Separation of gold or silver from impurities by melting the impure metal in a cupel (a flat, porous dish made of a refractory material) and directing a blast of hot air on it in a special furnace. The impurities, including lead, copper, tin, and other unwanted metals, are oxidized and partly vaporized and partly absorbed into the pores of the cupel.
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In this process, an alloy consisting of both noble and base metals is placed in a crucible. This mixture is then melted and allowed to freeze. When solidified, a button consisting of precious metals and lead can be removed from the slag of metal oxides and other materials. When assaying ores this slag layer can be quite massive, but when assaying karat gold, it is virtually non-existent. After cooling, the metals are placed in a special pot made of bone ash or clay called a cupel. Under high heat, lead turns to litharge, a lead oxide, which is absorbed by the cupel or lost to the atmosphere. At the end of the cupellation process, a button of pure gold and silver remains in the bottom of the cupel. The button is then placed in nitric acid to dissolve the silver, and the remaining pure (24k) gold weighed to determine the gold content present in the original starting material.
Loss of lead to the larger atmosphere creates an environmental hazard, and so this larger loss should be restrained by appropriate equipment and lead recovery means.