ASSAYING, the estimation of the amount of pure metal, and especially of the precious metals, in an ore or alloy. In the case of silver, the assay is either by the dry or by the wet process. The dry process is called cupellation, from the use of a small and very porous cup, called a cupel, formed of well burned and finely ground bone ash made into a paste with water. The cupel, being thoroughly dried, is placed in a fire clay oven. This oven, called a muffle, is set in a furnace, and when it is at a red heat the assay, consisting of a small weighted portion of the alloy wrapped in sheet lead, is laid upon the cupel. The heat causes the lead to volatilize or com bine with the other metals, and to sink with them into the cupel, leaving a bright globule of pure metallic silver, which gives the amount of silver in the alloy operated upon. In the wet process the alloy is dissolved in nitric acid, and to the solution are added measured quan tities of a solution of common salt of known strength which precipitates chlo ride of silver. The operation is concluded
when no further precipitation is obtained on the addition of the salt solution, and the quantity of silver is calculated from the amount of salt solution used. An alloy of gold is first cupelled with lead as above, with the addition of three parts of silver for every one of gold. After the cupellation is finished, the alloy of gold and silver is beaten and rolled out into a thin plate, which is curled up by the fingers into a little spiral, or cornet. This is put into a flask with nitric acid, which dissolves away the silver and leaves the cornet dark and brittle. After washing with water, the cornet is boiled with stronger nitric acid to remove the last traces of silver, well washed, and then allowed to drop into a small cru cible, in which it is heated, and then it is weighed.