Steel is a metal alloy of iron and a small amount of carbon (2.1% or less). The carbon, or other elements, acts as a hardening agent. The steel alloy has been known since before the Renaissance but was difficult to produce in quantity. With the invention of the "Bessemer Process" of steel production, which removes impurities from the iron by oxidation, it became cheaper and easier to produce steel in volume. Today steel is one of the most common manufacturing and building materials in the world.