Egyptian blue

There is no blue earth pigment anywhere on the planet. Red, yellow, brown and black are all lying in the ground in usable quantities; blue is not, and neither is a good green. So the oldest blue in the world is not a discovery, it is an invention, a piece of process chemistry worked out in Egypt in the early dynastic period and used continuously for something like four thousand years.

It spread across the Mediterranean and the Romans called it caeruleum. Vitruvius describes making it in the first century BC. Then the recipe was lost, somewhere around the fall of the Western Empire, and Europe spent the next thousand years without an affordable blue, which is a large part of why medieval blue is either ultramarine at gemstone prices or azurite that goes green.

The strangest part came later. Egyptian blue turns out to emit strong near-infrared light when you shine red light at it, brightly enough that a camera with the right filter can find microscopic traces of it on objects that look completely colourless to the eye. Conservators now use that to recover paintwork on Greek and Roman sculpture that everybody assumed had been white all along. The white classical statue is a mistake, and this pigment is one of the instruments proving it.

Tomb painting, coffin decoration, faience, sculpture, papyri, and the blue on almost any Egyptian object you have seen in a museum. It also painted Greek and Roman marble, which matters more than it sounds: it means the classical world was a coloured world, and the taste for bare white stone is an eighteenth-century idea projected backwards.

For anyone working now, the interesting part is the manufacturing story rather than the pigment. A brand that needs a colour nobody else has does not go looking for a rarer rock; it commissions the colour. That is the same move Yves Klein made in 1957 and the same move a paint company makes when it patents a shade. Egypt got there first by four and a half thousand years.