The rarest colour

Blue seems to be everywhere. It fills the daytime sky and colours the sea to the horizon, and for most people it is among the first colours they can name. Yet as a material, something that can be ground up, mixed and painted onto a surface, blue is strikingly rare. For most of human history it was the hardest colour to obtain, and the story of how people learned to make it runs alongside the story of trade, chemistry and art. To ask how a civilisation got its blue is to ask what it could mine, whom it could trade with, and how far its chemistry had come.

Part of the difficulty is that nature offers very little true blue to work with. The brilliant blue of a butterfly's wing or a jay's feather is usually not pigment at all but a trick of structure: microscopic ridges scatter light so that only blue reaches the eye, and if you crush the wing the colour vanishes. The same is true of most blue in the animal world, from the kingfisher to the morpho butterfly, whose colour is built from colourless structures. Even most blue flowers reach their colour by modifying red pigments rather than by producing a blue one outright. A painter who wanted blue could not simply gather it from the living world, as yellows and browns could be gathered from earth and ochre. Blue had to be won by other means.

The first people to solve the problem did so by manufacture. Around 2500 BCE, Egyptian workshops learned to make a blue pigment by heating sand, a copper compound and natron together at a high temperature. This substance, now called Egyptian blue, is generally regarded as the first synthetic pigment, a deliberate piece of chemistry produced long before anyone understood why it worked. The recipe spread around the ancient Mediterranean, but it was demanding to make, and after the fall of Rome the knowledge was lost for centuries, leaving later scholars to puzzle over a colour the ancients had made and they could not.

The blue that replaced it was natural but scarcely less troublesome. Ultramarine was made by grinding lapis lazuli, a stone mined in any quantity in only one remote region, in what is now Afghanistan. The raw stone had to be carried thousands of miles and then laboriously purified, for simple grinding produced only a dull grey. The best ultramarine was drawn out by kneading the ground powder in wax and washing it again and again, a slow process that threw away most of the material and kept just the purest blue. The finished pigment was, weight for weight, more expensive than gold. Medieval and Renaissance painters therefore used it sparingly and pointedly, reserving it for the most important passages of a picture; the robe of the Virgin Mary was so often painted in ultramarine that the colour itself came to carry meaning, a signal of devotion and expense at once.

A cheaper blue arrived by accident. In early eighteenth-century Berlin, a colour maker attempting to produce a red instead created a deep blue, the result of a contaminated batch of materials. Prussian blue, as it became known, was the first modern synthetic pigment, far less costly than ultramarine and soon available across Europe and beyond. It transformed both painting and printing, and the great wave in Hokusai's famous print owes its depth to this new and affordable blue, a colour no printmaker could have reached for a century earlier.

Ultramarine itself was finally tamed in the nineteenth century. In the 1820s a French society offered a prize to anyone who could reproduce the colour of lapis lazuli by artificial means, and within a few years chemists had done it, working out the make-up of the stone and manufacturing an identical pigment from cheap ingredients. Synthetic ultramarine cost a tiny fraction of the mineral it copied. Almost overnight, a colour that had signified wealth and importance for centuries became ordinary, available to any painter or printer who wanted it. The precious had become merely useful.

Even so, blue has never quite surrendered its difficulty. Genuinely new blue pigments remain rare events: as recently as 2009, chemists in the United States stumbled on a new and unusually durable blue while investigating quite different materials, and the discovery made news precisely because such finds are so uncommon. The colour that appears to fill the sky and the sea, and that children name so early, remains, as a material to be made and held in the hand, one of the hardest that human ingenuity has ever had to win.