Study of the alteration processes of Prussian blue in laboratory-prepared and genuine paint layers

The necessity of understanding degradation and alteration processes in a painting's materials is well established for preservation and art history issues. The task is complex because of the highly heterogeneous character of an ancient paint layer. In this context we focus on a particular pigment, Prussian blue. Prussian blue is a hydrated ferric ferrocyanide complex, first synthesized in 1704 in Berlin. It has been widely used by artists until the 1970s. However, the permanence of Prussian blue had already been questioned by the mid-eighteenth century, because it exhibits a tendency to fade in light and to turn green. To date, little attention has been devoted to the understanding of the degradation processes of Prussian blue in paint layers.
We induced discoloration upon light exposure in commercial and laboratory-synthesized Prussian blue watercolor and oil paint layers by accelerated ageing. Pure Prussian blue painted in a dark shade appears to be extremely light fast but fades when either painted in a lighter shade or mixed with white pigments.
We analyzed the paint layers by various techniques, i.e., UV-visible, Fourier transform infrared, Raman, Mössbauer and X-ray absorption spectroscopy. We attributed the fading of Prussian blue to a reduction of the iron(III) ions at the surface of the paint layers. We also observed a partial oxidation of Prussian in the entire paint layer. Finally we confirmed these results by analyzing works of art containing Prussian blue, i.e., a polychrome sculpture, wallpapers and mural decoration sample.