When treating of gold, it was mentioned that it and silver resist the action of heat and air, while the other metals with which they are alloyed, are oxidated at a high temperature. On this depends the assay of gold and silver alloys. For the complete success of this pro ccss, it is necessary that the quantity of base metal be large, otherwise the attraction between the noble metal and it prevents their separation. For this reason, a quantity of lead is always mixed with the alloy. The separation of gold and silver from other metals, by means of lead, is called cupellation, and the process is performed on a curie[ placed in a muffle. A cupel is a dish of a circular or pyramidal shape, made of bone a.:,hes, and having a small depression on its upper sur face, into which the alloy is put. A muffle is a vessel made of earthen ware, flat below, and arched above, open at one end, and shut at the other, and at the sides, except where it is perforated by small holes. The fur nace employed in cupellation is one in which a strong heat can be excited, and having a hole in its sides for the admission of the muffle. \Vhen the cupellation is to be performed, the muffle is placed in the furnace, to whin it is generally secured by luting The furnace is then kindled, and the mai.: and cupels are slowly heated. When these are red-hot, the alloy, beat out to a thin plate, and rolled up in a sheet of lead, obtained by the reduction of litharge, is put into the cupel. To prevent the air which enters the muffle from lowering the temperature of the metal, a quantity of burning charcoal is put at its mouth, by which the air, previous to its entrance, is heated. After 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 quan tity of lead that must be added to the alloy, for, if too little be used, the whole of the base metals are not ex tracted, 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 car ries along with it a minute quantity of the noble metal; accordingly, if the litharge be again subjected to cupel lation, it leaves a little silver on the cupel. To ascer tain the quantity of lead requisite, were formerly employed. These were bars of alloys, con taining 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 he 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 arc contain gold, this will be found along with the silver. These must therefore be separated by the process of parting, de scribed 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 arts.
Assay of Gold .-liloys.—The affinity between gold and silver is so strong that they cannot be separated entirely from each other by cupellation, unless a quantity of sil ver be added ; besides, gold is frcyiently alloyed with copper and silver, or with silver alone, as in some coins and trinkets. This makes the assay of gold more com plicated than that of silver, as the gold and silver must also be separated from each other. In general, the sil ver 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 proportions of lead and silver, is placed in a muffle, and the cupel lation is performed in the same way as with silver. When the process is finished, the button of metal is taken out, and kept in a state of fusion in a crucible, 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 extended into a thin leaf, between steel rollers, and coiled loosely up, in which state it is submitted to the process of parting, by which the gold and silver are separated.
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 higb, as the alloy of gold and silver is not volatilized.