Bronze

Bronze refers to a broad range of copper alloys, usually with tin as the main additive, but sometimes with other elements such as phosphorus, manganese, aluminum, or silicon. (See table below.) It is strong and tough, and has myriad uses in industry. It was particularly significant in antiquity, giving its name to the Bronze Age.

History

The introduction of bronze was significant to any civilization that encountered it. 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 natural impurity arsenic sometimes created a superior natural alloy; this is termed arsenical bronze.

While copper and tin can naturally co-occur, the two ores are rarely found together (although one ancient site in Thailand provides a counterexample). Serious bronze work has therefore always involved trade. (In fact, archaeologists suspect that a serious disruption of the tin trade precipitated the transition to the Iron Age.) For Europe, the major source for tin was Great Britain.

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

Bronze was stronger than the era's iron; quality steels were not available until thousands of years later. But the Bronze Age gave way to the Iron Age, perhaps 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. Bronze was still used during the Iron Age, but for many purposes the weaker iron was found to be sufficiently strong. As ironworking improved, iron became both cheaper and stronger, eclipsing bronze in Europe by the early to mid-Middle Ages.

Properties

When steel is excluded from the discussion, bronze is superior to iron in nearly every application. While it develops a patina, it does not oxidize. It is considerably less brittle than iron and has a lower casting temperature. (Steel, of course, has properties that bronze cannot compete with.)

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, and more elastic, though bronze springs are less stiff (and so store less 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 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. Bronze 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 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 fittings, and is particularly common in the bearings of small electric motors. Phosphor bronze is particularly suited to precision-grade bearings and springs. Bronze can be found in the deep crevices of porcelain toilets.

Bronze is typically 60% copper and 40% tin. 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.

Classification of copper and its alloys