ALLOY, a compound or mixture of two or more metals. When mercury is mixed with another metal, the compound is termed an amalgam. Alloys are di vided into three groups: (1) Those formed by the metals lead, tin, zinc, and cadmium, which impart to their alloys their own physical properties in the pro portions in which they themselves are contained in the alloy. (2) Those formed by almost all other metals. (3) Those which contain metals found in both these groups of alloys.
In every alloy the specific heat and the coefficient of expansion are always the means of those of its component metals. But in other physical proper ties a variation takes place. This is the case with specific gravity, which, in alloys of the first group, is the mean of their constituent metals; but in those of the second group it is always greater or less than the mean specific gravity of their constituents.
In some instances, when two melted metals are mixed together to form an alloy, an evolution of heat occurs which is believed to indicate that a chemical compound has been formed. This is the case with copper and zinc, copper and aluminum, platinum and tin. etc. The strength or cohesion of an alloy is gen erally greater than that of the mean cohesion of the metals contained therein or even of that of the most cohesive of its constituents.
The most useful alloy in the arts is brass. This compound metal is next to iron in importance.
There are some important alloys of copper and tin, among them bronze, gun metal, bell metal and speculum metal. In these the proportions vary from equal parts of copper and tin to 10 parts of copper with 1 of tin. The most cohesive, that is, the strongest of them, is a bronze consisting of 6 parts of cop per to 1 of tin. Phosphor bronze is an invention of recent years. The addi tion of from 0.25 to 2.5 per cent. of phosphorus to a bronze containing from 7 to 8 per cent. of tin gives it greater
hardness, elasticity, and toughness. This alloy is now much used for parts of machinery.
Pewter is a tin alloy which was more used formerly than now. Type metal is a compound of 50 parts of lead, 25 of antimony, and 25 of tin, but it varies slightly. Fusible metal melts at low temperatures; one kind is composed of 3 parts of tin, 5 of lead, and 8 of bis muth, and melts in hot water. This al loy is now a good deal employed in stereotyping, and in obtaining copies of woodcuts.
Aluminum bronze, very closely re sembling gold in appearance, is much used for pencil-cases, chains, and some larger objects. A compound of silver and aluminum is sometimes used for watch-springs, and for spoons and forks. Dentists use a very ductile alloy com posed of 2 parts by weight of silver and 1 of platinum.
When gold is to be used for coins, jew elry, or plate, it requires to be alloyed with copper or silver or with both, in order to harden it. Like silver, it is too soft when pure. There are five legal standards for articles made of gold i. e., alloyed gold apart from coin. These are called 22, 18, 15, 12, and 9 carat gold. That is to say, these figures represent the number of parts of pure gold in every 24 parts of the alloy used by the goldsmith or jeweler. English sover eigns are made of a mixture of 22 parts of gold to 2 of copper, and this is called 22-carat or standard gold. In Germany, Italy, and the United States, standard gold for the coinage is 21.6 carats.
In the United States, it is declared by law that the standard for both gold and silver coins shall be such, that of 1,000 parts, by weight, 900 shall be of pure metal and 100 of alloy.