The industrial importance of copper in the 20th Century has been extended by the ease with which it combines with other metals. Tin and zinc are and always have been the principal alloying elements, but there are now many others - aluminium, beryllium, chromium, manganese, etc. - which form alloys with special mechanical and physical properties. Copper alloys now in fact play an indispensable part in everyday life even for such unspectacular items as show-eyelets, 'gold' paint on cigarette packets, and for zip fasteners, costume jewellery and bird cages. These are but a few of the common things in which copper is a major constituent; in addition there are many even many more unobtrusive uses of copper, e.g. in large cast iron engine liners, as a 'flash' in electroplating, and as moulds for plastics: even the little bunches of bristles in tooth-and- nail-brushes are held firmly in place by tiny wedges of nickel silver which is a copper alloy.
The alloys containing copper fall into main types: these are copper-base alloys, such as brass, tin bronze and aluminium bronze, in which copper itself is the predominant element; and copper-bearing materials, such as certain aluminium alloys, high-duty alloys to resist severe corrosion, and steels and cast irons which are improved by small additions of copper. The proprietary alloy 'Monel', a mixture of copper and nickel in which the nickel predominates, occupies an intermediate field between these two main classes*.