Phase contrast tomography to elucidate the formation process of lead soap protusions in historical paintings

Many famous artworks from the 16-19th centuries, created with oil paint and lead containing pigments, show formation of protrusions: spherical bodies of lead soaps that initially form subsurface, but upon growing, break through the paint surface and significantly decrease its mechanical integrity. Minium (Pb3O4) is a pigment that has been hypothetically identified as an important factor triggering protrusion formation, where micrometer sized litharge (PbO) inclusions inside larger Pb3O4 pigment particles may act as nucleation centres of the protrusions. We propose to perform high-resolution phase contrast microtomography at ESRF-BM18 on a series of spontaneously formed and synthetically grown lead soap protrusions in order to study their growth in 3D with time and investigate the role of Pb3O4 and its PbO inclusions in their initial formation. The same samples on which 3D volume data is acquired will also be subjected to (2D) µ-XRPD mapping inside the Heritage BAG.