Cast iron is brittle, non-elastic, and the easiest fused of all iron, these properties varying directly with the amount of combined carbon and to a less degree with the amount of uncombined carbon contained. It can be cast into intricate forms and has the advantage of expanding upon cooling, but it cannot be forged, nor united by the usual welding process of heating and hammering. It it not malleable nor ductile, and cannot be hardened like steel, because it contains uncombined carbon. It has either a crystalline or a granular fracture, determined by rapidity of cooling, and melts at about 2100° F.