Prussian Blue (aka Iron stain, Perl's iron stain)

As a fabric dye, Prussian Blue was used in the manufacture of uniforms for the Prussian Army. The French (sensitive to national honor and the fact that the Prussians whipped them soundly in the war of 1870-71) are apparently quite fastidious about calling it "Paris Blue" instead, but it's the same pigment.

Although there may be technical variations from place to place, the basic technique is to use potassium ferricyanide at 60 degrees C(?) for 15 minutes, wash with H2O, stain with Safranin O for 2-5 minutes, rinse with 1% acetic acid, then dehydrate in increasing concentrations of alcohol then xylene.

It is used primarily to identify iron stores in bone marrow and to differentiate hemosiderin from other pigments. Positive staining is a rich blue color.