Thermal reconversion of oxidised lead white in mural paintings via a massicot intermediate

Lead white, a mixture of cerussite (PbCO 3 ) and hydrocerussite (2PbCO 3 ·Pb(OH) 2 ), is the most ancient and common white pigment used in mural paintings. However, it tends to blacken with time due to its oxidation to plattnerite (β-PbO 2 ). Chemical treatments were used but they can put the pictorial layers supports at risks. Hereby, we address the possibility of thermally reconverting black plattnerite to white lead carbonates via a massicot (β-PbO) intermediate. We first investigated the conditions (temperature, time, and environment) in which pure powders react, before studying mural painting samples. Experiments were made in ovens and thermogravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) characterisation were achieved. Litharge (α-PbO) and massicot were obtained from plattnerite, respectively, between 564 and 567 °C and at 650 °C. Lead carbonates, namely cerussite, hydrocerussite and plumbonacrite (3PbCO 3 ·Pb(OH) 2 ·PbO) formed from massicot in wet CO 2 below 100 °C in a few hours. Lastly, when heating plattnerite-based mural painting samples, lead species reacted with binders and mortar, yielding massicot, plumbonacrite but also lead silicate and calcium lead oxides. This demonstrates the viability of thermal reconversion of darkened lead in mural, while raising concerns about the formation of several lead species by reaction with mural painting constituents.