One of the first metals to be exploited by man because, like gold, it can be found in the native form, pure and requiring no smelting. It was later extracted from a variety of ores, the carbonate (malachite), oxides and sulphides. Shaping could be done by simple hammering, which served also to harden the metal. But where smelting produced the metal in molten form, the possibilities of casting must have soon been realised. An open mould would normally have been used as unflawed castings are very difficult to obtain in closed ones. Closed moulds were only introduced when alloying of the copper, particularly with tin to produce bronze, got over with difficulty.