Relative humidity, light, and inorganic fillers: Defining different roles on the ageing of modern oil paints through infrared spectroscopy

In the past two decades, increasing attention has been paid to oil-based modern (from the 20th century onwards) binding media and their complex conservation issues, often related to the insurgence of metal carboxylates and oxalates. Indeed, as repeatedly shown by case studies with unexpected degradation problems, predicting the behavior of modern oil paint films is not trivial. This can be linked to the complexity of tubes’ formulation, the pigment(s) present, and the various environmental parameters interacting, both separately and combined, with the paint layers. When designing accelerated ageing tests to study degradation mechanisms, balancing all these variables is challenging and hence approaches are not standardized, and results are not always easily comparable. The present work aims to study the impact of different variables on the ageing mechanisms of Prussian blue paint films containing two types of extenders, calcium carbonate, CaCO3, and hydromagnesite, Mg5(CO3)4(OH)2·4H2O. Prussian blue drying oil paints from two brands were studied alongside an alkyd resin one. Moreover, oil-based zinc white samples, well-known for easily leading to zinc carboxylates formation, were included. To discriminate the influence of visible light from that of relative humidity (RH%), paint mock-ups were aged at around 50 %, 70 %, and 90 % RH both in the dark and exposed to an irradiance of 60 W/m2. Chemical changes at the surface of the paint layers were tracked through micro attenuated total reflectance infrared spectroscopy (μ-ATR-FTIR), which allowed to assess variations in polymeric network related to hydrolysis and oxidation reactions. The possible formation of metal carboxylates and oxalates was also monitored through the spectroscopic technique. Overall, it is shown how the presence of hydromagnesite, more reactive than calcite in the testing conditions, may ease the formation of metal carboxylates and oxalates. Oxalates were always formed in the highly humid dark environment, thus questioning the role of light.