An alloy is a combination of two or more metals mixed together in varying proportions while melted. Alloys of various metals may be formed that have certain desirable qualities possessed by none of the metals separately. The alloying of metals is of great value to the commercial and the scientific world, because by scientific alloying metals can be produced that have certain special properties for special uses for instance, in the manufacture of automobiles, aeroplanes, electrical machinery, and scientific instruments metals are needed that possess a certain known degree of density, tenacity, hardness, color, malleability, or fusibility; or it may be that a metal of light weight or low cost is needed in the manufacture of some special machine. Scientific alloying will within a certain range produce metals with any reasonable requirement. As a simple illustration of the principle of alloying, if we melt together two parts of copper and one part of tin, the resulting alloy will be a metal that is of a yellowish gray color, more dense, and much harder than either of the metals of which it was made. The melting point of an alloy is, as a rule, lower than that of either of the metals of which it is composed.

Copper is probably the most used of all metals in the making of alloys. In bronze and brass alloys, copper is the preponderant metal, it being the metal with which we can best alloy smaller quantities of other metals. The principal function of copper in alloys is to impart strength and toughness. In ornamental work it imparts varying degrees of its red color.