Prussian blue

#1B3A6BFormula Fe₄[Fe(CN)₆]₃Earliest recorded use 1706

9 eras18 artists27 colors

The first modern synthetic pigment, and an accident. Prussian blue was made by mistake in a Berlin laboratory in 1706, and it changed what a blue could cost, which changed who could paint the sky.

Origin

The colourmaker Johann Jacob Diesbach was preparing a red lake from cochineal, and needed potash to precipitate it. He borrowed some from the alchemist Johann Konrad Dippel, not knowing it had been contaminated with animal oil distilled from blood and bone. Instead of red, the batch turned an intense blue. Nobody understood the chemistry — the compound is an iron-cyanide complex, and cyanide itself would not be isolated for another eighty years — but the recipe was reproducible, and by 1710 the pigment was being sold across Europe. It was the first blue in history that did not have to be dug out of a mountain. Its name comes from its adoption as the dye of Prussian military uniforms, and it is the direct ancestor of the blueprint: the cyanotype process precipitates the same compound on paper, which is why engineering drawings were blue for a century.

In art history

Prussian blue was perhaps a tenth the price of ultramarine and vastly stronger in tinting power, and its arrival is visible as a change in the look of European painting. Watteau used it almost immediately. Gainsborough built landscape distance with it. Its most famous journey, though, was eastward: imported Prussian blue reached Japan in the 1820s through Dutch traders at Nagasaki, and it made possible the deep gradated skies and the great wave of Hokusai's Thirty-Six Views of Mount Fuji — a European industrial accident became the defining colour of Japanese woodblock printing, which then travelled back to Europe and reshaped Impressionism. Later, Picasso's Blue Period leans heavily on it. It is also the blue in the shadows of a great deal of nineteenth-century landscape, where a touch of it cools an earth colour without turning it grey.

In the studio

The pigment has enormous tinting strength — a very small amount overwhelms a mixture, and beginners characteristically use ten times too much. It is transparent and dark in mass tone, almost black, and only reveals its blue when spread thin or mixed with white. Mixed with a yellow it gives the sharp cool greens that were impossible before, and mixed with chrome yellow it was sold ready-made as chrome green. Its weaknesses are two: it is destroyed by alkalis, so it cannot be used in fresco or mixed with lime, and it shows a curious reversible fading, losing colour in strong light and partly recovering in the dark, which conservators have documented in works that appear to change between exhibition and storage.

Afterlife

Prussian blue remains in production and in use, though phthalocyanine blue, introduced in 1935, has taken over most of its role: phthalo is even stronger, more transparent and completely lightfast. The old pigment found an unexpected second career in medicine — it binds thallium and radioactive caesium in the gut, and is on the World Health Organization's list of essential medicines as an antidote for heavy-metal and radiological poisoning. The pigment that gave Hokusai his wave is stockpiled for nuclear accidents.