Phase separation in lead-saponified drying oils: Implications for historical painting techniques and paint stability

Renaissance Masters often prepared siccative oils by heating linseed oil with siccatives, particularly lead oxide, inducing partial saponification and altering its properties. Our reconstructions show that lead-saponified oils naturally separate into two phases. In this study, we investigate the differences between these two phases through a comprehensive set of analytical methods, from macrolevel assessments (rheology) to microlevel characterizations (small and wide-angle x-ray scattering, optical microscopy, and scanning electron microscopy) and chemical analyses. The lower phase is enriched in free fatty acids and lead carboxylates, especially saturated species—both as free acids and metal soaps—prone to self-organization. As a result, the lower phase displays a pronounced lamellar organization with partial crystallization and exhibits viscoelastic and shear-thinning properties. In contrast, the unstructured upper phase behaves as a Newtonian fluid. The observed phenomena share similarities with the formation of soap-related defects such as protrusions, linked to saturated lead soaps.