Dyes had come from unexpected places. In the early 1700s, a paint manufacturer in Berlin, Johann Jacob Diesbach, was sharing the laboratory of Johann Konrad Dippel, an alchemist working to distil bone oil from animal blood mixed with potash, a salt. Diesbach tried to produce a red pigment by boiling cochineal, but he needed potash, and so he borrowed some from Dippel. Unbeknown to him, the substance was contaminated and meant for the trash, and so, instead of turning red, the mixture paled, and then, when Diesbach concentrated it, turned purple, and finally blue. At the time, there was no affordable or stable blue for dyers to use because, in those days, ultramarine, derived from lapis lazuli, came only from the mountains of Afghanistan and cost more than gold. ‘Prussian blue’, as the new pigment was called, was a 10th of the price of ultramarine, and it soon made its way to Paris, then around the world. Artists used Prussian blue for painting; it is the pigment in Vincent van Gogh’s The Starry Night (1889), and the Japanese imported it for woodblocks, most famously used in Hokusai’s The Great Wave off Kanagawa (1830-33). Prussian blue is related to prussic acid found in Zyklon B, the poisonous chemical that Nazis used in the gas chambers of their concentration camps.

It just so happened that purple was the most fashionable colour at that time, and previously had to be extracted from a kind of shellfish that lived only in the Mediterranean. Coal tar, on the other hand, was plentiful and, given the increasing number of gasworks in Britain and Europe, it was relied upon by a growing population for lighting, heating and cooking. The dye was not expensive, and the colour did not fade after washing, as others did. Perkin dubbed it ‘mauve’. When Empress Eugénie wore a silk mauve dress that summer, the fashionable ladies in Paris all imitated her. Magentas, yellows, blues and pinks were soon created. The German biologist Paul Ehrlich systematically studied these aniline dyes and forged relationships with representatives from the dyestuffs industry, expanding the histological palette. We tend to think of science as highly organised and planned but, in reality, it is often the story of earnest mistakes and those perspicacious enough to capitalise on them.