First combined application of photoacoustic and optical techniques to the study of an historical oil painting

Optical techniques are routinely applied in artworks analysis to obtain structural, morphological, and compositional information on the examined object in a non-invasive way. However, the complex structure of paintings, which can be sketched as multi-layer systems often including non-transparent layers, can hamper the penetration of the optical probe, making the non-invasive sectioning particularly challenging. Recently, cutting-edge photoacoustic (PA) methods have been tested on painted mock-ups to overcome the above-described detection limit of pure optical techniques, giving promising results. In this work, for the first time, PA measurements were taken on a nineteenth-century painting. Using a multi-modal approach, the effectiveness of the technique in revealing hidden features and providing cross-sectional measurements was assessed in comparison with other well-established pure optical techniques, namely multispectral Vis-NIR reflectography, UV imaging, fibre optics reflectance spectroscopy (FORS), optical coherence tomography (OCT), and laser scanning microprofilometry. Data integration allowed highlighting the capability of each method, proving the effectiveness of the proposed multi-modal approach. This successful application of the PA method on a real case study paves the way for its introduction among the other non-invasive techniques for paintings analysis.