The New Student's Reference Work/Assaying
Assay'ing is the art of finding how much of a given metal there is in a metallic ore or alloy—as, the amount of iron in a specimen of iron ore or the amount of silver in a silver dollar. The term is applied particularly to processes carried out chiefly by fusion or fire assaying; but the word is also applied as wet assaying to the commercial analysis of many things where chemical solvents and reagents are used. Assay processes vary with different metals, but the method used with silver ore will serve as an illustration. The first process is called scorification. One part by weight of ore is mixed with from ten to twenty times its weight of granulated lead and half its weight of borax. This mixture is put in a fire-clay dish, called the scorifier, and heated to redness in a furnace having a compartment or muffle open to the air, called a muffle furnace, until the substances are thoroughly melted. The surface of the molten led now shows in a circular space in the center of the scorifier, while the earthy materials form a slag which forms a ring around this circle. The heating is continued until a considerable part of the lead has been oxidized to lead oxide. This goes into the slag and increases its amount so that the slag finally covers the diminishing metallic lead. Then the melted mixture is poured into a mould, and, on cooling, a lead button is seen which can be detached from the slag. The lead has taken up the silver as well as any gold that may be present. The next process is called cupellation. The cupel is a small cup made of burnt bone and is porous. The lead button is put in this vessel, after the latter has been heated to redness in a muffle furnace. The lead and other base metals that may be present are burned or oxidized, and the oxides are absorbed by the porous mass of the cupel, or sent off in the shape of vapor. Silver and gold are not oxidized, hence they remain in the metallic state. Just before the assay is finished, rainbow colors come and go over the button, and a brilliant flashing up of color marks the end of the operation. The silver button left in the cupel is finally weighed. This is one of the simplest methods, but not the only method of assaying. A crucible is sometimes used instead of a scorifier.