Any artist who enjoys using Prussian blue will find it hard to imagine that such a beautiful blue was actually the result of an experiment gone wrong. The discoverer of Prussian blue, the colormaker Diesbach, was in fact not trying to make a blue, but a red. The creation of Prussian blue, the first modern, synthetic color was completely accidental.
Diesbach, working in Berlin, was attempting to create cochineal red lake in his laboratory. ("Lake" was once a label for any dye-based pigment; "cochineal" was originally obtained by crushing the bodies of cochineal insects.) The ingredients he needed were iron sulfate and potash. In a move that'll bring a smile to any artist's who's ever tried to save money by buying cheap materials, he obtained some contaminated potash from the alchemist in whose laboratory he was working, Johann Konrad Dippel. The potash had been contaminated with animal oil and was due to be thrown out.
When Diesbach mixed the contaminated potash with the iron sulfate, instead of the strong red he was expecting, he got one that was very pale. He then attempted to concentrate it, but instead of a darker red he was expecting, he first got a purple, then a deep blue. He'd accidentally created the first synthetic blue pigment, Prussian blue.
It's hard to imagine now, given the range of stable, lightfast colors we can buy, that in the early eighteenth century artists didn't have an affordable or stable blue to use. Ultramarine, which is extracted from the stone lapis lazuli, was more expensive than vermilion and even gold. (In the Middle Ages, there was only one known source of lapis lazuli, which means simply 'blue stone.' This was Badakshan, in what is now Afghanistan. Other deposits have subsequently been found in Chile and Siberia). Indigo had a tendency to turn black, was not lightfast, and had a greenish tinge. Azurite turned green when mixed with water so couldn't be used for frescoes. Smalt was difficult to work with and had a tendency to fade. And not enough was yet known about the chemical properties of copper to consistently create a blue instead of a green (it's now known that the result depends on the temperature it was made at).
Neither Diesbach nor Dippel was able to explain what had happened, but these days we know that the alkali (the potash) reacted with the animal oil (prepared from blood), to create potassium ferrocyanide. Mixing this with the iron sulfate, created the chemical compound iron ferrocyanide, or Prussian blue.
Diesbach made his accidental discovery sometime between 1704 and 1705. In 1710 it was described as being "equal to or excelling ultramarine". Being about a tenth of the price of ultramarine, it's no wonder that by 1750 it was being widely used across Europe. By 1878 Winsor and Newton were selling Prussian blue and other paints based on it such as Antwerp blue (Prussian blue mixed with white). Famous artists who have used it include Gainsborough, Constable, Monet, Van Gogh, and Picasso (in his 'Blue Period').
Prussian blue is a translucent (semi-transparent) color but has a high tinting strength (a little has a marked effect when mixed with another color). Originally Prussian blue had a tendency to fade or turn grayish green, particularly when mixed with white, but with modern manufacturing techniques, this is no longer an issue.