Material properties:
Iron is commonly found in the
Earth's crust in the form
of an ore, usually an iron oxide, such as magnetite or hematite. Iron is extracted from iron ore by removing the oxygen through its
combination with a preferred chemical partner such as carbon which is then lost
to the atmosphere as carbon dioxide. This process, known as smelting, was first applied to metals with lower melting points, such as tin,
which melts at about 250 °C (482 °F), and copper, which melts at about 1,100 °C
(2,010 °F), and the combination, bronze, which has a melting point lower
than 1,083 °C (1,981 °F). In comparison, cast iron melts at about
1,375 °C (2,507 °F).[4] Small quantities of iron were smelted
in ancient times, in the solid state, by heating the ore in a charcoal fire and then welding the clumps
together with a hammer and in the process squeezing out the impurities. With
care, the carbon content could be controlled by moving it around in the fire.
Unlike copper and tin, liquid or solid iron dissolves carbon quite readily.
All of these temperatures could be
reached with ancient methods used since the Bronze Age. Since the oxidation rate of iron
increases rapidly beyond 800 °C (1,470 °F), it is important that
smelting take place in a low-oxygen environment. Smelting, using carbon to
reduce iron oxides, results in an alloy (pig iron) that retains too much carbon to be
called steel. The excess carbon and other impurities are removed in a
subsequent step.