Continuous wave laser thermal restoration of oxidized lead-based pigments in mural paintings

Red lead and lead white are some of the most ancient and common pigments in mural paintings. However, they tend to blacken with time due to their oxidation to plattnerite (β-PbO2). The possibility to induce the reconversion reactions by CW laser heating is hereby discussed. A thermodynamic study by TGA showed that direct cerussite or hydrocerussite formation from plattnerite are not suitable reconversion routes, which was confirmed by laser irradiation trials under CO2 and CO2/H2O fluxes. Minium (Pb3O4) and subsequent massicot (β-PbO) formation from plattnerite were achieved (confirmed by SEM–EDS, XRD and micro-Raman) under Ar+, 810 nm diode and Nd:YAG lasers. The latter appears to be the most suited for restauration purposes, given the broad minium reconversion irradiance range. This is confirmed by successful trials on macroscopic areas of naturally darkened red lead containing samples.