Redox reactions in Prussian blue containing paint layers as a result of light exposure

Abstract

Prussian blue, a mixed valence pigment, typically KFe^III[Fe^II(CN)₆]·xH₂O, was the most widely used blue artistic pigment from ca. 1720 to the 1970's but, unfortunately, its paint layers, especially when used in conjunction with a white pigment, tend to fade or turn green upon extended exposure to light. In order to identify the mechanism underlying these changes, paint layers have been prepared with differing amounts of these white pigments and subjected to accelerated light exposure fading. The resulting unfaded and faded paint layers as well as both the Berlin white pigment, Fe₂^II[Fe^II(CN)₆], and the partially oxidized Berlin green pigment, {KFe^III[Fe^II(CN)₆]}_x{Fe^III[Fe^III(CN)₆]}_1−x, have been characterized by Raman and iron-57 Mössbauer spectroscopy. The results indicate that, upon fading, the Prussian blue pigment painted with a linseed oil binder and (PbCO₃)₂Pb(OH)₂ or ZnO, and to a lesser extent with TiO₂, undergoes a reduction at the exposed paint surface and an oxidation in the bulk of the paint layer. This combined reduction and oxidation disrupts, at least in part, the Fe^III–N–C–Fe^II intervalent electron transfer pathways in Prussian blue thus leading to pigment fading through a reduction in the intervalent electron transfer absorbance at about 700 nm.