No abstract

Yellow pyroantimonates Pb-Sb, Pb-Sb-Sn and Pb-Sb-Zn were synthesized by solid-state reactions at high temperature and characterized by X-ray diffraction and Raman spectroscopy. The lattice size of cubic pyrochlores increases with Sn and Zn doping and with Pb overstoichiometry, indicating the replacement of Sb 5+ by the larger cations. This fact permits the understanding of the corresponding Raman spectral modifications as a consequence of the changes in the local symmetry of the Sb-O polyhedra, justifying the exploitation of Raman spectroscopy to noninvasively identify structural modifications of pyroantimonate pigments in artworks.

The painter, Vincent van Gogh, and some of his contemporaries frequently made use of the pigment chrome yellow that is known to show a tendency toward darkening. This pigment may correspond to various chemical compounds such as PbCrO(4) and PbCr(1-x)S(x)O(4), that may each be present in various crystallographic forms with different tendencies toward degradation. Investigations by X-ray diffraction (XRD), mid-Fourier Transform infrared (FTIR), and Raman instruments (benchtop and portable) and synchrotron radiation-based micro-XRD and X-ray absorption near edge structure spectroscopy performed on oil-paint models, prepared with in-house synthesized PbCrO(4) and PbCr(1-x)S(x)O(4), permitted us to characterize the spectroscopic features of the various forms. On the basis of these results, an extended study has been carried out on historic paint tubes and on embedded paint microsamples taken from yellow-orange/pale yellow areas of 12 Van Gogh paintings, demonstrating that Van Gogh effectively made use of different chrome yellow types. This conclusion was also confirmed by in situ mid-FTIR investigations on Van Gogh's Portrait of Gauguin (Van Gogh Museum, Amsterdam).