Abstract
Modern oil paintings are affected by conservation issues related to the oil paint formulations and to the fact that they are often unvarnished, and in direct contact with the environment. Understanding the evolution of the molecular composition of modern oil paint during ageing, under the influence of environmental factors, is fundamental for a better knowledge of degradation phenomena and risk factors affecting modern art. We investigated for the first time the influence of relative humidity on the chemical composition of modern oil paints during curing and artificial ageing. For this purpose, modern oil paint layers naturally aged for 10 years were further artificially aged in low and high relative humidity conditions. Moreover, the influence of RH% on the curing of fresh paint layers was studied. The paints used in the experiments are from three suppliers (Old Holland, Winsor&Newton, and Talens), and contain cadmium or cadmium zinc sulfide as main pigment. The changes in the composition of extracts of paint samples were investigated by direct electrospray mass spectrometry with a quadrupole-time of flight mass analyser (ESI-Q-ToF). The obtained mass spectral data were interpreted by means of principal component analysis (PCA) operated on a data set containing the relative abundance of ions associated to significant molecules present in the extracts, and also by calculating the ratios between the signals relative to fatty acids, dicarboxylic acids and acylglycerols, related to hydrolysis and oxidation phenomena. The same paint samples were also analysed, in bulk, by pyrolysis gas chromatography mass spectrometry (Py-GC/MS), achieving chemical information on the total lipid fraction. High performance liquid chromatography (HPLC) ESI-Q-ToF was carried out for the characterisation of the profile of free fatty acids (FFA) and acylglycerols, defining the nature of the oils used in the paint formulations, and for the determination of the degree of hydrolysis. This study demonstrated that relative humidity conditions significantly influence the chemical composition of the paints. Ageing under high RH% conditions produced an increase of the formation of dicarboxylic acids compared to ageing under low RH%, for all paints, in addition to a higher degree of hydrolysis, followed by evaporation of free fatty acids.
Highlights
Drying oils have historically been used as binders in oil-based paints, thanks to their capability of forming a solid elastic film
A recently developed analytical method, based on reverse phase high performance liquid chromatography (HPLC) coupled with ESI-Q-ToF mass spectrometric detection[29], was used for the determination of free fatty acids (FFA) and of the different classes of acylglycerols (TAGs, DAGs, and MAGs) in the 2006 paint samples before artificial ageing, and in the fresh paint directly from the tube (Fig. 2a–f)
The study was focussed on modern oil paintings, which are proving unprecedented and challenging conservation problems, in order to provide fundamental knowledge to be used to prevent and slow-down the decay
Summary
Drying oils have historically been used as binders in oil-based paints, thanks to their capability of forming a solid elastic film. Cations in e.g. zinc, copper and lead containing pigments may react with free fatty acids and other carboxylic acid moieties present in the paint to form metal soaps agglomerates or ionomeric structures[9,16,17] Some of these reactions may contribute to the formation of water sensitive oil paint surfaces. The influence of relative humidity on a range of aged and young paint films still in the process of curing was studied for the first time This was done by monitoring the changes in composition of extracts of paint samples by direct electrospray mass spectrometry with a quadrupole-time of flight mass analyzer (ESI-Q-ToF). The degradation of this pigment has been object of several literature studies[23,24,25,26,27,28] whereas the degradation of the lipid binder has not been investigated in detail, to the best of our knowledge
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