118

GOLD.— Assaying.

Cnpellation of the Assay .—The process of cupella-tion on the large scale has been already described.When applied to assaying the principle is the same ;bnt in this case the whole of the melted litharge andother oxidized matters are removed by absorptioninto the cupel; whereas it has been shown that inoperating on large quantities the cupel is soonsaturated, and the greater part of the litharge andother oxides are allowed to ran away, or are expelledby the current of air which causes the oxidation.

The cupels for assaying purposes are formed ofthe same material as those employed for refining onthe large scale, and described at page 91; but theyare necessarily much smaller, and are considerablysimpler in their manufacture. They are made bypressing the moistened bone-earth into a mould ofcast iron or gun metal, which varies in construction.Two forms at present in use in the Royal Mint areshown in Figs. 1 and 8 on Plate III. one-half theiractual size. In the first of these, after the space inthe piece of metal, A, has been rather more thanfilled with bone-ash, the guide, b, is placed over it.The plunger, C, having been introduced into thisguide, the whole is placed under a common leverin press, by which means the bone-ash is compressedinto the form shown in Figs. 33 and 34, andFig. 1. on the Plate. Formerly theFi K . ss. plunger was struck several times witha hammer in order to give the requiredcompression. It is desirable to give aslight twisting motion to the plungeron removing it. The second form ofmould, shown in Fig. 3, has been intro-duced in order to diminish the labourFig. 34 . of the furnace manipulation. It willbe seen that each cupel is square, andis provided with four depressions. Each of theseis, however, rather smaller than the one alreadydescribed, the amount of metal operated uponin the two cases being about 7-5 and 16 grainsrespectively. The square cylindrical piece, b, havingbeen placed on the plate, a, is filled with bone-ash,and with the plunger, c, is introduced as before intothe press, the cupel when formed being five-eighthsof an inch deep, square, and slightly tapered towardsthe bottom. The cupels when formed should bekept in a dry place for several months, as it is foundthat when any method of hastily drying them isadopted they almost invariably crack when intro-duced into the muffle, and cause the assays to spurt.

Cupellation, like the process last described, isperformed in a muffle furnace, as shown at Fig. 2 ,Plate III. At the time of lighting the furnace, therequisite cupels are successively introduced into themuffle by a pair of light tongs of the form repre-

Fig. 35.

sented in Fig. 35, the floor of the muffle beingcovered with a thin layer of bone-ash before theintroduction of the cupels. In the furnaces of the !

Mint the number of assays that can be made atone time is forty-five when the large “ assay pound ”is used, and seventy-two with the small one, andit is usual to introduce an additional row of cupelswhen possible, to be employed in case of accident.The temperature of the muffle having been raised toan orange red heat, the assay pieces, either in the formof buttons mixed with lead as obtained by scorifica-tion, or as samples of alloy wrapped in lead-foil, asalready described, are successively introduced into thecupels, care being taken that they are so arranged as toavoid any possibility of confusion on their subsequentremoval. It is advisable that while the muffle isbeing thus “charged” the door should be partiallyclosed by means of a hot firebrick, in order to checkas much as possible the chilling of the cupels whichare near to it. The requisite amount of lead (whichwill be presently given) may be all introduced as awrapper of the assay piece. Of course when thealloy for assay consists of a button obtained by aprevious process of fusion or scorification, this addi-tion of lead is unnecessary, as a sufficient amount isalready contained in the button.

By the previous process of fusion with litharge, orthat of scorification, if deemed preferable, the earthyimpurities have been removed in the form of slag orscoria, and nothing now remains but the gold, or amixture of gold and silver, in combination with thelead and any other metals that may be present. Theprocess of cupellation, as previously stated, consistsin the oxidation of the lead and other oxidizablemetals, and their absorption by the cupel. It is, infact, a condition essential to the success of the assay-ing process that the whole of the oxides should beimbibed, and thus removed. But the power ofabsorption in a cupel varies according to its texture,and the care with which the material has been pre-pared. On an average, it may be calculated that itwill absorb about its own weight of fused litharge.This, though not strictly correct, will help to guidethe experimenter, as regards the amount of leadwhich ought to be mixed with the assay. It isevident, on the one hand, that this must not exceedthe absorptive power of the cupel. On the otherhand, the actual amount within this limit mustdepend on the nature of the alloy upon which thecnpellation is to be performed; but supposingthat the subject of assay is an alloy of gold andcopper, the following table will give the quantityof lead required for the separation of the copper.In treating an alloy of gold and silver with copper,the quantity of lead may be somewhat diminished;but in cupelling an alloy of copper with gold alone,a small excess of lead is not injurious, as, in conse-quence of the gold not being volatile, like silver, theheat can be raised and sustained for any length oftime, so as to oxidize the last portions of lead. Inthis table, which shows the proportion of lead recom-mended by D’Arcet, the first and second columnsexpress respectively the quantity of gold and ofcopper in 1000 parts of the alloy, and the thirdcolumn gives the number of parts of lead required forone part of the alloy according to its composition:—•

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