Chromium speciation studies for studying chemical reactivity of strontium and barium chromate-based pigments

The photo-degradation of strontium and barium chromate yellow pigments (SrCrO4, BaCrO4) from the 19th century remains unstudied. However, chrome-based yellow pigments are known for their tendency to undergo darkening when exposed to UVA-visible light. This darkening has been associated to the reduction of the Cr6+ into Cr3+ species for chrome yellow (PbCrO4) and zinc chromate (4ZnCrO4·K2O·3H2O) pigments [1, 2], a chromium speciation study for studying the chemical reactivity of SrCrO4 and BaCrO4 has yet to be conducted. To that end, we propose to perform a multi-technique investigation of artificially aged paint samples through the coupling of micro X-ray fluorescence (μ-XRF), and micro-X-ray absorption near edge structure (µ-XANES). The study of the photochemical-reactivity and the nature of the alteration products of these light sensitive pigments in oil medium will be key to improve the long-term conservation and restoration strategies of historical paintings.