An inorganic blue pigment that is a complex ferric ferricyanide that is produced by reacting a ferrous salt with potassium ferricyanide, produced as an intermediate in the production of Prussian blue but not the same compound. Turnbull's blue is not used as a paint pigment, but in making blueprints. Blueprint paper is sensitized with ferric ammonium citrate and potassium ferricyanide. When the paper is exposed to light, the ferric ions are reduced to ferrous ions.
