The compound NH4Fe[Fe(CN)6]·xH2O—a commercially available “Prussian blue” pigment—crystallizes in the Fm3m space group, a=10.232(1) Å, based on X-ray powder diffraction (XRPD) data. XRPD investigations of other commercially available “Prussian blue” pigments and oil paints were undertaken. Results for the pigments showed that the XRPD techniques were able to differentiate several different Prussian blue phases that differed only slightly in chemical compositions. Results for the oil paints allowed for the determination of the major crystalline phases used as fillers. However, on the basis of XRPD investigations of oil paints prepared in our laboratory containing a mixture of true Prussian blue Fe4[Fe(CN)6]3·14H2O and BaSO4 (a common filler), the pigment was detectable only in concentrations higher than 2%. This result suggests that XRPD may not be a preferred technique for the identification of Prussian blue in paintings and other works of art because the concentration of this pigment in such materials is commonly less than 2%.
