The fading of red lead pigment in wall paintings: tracking the physico-chemical transformations by means of complementary micro-analysis techniques

The phenomenon of red lead pigment fading in wall paintings was investigated through the study of an experimental fresco painting. Chemical modifications of the pigment induced a local fading of the pictorial layer after a 25-years natural ageing period. Representative features of the alteration phases, including composition and structural information, were obtained by applying complementary micro-analysis techniques to the study of a single paint sample. Focused X-ray diffraction patterns of small areas were collected using a highly sensitive detector, revealing the transformation of red lead pigment into both cerussite (lead carbonate) and anglesite (lead sulphate). The distribution of Pb, S, O and Ca elements within the cross-section was established using electron micro-probe analysis, and correlated to micro-Raman semi-quantitative mappings of minium (Pb3O4), cerussite (PbCO3) anglesite (PbSO4) and calcite (CaCO3) phases. The micro-structural characteristics of each lead-containing phase were investigated by means of scanning electron microscopy observations of the sample cross-section using backscattered electron imaging. The major role of atmospheric pollutants (SO2, CO2), together with water condensation on such a red pigment fading is emphasised.