c. 1706

Prussian Blue

Georg Ernst Stahl (1660–1734), Caspar Neumann (1683–1737)

Here’s something you may not have noticed: European paintings from before 1700 rarely feature much blue in them, and the color almost always accentuates the most exalted person in the frame. This is because the only durable blue pigment available for oil painting was ultramarine (from Latin, “beyond the sea”), made of wildly expensive lapis lazuli stones from Afghanistan. Smalt (ground cobalt glass) was available for blue ceramics, but oils caused it to discolor. Thus, lapis was the only nonfading blue since “Egyptian blue,” whose recipe had been lost when the Roman Empire disintegrated.

But a chance discovery changed all that. Sources disagree on some details, but it appears that a German dye-maker named Johann Jacob Diesbach was trying to make a red pigment from cochineal (obtained from crushed beetles) around the year 1706 when, to his surprise, he obtained a blue substance instead. His reagents were contaminated, as it turned out, and within two years the synthetic blue paint was on the market as Prussian blue, Berliner blue, and similar names. By 1724 the German-Polish chemist Caspar Neumann had leaked the recipe to the Royal Society of London, which published it. Apparently, the mixture of cochineal, alum, iron sulfate, and animal oil–contaminated potash (potassium carbonate) was what yielded the brilliant blue color.

No one was going to reverse-engineer Prussian blue from its chemistry, though. The idealized version has three iron atoms in the +2 oxidation state, each with six cyanides around it, and around these complexes are four irons in the +3 state. Older preparations had all sorts of impurities, complicating the structure further, so it was not until the 1970s that the full chemical details were worked out. Attempts to understand Prussian blue helped advance inorganic chemistry for over 250 years—long after its importance as a pigment had, well, faded. Further, the substance lent its name to prussic acid (hydrogen cyanide), and its complexing properties (the cyanides are arranged in space and point inward) make it a valuable drug in cases of acute metal poisoning, since it exchanges some of its iron atoms for thallium or other toxic metal ions, which can then be passed safely from the body.

Japanese artist Hokusai’s famous woodblock print The Great Wave off Kanagawa (c. 1830). For some time, such prints were thought by European collectors to use some strange Japanese blue pigment, but it turned out to be Prussian blue imported from Europe!