On account of the large supply, cheapness, and its many useful properties, iron ranks as the chief of metals. It is hardly, if ever, found in Nature in its pure state. It is extracted from its ores in the form of cast iron, which again when subjected to furnace treatment can either be converted into wrought iron or steel.

The main differences in the properties of cast and wrought iron and steel are chiefly due to the presence of carbon in the metal. Cast iron contains about 4.75 per cent, of carbon, steel up to about 1.75, and wrought iron from 0.01 up to about 0.2. The smallest variation in the amount of carbon present considerably alters the properties of the iron.

When iron contains an appreciable amount of sulphur it becomes brittle when heated and is called "hot short." If phosphorus is present the metal becomes "cold short."

The following table gives the percentage composition of several specimens of iron: -