2CuO, CO2, and H2O in several stages between 250 °C and 350 °C. The carbon dioxide and water are expelled into the atmosphere, leaving copper(II) oxide, which can be directly reduced to copper as described in the following section titled Reduction. Galena, the most common mineral of lead, is primarily lead sulfide (PbS). The sulfide is oxidized to a sulfite (PbSO3), which thermally decomposes into lead oxide and sulfur dioxide gas (PbO and SO2). The sulfur dioxide is expelled (like the carbon dioxide in the previous example), and the lead oxide is reduced as below. Reduction Reduction is the final, high-temperature step in smelting, in which the oxide becomes the elemental metal. A reducing environment (often provided by carbon monoxide, made by incomplete combustion in an air-starved furnace) pulls the final oxygen atoms from the raw metal. The required temperature varies both in absolute terms and in terms of the melting point of the base metal. Examples: Iron oxide becomes metallic iron at roughly 1250 °C (2282 °F or 1523.15 K), almost 300 degrees below iron's melting point of 1538 °C (2800.4 °F or 1811.15 K). Mercuric oxide becomes vaporous mercury near 550 °C (1022 °F or 823.15 K), almost 600 degrees above mercury's melting point of -38 °C (-36.4 °F or 235.15 K). Flux and slag can provide a secondary service after the reduction step is complete: they provide a molten cover on the purified metal, preventing contact with oxygen while still hot enough to readily oxidize. This prevents impurities from forming in the metal. Fluxes Metal workers use fluxes in smelting for several purposes, chief among them catalyzing the desired reactions and chemically binding to unwanted impurities or reaction products. Calcium oxide, in the form of lime, was often used for this purpose, since it could react with the carbon dioxide and sulfur dioxide produced during roasting and smelting to keep them out of the working environment. History Of the seven metals known in antiquity, only gold occurs regularly in its native form in the natural environment. The others – copper, lead, silver, tin, iron, and mercury – occur primarily as minerals, though copper is occasionally found in its native state in commercially significant quantities. These minerals are primarily carbonates, sulfides, or oxides of the metal, mixed with other components such as silica and alumina. Roasting the carbonate and sulfide minerals in the air converts them to oxides.