed paper. Transfer this piece to the left pan. Slightly release the beam, and it will be seen that the piece is too heavy. Arrest the beam. Remove the piece with the forceps and clip off a thin shaving. Again try the weight, and again clip if necessary till nearly the correct weight. The final adjustment is now made by firmly gripping the piece with the forceps in the left hand and gently applying a fine file with the right hand. In cold weather take care that the breath does not condense on the gold or balance and weights.

The student will find considerable difficulty in perfectly adjusting the gold to the weight. If time be valuable, let him note down the weight when nearly adjusted (e.g. 1.0012). Repeat the the process, obtaining a duplicate assay piece. The student has now weighed out two lots of the coin, each approximately 1000, or 5 gm. As a check, weigh out another piece, taking this time 9166 of pure proof gold. (See Proof Gold.

* In this case, of Australian coinage, i.e. a gold-silver alloy.

Several

students may combine in preparing proof gold. sovereigns, nine-tenths of which is recovered.

The outlay runs into one or two The cost therefore is small.) now to be added to bring the In samples 1 and 2 there is already required is (9166 × 2·5) — ·0834 = 9166 × 2.5 2.2915 gms.

=

To each of these three lots silver has proportion of silver to gold as 2.5:1. 0834 gin. silver; therefore the silver 2-2081. In sample 3 there is required Weigh out, then, two lots each of 2.20 silver and one lot of 2.29 silver. These instructions refer to Australian gold coinage alloyed with silver, but for English coinage alloyed with copper 2.29 gms. silver are required, and in the check 08 gm. copper should be included.

Weigh out three pieces of sheet-lead (containing no gold), each weighing 6 gm. (weigh on the pulp scales). The lead used varies in weight according to the contents of the bullion (see advanced text-books). Take each piece of lead and wrap it round the butt-end of a lead pencil, dumping in the projecting ends to make a secure packet. Charge each packet as follows:

If English coinage, alter the proportions as directed.

In charging each packet, roll up the gold, etc., so that it occupies little space; then transfer to the packet and close the packet carefully, compressing it in the fingers into a roughly spherical shape.

Take three bullion cupels (these are about " diameter, and their surfaces are faced with fine sifted bone ash); place them in the muffle at a bright red heat. After five minutes examine them, and if satisfactory, replace them in a row across the muffle. Place packet 3 with the check-piece in the centre cupel, and packets 1 and 2 in the side cupels. Cupel at a bright red heat. Allow the buttons to remain in for five minutes after the cupellation has apparently finished. Remove the cupels either gradually (taking three or four minutes), or rapidly by setting a hot cupel on top of each and then withdrawing. This must be done carefully, or the button may roll round the cupel and cause loss. When cool, carefully loosen each button on its cupel by means of strong pliers (this is easily done when only a little copper is present). Remove each button in turn, and brush off any adhering bone ash, gently tapping the edges with a light hammer-the button, turned on its edge, being held with the forceps. (This hammering is only necessary when the bone ash adheres very firmly to the button.) Replace the buttons on their respective cupels. Weigh them and note the weights.

The buttons must now be rolled out, and to secure uniform conditions they must all be rolled to the same thinness. A convenient size is 7.5 cm. by 1.2 cm. To roll out the button turn it on its edge, holding it in the forceps, and lengthen it by hitting a smart blow or two with a suitable hammer, the face of the

hammer and anvil being brightly polished. Now set the button on the anvil, with its original base down. Flatten to about 75 cm. in width with one blow on the centre, and then draw out the two ends, slightly wedge-shaped, by blows with a slight pulling motion. Fig. 96 shows the various stages from the button

to the 'cornet':-a, the button; b, the button after lengthening; c, after flattening; d, after being drawn out wedge-shaped; e, the cornet.

The button is now annealed by heating to dull redness on a cupel in the muffle or before the foot blowpipe. It is then introduced between the rolls, which must be set evenly apart from end to end, the screws being adjusted till the rolls grip the button well, but not too tightly. Pass the button through, and it should be now about one inch long. Bring the rolls closer together; anneal the button and pass through again; a third rolling should bring the button to the requisite thinness.

Auneal, and with the fingers roll the slip round a thin glass rod, keeping the bottom side (the dull side) out (see fig. 96 ƒ). The cornet is now ready for parting. The other buttons are now treated in the same way, taking care to note which is the 'check.'

Special platinum parting apparatus being expensive, the assays may be parted in beakers or in 5 cm. porcelain cups.

Place three small beakers on a sand bath in a fume chamber. In each beaker introduce 25 c.cs. No. 1 acid (116). Gently heat till 80° C. is reached. Then introduce the cornets, one in each beaker. Heat to boiling, and continue the boiling for ten minutes. Take each beaker in turn, holding it by the rim between the thumb and first two fingers, and remove it, and decant the acid down a glass rod into a 'waste silver' jar. Now add to each in turn 25 c.cs. boiling acid No. 2 (126). Replace and boil for ten minutes. Remove and decant as before. To each add about 25 c.cs. distilled water, decant, and wash each with two further lots of about the same quantity of water. Transfer each cornet to a small porcelain crucible by sliding it down the side of the beaker with the aid of gentle tapping. Remove most of the water adhering to the cornet and crucible by a small slip of filter paper. Gently dry the cups and contents on the sand bath. Then anneal in the muffle for a few minutes. Remove, cool, and carefully weigh the three cornets. Enter the results.

Calculations.—A concrete example will show the necessary calculations. Assume that

and assume also that you did not know the exact amount of gold in the coin. Then the check has gained in weight 917291660006. This quantity has therefore to be subtracted from the average of the two assays which is •9174 +9170 9172. Then 91720006·9166. And if 1.0012 (or other

2

=

weight of bullion taken) gives 9166 gold, what will 1000 bullion give?

Accuracy of the estimation.-The student will see that with care and the use of checks a high degree of accuracy may be obtained. With a balance sensitive to 10 mgm. and averaging three results, the fineness can be determined within ±0.02 parts per 1000; that is, if the true fineness be 9166, the results should range between 91662 and 91658. The student, however, with a balance sensitive to mgm., should obtain results correct within 3 parts per 1000; that is, the value of the bullion should be correct to a farthing.

Note. When the value of the bullion is unknown, a preliminary assay is done on a small cutting, which is treated for gold and silver like a bead from an assay. The results give data for calculating silver to add and for the gold, etc. in the check (see Rose's Gold).

CHAPTER X.

THE ASSAY OF SILVER IN ITS ORES AND PRODUCTS.

THE estimation of silver has, to a certain extent, been considered under gold ores and products in which the gold was supposed to predominate. Here such ores and products will be considered as generally contain higher values in silver than in gold. Most gold ores contain some silver, but not enough to bring them under this head.

The two materials here considered are

(a) Silver coinage (English).
(b) Galena.

(a) THE ESTIMATION OF SILVER IN COINAGE.

For practice, a convenient material to operate on is an English silver coin, which contains 925 parts of silver to 75 parts of copper by weight. The student, therefore, knows the composition of the alloy. When dealing with alloys of unknown composition the process is more complicated (see note appended).

Method. A given weight of the bullion is cupelled with lead sufficient to remove the copper. Any loss in cupellation is estimated by a 'check' assay and allowed for.

Apparatus.-The assay balance and weights, the muffle furnace and accessories, a fine watchmaker's file.

Reagents, etc.-Sheet lead, the silver contents of which have been estimated, electrolytic copper (sheet), and test silver.

Details of the Assay.-Weigh out, as before, four lots of 1000 each of a clean flattened silver coin, and two lots of 925 each of test silver, and two lots of '075 each of electrolytic copper, and six lots of 7 gms. sheet lead, the silver contents of which are known. Shape the lots of sheet lead into packets and arrange the packets as follows:

7 gms. Lead. (7 gms. Lead. [7 gms. Lead. (7 gms. Lead. (7 gms. Lead. (7 gms. Lead.

Nos. 2 and 5 are the checks; Nos. 1, 3, 4, 6 the assays.

Mark six bullion cupels with these numbers and place them in the muffle for five minutes at a temperature slightly below a bright red. Examine, and if satisfactory, replace in two rows across the muffle,-first row, 1, 2, row, 4, 5, 6; the checks being in the middle. Quickly introduce the six charges.

3;

second

Close the door till cupellation commences, and then open it and continue the cupellation so that 'feathers' just begin to form. It is rather difficult to obtain this heat, as the back row will be slightly hotter than the front; however, it may be closely approached by bringing the cupels forward in the muffle and keeping the two rows very close together, taking care, of course, that the charges do not freeze. When the blick (or coloured films) just disappears, close the doors for a few minutes and then remove the cupels, taking the usual precautions against the button spitting. When cool, clean and weigh the buttons, noting the results, and all details as to time of cupellation, temperature, etc. Calculations. Assume that the weights of the buttons are as follows :—

Add this to the average of the assays,

•9227+0026=9253.

That is, the fineness of the bullion is 925.3.

Accuracy. With good work, the cupellation loss should not be more than 6 parts per 1000, and on the average between 2 and 3 parts per 1000. This method, though not so accurate as the Volumetric method previously given, still is of a high degree of accuracy when checks are used. It further is of use in demonstrating to the student the necessity of careful cupellation when estimating silver.

Note. If the composition of the coin were unknown, a preliminary lot of 1.000 is cupelled with about 10 gms. lead. The result is weighed, and an equal quantity of silver is taken for the check: e.g. The preliminary gives 921 Ag. Make up checks of 921 Ag and 079 Cu (or other base metal if present). Then proceed as usual with the assay and

calculations.

(b) THE ESTIMATION OF SILVER IN GALENA.

The student has already assayed this ore for lead, and by cupelling the buttons so obtained he will obtain some idea as to the silver contents of the ore; but to obtain accurate results the ore must be specially treated either by crucible assay or by scorification. The student is advised to run four assays by the one method and four by the other on the same sample and compare the results.

(1) The Crucible Assay.

The principles of this method have already been discussed. Nails are used as before, and sufficient litharge is added to give a lead button of about 15 gm. Apparatus and Reagents. As before.

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Details of the Operation. Weigh out four charges of sampled galena (80 sieve) and fluxes, etc. as follows: