ASSAY. The proof or trial of the purity or fineness of metals,—particularly the pre cious metals, gold and silver.
2. By this proof the amount of pure metal in any homogeneous mass is ascertained. In the case of gold, the base metals, such as copper or tin, are removed by a method called eupellation, which is conducted in an assay furnace, in a cupel, or little cup composed of calcined bones. To the other metals lead is added,—this metal possessing the properties of oxidizing and vitrifying under the action of heat, of promoting, at the same time, the oxidation of any of the base metals which may be present, and of drawing such metals with it into the pores of the cupel, and thus •leav fr; behind the gold only, together with any amount of silver which may be present. The silver is separated from the gold by another process, founded on the property possessed by nitric acid of dissolving silver without acting upon gold.
3. The assay of silver is generally made by a method called the humid assay. The silver is dissolved in nitric acid, and a solu tion of common salt in water is added, by which the silver is precipitated in the form of a white powder, which is an insoluble chloride. It has been ascertained that one
hundred parts, by weight, of pure salt will Coavert into chloride of silver just one hun dred and eighty-four and one-fourth parts of pure silver. From this• theorem the fineness of the specimen operated upon is deduced from the quantity of salt used to convert int ) chloride a given amount of silver.
4. Assays at the mint are for two purposes. 1. To determine the value of the deposits of gold and of silver. 2. To ascertain whether the ingots prepared for coinage are of the legal standard of fineness. The standard gold of the United States is so constituted that in one thousand parts, by weight, nine hundred shall be of pure gold and one hundred of an alloy composed of silver and copper. The standard silver of the United States is com posed of nine hundred parts of pure silver and one hundred of copper. See ANNUAL ASSAY.