First Insights into the Formation of Metal Soaps in Alkyd-Based Paints: A Proof-of-Concept Investigation Using FTIR Spectroscopy.

The formation of metal soaps or carboxylates in oil paintings is a widely studied topic. Expanding upon the extant research on the subject, this work investigates the formation of metal soaps as pertaining to alkyd-based media. Especially popular in modern and contemporary art, alkyd paints are complex commercial formulations mainly containing a polyester backbone obtained by the condensation of glycerol and phthalic acids, where the third free alcoholic function is esterified with a blend of saturated and unsaturated fatty acids. The formulation may also contain cross-linking agents, dries, and catalysts. Compared to drying oils, alkyd systems have different stability and mobility, likely prompting different kinetics in the formation of metal soaps. This work explores the formation of metal carboxylates in mock-up paints prepared as mixtures of an alkyd binder with potassium hydroxide and three commonly used pigments (smalt, zinc white, and lead white) at ambient conditions and upon artificial aging. As a second step, samples from two contemporary works by Italian artists Franco Garelli and Luciano Minguzzi were investigated for comparison. The results confirm the formation of metal soaps in both mock-up and real paints, paving the way for future research, with significant implications especially for the conservation of modern and contemporary artworks.