The process of increasing the carbon content of the surface layers of a metal (often wrought iron) by heating the metal below its melting point with carbonaceous matter such as wood charcoal. The absorption of carbon into a metal surface. With an iron surface, the result is carbon steel. The introduction of carbon into the surface layer of a steel that has a low carbon content. The process is carried out by heating the components in a solid liquid, or gaseous carbon containing medium. The depth of penetration of carbon into the surface is controlled by the time and temperature of the treatment. After carburising it is necessary to harden the components by heating to a suitable temperature and quenching.