Reductive calcination, or smelting, is a process of heating ores to a high temperatures in the presence of a reducing agent such as carbon (coke), and a fluxing agent to remove the accompanying clay and sand (gangue). Smelting, which involves the reduction of metal oxides in ores to pure metal, is followed by separation of the gangue of the liquid metal and slag (i.e., the fused mixture of impurities and flux).

Iron ore is the ore most frequently smelted. The ore, typically containing 20% clay and sand, is first heated in an air blast furnace with coke and limestone (the fluxing agent) to a temperature above the melting point of iron and the slag. The coke is oxidized to carbon dioxide, which changes to carbon monoxide at high temperatures. The carbon monoxide reduces the ores, i.e., converts the metal oxides to metal, by taking on oxygen, which changes the carbon monoxide back to carbon dioxide. The difference in densities of the molten iron and molten slag allow each material to be removed separately from the furnace.

Secondary smelting is a related process used to recover nonferrous metals and alloys from new and used scrap and dross. Examples of materials that may be subjected to secondary smelting include scrap aluminum, babbitt, brass, bronze, copper, iridium, lead, magnesium, nickel, platinum-group metals, precious metals, tin, and zinc.