Wrought iron possesses one of the most valuable properties of metals; small masses of it will weld or unite into one. No other metals except platinum and aluminum possess this property. If two pieces of iron are heated to a white heat, they become sticky or viscous so that when hammered together they adhere and may be perfectly united by forging. This kind of iron is used by blacksmiths and, unlike cast iron, may be hammered into any desired shape. Because of its greater strength, wrought iron is used for making bars, plates, wires, structural material, and parts of machinery. It is tougher, stretches more, and gives more warning before fracture than does cast iron. Moreover, cast iron is not malleable or ductile like wrought iron. Neither cast nor wrought iron can be hardened and tempered.