Perls did not publish a detailed procedure other than indicating a dilute potassium ferrocyanide solution was applied to the tissue followed by hydrochloric acid.[2] Ferric iron deposits in tissue (present mostly as ferric iron within the storage protein ferritin) then react with the soluble ferrocyanide in the stain to form the insoluble Prussian blue pigment (a complex hydrated ferric ferrocyanide substance).