Journal
ANALYTICAL CHEMISTRY
Volume 85, Issue 2, Pages 860-867Publisher
AMER CHEMICAL SOC
DOI: 10.1021/ac3021592
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Funding
- ESRF [EC-799]
- Interuniversity Attraction Poles Programme-Belgian Science Policy [IUAP VI/16]
- BELSPO-SDD S2-ART project [SD/RI/04]
- Research Fund University of Antwerp, Belgium
- FWO (Brussels, Belgium) [G.0704.08, G.01769.09]
- EU FP7 programme CHARISMA [228330]
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Previous investigations about the darkening of chrome yellow pigments revealed that this form of alteration is attributable to a reduction of the original Cr(VI) to Cr(III), and that the presence of sulfur-containing compounds, most often sulfates, plays a key role during this process. We recently demonstrated that different crystal forms of chrome yellow pigments (PbCrO4 and PbCr1-xSxO4) are present in paintings by Vincent van Gogh. In the present work, we show how both the chemical composition and the crystalline structure of lead chromate-based pigments influence their stability. For this purpose, oil model samples made with in-house synthesized powders of PbCrO4 and PbCr1-xSxO4 were artificially aged and characterized. We observed a profound darkening only for those paint models made with PbCr1-xSxO4, rich in SO42- (x >= 0.4), and orthorhombic phases (>30 wt %). Cr and S K-edge micro X-ray absorption near edge structure investigations revealed in an unequivocal manner the formation of up to about 60% of Cr(III)-species in the outer layer of the most altered samples; conversely, independent of the paint models' chemical composition, no change in the S-oxidation state was observed. Analyses employing UV-visible diffuse reflectance and Fourier transform infrared spectroscopy were performed on unaged and aged model samples in order to obtain additional information on the physicochemical changes induced by the aging treatment.
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