New Insights into Radical Mechanisms in Natural Drying Oils: Paint Binders as Reactors for Nanoparticle Formation

Understanding radical processes in drying oils is essential for both heritage science and materials chemistry, as these reactions govern film formation and long-term stability in historical paintings. In this study, we investigate radical mechanisms in linseed oil, a traditional paint binder. Gold salts are used as probes to elucidate film-drying reactions. The introduction of gold salts into both raw and lead-treated linseed oils leads to the formation of gold nanoparticles, whose size, morphology, and distribution are monitored by spectroscopic methods and transmission electron microscopy (TEM). Under ambient conditions, spherical gold nanoparticles form during the drying of both oil types. Electron paramagnetic resonance (EPR) reveals oxygen-centered radicals (peroxyl (ROO●) or alkoxyl (RO●) radicals) involved in the oil-curing process. The addition of lead, following historical oil preparation recipes, increases radical activity and promotes the formation of additional species, suggesting more complex reaction pathways. In lead-treated oils, 3–4 nm lead-based nanoparticles are observed both in the oil bulk and in dried films, formed during the oil preparation.