Cupellation

Cupellation is an ancient assaying and refining process that separates precious metals such as gold and silver from base metals, especially lead, by melting the mixed metal in a small, shallow, porous vessel called a cupel and blowing a stream of air across the surface at high heat, so the base metals oxidize and are either absorbed into the cupel's own porous material or driven off, leaving behind a bead of the pure, unoxidized precious metal. Archaeological and textual evidence traces cupellation back to at least the third millennium BCE in the ancient Near East, and the technique became the standard method both for extracting silver from lead ores through a related smelting process and for assaying the purity of gold and silver coinage and bullion in cultures across the ancient and medieval world. Cupellation remains in use today largely unchanged in principle, particularly in the fire assay method still used to determine the precious-metal content of ore samples in modern mining laboratories.