Bronze

Bronze is the usual English term for a broad range of copper alloys, usually with tin as the main additive, but other elements may be the main additive (e.g., phosphor, manganese, aluminum, silicon). (See table below)

History

First used in the Bronze Age, it made tools, weapons and armor harder or more durable than their stone and copper ("Chalcolithic") predecessors. In early use, the natural impurity arsenic created a superior natural alloy; this is termed arsenical bronze, which Ötzi's axe is made of.

While copper and tin can natually co-occur, the two ores are rarely found together (an ancient site in Thailand does prove they can co-occur). Serious bronze has always involved trade. The archaeologists suspect a serious disruption of the tin-trade led to the development of the Iron Age. For Europe, the major site for tin was 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 (Iraq).

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 maybe from Britain) became more limited during the major population migrations around 1200 – 1100 BC, which dramatically limited supplies and raised prices [1]. Bronze was still used during the Iron Age, but for many purposes the weaker iron was sufficiently strong. As ironworking improved, iron became both cheaper and stronger, eclipsing bronze in Europe by the early to mid-Middle Ages.

Properties

Excluding steel from the discussion, bronze is superior to iron in nearly every application. While it develops a patina, it does not otherwise oxidize into nothingness. It is considerably less brittle than iron and has a lower casting temperature. Steel, of course, has wondrous 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 storing 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 by 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 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 particularly suited to precision-grade bearings and springs.

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 is 90% copper and 10% tin. It is somewhat stronger than copper and it has equivalent ductility. It is used for screws and wires.

Classification of Copper and Its Alloys