Prussian blue

Prussian blue, category of deep-blue pigments containing ferrocyanide. It is used to color paint, enamel, lacquer, printing ink, and carbon paper. Some Prussian blue is prepared from sodium ferrocyanide that has been oxidized in the presence of sodium chlorate, sodium chromate, or other reagent. Others are made by oxidizing a mixture of ferrous sulfate and potassium ferrocyanide. Prussian blue was first made in 1704.