Plumbonacrite identified by X-ray powder diffraction tomography as a missing link during degradation of red lead in a van gogh painting

Abstract

Red lead, a semiconductor pigment used by artists since antiquity, is known to undergo several discoloration phenomena. These transformations are either described as darkening of the pigment caused by the formation of either plattnerite (β-PbO2) or galena (PbS) or as whitening by which red lead is converted into anglesite (PbSO4) or (hydro)cerussite (2 PbCO3 Pb(OH)2; PbCO3). X-ray powder diffraction tomography, a powerful analytical method that allows visualization of the internal distribution of different crystalline compounds in complex samples, was used to investigate a microscopic paint sample from a Van Gogh painting. A very rare lead mineral, plumbonacrite (3 PbCO3 Pb(OH)2 PbO), was revealed to be present. This is the first reported occurrence of this compound in a painting dating from before the mid 20th century. It constitutes the missing link between on the one hand the photoinduced reduction of red lead and on the other hand (hydro)cerussite, and thus sheds new light on the whitening of red lead. Unravelling minium degradation: The very rare lead carbonate mineral plumbonacrite was identified in a Van Gogh painting using combined microscopic X-ray fluorescence and X-ray powder diffraction mapping and tomography methods. Thanks to the high spatial resolution and specificity of this technique an additional step in the photochemical degradation pathway of red lead (minium) is demonstrated.