Which metal was more valuable than gold until an 1886 electrolytic process made it cheap?

Aluminium - the third most abundant element in the crust and, until the 1880s, one of the most expensive substances on Earth, because it clings to oxygen too tightly to be smelted the way iron is. Napoleon III is said to have reserved aluminium cutlery for his most honoured guests and given the rest gold.

The Washington Monument was capped in 1884 with a cast aluminium pyramid weighing under three kilograms, displayed in a jeweller's window before installation because it was a precious metal. Two years later Charles Hall in Ohio and Paul Héroult in France, both 22, independently found the same electrolytic process, and the price collapsed by orders of magnitude within a decade.

It still takes a great deal of electricity - smelters are sited next to hydroelectric dams for that reason, and aluminium is sometimes described as solid electricity. Recycling it needs about 5 per cent of the energy of making it new, which is why the recycling economics work so much better than for most materials, and why a discarded can is worth genuinely chasing.