Complementary use of Optical Coherence Tomography (OCT) and Reflection FTIR spectroscopy for in-situ non-invasive monitoring of varnish removal from easel paintings

Although the solvent removal of unwanted layers (e.g. aged varnishes, overpaints, oxalate patinas, dirt) from easel paintings is one of the most frequently performed restoration treatments, it is questioned regarding its controllability and safety to the artwork. There is thus a need for developing suited diagnostic methodologies able to inform restorers on the chemical, optical and morphological effects of cleaning. To this aim, we have explored the possibility of complementary use of Optical Coherent Tomography (OCT) and reflection FTIR. On the one hand, the OCT technique provides cross-sectional images with varnish layers visible and thus permits for measurement of their thicknesses. Combining many cross-sections into 3D data provides insight into the varnish thickness distribution over a given area of the paintings. On the other hand, reflection FTIR allows for a chemical characterization of the surface compounds revealing the nature of the varnish and monitoring its removal. Artificially aged model samples and two historical paintings were investigated at different steps of solvent cleaning tests and the results of the two techniques compared highlighting the benefits of their combined exploitation.