ASSAYING, is a term particularly ap plied to the separation of gold or silver from other metals. In its more extended meaning, it is used for the determination of the quantity of any metal whatsoever in, composition with any other metal or mineral.
The assaying of gold or silver is divi ded into two operations; by the first they are separated from the imperfect metals, or those easily oxyded ; by the second they are parted from the metals which resist oxydation by simple exposure to air, and which are therefore called the perfect metals ; this second process gene rally consists in parting gold and silver from each other, as the third perfect me tal, plating, is but seldom found united to them.
The basis of the 'method of separating gold or silver from the imperfect metals is fbunded on the facility with which the lat ter imbibes oxygen ; and the process is cal culated to accelerate this operation as much as possible; hence the oxyde of lead or litharge, is generally considered as the most powerful purifier of the perfect me tals, from the ease with which it parts with its oxygen to the imperfect metals united with them; but oflate, oxyde of manganese has been found superior to it, in several instances, for this purpose. In the che mical analyses of metals, the oxyde of lead is generally preferred for the above pur pose ; but in the assays performed by authority, metallic lead is always used, probably from the ease it is supposed to afford in determining the weight of the different ingredients by calculation. The lead in the process first becomes oxyded. Then yields some of its oxygen to the other imperfect metals, and afterwards becomes vitrified, in conjunction with the other oxydes so formed, and carries them off along with it, leaving the perfect me tals pure. The above operation is called cupellation, and is performed on a flat round cake of bone ash, compressed within an iron ring, that is named a cupel;. this is placed in a vessel called a muffle, which resembles an oven in miniature, that is fixed in a furnace capable of giving a heat sufficient for the fusion of gold, so that its mouth may come in contact with the door, at the side to which it is luted, to separate it from the peal ; there are small slits formed in the sides of the muf fle, to afford a passage for the air.
When the muffle and empty cupels are heated red hot, a little powdered chalk is put on the floor of the muffle, to prevent the cupels from adhering to it after the operation. Cupels should be always of size proportinate to the lead to be used, as they cannot absorb a weight of litharge, at the utmost, more than their own.
The assay of silver is performed in this country on a piece of metal not exceeding thirty-six grains, if the alloy appears con siderable ; which piece is laminated, and weighed with extreme accuracy in a very sensible balance. It is then wrapped up in the requisite quantity of lead, rolled out into a sheet, which is revived from litharge, that it may be free from the silver which lead in general contains naturally.
The silver and lead are put on the en pel when it and the muffle are red hot. The metal immediately melts, and begins to send off dense fumes, and a minute stream of red fused matter is seen per petually flowing from the top of the glo bule, down its sides, to the surface of the cupel, where it sinks ; the fume consists of lead in vapour, and the red stream of vitrified lead which carries down with it the copper, or other alloy of silver, in to the cope]. As the cupellation ad vances the melted button becomes round er, its surface becomes streaky, with large bright points of the fused oxyde, which move with increased rapidity ; the last portions of the litharge on the surface quickly disappear, shewing the melted metal with bright iridescent colours, which directly after becomes opaque, and then suddenly appears brilliant, clean, and white, as if a curtain had been with drawn from it ; at which time the assay ers say it lightens. The silver is now left pure, and the cupel is allowed to cool gradually till the globule of silver is fix ed, when it is taken out while still hot, and, when cold, weighed with as much ac curacy as at first. The difference be tween the weight of the globule, and that of the silver first put in, spews the quan tity of alloy. if the globule is cooled too quickly, the outward surface contracts so suddenly as to force out the fluid metal at the centre in arboresccnt shoots, by which some portion is lost, and the assay spoiled.