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Removal of old deteriorated varnish layers from an easel painting is one of the most common but also most delicate restoration treatments. Apart from purely mechanical methods various solvents or mixtures of solvents have been used for this task over the last hundred years. Most often these chemicals were applied mechanically by swabbing, but also in a form of gels. However, the resulting formation of secondary and detrimental effects (such as swelling, leaching, solvent retention as well as high toxicity to the user) forced the development of new cleaning methodologies, aimed to overcome these drawbacks, guaranteeing a safe and effective removal of the damaged material. The aim of this paper is to discuss the applicability of optical coherence tomography for a multi-faceted and complementary assessment of different varnish removal processes, including the application of a novel nanogel system, the PHB-GVL. Since some effects, like swelling, may develop in a time span of hours, which is not a typical examination time for OCT, long-lasting stability of the measurement system had been ensured. For this task, a lab-developed high-resolution OCT instrument (3 µm axial resolution in the air) was adopted and the repeatability of surface recognition of 1.5 μm over 17 hours was achieved. This permitted for tracing of swelling effects in a multi-hour scale. The most desirable effect of such a restoration treatment is the removal of varnish without any alteration to the paint layer underneath. With the use of OCT, it was possible to trace this process by a direct comparison of the topography of the paint layer under varnish (before treatment) and after treatment. The results pointing at superior properties of the PHB-GVL nanogel applied with an electrospun polymeric tissue will be presented.
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