Fading of modern Prussian blue pigments in linseed oil medium

Abstract

The fading of modern laboratory-synthesized and commercial Prussian blue, iron(III) hexacyanoferrate(II), based pigments in a linseed oil medium during exposure to light has been investigated. The Prussian blue pigments were painted with linseed oil, as a pure pigment and mixed with white lead, (PbCO₃)₂Pb(OH)₂, zinc white, ZnO, or titanium white, TiO₂, pigment. The samples were subjected to accelerated ageing for 800 h and the light fastness of the Prussian blue pigment was evaluated by reference to blue wool standards. Pure Prussian blue is extremely light fast whilst it strongly fades when mixed with a white pigment, especially with lead white or zinc oxide. The painted samples were studied by UV-visible, iron K-edge X-ray absorption, iron-57 transmission Mössbauer, and attenuated total reflectance infrared spectroscopy. X-ray absorption results reveal a decrease in the iron coordination number in aged samples in the presence of white pigment. The Mössbauer spectra of the pure Prussian blue and the unaged and aged mixtures of Prussian blue and lead white or zinc oxide at 1 : 100 and 1 : 10 dilution ratios, respectively, indicate the presence of iron(II) and iron(III) in a ratio close to one as expected for the bulk stoichiometric KFe^III[Fe^II(CN)₆]; no change in the spectral parameters was observed upon ageing. Combined with the X-ray near edge absorption and infrared studies, these results suggest reduction of the surface iron ions in the Prussian blue with ageing upon exposure to light.