Copper

Copper is Venus in the alchemical scheme, marked with a circle set above a cross. The same sign is used in biology and astronomy for the female and for the planet, and its survival in that role is one of the more visible remnants of alchemical notation in ordinary modern use.

The reasons given for the association are worth separating from the association itself. The link is attested from late antiquity and stable by the medieval period, in the same body of texts that fix the other six. The explanations offered are later, or reconstructed: that polished copper and bronze made the mirrors of the ancient world, and the mirror is Venus's attribute; that Cyprus, the island the metal is named for, held a major cult of Aphrodite; that copper's characteristic corrosion products are the green of vegetation and growth. All three are plausible, none is documented as the original reason, and the honest position is that the correspondence is well attested while its origin is not.

The Cyprus connection at least is solid on the linguistic side. The Latin cuprum is a contraction of aes cyprium, the metal of Cyprus, which supplied a great deal of the Roman world's copper.

Copper's compounds carry their colour strongly. Verdigris, a copper acetate, and the mineral malachite, a copper carbonate, gave painters and dyers a range of greens; azurite gives blue. Egyptian blue, the first synthetic pigment, made from about the third millennium BCE, is a calcium copper silicate. The green patina on old bronze and on copper roofs is a mixture of carbonates and sulfates, and it is protective, which is why it is left in place.

Copper occurs native, as metal, in nuggets and masses, most famously around Lake Superior. That is why it was first. It required no smelting, no furnace and no theory: a nugget could be hammered into shape cold, and copper beads and awls from the Near East date to the ninth and eighth millennia BCE.

Smelting, reducing copper ore with charcoal in a sufficiently hot fire, is a genuine technological threshold and appears in the fifth millennium BCE. It means the supply is no longer limited to what nature has already refined.

Pure copper is soft. Alloyed with about ten per cent tin it becomes bronze, which is far harder, casts more cleanly and holds an edge. That combination defines an age. Its difficulty is that copper ore and tin ore rarely occur in the same places, so bronze required long-distance trade in a way that nothing before it had: Cornish and Iberian tin moved across Europe to Mediterranean copper, and the eastern Mediterranean drew tin from as far away as central Asia. The collapse of those networks around 1200 BCE is one of the standard explanations offered for the end of the Late Bronze Age.

Brass, copper with zinc in place of tin, came later, made in quantity in the Roman world from about the first century BCE by heating copper together with zinc ore and charcoal, without the zinc ever being isolated as a metal.

Silver conducts electricity better than copper, and nothing else does. Silver is far too expensive to wire a city with, so the entire electrical world is built from the second-best conductor available, and has been from the beginning.

That is not a small statement. Every generator winding, every transformer, every motor, every mains cable, every circuit board trace and, until fibre displaced it, every telephone line is copper. An ordinary car contains a kilometre or so of it; an electric car contains several times that, and projected copper demand is one of the harder physical constraints on electrification.

Copper also conducts heat well, resists corrosion in water, and is easy to draw, roll and join, which is why plumbing and roofing use it. It is among the most recycled of all metals, because scrap copper is easy to identify, easy to remelt, and loses nothing in the process.

Biologically it is essential in small amounts, sitting in the enzymes that handle oxygen. In molluscs and arthropods, copper rather than iron carries oxygen in the blood, in a protein called haemocyanin, which is why lobster and horseshoe crab blood is blue.

It is also antimicrobial. Copper and its alloys kill bacteria on contact, an effect noted empirically in Egyptian and Greek medical texts for wound treatment and since confirmed in detail in the modern literature. Brass door handles and copper touch surfaces in hospitals are a deliberate application of a property that people relied on for millennia without knowing the mechanism.