ASSAY, ASSAYING. A branch of chemical analysis. It is a process by which the quality of gold or silver bul lion, trinkets, plate and coin is ascertained with precision, or by which the quantity of either of these precious metals may be determined in an alloy. It is now ex tended to determine the quantity of pla tinum and palladium in certain bullion and gold dust from Brazil. The art of assaying depends upon the fact that gold and silver have but a very feeble affinity for oxygen in comparison with copper and tin, and these latter, when oxydized, unite with lead, and sink with it into any por ous earthen vessel. This vessel, or eupel, may be made of leached wood-ashes, or of burned bones, well powdered.
Oupellation, as this operation is called, is usually carried on in a small furnace, capable of being heated so as to melt gold. In the annexed cut, fig. 1 represents the fur nace, which is lighted with charcoal when an assay is to be made.
In the middle of this fur nace when heated, is placed an earthen vessel called a muffle, fig. 2 (comparatively enlarged), which is of an oven form vaulted above and flat below, open at one end and closed elsewhere, exce_pt by a few narrow slits at the top and aides. The body of the muffle is surrounded with coals, and before cupellation is gradually heated to a cherry red. Its use is to protect the small cupela ranged on its floor from any impurities of fuel, and at the seine time to afford it plenty of air to oxydize the metals. The eopels are little bone dishes, made of the ma terials stated previously moistened into a paste, and pressed into a shape by a cast-iron mould, which fig. 3. represents.
From 12 to 85 grains is the pro per quantity of alloy to be weighed out for examination. It is wrap ped up in lead-foil or paper, and surrounded by the thin plate of lead, they are then placed upon the cupel and laid in the muffle iying in the furnace, heated red hot, when they immediately melt. The lead oxidizes, and the oxide of lead formed, melts and runs down the sides of the mass into the body of the eupel. The button of alloy
becomes smaller and brighter, and ulti mately leaves the silver in a state of great purity. The quantity of lead necessary to carry out this experiment is a matter of nicety; if too much be used, some silver will be lost ; if too little, the cop per will not be removed from the alloy. In every assay, by this way, a little silver does seem to be lost. The operation be ing now finished, the &mei is cooled, and the button of pure metal weighed. The difference between it and the original weight gives the amount of alloy present.
A gold assay is more complex than a silver one. The copper alloyed with gold cannot well be separated by heat alone : some silver requires to be added for this purpose, which entails the necessity of a subsequent operation to remove the silver. In coins and manufacture, gold is often mixed with silver, and the opera tion of separating them is termed parting, which consists in treating the mass with dilute nitric acid, which dissolves the sil ver out and leaves the gold. If the amount of silver he small, it is necessary to add some silver to it, for in that case the gold preponderating, the silver is protected from the action of the acid. After the acid has removed all the silver and copper, the insoluble metal remain ing is placed on the impel and heated in the furnace, it is then cooled and weighed, and the weight indicates the absolute quantity of gold in the sample.
Assaying is extended to copper ores, to determine the value of a sample, as to the amount it should produce in the smelting furnace, or wheiner it be worth working. Assays may be conducted by the assistance of heat and fluxes, or in the dry way as it is termed, or else in the moist way, as by acids or other re agents ; the former is the readier mode, the latter the more correct. Berthiefs work on "Assays by the Dry Way," and Mitchel's "Manual of Practical Assaying" may be consulted for further information.