text: 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 carbon source acts as a chemical reactant to remove oxygen from the ore, yielding the purified metal element as a product. The carbon source is oxidized in two stages. First, carbon (C) combusts with oxygen (O) in the air to produce carbon monoxide (CO). Second, the carbon monoxide reacts with the ore (e.g. FeO) and removes one of its oxygen atoms, releasing carbon dioxide (). After successive interactions with carbon monoxide, all of the oxygen in the ore will be removed, leaving the raw metal element (e.g. Fe). As most ores are impure, it is often necessary to use flux, such as limestone (or dolomite), to remove the accompanying rock gangue as slag. This calcination reaction emits carbon dioxide.
  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 K), almost 300 degrees below iron's melting point of 1538 °C (2800 °F or 1811 K).
  * Mercuric oxide becomes vaporous mercury near 550 °C (1022 °F or 823 K), almost 600 degrees above mercurys melting point of -38 °C (-36.4 °F or 235 K), and also above mercurys boiling point.