Advanced X-ray techniques to study the alteration of pigments in paintings

The application of X-ray methods (using conventional sources or synchrotron radiation) for investigating degradation phenomena in paintings has significantly increased in the last two decades. This rise is due to their ability to provide spatially resolved elemental, molecular, and structural information from the macroscopic to the nanoscopic levels. This review will focus on the application of latest-generation X-ray techniques, including X-ray fluorescence (XRF), X-ray absorption spectroscopy (XAS), and X-ray diffraction (XRD), to study the alteration processes of pigments in paintings. The first part outlines the fundamentals of XRF, XAS, and XRD techniques and then describes the corresponding instrumental set-ups used for non-invasive macro-scale mapping of paintings and synchrotron radiation-based X-ray analysis of paint micro-samples. Subsequent sections will cover advancements in X-ray data analysis software, workflow management systems, Open Science and FAIR data initiatives, alongside practical aspects of sample preparation and issues concerning X-ray-induced damage to paints. The final section will review degradation phenomena resulting from chemical changes of selected classes of pigments. This will involve describing key findings obtained from paintings, related micro-samples, and artificially aged paint mock-ups. The outcomes discussed in this review highlight their crucial role in developing effective monitoring and preventive conservation strategies for artworks highly susceptible to degradation within heritage sites and museums.