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A mixture of two or more metals is called an alloy. Alloys are distinguished from composite metals in that alloys are thoroughly mixed, creating, in effect, a synthetic metal. In metal composites, the introduced metal retains its identity within the matrix in the form of fibers, beads, or other shapes.

Alloys can be created by mixing the metals while in a molten state or by bonding metal powders. Various alloys have different desired properties such as strength, visual attractiveness, or malleability. The number of possible alloy combinations is almost endless since any metal can be alloyed in pairs or in multiples.

An entire period of human prehistory is named for the earliest known alloy—bronze. During the Bronze age (c.3500-1000 B.C.) humans first fashioned tools and weapons from something other than basic materials found in nature. Humans combined copper and tin to form a strong metal that was still easily malleable. Modern bronze contains a 25:75 ratio of tin to copper. The use of bronze in early times was greatest in nations where tin deposits were most plentiful, like Asia Minor, and among countries that traded with tin-mining nations.

Brass is an alloy of copper and zinc. It is valued for its light weight and rigid strength. It has a ratio of about one-third zinc to two-thirds copper. The exact ratio of metals determines the qualities of the alloy. For example, brass, having less than 63% copper, must be heated to be worked. Brass is noted for its beauty when polished. Brass was perhaps first produced in Palestine from 1400 to 1200 B.C. It was later used by the Romans for coins. Many references to brass in the Bible and other ancient documents are really mistranslations of mentions of bronze.

Pewter is an alloy of copper, tin, and antimony. It is a very soft mixture that can be worked when cold and beat repeatedly without becoming brittle. It was used in Roman times, but its greatest period of popularity began in England in the fourteenth century and continued into the eighteenth. Colonial American metalworks produced some notable pewter work. As a cheaper version of silver, it was used in plates, cups, pitchers, and candelabras.

The various types of steel and iron are all alloys classifiable by their content of other materials. For instance, wrought iron has a very small carbon content, while cast iron has at least 2% carbon.

Steels contain varying amounts of carbon and metals such as tungsten, molybdenum, vanadium, and cobalt, giving them the strength, durability, and anti-corrosion capabilities required by their different uses. Stainless steel, which has 18% chromium and 8% nickel alloyed to it, is valued for its anti-corrosive qualities.

Duraluminum contains one-third steel and twothirds aluminum. It was developed during World War I for the superstructures of the Zeppelin airships built in Germany.

Many alloys add function to physical beauty. For example, sterling silver is made with 8% copper to add strength so that it can be made into chalices and silverware.

All American coins are made from copper alloy, sometimes sandwiched between layers of silver.

Alloys greatly enhance the versatility of metals. Without them there would be total dependency on pure metals, which would affect their cost and availability. Alloys are a very important part of humankind's past and future.

See also Metallurgy.

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