Ageing of Prussian blue oil paints: the effect of the pigment studied using attenuated total reflection Fourier transform infrared spectroscopy

Degradation phenomena such as cracking and paint loss have been observed in dark blue paint strokes in Vincent van Gogh's paintings. Analysis of several samples taken from such paint strokes has confirmed that the paint contains Prussian blue pigment, linseed oil and a small amount of beeswax. Research into the chemical and mechanical processes taking place during the ageing of this paint is required to understand the cause behind the degradation phenomena. Therefore, attenuated total reflection Fourier transform infrared spectroscopy was used to study the chemical changes during the artificial ageing of oil paint reconstructions and correlate those changes with visual observations of the paints’ degradation. The influence of different pigments, as well as the pigment volume concentration and the presence of beeswax on the ageing of the reconstructions was investigated. The tested pigments include both soluble and insoluble Prussian blue prepared according to historical 19 th century recipes, modern Prussian blues, and a cobalt blue pigment. Ester hydrolysis was observed during artificial ageing for both cobalt blue and Prussian blue paints, but this effect was much larger for the latter. Additionally, NH 3 adsorption and its subsequent conversion to NH 4 + were observed in most Prussian blue reconstructions, which is expected to contribute to the increased ester hydrolysis rate in these paints and the formation of free fatty acid efflorescence. The type of Prussian blue pigment was found to influence the capability of NH 3 adsorption and the formation of fatty acid efflorescence, as this was not found in all reconstructions. • Artificial ageing and analysis of Prussian blue oil paints oil paint reconstructions • ATR-FTIR spectroscopy shows increased ester hydrolysis in Prussian blue oil paints • Prussian blue oil paints are capable of NH 3 adsorption and conversion to NH 4 + • Fatty acid efflorescence is observed on most Prussian blue oil paints