66 



METALLURGY. 



Metalkir- the temperature of the metal, a quantity of burning 

 fj- charcoal is put at its mouth, by which the air, previous 

 "Y"" to its entrance, is heated. Alter the metal is put in, it 

 very soon melts, and emits white fumes, and a quantity 

 of a reddish substance is formed, which is absorbed by 

 the cupel. This is the lead oxidated, carrying along 

 with it the metal with which the silver is alloyed. As 

 the process proceeds, the fused metal acquires a globu- 

 lar form, and its surface is studded over with melted 

 litharge; at last the silver acquires a beautiful bril- 

 liancy, which is a proof that the cupellation is finished. 

 The cupel is then allowed to cool slowly, to prevent the 

 contraction of the outer parts of the metal from scatter- 

 ing about the internal unconsolidated portion. The 

 silver is then taken out and carefully weighed. 



It is of material consequence to know the exact 

 quantity of lead that must be added to the alloy, for 

 if too little be used, the whole of the base metals 

 are not extracted, which is known by the metal left in 

 the cupel being flat,of a dull colour, and adhering to the 

 dish. If too much lead be employed, the litharge 

 formed carries along with it a minute quantity of the 

 noble metal ; accordingly, if the litharge be again sub- 

 jected to cupellation, it leaves a little silver on the cu- 

 pel. To ascertain the quantity of lead requisite, touch- 

 needles were formerly employed. These were bars of 

 alloys, containing different known quantities of silver 

 and copper, with which the alloy to be assayed was 

 compared. This mode is however almost abandoned, 

 the assay-master trusting to the appearance of the alloy, 

 the ease with which it is cut, its malleability, and the 

 colour of its surface when heated. It is of very great 

 importance also to attend to the heat required. If the 

 heat be too strong, part of the silver is volatilized; if 

 too low, the litharge is not absorbed by the cupel. 



If the process of cupellation be well performed, the 

 button of metal left adheres slightly to the cupel, its 

 surface is brilliant, and quite clean, and it has a globu- 

 lar shape. The silver also sometimes presents a lami- 

 nated structure, which, when viewed through a mi- 

 croscope, appears composed of scales of a pentagonal 

 shape. This is a good test of the purity of the metal ; 

 for when any of the alloying metal is left, the surface 

 is quite smooth. Where great delicacy is required in 

 the assay, two portions ought to be subjected to cupel- 

 lation at the same time ; and should the results not 

 agree, the operation must be repeated. 



The assay of silver ores, with the view of ascertain, 

 ing the quantity of silver, is performed nearly in the 

 same way as the assay of the alloy. For this purpose 

 the ore is roasted, and then mixed with litharge, and 

 quickly fused. The product is then fused with black 

 flux, by which the litharge is reduced, and the lead 

 falls to the bottom, carrying with it the other metals. 

 The metallic button is afterwards mixed with the 

 proper quantity of lead, and subjected to cupellation, 

 by which the silver is obtained pure. Should the ore 

 contain gold, this will be found along with the silver. 

 These must therefore be separated by the process of 

 parting, described under the assay of gold. 



The analysis of the ores of silver, with the view of 

 ascertaining the quantity of all the ingredients, is per- 

 formed chiefly by the action of nitric acid. That just 

 described is, however, the one usually followed in the 



Assay of ar ' S- 



Gold Al- Assay of Gold Alloys. The affinity between gold and 



lojs. silver is so strong that they cannot be separated entirely 



from each other by cupellation, unless a quantity of sil- Mstallur- 

 ver be added ; besides, gold is frequently alloyed with gy- 

 copper and silver, or with silver alone, as in some coins 1< ""~v^ < "* 

 and trinkets. This makes the assay of gold more com- 

 plicated than that of silver, asthegold and silver must al- 

 so be separated from each other. In general, the silver 

 added to the alloy, amounts to about three times the 

 quantity of gold, though some recommend that only 

 twice the quantity should be employed. 



The alloy being mixed with the requisite propor- 

 tions of lead and silver, is placed in a mufflle, and 

 the cupellation is performed in the same way as with 

 silver. When the process is finished, the button of me- 

 tal is taken out, and kept in a state of fusion in a cru- 

 cible for some time, by which the whole of the lead is 

 expelled. It is, when cold, beat into a plate, again 

 made red hot, and slowly cooled. It is afterwards ex. 

 tended into a thin leaf, between steel rollers, and coil- 

 ed loosely up, in which state it is submitted to the pro- 

 cess of purling, by which the gold and silver are se- 

 parated. 



In the cupellation of gold, the heat required is 

 stronger than is requisite for that of silver ; and there 

 is little risk of making it too high, as the alloy of gold 

 and silver is not volatilized. 



As in the cupellation of silver, it is necessary that 

 the due proportion of the metals with which the alloy 

 is to be mixed, be employed. This is ascertained in 

 different ways. A tolerably accurate idea of the quan- 

 tity of gold in an alloy is acquired by the use of Touch 

 needles. Of these there are four different kinds : 1st, 

 gold alloyed with silver ; 2d, gold and copper ; 3d, 

 gold, with two of silver and one of copper ; 4/7; , gold,, 

 with two of copper and one of silver. These have dif- 

 ferent quantities of gold alloyed with them ; they are 

 not, however, much employed, more particularly those 

 containing copper, as the colour communicated by this 

 metal to gold does not differ much from that of the 

 different kinds of copper. 



Another method by which the quantity of gold is 

 estimated, is by the streak which the alloy gives on 

 the darker sorts of basalt, or on black pottery. The 

 streak is compared with those given by touch needles, 

 which is, in general, made more distinct by the stone 

 being previously wetted. Occasionally a drop of ni- 

 tric acid is put on the streak, and by the effects produ- 

 ced on it, the quantity of gold is estimated. By the 

 different colours communicated, the workman acquires, 

 by experience, an idea of the quantity of gold present. 

 When the proportion of gold is large, it is necessary to 

 add a little muriatic acid to the nitric acid, otherwise the 

 streak is not affected ; the gold seemingly protecting the 

 other metals from the action of the acid. According to 

 Vauquelin, the best acid is composed of 98 nitric acid, of 

 specific gravity 1340, 2 of muriatic acid 1173, and 25 

 of water. When this acid is used, it must not remain 

 long on the streak. 



The quantity of lead added to the alloys of silver 

 and gold is differently stated by different assay ers. Ac- 

 cording to the experiments of Tillet, Hellot, and Mac- 

 quer, 



1 copper, 3 silver, require 40 of lead. 

 15 48 



1 23 96 



The quantity of lead necessary increasing with the sil- 



