Copper

Copper was the first metal people smelted from ore, and it named an age: the Bronze Age is the age of copper alloyed with tin. It remains the second most important technical metal after iron, and for the same reason as then — availability combined with useful properties.

The chief of those properties is electrical conductivity: only silver beats it, and silver costs incomparably more.

Native copper exists, but most is mined from sulfide ores — chalcopyrite CuFeS₂, chalcocite Cu₂S, bornite. There are oxidised ores too: malachite, azurite, cuprite.

Malachite is worth singling out. It is a basic copper carbonate, and its characteristic green is the same green as the patina on bronze monuments and copper roofs. Patina forms under the action of moisture, carbon dioxide and traces of acidic gases, and unlike rust it is dense and protects the metal beneath it.

Copper has been known for at least nine thousand years. Smelting from ore was mastered around the fifth millennium BC: copper ores are often brightly coloured and easy to recognise, and the temperature needed to reduce copper with charcoal is reachable in an ordinary fire with a draught.

The Latin cuprum derives from the name of Cyprus, the largest supplier of copper in the ancient Mediterranean.

Copper stands to the right of hydrogen in the reactivity series, so it does not displace hydrogen from acids: hydrochloric and dilute sulfuric acid have practically no effect on it. It is dissolved by oxidising acids — nitric and concentrated sulfuric — and the product depends on the conditions.

The main oxidation states are +1 and +2. Copper(II) compounds are usually coloured: the sulfate is blue in solution, the oxide black, the hydroxide pale blue. A characteristic reaction is with ammonia: the hydroxide precipitate dissolves to give a deep blue complex, and copper is easy to identify by that colour.

Copper and its alloys have a marked antimicrobial action, which is why copper door handles and handrails are returning to hospitals.

More than half of all copper goes into electrical engineering: wires, windings, busbars, printed circuit boards. Then plumbing and roofing, heat exchangers, and alloys — brass with zinc, bronze with tin, cupronickel with nickel.

Copper compounds are used in agriculture: Bordeaux mixture, copper sulfate with lime, remains one of the oldest fungicides and has been in use since the late nineteenth century.

Copper is part of the enzymes of the respiratory chain and of enzymes that handle oxygen. In some animals — cephalopods, crustaceans, spiders — it plays the role iron plays in vertebrates: their oxygen is carried by haemocyanin, which is why their blood is blue.

For humans copper is an essential trace element in very small amounts; in larger ones its compounds are toxic.