ASSAY', or ASSAYING, is the process employed in determining the proportion of pure metal in a metallic ore or in an alloy. This method of analysis is more generally followed in the examination of compounds of silver and gold, but is likewise resorted to in the investigation of ores of iron, copper, tin, zinc, bismuth, antimony, mercury, and lead. I In manufactured articles, also, such as silver-plate and gold-plate, some foreign metal (generally copper) is present, to impart hardness to the metal; and in Great Britain, each article is assayed at the Goldsmiths' hall, previously to being sold, so as to determine the exact richness of the metal whereof it is made. In the A. of compounds containing silver, the apparatus employed is a repel—a small basin-shaped vessel made of bone-ash; and a muffle, composed of fire-clay, about 8 in. in length and 3 to 4 in. in diameter, shaped like a miniature railway tunnel, open at one end, closed at the other end, and having numerous, slits or along the side. The more simple A. of silver con sists in the examination of argentiferous lead ore. By a preliminary process, the sulphur is separated (see LEAD); and weighed fragments of the mixed lead and silver being placed on enpels, the latter arc introduced into the muffle, which has been previously heated iu a furnace, where it still remains. The fire is then increased, and air being admitted to the muffle, the oxygen of the air unites with the lead, forming oxide of lead (PbO), which in part volatilizes through the openings in the side of the muffle, and in other part sinks into the porous bone-earth of which the cupel is made. Whilst the lead is thus carried away, the silver remains behind as a molten metallic globule, and when the last traces of lead-fumes leave the silver bead, the latter suddenly lightens, and immediately thereafter becomes brilliant and white. On being slowly allowed to cool, the globule of silver may be weighed, and the amount of pure metal thus determined. The use of the cupel during this process has led to the term cupellatian being employed in place of A. When silver contains copper, which it does in ordinary
coinage and silver-plate, it becomes necessary to mix lead with the alloy before attempt ing to separate the copper. The manner in which time lead is generally added is to roll the alloy of silver and copper in a piece of sheet-lead or lead-foil, and place the whole package on the cupel. During the heating in the muffle, the lead oxidizes as usual, and in part passing into the bone-earth of the cupel, carries the copper with it. The amount of lead required to effect the separation of copper from silver in this way is given in the following table: The metallurgic chemist, while performing an A., can determine, by the examination of the stains on the cupel after the process has been finished, what metal may have accompanied, and been separated from, the silver, even in minute quantity. Thus, lead alone imparts a straw-yellow or orange stain; copper, a gray.or dark-brown tint; and iron, a black stain.
During the A. of silver by the foregoing or dry method, a certain loss of metal generally occurs, which averages 2 parts in 1000; and this circumstance has induced the authorities in the mints of Great Britain, France, and other European kingdoms, as well as the United States, to adopt a humid process for the A. of silver, which will determine the value of a silver alloy to within 0.5 (or half a part) in 1000. The humid or wet A. consists in dissolving the compound of silver in nitric acid of density 1.25, and there after adding a solution of common salt (chloride of sodium, NaCI), which causes the precipitation of the chloride of silver (AgCI) in white flocculi. T.hc common salt is made of a definite strength, and is poured out of a measured or graduated vessel, till all further precipitation of the silver ceases, when the amount required of the solution of common salt is read off, and by a simple calculation its equivalent in pure silver is obtained.