Assay
The determination of the quantity of any particular metal in an ore, alloy, or other metallic compound, more especially of the quantity of gold or silver in coin or bullion. It differs from analysis thus: The component parts of the mineral or alloy are, by analysis, separated, and an estimate made of their respective quantities; while by assay, it is only the valuable metals that are sought for; as, in the case of silver and gold alloys the inferior metals are dispersed, the quantity being determined by the loss of weight. A gold alloy is assayed by obtaining a certain number of grains, which, after being carefully weighed, are wrapped in sheet-lead and exposed to intense heat in a cupel placed under a muffle. Cupels for this purpose must be very porous, and are simply a small block with a cavity on the upper side to receive the metal. When fusion takes place, the lead is converted into a vitreous oxide, which, acting as a flux, acts powerfully to oxidize and vitrify the inferior metals contained in the alloy, which being changed, are absorbed by the porous cupel, leaving a globule of unoxidable metal behind. The globule will be silver or gold, or a compound of both, which may be separated by the method shown at "Separating Metals from their Alloys." Another method of assaying is described at "Touch-needle." See MUFFLE.