Characterization of the reconversion products of darkened lead white and development of an innovative approach for its stabilization

In wall paintings, the darkening of lead-based pigments is a widespread alteration process due to the oxidation of lead Pb(II) into Pb(IV) in the form of lead dioxide (primarily plattnerite, β-PbO₂). In the case of lead white, it consists of lead carbonate transforming by oxidation into plattnerite. This phenomenon can severely affect the esthetical appearance of a painting and seems related to several factors such as alkaline environment, humidity, light, oxidizing agents, and microorganisms. These elements have been investigated as potential responsible for the darkening, but without firmly establish the causes, mechanisms, and kinetics of the reactions. Nevertheless, the desire of recovering the original colors encouraged conservators to use a chemical redox treatment known as “reconversion”, based on the use of hydrogen peroxide and diluted acetic acid. Although never analytically confirmed, this chemical treatment was successfully applied on darkened portions of several wall paintings proving the recovery of the white appearance. Unfortunately, in some cases, particularly those in humid environments, the reconversion was not stable and darkening reoccurred over time. This evidence highlights the need to ensure the stability of the recovered white guaranteeing the long-term efficacy of the reconversion. This study illustrates the results of in vitro laboratory tests, performed on pure powdered plattnerite, to investigate the chemical nature of the reconversion products and to develop an innovative stabilization method based on the use of di-ammonium phosphate (DAP). The attenuated total reflection mode infrared analyses (ATR-FTIR) and the X-ray powder diffraction (XRD) data showed that the addition of DAP leads to the formation of a white compound, namely hydroxypyromorphite [Pb₅(PO₄)₃OH], which thanks to its extremely low solubility, could prevent the occurrence of redarkening of the treated painted surface.

Graphical Abstract