On two letters from Caspar Neumann to John Woodward revealing the secret method for preparation of Prussian blue.

Prussian Blue (ferric hexacyanoferrate(II)) was discovered accidentally by Johann Jacob Diesbach (1) and Johann Konrad Dippel (2) in Berlin in 1706 (3). As the first synthetic pigment, Prussian Blue received its name from the country in which its discovery occurred. It revolutionized the use of the color blue in artwork because it was less expensive and/or more stable than other blue pigments of the time.