Prussian blue (also known as Berlin blue, Brandenburg blue, Parisian and Paris blue) is a dark blue pigment produced by oxidation of ferrous ferrocyanide salts. Turnbull’s blue is essentially identical chemically, excepting that it has different impurities and particle sizes—because it is made from different reagents—and thus it has a slightly different color.
Prussian blue was created in the early 18th century and is the first modern synthetic pigment. It is prepared as a very fine colloidal dispersion, because the compound is not soluble in water. It contains variable amounts of other ions and its appearance depends sensitively on the size of the colloidal particles. The pigment is used in paints, it became prominent in 19th-century aizuri-e Japanese woodblock prints, and it is the traditional ‘blue’ in technical blueprints.
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The Daily Omnivore 