Platinum

Platinum was worked in South America long before Europeans could do anything with it. Objects combining platinum and gold, made by sintering rather than melting, come from the coast of what is now Ecuador and are associated with the La Tolita culture, from roughly the last centuries BCE into the early centuries CE. Melting was out of the question: platinum does not soften below about 1768 degrees C, far beyond any furnace of the period anywhere in the world.

Spanish colonists in the gold workings of the Choco region of Colombia encountered it as an unwanted heavy white grit that could not be separated from the gold and could not be melted out. They called it platina, a diminutive of plata, silver, meaning something like inferior little silver, and there are accounts of it being thrown back into the rivers.

Antonio de Ulloa, on the Spanish and French geodesic expedition to South America, described it in his published account of 1748, which introduced the metal to European science. Charles Wood had independently brought samples to England from Jamaica in 1741. Working it required new techniques, and it was not routinely melted until the oxy-hydrogen torch and related methods of the nineteenth century.

The point for a book of correspondences is a negative one, and it should be stated plainly. Platinum arrived in the European scientific world in the middle of the eighteenth century, more than two centuries after the seven-metal scheme had already been broken by antimony, bismuth and zinc, and at the moment when the number of known metals was about to multiply many times over. No alchemical tradition assigned it anything. Any planetary or esoteric attribution of platinum is a modern construction, and dating it honestly means dating it to the twentieth century or later.

Platinum's industrial value rests on catalysis. A catalyst provides a surface on which reacting molecules are held in the right orientation and their bonds weakened, so that a reaction proceeds at a temperature at which it otherwise would not, while the catalyst itself is left unchanged. Platinum is extraordinarily good at this for reactions involving hydrogen and oxygen.

The largest single application is the vehicle catalytic converter, introduced in the United States from 1975, in which platinum together with palladium and rhodium converts carbon monoxide, unburned hydrocarbons and nitrogen oxides into carbon dioxide, water and nitrogen. Roughly half of platinum demand is automotive. The Ostwald process, which oxidises ammonia to nitric acid over a platinum-rhodium gauze, supplies the world's nitrates for fertiliser and explosives. Hydrogen fuel cells use platinum electrodes, and the cost and scarcity of the metal is one of the central obstacles to their wider adoption.

Platinum is chemically almost inert, unaffected by acids other than aqua regia, and its melting point is high, which made it the material of choice for laboratory crucibles and for physical standards. From 1889 until 2019 the kilogram was defined as the mass of a particular cylinder of platinum-iridium alloy kept at Sevres near Paris, and the metre was likewise defined by a platinum-iridium bar until 1960. The kilogram prototype was the last physical artefact to define an SI unit, and it was retired because it was observed to drift relative to its own official copies by a few tens of micrograms over a century.

Platinum's scarcity is real: about five parts per billion in the crust, and roughly three quarters of world supply comes from a single geological formation, the Bushveld Complex in South Africa.