Bronze

Bronze is an alloy consisting primarily of copper, commonly with about 12–12.5% tin and often with the addition of other metals (including aluminium, manganese, nickel, or zinc) and sometimes non-metals (such as phosphorus) or metalloids (such as arsenic or silicon). These additions produce a range of alloys some of which are harder than copper alone or have other useful properties, such as strength, ductility, or machinability.

The archaeological period during which bronze was the hardest metal in widespread use is known as the Bronze Age. The beginning of the Bronze Age in western Eurasia is conventionally dated to the mid-4th millennium BCE and early 2nd millennium BCE in China; elsewhere, it gradually spread across regions. The Bronze Age was followed by the Iron Age, which started in about 1300 BCE and reached most of Eurasia by about 500 BCE, although bronze continued to be much more widely used than it is in modern times.

Because historical artworks were often made of bronzes (alloyed with tin) and brasses (alloyed with zinc), and old documentation sometimes confused the two terms, modern museum and scholarly descriptions of older artworks increasingly use the generalized term "copper alloy" instead of the names of individual alloys. This is done (at least in part) to prevent database searches from failing merely because of errors or disagreements in the naming of historic copper alloys.

Etymology

The word bronze (1730–1740) is borrowed from Middle French (1511), itself borrowed from Italian (13th century, transcribed in Medieval Latin as ) from either:

, back-formation from Byzantine Greek (, 11th century), perhaps from ( ), reputed for its bronze;Originally or originally: in its earliest form from Middle Persian brinǰ⁠ (also bring⁠) (Iranian Persian ),Originally from which Georgian (), Turkish , and Armenian (), all meaning or , as well, have also been borrowed. A Dictionary of the English Language (1746) offers: "a composition of copper and tin makes bell metal; and copper and brass; melted in equal quantities, produces what the French call bronze, used for figures and statues. Chambers".

Webster's Dictionary (1828) offers: "Tin united with copper in different proportions, forms bronze, bell-metal, and speculum-metal. D. Olmsted".

Chambers Dictionary (1872) offers: "Bronze, bronz, n. an alloy of copper and tin, of a brown or burned colour;"

History

The discovery of bronze enabled people to create metal objects that were harder and more durable than had previously been possible. Bronze tools, weapons, armor, and building materials such as decorative tiles were harder and more durable than their stone and copper ("Chalcolithic") predecessors.

The earliest tin-copper-alloy artifact has been dated to , in a Vinča culture site in Pločnik (Serbia), and believed to have been smelted from a natural tin-copper ore, stannite. Other early examples date to the late 4th millennium BCE in Egypt, Susa (Iran) and some ancient sites in Luristan (Iran), Tepe Sialk (Iran), Mundigak (Afghanistan), and Mesopotamia (Iraq).

Copper–arsenic alloys, from naturally or artificially mixed ores of those metals, forming arsenical bronze, were found to result in a metal of a silver hue, with similarly increased strength, a lower melting point, and simpler to work, than pure copper. The earliest known arsenic–copper alloy artifacts come from a Yahya Culture (Period V 3800–3400 BCE) site, at Tal-i-Iblis on the Iranian plateau, and were smelted from native arsenical copper and copper-arsenides, such as algodonite and domeykite.

Tin became the major non-copper ingredient of bronze in the late 3rd millennium BCE. was superior to arsenic copper in that the alloying process could be more easily controlled, and the resulting alloy was stronger and easier to cast. Also, unlike those of arsenic, metallic tin and the fumes from tin refining are not toxic.

Ores of copper and tin, which is much rarer, are not often found together (exceptions include Cornwall in the United Kingdom, one ancient site in Thailand and one in Iran), so serious bronze work has always involved trade with other regions. Tin sources and trade in ancient times had a major influence on the development of cultures. In Europe, a major source of tin was the British deposits of ore in Cornwall, which were traded as far as Phoenicia in the eastern Mediterranean. In many parts of the world, large hoards of bronze artifacts are found, suggesting that bronze also represented a store of value and an indicator of social status. In Europe, large hoards of bronze tools, typically socketed axes (illustrated above), are found, which mostly show no signs of wear. With Chinese ritual bronzes, which are documented in the inscriptions they carry and from other sources, the case is clear. These were made in enormous quantities for elite burials, and also used by the living for ritual offerings.

An analysis of 324 metallic finds of the Indus Valley Civilisation (2600–1900 BCE), in the 1990s, found 67 (20%) to have some tin content, to be considered bronze artifacts, with 26 (8%) of the artifacts having a tin content in excess of 10%, permitting casting; the source of the tin is unknown, but has been speculated to be from Bukhara or Samarkand, in Uzbekistan.

Transition to iron Though bronze, whose Vickers hardness is 60–258, is generally harder than wrought iron, with a hardness of 30–80,Smithells Metals Reference Book, 8th Edition, ch. 22 the Bronze Age gave way to the Iron Age after a serious disruption of the tin trade: the population migrations of around 1200–1100 BCE reduced the shipment of tin around the Mediterranean and from Britain, limiting supplies and raising prices. As the art of working in iron improved, iron became cheaper and improved in quality. As later cultures advanced from hand-wrought iron to machine-forged iron (typically made with trip hammers powered by water), blacksmiths also learned how to make steel, which is stronger and harder than bronze and holds a sharper edge longer. Bronze was still used during the Iron Age and has continued in use for many purposes to the modern day.

Composition

There are many different bronze alloys, but typically modern bronze is about 88% copper and 12% tin.Knapp, Brian. (1996) Copper, Silver and Gold. Reed Library, Australia. 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. Historical "bronzes" are highly variable in composition, as most metalworkers probably used whatever scrap was on hand; the metal of the 12th-century English Gloucester Candlestick is bronze containing a mixture of copper, zinc, tin, lead, nickel, iron, antimony, arsenic and an unusually large amount of silver – between 22.5% in the base and 5.76% in the pan below the candle. The proportions of this mixture suggest that the candlestick was made from a hoard of old coins. The 13th-century Benin Bronzes are in fact brass, and the 12th-century Romanesque Baptismal font at St Bartholomew's Church, Liège is sometimes described as bronze and sometimes as brass.

During the Bronze Age, two forms of bronze were commonly used: "classic bronze", about 10% tin, was used in casting; "mild bronze", about 6% tin, was hammered from ingots to make sheets. Bladed weapons were primarily cast from classic bronze while helmets and armor were hammered from mild bronze. Early black powder firearms such as cannon barrels were made from bronze. During the Austro-Hungarian North Pole expedition, the firearms onboard the Arctic exploration ship Admiral Tegetthoff were made completely of bronze as they were seen as an advantage over iron turning brittle at cold temperatures and deemed resistant to saltwater corrosion.

Modern (90% copper and 10% zinc) and architectural bronze (57% copper, 3% lead, 40% zinc) are more properly regarded as brass alloys because they contain zinc as the main alloying ingredient. They are commonly used in architectural applications. Plastic bronze contains a significant quantity of lead, which makes for improved plasticity, and may have been used by the ancient Greeks in ship construction. has a composition of Si: 2.80–3.80%, Mn: 0.50–1.30%, Fe: 0.80% max., Zn: 1.50% max., Pb: 0.05% max., Cu: balance. Other bronze alloys include aluminium bronze, phosphor bronze, manganese bronze, bell metal, arsenical bronze, speculum metal, bismuth bronze, and cymbal alloys.

Properties Copper-based alloys have lower melting points than steel or iron and are more readily produced from their constituent metals. They are generally about 10 percent denser than steel, although alloys using aluminium or silicon may be slightly less dense. Bronze conducts heat and electricity better than most steels. Copper-base alloys are generally more costly than steels but less so than nickel-base alloys.

Bronzes are typically ductile alloys and are considerably less brittle than cast iron. 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 low-friction properties of bearing bronze, which has a high lead content of 6–8%, the resonant qualities of bell bronze (20% tin, 80% copper), and the resistance to corrosion by seawater of several bronze alloys.

The melting point of bronze is about but varies depending on the ratio of the alloy components. Bronze is usually nonmagnetic, but certain alloys containing iron or nickel may have magnetic properties. Bronze typically oxidizes only superficially; once a copper oxide layer is formed, the underlying metal is protected from further corrosion. This can be seen on statues from the Hellenistic period. If copper chlorides are formed, a corrosion-mode called "bronze disease" will eventually destroy it completely.

Uses

Bronze, or bronze-like alloys and mixtures, were used for coins over a longer period. Bronze was especially suitable for use in boat and ship fittings prior to the wide employment of stainless steel owing to its combination of toughness and resistance to salt water corrosion. Bronze is still commonly used in ship propellers and submerged bearings. In the 20th 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. Sculptors may prefer silicon bronze because of the ready availability of silicon bronze brazing rod, which allows color-matched repair of defects in castings. Aluminium is also used for the structural metal aluminium bronze.

Unlike steel, bronze struck against a hard surface will not generate sparks, so it (along with beryllium copper) is used to make hammers, mallets, wrenches and other durable tools to be used in explosive atmospheres or in the presence of flammable vapors. Bronze is used to make bronze wool for woodworking applications where steel wool would discolor oak. Phosphor bronze is used for ships' propellers, musical instruments, and electrical contacts.

Bronze parts are tough and typically used for bearings, clips, electrical connectors and springs. Bronze also has low friction against dissimilar metals, making it important for cannons prior to modern tolerancing, where iron cannonballs would otherwise stick in the barrel.

Architectural bronze