Metallic Copper: Quick Silver & Gold Assay Method

Metallic Copper: Quick Silver & Gold Assay Method

Under the above title it is desired to bring to the attention of assayers a short-cut method which has been perfected by Mr. A. S. Warren, of Buffalo, and the writer; has been used several years at three “ Lake copper ” refineries in occasional alternation with the regular “ combination,” or “ wet and fire,” assay; and has proved more rapid, and rather more accurate than the latter on all refined copper—namely, the direct cupellation of the silver chloride (4-gold) precipitate, without scorification.

No claim of priority is made for this direct method, as it has suggested itself to many others, and was mentioned by Mr. Warren in the symposium of assayers’ results brought together by the Secretary, after the plan and suggestions of Dr. A. R. Ledoux.

Lately, this important subject of assaying copper-materials seems to have been re-opened for discussion.

The shortening of time by the direct method is so considerable (nearly 20 minutes on a batch)—to say nothing of the saving of material and the confinement of possible errors in regulation of furnace-heat, etc., to one operation instead of two—that it seems strange that this quick process has not received in print, at least, the attention it deserves.

The fact that an occasional attempt to cupel directly the silver chloride showed loss by “ spitting ” may have led some assayers to conclude that a preliminary scorification was necessary.

The writer proposes to prove, by the tabulated results of experiments:

  1. That although some little time may possibly be required to acquire the proper details of manipulation, yet there need be no loss by spitting if the operation is properly conducted.
  2. That the amount of lead required is only from one-third to one-half that required for the old method.
  3. That direct cupellation of fairly pure silver chloride gives, if anything, a little less loss than the process involving preliminary scorification, and may be used at will, interchangeably with that process, on all metallic copper which does not leave, on solution in nitric acid, too much residue of silica or sulphur. In the writer’s experience, it has been necessary to reject very few check-assays fired by cupellation, in the regular work of the office.
  4. That a quantity of insoluble antimony oxides containing over 0.1 gramme of metallic antimony may be easily removed, under the reducing conditions provided for by the writer, without any preliminary scorification. A small part slags off, but the greater part seems to be reduced enough to volatilize.

It is generally admitted, not only by assayers, but by buyers and sellers of metal, that the fire-assay for silver is not a strictly scientific and accurate process, but a commercial one, involving a certain loss in precious metals, which we can only strive to keep within narrow limits by proper manipulation. Accordingly, anything which can save a little time and reduce chances for error seems worthy of careful experiment and detailed description.

The investigation here recorded involved a comparison of the losses sustained by weighed samples of pure metallic silver cupelled with lead, with the corresponding losses on silver chloride treated in three ways—namely, by direct cupellation with the powdered lead; by such direct cupellation in the presence of insoluble oxides of antimony ; and by scorification with 15 grammes of powdered lead and 5 grams of litharge before cupellation.

The experimental assays were made with three separate quantities of silver (20+, 50+, and 150+ milligrammes respectively). The writer used a sheet of silver foil purchased from dealers, and found by wet assay that it was pure enough for the purpose. Such foil is sometimes not strictly of the same degree of fineness throughout; but only slight variations could result from this under the conditions.

Of the powdered lead, 100 grammes yielded only 0.02 milligrams of silver, which could not affect results. The lead used should be granulated to pass through a sieve of 30 meshes to the linear inch (2.5 centim.).


The assays were cupelled in batches of four, in a single row, one-fourth to one-half inch (6 to 12 centim.) from the front edge of the muffle of a No. 2 Hoskins gasolene furnace, the position being regulated according to the heat existing at the time. Each single row of four 1.25 in. cupels was planned to contain : (a) one sample of metallic silver, as a proof; (b) one of silver chloride; (c) one of silver chloride with oxides of antimony; and (d) one button of lead and silver from the previous scorification of silver chloride.

The loss in the samples b, c and d of any batch may thus be compared with the loss at the same heat in the “ proof a.”

The order of the several cupels in each batch of assays was varied, each line, read horizontally from left to right, representing a row of cupels:


Wet assays of the standard silver (by precipitation as chloride—or bromide—and ignition in porcelain crucibles) were also made, to prove that the losses, shown in furnace-work, were not due to any error in solution and precipitation of silver chloride or want of fineness in the silver employed.


Mr. Warren says, in a letter, that he has found the use of bromides instead of chlorides, as recommended by Cabell Whitehead, of great advantage in the wet “ mint ” assay of silver bullion, shortening the required time nearly one-half.

More details of the method used for direct cupellation of the chloride, as obtained from a solution of copper borings, will be given to indicate the essential points.

When the determination of gold is desired it is filtered off first. The writer recommends the plan adopted by assayer “ Q ” in the symposium, until some better collector is found. He obtained high results on gold, and avoided loss from excess of chlorine, by adding only one drop of solution of chlorides to the nitric acid solution of copper and silver, to collect the gold. The dried filter may then be held in a loop of platinum wire and carefully charred, allowing the half-burned ash to drop into the filter containing the silver-chloride.

Any globule of sulphur might be picked out of the solution before precipitation, burned off, and the residue replaced in solution. The chloride is filtered off on a 7-centim. S. and S. washed filter, No. 589.

A solution of one assay-ton of copper-borings may be filtered and washed in less than ten minutes by using a large-stemmed 2-inch funnel, placing under the filter a platinum cone with very coarse perforations (about 1 mm. in diameter), and keeping the stem full of filtrate.

Portions of 6 grammes of lead are placed on little rectangular pieces of sheet-iron (2.5 by 1.5 in.), with the corners turned up a little at one end, to form a small scoop which can be quickly handled by the cupel tongs.

The filters, still wet, are taken from the funnels in order of number, and from 1.5 to 2 grammes of the powdered lead are dusted over the inside. The top of the filter is closed together and the filter folded down upon itself, so that the side of double thickness shall be the outside.

The red-hot cupel is quickly drawn forward and the remainder of the lead spread in it, the folded filter placed in, point down, and the cupel shoved back into the hottest part of the muffle.

The reduction of the silver chloride should be quick, and completed before the outside envelope has been burned off; and the door of the muffle may have to be closed as soon as the cupels are filled.

This description applies to muffles without forced draft. Mr. Warren says that, with natural gas and forced draft, he has thought best to bring cupels forward, outside the furnace, while burning the paper, and then to cover with lead and cupel.

As soon as the lead is hot the cupel may be tipped a trifle, if any little globule of lead is caught just above the edge of the molten bath, and is then brought to the front of the furnace, and the operation finished at low heat with the usual precautions—production of feather-litharge, etc.

Assay Results

Table I. shows the losses of silver resulting from the separate conditions specified.

Assays made under the same conditions (although at different times) were grouped together in the table, so that the averages might be exhibited.

Bringing together the average losses, under varying conditions, we have the results shown in Table I.

This table contains some figures on silver-loss in cupelling, taken from Table II. in Furman’s paper, the tests of Mason and Bowman, and the results of Godshall. The percentage- loss of silver seems to be a trifle less, the larger the silver button; hence only those results could be compared which were the result of tests on the same weights of silver.



Furman used rather more lead than the others. Mason and Bowman did not give any particulars. The agreement is really very close.

The effect of an excessive quantity of antimony is to increase the losses and form a ring of slag in the cupel, even though the writer dusted some lead chloride into the filter. This salt appeared to assist in the removal of an excessive amount of oxides of antimony. The oxides resulting from 0.1 gramme of this metal are as much as would ordinarily be found even in a casting copper, and there is no difficulty in removing that amount from the silver by direct cupellation of the chloride with 6 grammes of lead, observing the precautions given.

It is our experience that after one acquires the knack or cupelling the chloride precipitate without “ spitting,” the method rarely fails to show good agreement in check-assays.

As the question of solubility of gold in the nitric-acid solution of copper is another matter, and a special problem not yet satisfactorily solved, it has not been discussed here.

Out of 274 pairs of original check-assays of silver in various Lake coppers, taken consecutively without selection, and fired by two operators, 240 pairs agreed within 0.5 oz., 21 more were within 0.7 oz., and the remainder of 13 within 1 oz., Troy, per ton.

Of the 13 mentioned, it was found that a few of the sample- plates cast from the molten charges were uneven, probably from pouring too cold. Sample-plates 6 in. square and 0.5 in. thick were used to avoid the bad effects of the segregation of silver on cooling—a question which was ably treated, some time ago, by Edward Keller, of Baltimore.

Considering that the direct cupellation of the silver (and gold) chloride precipitate saves time, confines the chances of errors in judgment of temperature, etc., to one operation, and reduces to one-third or one-half the amount of lead required, we recommend that assayers should try this quick assay for refined coppers and anodes, reserving the usual “ combination ’’ method for mattes and crude copper, which leaves so much siliceous residue, etc., that a slagging melt in the scorifier is necessary before cupellation.

The short-cut process which has been so carefully tested by Mr. Heath has, of course, suggested itself to nearly all assayers who have determined silver in copper material by the “ combination ” process. It was tried in my laboratory several years ago, and our results entirely confirm the conclusions of Mr. Heath. Where one is handling refined copper, such as anodes or the Lake product, it is safe to cupel the chloride without preliminary scorification. But on converter-copper, or any impure form of pig, it cannot be employed, for the reason which Mr. Heath states : namely, there is too much sand, sulphur and other insoluble matter in such products.