Illuminating Prussian blue fading in oil paintings: a multi-scale and multi-technique approach to mitigate synchrotron radiation X-ray induced iron photoreduction

Resolving chemical species at the submicrometer scale is crucial in Heritage Science, where synchrotron radiation (SR)-based X-ray methods, including µ-XRF mapping and µ-XANES spectroscopy, offer unique insights into the composition of heterogeneous and opaque materials, such as degraded inorganic pigments in paintings. However, the high intensity and brightness of SR beams pose significant risks of radiation damage, which can compromise both sample integrity and data interpretation. This challenge is notably evident in Prussian blue (a ferric hexacyanoferrate pigment), where X-ray induced photoreduction of Fe3+ to Fe2+ interferes with speciation analysis in faded artworks, necessitating rigorous experimental strategies to ensure data reliability. To address this, we present a multi-scale and multi-technique approach aimed at safely investigating the light-induced degradation of Prussian blue in oil paintings via SR-based X-ray micro-spectroscopic techniques. The study focuses on the 1802 painting Pamphilus and his Servant Davus by the Danish artist Nicolai Abildgaard (Copenhagen, 1743-Frederiksdal, 1809), which exhibits various degrees of fading in the Prussian blue-based paints, particularly evident when comparing light-exposed areas to those protected by the frame. After evaluating the paint composition and fading at the macro-scale using a set of non-invasive MOLAB techniques (namely colorimetry, reflectance UV–VIS–NIR and external reflection mode FT-IR spectroscopies), two micro-samples from light-exposed and unexposed areas were selected for SR-based µ-XRF mapping and µ-XANES spectroscopy at the Fe K-edge. In parallel, a series of Prussian blue-based oil paint mock-ups were prepared, photoaged, and analyzed in advance of historical samples to establish optimal conditions for mitigating SR X-ray induced Fe3+→Fe2+ reduction during measurements, achieved by systematically varying fluence, dose, and temperature. Under these optimized, non-damaging conditions, we successfully mapped the stratigraphic distribution of iron compounds at submicrometric resolution in the historical cross-sections. The results, unaffected by analytical artifacts, revealed the nature of the secondary products responsible for the observed fading in Abildgaard’s painting. Consequently, this research offers a validated experimental approach for future SR-based X-ray micro-spectroscopy studies of Prussian blue degradation in other artworks.