Bronze

  Bronze is any of a broad range of copper alloys, usually with tin as the main additive, but sometimes with other elements such as phosphorus, manganese, aluminium, or silicon. (See table below.) It was particularly significant in antiquity, giving its name to the Bronze Age. That name, in turn, is perhaps ultimately taken from the Persian word "birinj," meaning "copper".^[1]

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History of Bronze

  Bronze was significant to any culture that encountered it. It was one of the most innovative alloys of mankind. Tools, weapons, armor, and various building materials like decorative tiles made of bronze were harder and more durable than their stone and copper ("Chalcolithic") predecessors. In early use, the impurity arsenic sometimes created a superior alloy; this is termed arsenical bronze.

The earliest tin-alloy bronzes date to the late 4th millennium BC in Susa (Iran) and some ancient sites in Luristan (Iran) and Mesopotamia (Iraq).

While copper and tin can naturally co-occur, the two ores are rarely found together (although one ancient site in Thailand and one in Iran provide counterexamples). Serious bronze work has therefore always involved trade (and the compelling idea that there were really traders in such goods). In fact, archaeologists suspect that a serious disruption of the tin trade precipitated the transition to the Iron Age. In Europe, the major source for tin was Great Britain, where significant deposits of ore could be found in Cornwall. Phoenician traders visited Great Britain to trade goods from the Mediterranean for tin.^{[citation needed]}

  Bronze is stronger (harder)^[2] than wrought iron, but the Bronze Age gave way to the Iron Age. That may have been because the shipping of tin around the Mediterranean (or from Great Britain) became more limited during the major population migrations around 1200 – 1100 BC, which dramatically limited supplies and raised prices.^[3] Bronze was still used during the Iron Age, but for many purposes the weaker wrought iron was found to be sufficiently strong. As ironworking improved, iron became cheaper, and people figured out how to make steel, which is stronger than bronze, holding a sharper edge longer.^[4]

Properties

Bronze only oxidizes superficially; once the surface oxidizes, the thin oxide layer protects the underlying metal from further corrosion. Copper-based alloys have lower melting points than steel and are more readily produced from their constituent metals. They are generally about 10 percent heavier than steel, although alloys using aluminium or silicon may be slightly less dense. Bronzes are softer and weaker than steel, bronze springs are less stiff (and so store less energy) for the same bulk. It resists corrosion (especially seawater corrosion) and metal fatigue better than steel and also conducts heat and electricity better than most steels. The cost of copper-base alloys is generally higher than that of steels but lower than that of nickel-base alloys such as stainless steel.

Copper and its alloys have a huge variety of uses that reflect their versatile physical, mechanical, and chemical properties. Some common examples are the high electrical conductivity of pure copper, the excellent deep-drawing qualities of cartridge case brass, the low-friction properties of bearing bronze, the resonant qualities of bell bronze, and the resistance to corrosion by sea water of several bronze alloys.

In the twentieth century, silicon was introduced as the primary alloying element, creating an alloy with wide application in industry and the major form used in contemporary statuary. Aluminium is also used for the structural metal aluminium bronze.

  Bronze is the most popular metal for top-quality bells and cymbals, and more recently, saxophones. It is also widely used for cast metal sculpture (see bronze sculpture). Common bronze alloys often have the unusual and very desirable property of expanding slightly just before they set, thus filling in the finest details of a mould. Bronze parts are tough and typically used for bearings, clips, electrical connectors and springs.

Bronze also has very little metal-on-metal friction, which made it invaluable for the building of cannons where iron cannonballs would otherwise stick in the barrel. It is still widely used today for springs, bearings, bushings, automobile transmission pilot bearings, and similar fittings, and is particularly common in the bearings of small electric motors. Phosphor bronze is particularly suited to precision-grade bearings and springs.

Bronze is typically 88% copper and 12% tin.^[5] Alpha bronze consists of the alpha solid solution of tin in copper. Alpha bronze alloys of 4–5% tin are used to make coins, springs, turbines and blades.

Commercial bronze (otherwise known as brass) is 90% copper and 10% zinc, and contains no tin. It is stronger than copper and it has equivalent ductility. It is used for screws and wires.

Unlike steel, bronze struck against a hard surface will not generate sparks, so it is used to make hammers, mallets, wrenches and other durable tools to be used in explosive atmospheres or in the presence of flammable vapours.

Classification of copper and its alloys

See also

• Aluminum bronze
• Brass, a subset of the copper alloys in which zinc is the principal additive
• Bronze medal
• Bronze sculpture, the use of bronze for artistic representations
• Bronzing, a process by which an object is coated in bronze
• Cupronickel, an alloy used on ships
• Florentine bronze, an alloy which is not standardised (in proportions) worldwide
• Gunmetal, various copper-zinc-tin alloys, some including phosphorus
• Lost-wax casting
• Luristan bronze - Bronze Age artifacts recovered from areas of present day Iran
• Phosphor bronze, with properties useful in making corrosion-resistant springs
• Seagram Building, with a record 3.2 million pounds of bronze used in its façade
• Speculum metal, a high-tin bronze

References