Ageing-induced changes in varnish-coated oil paints: Surface and subsurface multi-technical analysis

Preventing varnish degradation is a major challenge in painting conservation. Despite extensive research, the mechanisms behind varnish alteration are not yet fully understood due to the complexity of the concurrent processes involved. Moreover, the extent to which organic and inorganic pigments, dispersed in various binding media within the paint substrate, influence this process remains unclear. Due to this complexity, detecting early signs of varnish degradation is a crucial step in protecting the underlying painting from irreversible damage. This study aimed to evaluate the effects of thermo-hygrometric and photochemical ageing ─ at both surface and subsurface levels ─ on natural (dammar resin) and synthetic (acrylic resin) coatings. These were analysed both as standalone layers and in combination with oil paint materials containing two common inorganic pigments: titanium dioxide and yellow ochre. A comprehensive suite of complementary, non-invasive techniques, spanning nano- to macroscale resolution, was employed. This optimized monitoring protocol, with potential in situ applicability, enabled high-resolution mapping of varnish degradation patterns, elucidating the complex interplay between compositional factors, environmental ageing conditions, and substrate-paint interactions. This study establishes a robust framework for evaluating material stability through the identification of early degradation markers, which can directly inform conservation strategies for painted artworks. Furthermore, it provides scientific support for varnish selection criteria, condition assessment protocols, and predictive conservation models.