Revealing the distribution of metal carboxylates in oil paint from the micro- to the nanoscale
Xiao Ma, Victoria Beltran, Georg Ramer, Georges Pavlidis, dilworth parkinson, Mathieu Thoury, Tyler Meldrum, Andrea Centrone, Barbara H. Berrie
Oil paints comprise pigments, drying oils, and additives that together confer desirable properties, but can react forming metal carboxylates (soaps) that, over time, may damage artworks. Despite substantial research to understand these phenomena, soap formation and aggregation are not well-understood processes. Here, composition-sensitive techniques are employed to determine the distribution of soaps in oil paint from the microscale to the nanoscale. A 23-year old commercial sample of known composition and no treatment history provides a link between laboratory mock-ups and artists paintings and enable inferring the mechanisms of soap formation. Infrared microscopy reveals gradients in the distribution of Zn- stearate, a Zn-carboxylate complex characterized by a broad IR peak, and in the molecular conformation (order) of species through the paint layer. Infrared nano-spectroscopy reveals a complex and heterogeneous distribution of soaps at the nanoscale. Zn-stearate is dispersed in nano-sized aggregates throughout the paint film; Al-stearate, not easily identified with conventional techniques, is intermixed with a minor fraction of Zn-stearate and a phase characterized by the broad Zn-carboxylate absorption peak.