ASSAYING. 



lies on the surface as a red scoria. The 

 heat is known t.o be too great, when the 

 cupel can scarcely be distinguished from 

 the muffle, and the ascending fume can 

 scarcely be seen for the dazzling heat. 

 Towards the end of the operation the fire 

 should be gradually increased, for in pro- 

 portion as the lead is abstracted from the 

 alloy, it becomes less easy effusion ; and 

 at last a heat fully equal to the melting 

 of pure silver is required. 



As the cupellation requires a free ac- 

 cess of air, as well as an high degree of 

 heat, the stopper of the muffle is always 

 removed as soon as the metal is put into 

 the hot cupel, to allow a current of air 

 to pass through the muffle : but to prevent 

 this from cooling the muffle too fast, 

 several round pieces of charcoal are 

 heaped up, in front of the muffle, on an 

 iron plate placed there to hold them, 

 which burn with sufficient force to heat 

 the air as it passes to the cupels. The 

 furnace should be made so that the heat 

 of the fuel within may be readily increas- 

 ed or diminished, but at the same time 

 so that it can be kept up with steadi- 

 ness. 



The time taken up in making one assay 

 of silver, is generally from 15 to 25 mi- 

 nutes. The proportioning of the lead to 

 the supposed alloy in the silver to be as- 

 sayed is of great importance ; if too little 

 is employed, some of the alloy will remain 

 in the mass; but if too much is used, some 

 of the silver will be wasted ; for Mr. Til- 

 let has found, that when the proper quan- 

 tity of lead is used, it carries down a por- 

 tion of the silver into the cupel, which he 

 has ascertained by accurate experiments 

 to amount to _|_ of the lead in the cu- 

 pel ; whereas the natural admixture of 



silver in lead is only 1 . But when an 



excess of lead is employed for cupella- 

 tion, this loss of silver is somewhat great- 

 er, though it does not increase in the ra- 

 tio of the excess of lead ; for ten parts 

 of lead to a given alloy will not carry down 

 twice as much silver as five parts, though 

 the difference of loss will be very sensi- 

 ble. When the litharge carried into the 

 cupel is reduced to reguline lead, on be- 

 ing cupelled a second time, it will yield a 

 button of silver, fully equal to the loss of 

 this metal in the first assay. In all these 

 reductions the silver appears equally dis- 

 tributed through the lead, for Mr. Tillet 

 found that separate globules of the lead, 

 spurted out by accident upon an empty 

 cupel in the muffle, each left a minute 

 atom of silver lying upon the spot where 

 the globules had scorified. 



Bismuth will serve the same purpose 

 as lead in cupellation ; but, besides being 

 dearer, it is found to carry down with it 

 into the cupel somewhat more of the sil- 

 ver than the same quantity of lead does. 



To estimate the quantity of alloy in 

 silver, the ancient assay ers used touch- 

 needles, or small slips of silver, alloyed 

 with known proportions of copper, in a 

 regularly increasing series, from the least 

 to the greatest proportion ever used. 

 The silver to be assayed was compared 

 with these, and itsaHoy estimated by that 

 of the needle, to which it shewed the 

 closest resemblance. But an experienced 

 assayer is at the present time able to 

 judge of the alloy with sufficient exact- 

 ness, by the ease or difficulty with which 

 the silver is cut, by the colour and grain 

 of a fresh cut surface, the malleability, 

 the change of surface when made red hot, 

 and the general appearance. 



The assay of gold is more complicated 

 than that of silver. The baser metals 

 may be separated from it by cupellation 

 in the same manner as from silver, ex- 

 cept copper, which has so strong an affi- 

 nity for gold, that it can scarcely be over- 

 come by this method, unless silver is first 

 combined with the mass ; and this makes 

 the second operation necessary, mention- 

 ed before, namely, the parting of the 

 gold from the silver. 



The process of parting is performed 

 by the aquafortis of commerce, which dis- 

 solves the silver, and leaves the gold un- 

 touched. But in this operation it is found, 

 that when the gold exceeds a certain pro- 

 portion in the mixture, it so much pro- 

 tects the silver from the acid, as more or 

 less to prevent its action. Therefore, 

 when the gold is in excess, it becomes 

 necessary to add so much silver as to give 

 this metal the predominance. The pro- 

 portion of silver generally used is three 

 parts to one of the gold, from whence 

 the process obtained the name of quarta- 

 tion. Several good assayers think two 

 parts of silver are sufficient. More than 

 three parts also may be used, but then it 

 will protract the process needlessly. 



Though, when copper also as well as 

 silver is present, the parting may be pro- 

 ceeded to, as this metal is likewise solu- 

 ble in a^quafortis ; yet it is found to have 

 some advantages to cupel the mixture 

 first with lead ; and likewise even when 

 no copper is combined with the gold. 



The cupellation of gold is thus con- 

 ducted ; the portion of the alloy of silver 

 being estimated by touch-needles, as 

 much silver is added as will make the en- 



