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For other uses, see the metal alloy.
Bronze Chola statue of Nataraja at the Metropolitan Museum of Art, New York City

Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive, but sometimes with other elements such as phosphorus, manganese, aluminium, or silicon. It is hard and brittle, and it was particularly significant in antiquity, giving its name to the Bronze Age. Bronze derives from the Italian: bronzo and, in turn, is perhaps ultimately taken from the Persian word birinj ("bronze"). It is also believed that the word may have come from the Nordic word Brongru (brown).[1]

History

Chinese pu vessel with interlaced dragon design, Spring and Autumn Period (722 BC-481 BC)

The discovery of bronze enabled people to create better metal objects than they previously could. 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. Initially bronze was made out of copper and arsenic to form arsenic bronze. It was only later that tin was used, becoming the sole type of bronze in the late 3rd millennium BC. Tin bronze was superior over arsenic bronze in that the alloying process itself could more easily be controlled (as tin was available as a metal) and the alloy was stronger and easier to cast. Also, unlike arsenic, tin is not toxic.

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).

Copper and tin ores are rarely found together (exceptions include one ancient site in Thailand and one in Iran), so serious bronze work has always involved trade. In Europe, the major source for tin was Great Britain's deposits of ore in Cornwall, which were traded as far as Phoenicia in the Eastern Mediterranean.

Though bronze is generally harder than wrought iron, with Vickers hardnesses of 60-258[2] vs 30-80[3], the Bronze Age gave way to the Iron Age; this happened because iron was easier to find. Bronze was still used during the Iron Age, but, for many purposes, the weaker wrought iron was found to be sufficiently strong. Archaeologists suspect that a serious disruption of the tin trade precipitated the transition. The population migrations around 1200 – 1100 BC reduced the shipping of tin around the Mediterranean (and from Great Britain), limiting supplies and raising prices.[4] As ironworking improved, iron became cheaper; and as cultures advanced from wrought iron to forged iron, they learned how to make steel, which is stronger than bronze and holds a sharper edge longer.[5]

Properties

Assorted ancient Bronze castings

Bronze is considerably less brittle than iron. Typically bronze only oxidizes superficially; once a copper oxide (eventually becoming copper carbonate) layer is formed, the underlying metal is protected from further corrosion. However, if copper chlorides are formed, a corrosion-mode called "bronze disease" will eventually completely destroy it.[6] 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 heavier than steel, although alloys using aluminium or silicon may be slightly less dense. Bronzes are softer and weaker than steel, e.g. bronze springs are less stiff (and so store less energy) for the same bulk. Bronze 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.

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.

Uses

Ewer from 7th century Iran. Cast, chased, and inlaid bronze. New York Metropolitan Museum of Art

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 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.

It is also widely used for cast bronze sculpture. Many common bronze alloys 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.

Spring bronze weatherstripping comes in rolls of thin sheets and is nailed or stapled to wood windows and doors. There are two types, flat and v-strip. It has been used for hundreds of years because it has low friction, seals well and is long lasting. It is used in building restoration and custom construction.

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.[citation needed] 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. It is also used in guitar and piano strings.

Bronze is typically 88% copper and 12% tin.[7] 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 (90% copper and 10% zinc) and Architectural bronze (57% Copper, 3% Lead, 40% Zinc) are actually brass alloys because they contain zinc as the main alloying ingredient. They are commonly used in architectural applications. [8][9]

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 vapours.

Bronze statues

See also: Bronze sculpture

Indian Hindu artisans from the period of the Chola empire in Tamil Nadu, used bronze to create intricate statues via the lost wax casting method with ornate detailing depicting the Gods of Hinduism mostly, but also the lifestyle of the period. The art form survives to this day, with many silpis, craftsmen, working in the areas of Swamimalai and Chennai.

In antiquity other cultures also produced works of high art using bronze. For example: in Africa the bronze heads of the Kingdom of Benin, in Europe; Grecian bronzes typically of figures from Greek mythology, in east Asia; Chinese bronzes of the Shang and Zhou dynasty — more often ceremonial vessels but including some figurine examples.

Bronze continues into modern times as one of the materials of choice for monumental statuery.

Musical instruments

Bronze is the most popular metal for top-quality bells, particularly bell metal, which is about 23% tin.

Nearly all professional cymbals are made from a bronze alloy. The alloy used in drum kit cymbal bronze is unique in the desired balance of durability and timbre.

According to a legend (on the Zildjian cymbals website), in 1623, an Armenian man in Turkey named Avedis Zildjian, an alchemist, was attempting to form base metals into gold. Upon dropping an ingot on the ground, he was amazed at how well it rang. He was given the title Zildjian ("son of cymbal maker") by the Turkish Sultan. Today, the Avedis Zildjian Corporation is the largest maker of cymbals in the world.[10]

Modern cymbals consist of several types of bronze, the most common being B20 bronze, which is roughly 20% tin, 80% copper, with traces of silver. Zildjian and Sabian use this alloy for their professional lines. A Swiss company, Paiste, uses a softer B8 bronze which is made from 8% tin and 92% copper in nearly all of their cymbals. Zildjian and Sabian use this metal too, in their budget priced cymbals.

As the tin content in a bell or cymbal rises, the timbre drops.[11] As well as B8 and B20, Meinl Percussion uses B10 (10% tin) and B12 (12% tin) alloys for cymbals, which have timbres roughly between B8 and B20.[12]

Bronze is also used for the windings of steel strings of various stringed instruments such as the double bass, piano, harpsichord, and the guitar, replacing former gut and nylon strings. Bronze strings are commonly reserved on pianoforte for the lower pitch tones, as they possess a superior sustain quality to that of high-tensile steel[13]

Bronzes of various metallurgical properties are widely used in struck idiophones around the world, notably in South East Asia, and most famously for the Javanese gamelan and other glockenspiel-like musical instruments. The earliest bronze archeological finds in Indonesia date from 1-2 BCE, including flat plates probably suspended and struck by a wooden or bone mallet.[13][14]

Some companies are now making saxophones from phosphor bronze (3.5 to 10% tin and up to 1% phosphor content).

See also

Fragment of the grave of Cyprian Kamil Norwid in the Bards' crypt in Wawel Cathedral, Kraków, Poland by sculptor Czesław Dźwigaj

References

  1. ^ Bronze at the Online Etymological Dictionary
  2. ^ http://www.allaboutgemstones.com/metal_jewelry_bronze.html
  3. ^ Smithells Metals Reference Book, 8th Edition, ch. 22
  4. ^ http://www.claytoncramer.com/Iron2.pdf
  5. ^ Ancient Blacksmiths, the Iron Age, Damascus Steels, and Modern Metallurgy
  6. ^ http://proteus.brown.edu/greekpast/4867
  7. ^ Knapp, Brian. (1996) Copper, Silver and Gold. Reed Library, Australia.
  8. ^ http://www.copper.org/applications/architecture/arch_dhb/copper_alloys/intro.html
  9. ^ http://www.copper.org/resources/properties/standard-designations/introduction.html
  10. ^ Background of Zildjian
  11. ^ Von Falkenhausen, Lothar (1993). Suspended Music: Chime-Bells in the Culture of Bronze Age China. Berkeley and Los Angeles: University of California Press. p. 106.
  12. ^ MEINL CYMBALS... INSPIRE: 4 IS MORE
  13. ^ a b McCreight, Tim. Metals technic: a collection of techniques for metalsmiths. Brynmorgen Press, 1992. ISBN 0961598433
  14. ^ LaPlantz, David. Jewelry - Metalwork 1991 Survey: Visions - Concepts - Communication: S. LaPlantz: 1991. ISBN 0942002059
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Wikisource has the text of the 1911 Encyclopædia Britannica article "Bronze".
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