Confocal and angular-resolved XRF: A comparative study of mobile techniques for the non-invasive analysis of layered structures in artworks

Cultural heritage artworks are complex heterogeneous objects, often with intricate stratigraphies. The study of such intricate strata traditionally relies on invasive cross-sectional analyses involving microsampling. These destructive approaches, nevertheless, raise conservation concerns and should be minimized. Nonetheless, and although a growing demand for mobile non-invasive methods exists in heritage science, few are able to directly and non-destructively analyze thin superimposed layers. Two X-ray Fluorescence (XRF) techniques may prove vital to solve this: Confocal XRF (CXRF), and Angle-Resolved XRF (AR-XRF). While CXRF has proven highly effective for the study of museum paintings and cave art, AR-XRF remains less explored in heritage studies. Here, we present a first comparative evaluation of both non-invasive approaches for cultural heritage using high-performance mobile instruments recently developed at the XRAYLab in Catania. The CXRF scanner provides vacuum-free, low-powered, fast 3D mapping capabilities (dwell times as low as 200 ms; 8.8 µm (FWHM) confocal depth resolution at Zr Kα), while also supporting 2D micro-XRF scanning. The compact AR-XRF vacuum-free instrument, equipped with a low-power monochromatic source, has been designed for portability and versatility (supports GI-XRF, AR-XRF, GE-XRF, and TXRF) and offers sub-ppm detection limits for surface elements. Both devices are compared here on thin-film metallic standards, paint-stratigraphy mock-ups, and real artwork stratigraphies, showcasing their complementarity, highlighting their respective capabilities, and demonstrating their power for future revolutionary heritage studies.