Assorted bronze castings found as part of a cache, probably intended for recycling.
Bronze is the traditional name for a broad range of alloys of copper. It is usually copper with tin as the main additive. First used during the Bronze Age, to which it gave its name, bronze made tools, weapons and armor that were either harder or more durable than their stone and copper predecessors. During the Bronze Age, arsenic was often included in the bronze (mostly as an impurity), which made the alloy even harder. The earliest bronzes date to the late 4th millennium BC, and are found in the context of Susa and some ancient sites in Luristan and Mesopotamia.
Bronze was also stronger than iron, another common metal of the era, and quality steels were not available until thousands of years later. Nevertheless the Bronze Age gave way to the Iron Age as the shipping of tin around the Mediterranean ended during the major population migrations around 1200 – 1100 BC, which dramatically limited supplies and raised prices. Bronze was still used to a considerable extent during the Iron Age, but for many purposes the weaker iron was sufficiently strong to serve in its place. As an example, Roman officers were equipped with bronze swords while foot soldiers had to make do with iron blades.
Copper-based alloys have lower melting points than steels and are more readily produced from their constituent metals. They are comparable to steel in density, most copper alloys being only about 10 percent heavier, although alloys using much aluminium or silicon may be slightly less dense than steel. Bronzes are softer and weaker than steel, and more elastic, though bronze springs are less stiff (lower energy) for the same bulk. Bronzes resist corrosion (especially seawater corrosion) and metal fatigue better than steel. Bronzes also conduct 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.
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 by several bronze alloys.
Starting in the twentieth century silicon was introduced as the primary alloying element with copper. Silicon bronze is used in a wide variety of industrial applications, and largely represents the bronze used in the production of 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 also 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 also has very little metal-on-metal friction, which made it invaluable for the building of cannon where iron cannonballs would otherwise stick in the barrel. Bronze is still widely used today for springs, bearings, bushings and similar roles, and is particularly common in the bearings on small electric motors. Phosphor bronze is thought particularly suited to precision-grade bearings and springs.
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