How did a color experiment in Berlin create a blue painters had never known?

Less, and more slowly, than the legend says. Its crystal structure traps iron in two oxidation states, and electrons moving between them absorb light so the eye sees an intense blue. The pigment arose in Berlin early in the eighteenth century, and a study following it through Paris shows the route to the palette ran through chemists, manufacturers and color merchants before any studio.

Before the eighteenth century a deep and durable blue was hard and expensive to obtain, so it was used sparingly and for important details alone.

In Berlin a new pigment came out of a reaction between iron salts and ferrocyanide compounds, raw materials that were available and cheap and that depended on no distant mine or trade route.

Its color arises by an unusual mechanism. The crystal structure traps iron in two oxidation states, and electrons moving between them absorb light in a particular range, so what reaches the eye is an intense blue. That is also the source of its stability.

The familiar story is that the new pigment transformed the palette at once. A study following its route through Paris describes a longer chain: laboratory, factory, color shop and studio. The tale of the discovery as one famous accident also rests on later testimony, and the details of the original experiment are not entirely certain.