Discolouration of historical pigments: Investigating the whitening of red lead tempera-based mock-ups after natural weathering

Abstract

Red pigments Pb3O4 (minium or artificial red lead) can undergo mineralogical transformations due to sulphation, oxidation, or carbonation which trigger chromatic alterations such as a blackening or whitening of the painting surface. Mineral phases neoformed during blackening (e.g., galena -PbS-, plattnerite -PbO2) and whitening processes (e.g., cerussite -PbCO3-, hydrocerussite -(PbCO3)2·Pb(OH)2-, anglesite -PbSO4) are well-known, however more information is needed concerning the mechanisms, especially regarding the later. To understand the whitening process in Pb3O4-based tempera, mock-ups were prepared mixing red lead with either egg yolk or rabbit glue as binders, and outdoor in the city of Granada (S Spain) during 30 months. Evolution of degradation was evaluated at 6, 12, 24 and 30 months of exposure. Surface characteristics (colour, roughness and microtexture) were studied in reference and degraded paints, as well as mineralogy and molecular composition using micro-X-ray Diffraction and Attenuated Total Reflectance Fourier–transform Infrared Spectroscopy. Moreover, micromorphological changes and chemical composition were analysed with scanning electron microscopy with energy-dispersive X-ray spectroscopy. Results indicate that in all paints carbonation process happened, yielding different white lead carbonates. However, depending on the painting’s binder, other processes take place such as sulphation (egg yolk) and chlorination (rabbit glue) revealing the roll of pigment-binder interaction in the paint susceptibility to physical or chemical changes.