Copper

The red metal

Copper is one of only two common metals that are not silver-grey; the other is gold. Its colour and its conductivity come from the same place: a full set of ten d electrons under a single loose s electron, [Ar] 3d¹⁰ 4s¹, free to move through the metal and carry charge and heat. Only silver conducts either better. It is soft and ductile — a kilogram can be drawn into kilometres of fine wire — and it hardens when worked and softens again when heated. In dry air it keeps its colour; in damp air it slowly grows a green skin of basic carbonate and sulphate, the verdigris that shields the metal beneath and turns old roofs and statues green. It dissolves in nitric acid but not in hydrochloric, and its salts, blue and green, are as coloured as the metal is red.

Ore, smelter and refinery

The crust holds about 60 parts per million of copper, mostly as sulphides — chalcopyrite, CuFeS₂, above all — with green malachite and blue azurite where the ore has weathered. Lumps of native metal turn up in a few places, most famously in Michigan. Modern mines work rock with well under 1 % copper: the ore is crushed, the sulphide floated off in a froth, smelted to a matte, blown with oxygen to blister copper and finally refined by electrolysis to 99.99 % cathode copper, the gold and silver from the ore collecting in the slime under the anodes. The world mines over 20 million tonnes a year, nearly a quarter of it in Chile, and recovers roughly a third of what it uses from scrap: copper can be recycled again and again without loss.

Wire, bronze and blood

More than half of all copper goes to electrical use — wiring, the windings of motors and generators, transformers, printed circuits — and electrification since the 1880s has been, in a material sense, a matter of copper. Much of the rest is plumbing, roofing and alloys: bronze with tin, brass with zinc, cupronickel in coins, and the small share that hardens sterling silver and reddens rose gold. Copper sulphate in Bordeaux mixture has kept mildew off vines since 1885, and copper surfaces kill bacteria on contact, one reason door handles were long made of brass.

The body needs about a milligram a day for the enzymes that handle oxygen, build connective tissue and pigment, and load iron into the blood; an adult holds about 70 mg. Snails, squid and lobsters go further: their oxygen carrier is haemocyanin, a copper protein, and their blood runs blue.

History and name

From Shanidar to Cyprus

Copper was the first metal worked, because it lay about as metal: a pendant hammered from native copper, found at Shanidar in Iraq, is about 10,700 years old. Smelting it from ore began in the Balkans by 5000 BC, the oldest metallurgy yet found, and Ötzi, the man frozen in the Alps around 3300 BC, carried a copper axe. Adding tin gave bronze, harder and easier to cast, and from about 3300 BC the Bronze Age spread from Mesopotamia across the Old World. The Romans took much of their copper from Cyprus and called it aes Cyprium, Cyprian metal, later shortened to cuprum — the source of both the symbol Cu and the English name. The modern age of copper began with Volta's pile of 1800, which alternated zinc with copper or silver, and with Faraday's spinning copper disc of 1831, the first generator.