Bronze

• 1. Introduction

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  Bronze is an alloy made primarily of copper, usually carrying about 12 to 12.5 percent tin. That small dose of a second metal changed what humanity could build. Tools, weapons, armor, and even decorative tiles suddenly came out harder and more durable than the stone and copper objects that came before. An entire archaeological period takes its name from this material: the Bronze Age, the stretch when bronze was the hardest metal in widespread use. The earliest known tin-copper artifact has been dated to around 4650 BCE, at a Vinca culture site in Plocnik, in present-day Serbia. From there the questions multiply. Why did a metal this useful depend on trade routes stretching from Cornwall to the eastern Mediterranean? How did a corrosion-resistant alloy end up wrapping a Park Avenue skyscraper, ringing in cathedral bells, and standing in for third place at the Olympics? And why do modern museums increasingly refuse to call old bronzes bronze at all?

• 2. A Question Of Recipe

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  About 88 percent copper and roughly 12 percent tin is the rough formula for modern bronze, but the word covers a sprawling family of mixtures. Other metals join the blend, including aluminium, manganese, nickel, or zinc, and sometimes non-metals like phosphorus or metalloids like arsenic and silicon. Each addition buys a property: greater hardness, strength, ductility, or easier machining than copper alone offers.

  Alpha bronze, the solid solution of tin in copper at about 4 to 5 percent tin, goes into coins, springs, turbines, and blades. During the Bronze Age itself, metalworkers leaned on two practical recipes. Classic bronze, around 10 percent tin, was poured into casts for bladed weapons. Mild bronze, around 6 percent tin, was hammered from ingots into sheets for helmets and armor.

  Historical bronzes are wildly inconsistent, because most metalworkers simply used whatever scrap was on hand. The 12th-century English Gloucester Candlestick proves the point. Its metal blends copper, zinc, tin, lead, nickel, iron, antimony, and arsenic, plus an unusually large amount of silver, ranging from 22.5 percent in the base to 5.76 percent in the pan below the candle. Those proportions hint that the candlestick was melted down from a hoard of old coins.

• 3. The Naming Problem

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  Brass is an alloy of copper and zinc, and that distinction has quietly upended how scholars catalogue old artworks. Because historical pieces were cast from bronzes and brasses of two very different compositions, museums increasingly use the generalized label copper alloy rather than naming an individual alloy. The reason is practical: a database search should not fail simply because experts disagree on what to call a historic copper alloy.

  The 13th-century Benin Bronzes illustrate the trap. Despite the name, they are in fact brass. The medieval and early-modern castings of the Benin Empire, spanning the 13th to 19th centuries CE and often labelled bronzes, are technically cast from an often leaded brass alloy. The 12th-century Romanesque baptismal font at St Bartholomew's Church in Liege gets described sometimes as bronze and sometimes as brass.

  Even everyday objects blur the line. Modern bronze of 90 percent copper and 10 percent zinc, and architectural bronze of 57 percent copper, 3 percent lead, and 40 percent zinc, are more properly brass alloys, since zinc is the main alloying ingredient. The word bronze itself has a tangled lineage, borrowed into English between 1730 and 1740 from Middle French bronze of 1511, which came from the Italian bronzo recorded in the 13th century.

• 4. Trade And Treasure

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  Ores of copper and tin are rarely found together, and tin is much the rarer of the two. A handful of exceptions exist, including Cornwall in the United Kingdom, one ancient site in Thailand, and one in Iran. Everywhere else, serious bronze work demanded trade with distant regions, and that dependence shaped whole cultures. In Europe, the British tin deposits of Cornwall were traded as far as Phoenicia in the eastern Mediterranean.

  Large hoards of bronze artifacts turn up across the world, suggesting the metal doubled as a store of value and a marker of social status. In Europe, hoards of bronze tools, typically socketed axes, are found showing no signs of wear. Chinese ritual bronzes make the point unmistakable. Documented in the inscriptions they carry, they were produced in enormous quantities for elite burials and used by the living for ritual offerings.

  The Indus Valley Civilisation, dated 2600 to 1900 BCE, left numbers to study. An analysis in the 1990s of 324 metallic finds turned up 67, or 20 percent, with enough tin to count as bronze. Of those, 26, or 8 percent, carried tin content above 10 percent, enough to permit casting. The source of that tin is unknown, though it has been speculated to come from Bukhara or Samarkand, in Uzbekistan.

• 5. When Iron Won

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  Bronze measures 60 to 258 on the Vickers hardness scale, comfortably harder than wrought iron at 30 to 80. By that measure bronze should have held its ground, yet the Bronze Age still gave way to the Iron Age. The cause was disruption, not inferiority. Population migrations around 1200 to 1100 BCE choked the shipment of tin around the Mediterranean and from Britain, shrinking supplies and driving prices up.

  Iron answered that crisis on economics. As smiths improved their craft, iron grew cheaper and better in quality. Later cultures advanced from hand-wrought iron to machine-forged iron, typically shaped with trip hammers powered by water. Along the way blacksmiths learned to make steel, which beats bronze on strength and hardness and holds a sharper edge longer.

  The transition was gradual rather than total. The Iron Age started about 1300 BCE and reached most of Eurasia by about 500 BCE. Bronze never disappeared during it, and has continued in use for many purposes right up to the present day.

• 6. Properties Of A Survivor

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  Around 950 degrees Celsius is the rough melting point of bronze, lower than steel or iron, and it shifts with the ratio of the alloy components. That low melting point, paired with easy production from its constituent metals, made bronze workable long before furnaces could tame harder metals. Copper-based alloys run about 10 percent denser than steel, though versions using aluminium or silicon come out slightly lighter, and they conduct heat and electricity better than most steels.

  Ductility is the quiet virtue here. Bronzes bend rather than shatter, far less brittle than cast iron, and their properties can be tuned for specific jobs. Bearing bronze carries a high lead content of 6 to 8 percent for low friction. Bell bronze, 20 percent tin and 80 percent copper, is prized for resonance. Several bronze alloys resist corrosion by seawater.

  That corrosion resistance comes from chemistry visible on ancient statues. Bronze oxidizes only at the surface, and once a copper oxide layer forms it shields the metal beneath, an effect seen on statues from the Hellenistic period. There is a darker counterpart. If copper chlorides form instead, a corrosion mode called bronze disease will eventually destroy the object completely.

• 7. Bells, Cymbals And Strings

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  Bell metal is a high tin bronze, typically about 23 percent tin, and it is the preferred metal for bells. The tin proportion is not decorative. As the tin content in a bell or cymbal rises, the timbre drops, so the recipe tunes the sound itself.

  Nearly all professional cymbals are made from bronze for its balance of durability and timbre. B20 bronze, roughly 20 percent tin and 80 percent copper with traces of silver, is common, while the tougher B8 bronze uses 8 percent tin and 92 percent copper. Bell bronze, also called B20, shapes the tone rings of many professional banjos. That tone ring is a heavy folded or arched metal ring, usually 3 pounds, attached to a thick wood rim under a stretched skin or plastic head, and it gives the banjo a crisp powerful lower register and a clear bell-like treble.

  Strings carry bronze too. It forms the windings on steel and nylon strings for the double bass, piano, harpsichord, and guitar. On the pianoforte, bronze strings are reserved for the lower pitch tones, where they sustain better than high-tensile steel. The instrument family stretches across Asia, from Tibetan singing bowls and the Javanese gamelan to gongs, with the earliest bronze finds in Indonesia dating from 1 to 2 BCE and ancient bronze drums from Thailand and Vietnam reaching back 2,000 years.

• 8. Cast In Permanence

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  Common bronze alloys expand slightly just before they set, an unusual property that lets molten metal fill the finest details of a mould. As the bronze then cools it shrinks a little, making it easier to free from the mould. That behavior made bronze the material of choice for sculpture across civilizations. The Assyrian king Sennacherib, who reigned from 704 to 681 BCE, claimed to be the first to cast monumental bronze statues of up to 30 tonnes, using two-part moulds instead of the lost-wax method.

  Greek artists ranked bronze statues as the highest form of sculpture, yet survivals are few, because bronze objects were repeatedly melted down for reuse throughout the Classical period. Many of the most famous Greek bronzes are known only through Roman copies in marble, which stood a better chance of lasting. In India, Hindu artisans of the Chola empire in Tamil Nadu used the lost-wax method to cast intricate deities, an art form that survives among craftsmen called silpis working in Swamimalai and Chennai.

  The modern era kept finding new canvases for the metal. In 1958 the Seagram Building rose on New York City's Park Avenue, the first time an entire building was sheathed in bronze, designed by Mies van der Rohe. The General Bronze Corporation fabricated 3,200,000 pounds, or 1,600 tons, of bronze for the 38-story, 516-foot tower at its plant in Garden City, New York. At 36 million dollars it was the most expensive building of its time, and even the screws holding its fixed windows were made of brass.