Prussian blue was the first major synthetic pigment that created an affordable alternative to the expensive mineral-based pigment, ultramarine.
Ever wondered why religious paintings of the Middle Ages featured the color blue so strongly? Ecclesiastical patrons wanted only the best and most expensive art supplies to depict religious figures, and back then the best pigment for blue paint cost as much as gold leaf. Ultramarine, the strongest and most intense blue pigment, was made by crushing the semi-precious stone lapis lazuli. Lapis was found in Afghanistan, and the laborious overland journey added considerable cost to the acquisition of the pigment.
Another commonly used blue pigment was azurite, also known as “citramarine” meaning, “from this side of the seas,” in contrast with ultramarine (oltramarino), which indicates its origin “from beyond the seas.” Conventionally, ultramarine was used to paint the heights of the heavens, and painters used the more greenish azurite for the depths of the oceans.
Finding a less expensive means of producing blue pigment was a serendipitous accident. A Berlin colormaker named Diesbach first synthesized Prussian blue, or iron ferrocyanide, in the early 1700s in the process of making another pigment, cochineal red lake. Impurities in the starting materials for a particular batch of the cochineal produced a pale pigment that became intensely blue when it was concentrated. Prussian blue represented the first major breakthrough in making pigments from a chemical process rather than from a mineral, a plant, or an animal, and so began the mass production of an intense blue colorant for making paints. Prussian blue also found use as a printing ink and for dying silks and it remains an inexpensive blue pigment.
More information about Prussian blue may be found at http://www.webexhibits.org/pigments/indiv/overview/prussblue.html.