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

TL;DR

This study explores laser heating techniques to reverse oxidation in ancient lead-based mural pigments, successfully converting darkened lead oxides back to their original bright forms using specific laser treatments.

Contribution

It demonstrates the feasibility of using continuous wave laser heating to restore oxidized lead pigments in murals, identifying optimal laser parameters for effective reconversion.

Findings

Laser irradiation can revert plattnerite to minium and massicot.

Nd:YAG laser is most effective for pigment restoration.

Successful restoration on natural darkened mural samples.

Abstract

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 (\b{eta}-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 (\b{eta}-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.