ALLOYS, a term applied in its broadest sense to any metallic substance which is not a chemical element. If this definition is taken literally all of the metals of commerce could be regarded as alloys, since even the purest ones contain some foreign sub stance. It is the practice, however, to classify the metallic materials of commerce into two main groups—pure or com mercially pure metals and alloys. The commercial metals are often graded with respect to the nature and amount of "im purity" present, thus giving some justification for differentiating between a "commercially pure" metal and an alloy. It is inter esting to note a use of the term alloy applying to "impurities" in connection with gold. For several thousands of years gold was the most valuable metal known and any other metal mixed with it decreased the value for a given weight. The art of alloying cheaper metals with gold was practiced so fraudulently that "alloy" became associated with "impurity" or an addition which detracted from the value. This usage is standard in general Eng lish literature. It is not intended to convey the idea that the art of alloying was used formerly only to degrade or debase, but such practices became common knowledge, whereas the con structive practices did not. The great efforts in the 19th century toward modifying the properties of one metal by the addition of others served not only to popularize the benefits of alloying, but also contributed generously to the great industrial 'advances of the last few generations.
A definition of alloy is given by the American Society for Testing Materials, as follows: "A substance having metallic properties, consisting of two or more metallic elements or of metallic and non-metallic elements, which are miscible with each other when molten, and have not separated into distinct layers when solid." Although this definition suggests what should obtain in the molten condition—and most alloys are prepared by melting —they may be made in other ways. One of the oldest alloys is steel, composed essentially of iron and carbon. The old method of adding carbon was by heating solid iron in charcoal or in the reducing portions of wood fires, similar in principle to the present methods of adding carbon to solid iron or steel by carburization. Also alloys may be formed by mixing metal powders and heating to a temperature below the melting point, and they may be formed directly by electro-deposition. The art of preparing alloys is very old. In ancient times a number of alloys were produced by direct and simultaneous reduction of two or more metals from the ores. All of the material in a metallic substance need not be part of the alloy. For example, puddled iron or wrought iron contains some non-metallic material called slag. Wrought iron is not regarded as an alloy of iron and slag but as a relatively pure iron because the metallic constituent is relatively pure. The slag is regarded as an "inclusion." On the other hand, particles of cuprous oxide in copper may technically be regarded as part of the alloy because molten copper dissolves several per cent of this oxide.
Gradually the metallurgical art has advanced until nearly all of the metallic elements can be produced in a more or less pure state. Tens of thousands of combinations of so or more of the metallic elements with one another and with non-metallic ele ments have been prepared in recent times in the search for useful mixtures. It is estimated that more than 5,000 alloy compositions are in current use. This number does not include small variations such as may be expected in attempts to produce the same com position. In some respects the most common of the alloys are those used in coinage and jewellery. The so-called copper coins are really "bronze" alloys containing usually 95% copper, 4% tin and 1% zinc. Silver coins in Great Britain until 192o were made of "sterling" silver containing 92.5% silver and 7.5% copper. All U.S. silver coins Contain 9o% silver and o% copper ; the five-cent piece contains 75% copper and 25% nickel. British gold coins contain 91.67% gold and 8.33% copper, whereas U.S. gold coins contain 9o% gold and io% other metals, mostly copper. Platinum, gold and silver used in the manufacture of jewellery are nearly always alloyed with other metals to modify colour, improve strength, hardness and wearing qualities or to effect economy. The purity of gold jewellery is indicated by the "karat," 24-karat being pure gold. Eighteen-karat gold containing 75% gold and 25% silver is green, 75% gold, 12.5% silver and 12.5% copper is yellow, and 75% gold and 25% copper is red. White gold can be mad, by the addition of nickel. These precious metal alloys comprising many specific compositions also have other important uses. For example, platinum alloys are used in thermo-couples and electric contacts of magnetos, gold alloys in dentistry and silver alloys for table ware.
Steels are made in the form of castings, and all kinds of wrought shapes such as forgings, bars, rods, plates, sheets, structural shapes, tubing and wire. Cast irons not used in the wrought form are also included in the ferrous alloys. These are classified in three main groups : grey, white and malleable cast iron. Grey iron is es sentially an iron alloy containing 2 tO 4.5% carbon and sufficient silicon to cause the major part of the carbon to separate as graphite on solidification and cooling. White iron contains so little silicon or is solidified and cooled so rapidly that the carbon does not separate as graphite. It is hard and has a white fracture, whereas grey iron is relatively soft and machinable and has a grey fracture. Malleable iron is obtained by annealing white iron in such a manner as to eliminate the carbon or more commonly to cause its separalion into small and more or less spherical par ticles of graphite resulting in relative softness and absence of brittleness.