The first bronze implements were made about 2500 B.C. when the Sumerians exhausted their sources of tin to make the useful alloy. Bronze is a copper alloy, originally composed of roughly 90 percent copper to 10 percent tin. Bronze becomes stronger as more tin is added but loses much of its toughness and malleability. Contemporary bronzes are copper alloys that can contain iron, aluminum, silicon, manganese, lead and other elements.
Bronze’s malleability is one of its greatest assets. The piping system in Egypt was constructed with copper and its alloys and still remains in good condition. Bronze can be shaped, hammered and cast into almost any shape. Thanks to its resistance to corrosion and excellent conductivity of electricity and heat, bronze is used for everything from miniature machine bearings to giant ship propellers.
Pure copper is never pure as an ore. Impurities of lead, arsenic, antimony, nickel, bismuth and other materials can always be found in naturally occurring copper ore. Bismuth impurities in copper produce brittleness and lead impurities produce softness. Because bronze is an alloy, these impurities are eliminated in the copper, and only controlled amounts of other materials are allowed to compose the alloy. The addition of 1 percent phosphorus makes bronze harder and stronger.
Bronze, as an alloy, is harder than copper and therefore can be applied to more rigorous uses. The hardness of a metal is designated with a numerical value using the Rockwell hardness scale. Silicon bronze has HRB 50, where B is the class of hardness and 50 is the specific value. Phosphor bronze has HRB 45 and manganese bronze has HRB 80.
Bronze replaced pure copper in casting pots, weapons and other mold-made items because it is easier to cast. The molten metal flows more smoothly because of its lower melting point and creates fewer imperfections in the finished product. A typical bronze for casting statues and other artistic pieces is made with less than 10 percent tin and has lead and zinc mixed in for their workable qualities. Cast bronze also has the chemical properties to stand up to weathering and corrosion. Bronze oxidizes externally. This protects the inner layers of metal from completely deteriorating.
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