An ore is a complex mixture of a metal-containing mineral and economically worthless material called gangue (pronounced “gang”), consisting of sand, clay, and other impurities. The extraction of a metal from its ore requires several steps: (1) concentration of the ore and, if necessary, chemical treatment prior to reduction; (2) reduction of the mineral to the free metal; and (3) refining or purification of the metal. These processes are a part of metallurgy, the science and technology of extracting metals from their ores. Another aspect of metallurgy is the making of alloys, solid metallic solutions composed of two or more elements. Steels and bronze are examples.