Damages induced by synchrotron radiation-based X-ray microanalysis in chrome yellow paints and related Cr-compounds: assessment, quantification and mitigation strategies

Synchrotron radiation (SR)-based X-ray methods are powerful analytical tools for several purposes and we widely use them for probing the degradation mechanisms of inorganic artists’ pigments in paintings, including chrome yellows (PbCr1-xSxO4; 0 ≤x≤0.8), a class of compounds often found in Van Gogh masterpieces. However, the high intensity and brightness of SR beams raise important issues regarding potential damages of the analyzed samples. A thorough knowledge of the SR X-ray sensitivity of each class of pigment in the painting matrix is therefore required to find analytical strategies that contribute to minimize the damage for preserving the integrity of the analyzed sample and to avoid misinterpretation of the data. Here, we employ a combination of Cr K-edge X-ray absorption near edge structure (XANES) spectroscopy, Cr-Kβ X-ray emission spectroscopy (XES) and X-ray diffraction (XRD) to monitor and quantify the effects of SR X-rays on the stability of chrome yellows and related Cr-compounds and to define strategies for mitigating their damage. We found that the SR X-ray beam exposure induces changes in the oxidation state and local coordination environment of Cr-ions and leads to a loss of the compound’s crystalline structure. The extent of X-ray damage depends on some intrinsic properties of the samples (chemical composition of the pigment as well as the presence/absence and nature of the binder) and it can be minimized by optimizing the overall fluence/dose released to the samples and by working in vacuum and cryogenic conditions.