Alkyd artists' paints: Do pigments affect the stability of the resin? A TG and DSC study on fast-drying oil colours

We studied ten alkyd artists' paints from the Griffin Alkyd, "fast drying oil colours" series (Winsor & Newton) in order to evaluate the effect of pigment-binder interactions on the stability of the alkyd resin. The literature on alkyd paints has focused above all on the characterization of the paints and their ageing processes using various techniques, however, to the best of our knowledge, research on pigment-resin interaction is still lacking. We selected four alkyd colours (red, blue, yellow and green) in two formulations based on inorganic or organic pigments together with titanium white and black formulated with carbonized bones and we applied a combined differential scanning calorimetric (DSC) and thermogravimetric (TG) approach. The pure pigments and the whole paint replicas were analyzed, and the effect of natural ageing on the paints was monitored for eight months. Artificial ageing in an acetic acid atmosphere was also studied. We found that organic and metallorganic pigments interacted the strongest with the alkyd resin, while the inorganic pigments appear to act more as dispersants. The chemical drying of the paint on the other hand, via auto-oxidation, is almost independent of the pigment and only depends on the alkyd content. The paint is fully dried after eight months. Paint samples from two hyper-realistic artworks, "Salto di qualità", 2003, and "Senza nome", 2007, by the Italian painter Patrizia Zara were investigated by DSC in order to evaluate the effects of several years of natural ageing on alkyd fast-drying oil colours. At the end of chemical drying, the alkyd colours seemed very stable. Artificial acid ageing visibly damaged the paint replicas, but appeared to act mostly on the pigment rather than on the resin network. © 2014 Elsevier Ltd. All rights reserved.