Early formation and detection of lead-based products from PbO-oil paint systems

Leaded oil medium, historically used by artists to modify paint properties, was prepared by treating oil with litharge (α-PbO), massicot (β-PbO), or minium/red lead (Pb3O4), forming lead soaps. Alongside lead soaps, recent synchrotron-based structural micro-probe analyses revealed presence of lead formate (Pb(HCOO)2), lead formate hydroxide (Pb(HCOO)(OH)), shannonite (Pb₂OCO₃), plumbonacrite (Pb5(CO3)3O(OH)2) and neo-formed hydrocerussite (Pb3(CO3)2(OH)2). This study reviews their occurrence in historical and model systems and explores their early formation and distribution in leaded oil model systems. Model systems were analyzed using optical microscopy, GC-MS, ATR-FTIR, µ-FTIR, and SR-µ-XRPD. This study shows that, beyond lead soaps, various lead-based compounds form heterogeneously in leaded oil systems. Lead formate crystallizes and migrates within the first days of curing, while lead formate hydroxide and lead carbonates (shannonite, plumbonacrite and neo-formed hydrocerussite) tend to concentrate in the globule areas. This paper proposes potential chemical formation pathways for these unusual lead-based products in leaded oil systems.