Van Gogh Yellow (VGY) is the name given to a family of yellow inorganic pigments used by numerous painters, and, in particular, by Vincent Van Gogh in his late period. VGY is basically composed by lead chromate PbCrO4, with sulphur added in different amounts to form paler shades of yellow. The resulting VGY PbCr(1-x)SxO4 solid solution tends to darken on exposure to sunlight: brownish deteriorated areas are evident in some versions of Van Gogh’s famous Sunflowers. A series of recent studies have shown that the degradation occurs by formation of greenish Cr2O3 and that the degradation rate increases with sulphate concentration [1,2]. This work aims at answering the following open questions: Why sulphur induces degradation? To what extent degradation is a purely surface process or depends on the underlying bulk structure of VGY solid solution? On one hand, our objective is to establish an ab initio (DFT) computational strategy for the accurate prediction of structure-property relationships in inorganic pigments of interest for cultural heritage. On the other, we aim at elucidating the origin of crystalline structures, stability and optical properties of VGY. From our calculations, we find that dispersion forces play a crucial role in determining the correct crystal structure of VGY, while trends in electronic band-gaps with increasing sulphur content can be determined with affordable PBE and PBE+U approaches. We also find that VGY PbCr(1-x)SxO4 solid solution is not stable for any Cr:S ratio. Thus, we can predict that the presence of sulphur represents a cause of thermodynamic instability leading to long-term phase separation of VGY into PbCrO4 and PbSO4. Since PbCr(1-x)SxO4 results to be yellow, even for the highest sulphur content explored, we can state that the paler hue of sulphur-containing VGY arises not from the color of the solid solution itself, but from mixing of VGY with white pure sulfate. All in all, we can state that phase separation and ultraviolet adsorption of segregated pure PbSO4 are the initial steps of VGY browning process. This study paves the route toward the characterization of VGY surface chemistry and degradation reactions. [1] L. Monico et. al. Anal. Chem. 86 (2014) 10804-10811. [2] H. Tan et. al. Angew. Chem. Int. Ed. 52 (2013) 11360-11363.

Exploring degradation of Van Gogh yellow from the inside: A computational study on the PbCr(1-x)SxO4 solid solution

251st National Meeting (2016)

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