The ageing of model pigment/linseed oil systems studied by means of vibrational spectroscopies and chemometrics

We used Fourier Transform - Near Infrared (FT-NIR) and micro-Raman spectroscopies to follow the changes occurring in a thin layer of linseed oil in the presence of lead white (basic lead carbonate) and zinc white (zinc oxide), which were due to the natural ageing process. The curing of linseed oil is a very complicated process, owing to different mechanisms that occur simultaneously, ones which may be further modified by the presence of pigments and by environmental conditions (e.g. light, humidity, temperature). Both lead white and zinc white pigments affect this process, leading to the formation of films with different properties. Previous studies have already characterised the ageing of binders alone, including linseed oil. We investigated, by means of a combined analysis of the results of two different vibrational spectroscopy methods, the role of these two white pigments on the ageing of a linseed oil film. Model samples were prepared, and we followed their evolution over a 24-month period. Two ten-year-old oil mock-ups, which were prepared by using lead and zinc white pigments (comparable to contemporary oil painting), were analysed within the framework of the present results. Lastly, we exploited the potentiality of Principal Component Analysis (PCA) on the combined FT-NIR and Raman spectra in order to define a possible ageing trend for two oil-white mixtures. Furthermore, we tested the statistical models by comparison with spectra related to the two ten-year-old mock-ups.