The separation of a valuable metal from its alloy or impurities. The art of assaying differs from that of analysis in this respect: by analysis the various component parts of the mineral or alloy are separated, and their respective quantities estimated; by assay only the valuable metal or petals is sought for. In the assay of gold or silver alloys, for example, the inferior metals are dissipated, and the quantity estimated only by the loss of weight. There are two modes by which the art of assaying is performed, and sometimes one is employed to corroborate the other. The one is called the humid process, by which a solution of the metals is effected by means of acids, after which those sought for are precipitated by proper re-agents; the other is called the dry process, and is performed by the agency of fire. In the mining districts, the comparative value of some ores is roughly estimated by the dry process of assaying. In London, the mode is seldom resorted to, except for the purpose of estimating the quantity of gold or silver in an alloy.
The quantity of these metals, or of platinum, in any alloy, may be as correctly estimated by the dry, as by the humid, process; we shall therefore describe the mode of assaying those alloys in this article, and refer the reader to the analysis of ores (see Ore) for the most correct mode of assaying other metallic compounds. An alloy of gold is assayed by detaching from different parts of the article a small portion, by a knife or a file, until the requisite number of grains for the experiment is obtained. These being carefully weighed, are wrapped in a piece of sheet lead, and placed in the bowl of a cupel, and exposed under a muffle to an intense beat. The cupel is a small cubic or circular solid, with a cavity on the upper surface to receive the metal; it is made of a very porous material; the ashes of burnt bones, made into a paste with water and slowly dried, are generally employed for this purpose. When the alloy and lead are exposed to intense heat, as before described, fusion of the whole ensues, but the lead speedily becomes converted into a vitreous oxide or glassy fluid; this powerfully promotes the oxidation and vitrification of any inferior metal contained in the alloy, and when they are thus changed, they percolate through the porous cupel, and leave only a globule of metal not oxidable by heat.
This globule, therefore, will be of gold or silver, or a compound of them, these metals, as well as platinum, not being affected by the action of fire and air. To separate the silver from the gold, the alloy is hammered or rolled into thin plates, and digested with dilute nitric acid; this dissolves the silver, but does not act on the gold. When the first solution is poured off, another portion of nitric acid is added, to effect a perfect solution of all the silver. The gold is now left as a porous spongy mass, and when washed and dried, its quantity is ascertained by weighing. If the quantity of silver in the alloy be small, the excess of gold defends it from the action of the nitric acid; the process called quartation must therefore be resorted to. This consists in adding three parts of silver to the mass, and fusing them together. The silver, then, being in excess, is all separated by the mode before described. The quantity of silver may be ascertained by precipitating it from the solution with muriate of soda. An insoluble substance, chloride of silver, is thus formed, which, when carefully washed, dried, and weighed, will indicate the quantity of the metal, 100 parts of the chloride containing 751/2 of silver.
In estimating the commercial value of gold and silver articles, they are said to be so many carats fine: the carat does not denote any specific weight, but a part. Each article is said to contain 24 carats, whatever may be its weight; and the quantity of alloy in carats, or parts, is deducted from the whole. Thus, if an alloy contain 4 parts of inferior metal, it is said to be 20 carats fine; if it contain 6, it is of 18 carats fine, which is the standard of jewellery in England. In France, where every small article of gold is assayed before it is permitted to be sold, a different mode of assay is adopted. The trinket is rubbed on a black touchstone, formed of the black basaltes; black flint or pottery will answer the purpose as well. The jeweller states the quantity and nature of the alloy employed in the article, and the mark it makes on the touchstone is compared with a similar mark made by a needle composed of the same metals and the same proportions. If they correspond in appearance, and are not differently affected by nitric acid or heat, the alloy is pronounced to be of a similar kind to the needle.
A great number of assay needles, formed of different proportions of alloys, are necessary for this mode of assay, and long practice and experience in the artist are indispensable. He is guided in the operation, not merely by the appearance of the stroke on the touchstone, but also by the comparative roughness or smoothness, dryness or greasiness, which is observed in rubbing. The gold coin of Great Britain is 22 carats fine: silver coin is composed of 121/3 silver and 1 of copper. An alloy of silver with an inferior metal is assayed by cupellation alone, the process of quartation not being necessary. A given portion is placed in the cupel with a sufficient quantity of lead, and exposed under a muffle to intense heat, until the lead and other inferior metals are vitrified and absorbed by the cupel. The workman is guided in this operation by the appearance of the melted globule. Until the last portions of the alloy have passed through the cupel, the mass appears to be in a state of commotion or ebullition; but when they are absorbed, the silver becomes quiescent, and exhibits brilliant prismatic colours.