Bronze is an alloy consisting primarily of copper. The addition of other metals, produces
an alloy much harder than plain copper. The historical period where the archeological
record contains many bronze artifacts is known as the Bronze Age.
Because historical pieces were often made of brasses and bronzes with different compositions,
modern museum and scholarly descriptions of older objects increasingly use the more inclusive
term "copper alloy" instead. The word bronze is borrowed from French bronze,
itself borrowed from Italian bronzo "bell metal, brass", from either:
Ravenna *bróntion, back-formation from Byzantine Greek brontēsíon, perhaps from Brentḗsion
‘Brindisi’, reputed for its bronze; or early Persian birinj, biranj "brass", piring
"copper", from which also came Serbo-Croatian pìrinač "brass", Georgian brinǰao "bronze",
Armenian płinj "copper".
History
The discovery of bronze enabled people to create metal objects which were harder and
more durable than previously possible. Bronze tools, weapons, armor, and building materials
such as decorative tiles were harder and more durable than their stone and copper predecessors.
Initially, bronze was made out of copper and arsenic, forming arsenic bronze, or from naturally
or artificially mixed ores of copper and arsenic. It was only later that tin was used, becoming
the major non-copper ingredient of bronze in the late 3rd millennium BC. Tin bronze
was superior to arsenic bronze in that the alloying process could be more easily controlled,
and the resulting alloy was stronger and easier to cast. Also, unlike arsenic, tin is not
toxic. The earliest tin-alloy bronze dates to 4500
BCE in a Vinča culture site in Pločnik. Other early examples date to the late 4th
millennium BC in Susa and some ancient sites in China, Luristan and Mesopotamia.
Ores of copper and the far rarer tin are not often found together, so serious bronze work
has always involved trade. Tin sources and trade in ancient times had a major influence
on the development of cultures. In Europe, a major source of tin was England'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 hardness of 60–258 vs.
30–80, the Bronze Age gave way to the Iron Age because iron was easier to find and easier
to process into a usable grade of metal. Pure iron is soft, and the process of beating and
folding sponge iron to make wrought iron removes carbon and other impurities from the metal
which need to be re-introduced to improve hardness. Careful control of the alloying
and tempering eventually allowed for wrought iron with properties comparable to modern
steel. Bronze was still used during the Iron Age.
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,
limiting supplies and raising prices.
As the art of working in iron improved, iron became cheaper, and as cultures advanced from
wrought iron to machine forged iron, the blacksmiths learned how to make steel, which is stronger
than bronze and holds a sharper edge longer. Composition
There are many different bronze alloys, but typically modern bronze is 88% copper and
12% 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. 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 with 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 may suggest that the candlestick was made from a hoard
of old coins. The Benin Bronzes are really brass, and the Romanesque Baptismal font at
St Bartholomew's Church, Liège is described as both bronze and brass.
In the Bronze Age, two forms of bronze were commonly used: "classic bronze", about 10%
tin, was used in casting; and "mild bronze", about 6% tin, was hammered from ingots to
make sheets. Bladed weapons were mostly cast from classic bronze, while helmets and armor
were hammered from mild bronze. Commercial bronze and architectural bronze
are more properly regarded as brass alloys because they contain zinc as the main alloying
ingredient. They are commonly used in architectural applications.
Bismuth bronze is a bronze alloy with a composition of 52% copper, 30% nickel, 12% zinc, 5% lead,
and 1% bismuth. It is able to hold a good polish and so is sometimes used in light reflectors
and mirrors. Plastic bronze is bronze containing a significant
quantity of lead which makes for improved plasticity possibly used by the ancient Greeks
in their ship construction. Other bronze alloys include aluminium bronze,
phosphor bronze, manganese bronze, bell metal, arsenical bronze, speculum metal and cymbal
alloys. Properties
Typically bronze only oxidizes superficially; once a copper oxide 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. 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—bronze springs, for example, are less stiff for the same bulk. Bronze resists
corrosion and metal fatigue more than steel and is a better conductor of heat and electricity
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 low-friction properties
of bearing bronze, the resonant qualities of bell bronze, and the resistance to corrosion
by sea water of several bronze alloys. The melting point of bronze varies depending
on the ratio of the alloy components and is about 950 °C. Bronze may be nonmagnetic,
but certain alloys containing iron or nickel may have magnetic properties.
Uses
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. 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 mold. Bronze parts are
tough and typically used for bearings, clips, electrical connectors and springs.
Bronze also has very low metal-on-metal friction, which made it invaluable for the building
of cannon where iron cannonballs would otherwise stick in the barrel. 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. Unlike steel, bronze struck against a hard
surface will not generate sparks, so it 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.
Bronze statues
In India, bronze sculptures from the Kushana and Gupta periods and later periods have been
found. 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 deities 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.
The Assyrian king Sennacherib claims to have been the first to cast monumental bronze statues
using two-part moulds instead of the lost-wax method.
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 sculptures, although
known for their longevity, still undergo microbial degradation; such as from certain species
of yeasts. Bronze continues into modern times as one
of the materials of choice for monumental statuary.
Musical instruments
Bronze is the preferred metal for top-quality bells, particularly bell metal, which is about
23% tin. Nearly all professional cymbals are made from
bronze, which gives a desirable balance of durability and timbre. Several types of bronze
are used, commonly B20 bronze, which is roughly 20% tin, 80% copper, with traces of silver,
or the tougher B8 bronze which is made from 8% tin and 92% copper. As the tin content
in a bell or cymbal rises, the timbre drops. Bronze is also used for the windings of steel
and nylon strings of various stringed instruments such as the double bass, piano, harpsichord,
and the guitar. 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.
Bronzes of various metallurgical properties are widely used in struck idiophones around
the world, notably bells, singing bowls, gongs, cymbals and other idiophones from Asia. Examples
include Tibetan singing bowls, temple bells of many sizes and shapes, gongs, Javanese
gamelan and other bronze 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. Ancient bronze drums from Thailand and Vietnam date back 2,000
years. Bronze bells from Thailand and Cambodia date back to 3,600 BCE.
Some companies are now making saxophones from phosphor bronze. Bell bronze is used to make
the tone rings of many professional model banjos. The tone ring is a heavy folded or
arched metal ring attached to a thick wood rim, over which a skin, or most often, a plastic
membrane is stretched - it is the bell bronze that gives the banjo a crisp powerful lower
register and clear, bell-like treble register-especially in bluegrass music.
Medals
Bronze has been used in the manufacture of various types of medals for centuries, and
are known in contemporary times for being awarded for third place in sporting competitions
and other events. The later usage was in part attributed to the choices of gold, silver
and bronze to represent the first three Ages of Man in Greek mythology: the Golden Age,
when men lived among the gods; the Silver age, where youth lasted a hundred years; and
the Bronze Age, the era of heroes, and was first adopted at the 1904 Summer Olympics.
At the 1896 event, silver was awarded to winners and bronze to runners-up, while at 1900 other
prizes were given, not medals. Industrial
Various kinds of bronze are used in many different industrial applications.
Phosphor bronze is used for ships' propellers, musical instruments, and electrical contacts.
Bearings are often made of bronze for its friction properties. It can be filled with
oil to make the proprietary Oilite and similar material for bearings.
Aluminium bronze is very hard and is used for bearings and machine tool ways.
See also
Bronze Age List of copper alloys
Art object Bronze and brass ornamental work
Bronze medal Bronze sculpture
Bronzing Chinese bronze inscriptions
French Empire mantel clock Seagram Building
Dezincification Resistant Brass UNS C69100
Yoruba traditional art References
External links Bronze bells
"Lost Wax, Found Bronze": lost-wax casting explained
"Flash animation of the lost-wax casting process". James Peniston Sculpture. Retrieved 2008-11-03.
Viking Bronze – Ancient and Early Medieval bronze casting