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Bronze

Copper and tin, harder together

Date
c. 3,300 BCE
Era
Prehistory
Rebuild stage
III Crafts & cities
Difficulty

Plate

Legend
  1. 1Crucible of molten bronze
  2. 2Pot bellows
  3. 3Clay nozzle (tuyère)
  4. 4Outer clay mould
  5. 5Clay core
  6. 6Pouring cup and channel

Anatomy

Bronze, glowing while hot: in the crucible, the stream and the mouldClay mould and core, cut along the joint; the crucibleSand and ground, cut; the green withyThe mould is two clay halves; the front one is taken away to show the joint face of the other. The clay core fills the socket of the axe. It hangs from a bar bedded in notches cut in the rims of both halves, which are bound together, so it can neither sink nor float up in the heavy metal. Metal poured from the crucible runs down the channel beside the core, reaches the cutting edge first and fills the cavity from the bottom up, while the air it pushes out leaves through the vent scratched in the joint and through the cup. The thick metal in the cup and channel freezes last and feeds the axe as it shrinks; it is broken off afterwards. Pure copper melts at 1,085 °C. Tin lowers the temperature at which the whole charge is liquid, to about 1,005 °C at 10 % tin and 875 °C at 20 %, from data on cast tin bronzes and the copper–tin phase diagram. The figure pours every alloy at 1,100 °C, the heat used in experimental casts of 10 % bronze into copies of a Bronze Age axe mould. The more degrees the metal has above its liquidus, the longer it runs before it freezes and the thinner the section it can fill. The figure works this out from one simplified rule of its own: a 1 mm edge needs about 75 degrees. So 10 % bronze, some 95 degrees above, fills the edge, while copper, only 15 degrees above its melting point, stops about 16 mm short. As cast, the metal gets harder with tin, from about 50 on the Brinell scale for copper to 95 at 10 % and 170 at 20 %, and its colour goes from copper red to gold, then paler toward silver, as measured on test castings. Percentages are by mass. Time is shortened: a real axe takes minutes to cool.

Bronze is copper alloyed with a little tin, usually about a tenth. It is harder than copper, melts at a lower temperature and fills a mould well, and from the fourth millennium BCE it gave its name to an age of tools, weapons, vessels and statues.

What it is

Bronze is an alloy, a metal made of two or more elements, here mostly copper with a smaller share of tin; the classic recipe is about nine parts copper to one of tin by weight. Surviving ancient objects range from about two thirds to 95% copper, and other metals such as arsenic, lead or zinc were sometimes present.

Tin makes copper harder, so that a blade keeps its edge better. Bronze melts over a range around 950 °C, depending on the tin content, while pure copper melts at 1,085 °C; the lower temperature is easier to reach with a charcoal fire and bellows. Molten bronze also runs freely into a mould.

How it works

Pure copper is made of crystals whose layers of atoms slip easily over one another, which is why it bends and dents. Tin atoms are larger. Scattered through the copper, they jam those layers, so the metal resists bending and denting. Hammering the cold metal hardens it further, so a bronze blade could be cast, then beaten along the edge to harden it.

A large flat copper ingot with four projecting corners beside a round grey tin ingot and a piece cut from another Raw materials of the Bronze Age trade: a four-handled copper “oxhide” ingot and pieces of tin.

The tin content decides how the metal can be used. Bronze with about 10% tin was the standard for casting tools and weapons; softer bronze with about 6% was hammered into sheet. Bell metal has more than 20% tin: it is hard and rings well but breaks if dropped. Too much tin makes any bronze brittle.

Copper and tin ores rarely occur in the same place. Copper was mined in many regions, Cyprus among the richest; tin came from far fewer places, such as Cornwall, the Taurus Mountains and Afghanistan or its neighbours. Bronze therefore depended on long trade routes, and a disruption in the tin supply could stop production far away.

An open two-piece stone mould with an axe-shaped hollow in each half, and the cast bronze axe beside it A two-piece stone mould: closed, the two halves form the axe, and the metal is poured in at the narrow end.

Simple shapes such as flat axes were cast in open moulds or in two-piece moulds of stone or clay. Hollow or detailed objects could be made by the lost-wax method. The founder modelled the object in wax around a clay core, added wax rods that would become the pouring channel, and covered everything in fine clay. Baking the mould melted the wax out. Molten bronze poured into the hot mould filled the space the wax had left, and the mould was broken away once the metal had cooled. Modern art foundries still work this way: they pour bronze at well over 1,000 °C into a mould heated beforehand to several hundred degrees, so that it does not crack.

How to build it

Casting bronze means working with metal at more than 1,000 °C. Learn it in a foundry course or a school workshop with an experienced caster before trying it anywhere else. The steps below describe a small lost-wax casting.

Make the model. Shape a small object in casting wax, solid and no thicker than a finger at any point. Attach a wax rod and a wax cup at the thickest part: they will form the pouring channel.

Invest it. Fix the wax cup on a rubber sprue base, set a steel flask over it and fill the flask with an investment sold for bronze casting, a mix of plaster and silica; when the base is removed, the cup forms the pouring opening. Follow the maker’s instructions for mixing, setting and drying exactly: each system has its own.

Burn out the wax. Heat the mould upside down in a kiln on the burnout schedule given for the investment, so that the wax runs out and the mould is fully dry. Pour while it is still hot: a cold or damp mould can crack or blow out metal.

Melt the metal. Use ready-made casting bronze (copper with about 10% tin) or weigh out clean copper and tin in a 9 to 1 ratio. Melt it in a clay-graphite crucible in a furnace with forced air, add the tin last if you alloy it yourself, and skim the dross.

Pour and finish. Lift the crucible with proper tongs and pour in one steady stream until the cup is full. Leave it to cool, break off the mould, cut away the channel and file, chase and polish the surface.

Safety. Molten metal causes severe burns and fires. Wear protective clothing made for molten-metal splash, a face shield over safety glasses, foundry gloves and boots, with trousers over the boots so that metal cannot run inside, and keep onlookers away. Every tool, mould and ingot must be bone dry: a drop of water under molten metal turns to steam and throws metal out. Pour over dry sand, never over concrete, and work outdoors or under strong extraction. Never melt scrap of unknown composition: brass gives off zinc fumes, and some old alloys contain lead or other toxic metals. Investment and broken moulds release silica dust: mix and break them out wet or under dust extraction, and wear a certified, properly fitted respirator for fine silica dust.

History and evolution

Copper alloyed with arsenic was made on the Iranian plateau, at sites such as Tal-i Iblis, around 3800 to 3400 BCE. Arsenical copper is hard, but its fumes are poisonous. The oldest tin bronze known so far is a foil from Pločnik in Serbia, which its excavators date to about 4650 BCE, although the context of the find has been challenged; it was probably smelted from an ore that already contained both metals. In the Near East the Bronze Age is dated from the middle of the fourth millennium BCE, at first mostly with arsenical copper; tin bronze became the usual alloy there during the third millennium. In China the Bronze Age began in the early second millennium BCE.

A Chinese bronze ritual cauldron with three legs and two upright handles, its body decorated with a band of stylised animal masks A Shang ritual cauldron of the ding type. Chinese founders cast such vessels in moulds assembled from many clay pieces.

The Uluburun wreck, which sank off southern Turkey around 1300 BCE, shows the scale of the trade. Its hold carried about 10 tonnes of Cypriot copper, including some 350 oxhide ingots, and about a tonne of tin, roughly the ten-to-one proportion of bronze. In China, the Shang and Zhou courts had ritual vessels cast for offerings and burials, using a piece-mould method with no parallel elsewhere.

Around 1200 BCE, wars and migrations around the eastern Mediterranean broke many trade routes; one explanation is that tin became scarce, although the evidence varies from region to region. Iron, whose ore is found almost everywhere, slowly replaced bronze for tools and weapons as smiths learned to work it. Bronze stayed in use for bells, statues, mirrors, coins and ship fittings, and today for bearings, springs and marine parts that must resist seawater.

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