Nonlinear imaging microscopy for assessing structural and photochemical modifications upon laser removal of dammar varnish on photosensitive substrates

<p>Varnish layers are commonly used to protect painted surfaces from atmospheric pollution, oxidation<br> and improve the aesthetic appearance of the artwork by providing an even surface finish, brilliance and<br> depth to the colours. However, the outer varnish layers suffer from progressive deterioration due to<br> aging and the continuous exposure to aggressive environmental conditions, imposing the need of their<br> removal for rectifying the optical and aesthetic properties of the painting and extend its lifetime. The<br> removal of the surface varnish layer, without affecting the painting substrate, comprises a delicate<br> intervention in cultural heritage (CH) conservation.<br> The main objective of this study is to determine by nonlinear imaging microscopy (NLM) the extent of<br> the photochemical damage that could be induced on underlying painting layers by laser removal of<br> varnish protective coatings. This will lead to the identification of the optimal laser cleaning conditions<br> that produce the minimum collateral damage to the painting layers.<br> The current study is undertaken using model samples constituted by bilayers, where the top varnish<br> layer (dammar) coats a bottom layer constituted by a doped synthetic polymer (polymetilmetacrilate,<br> PMMA doped with POPUP) film, the latter mimicking a paint layer. The target is to determine the<br> affected region as a function of depth of the doped polymer layer induced by laser ablation of the<br> varnish. To this aim we use the non destructive NLM imaging modalities of third harmonic generation<br> (THG) and multiphoton excitation fluorescence (MPEF) as novel diagnostic tools and a number of<br> laser conditions for varnish removal, namely different ultraviolet (UV) wavelengths and pulse durations.<br> Characterization of the samples by NLM is complemented by spectroscopic micro-Raman and laser<br> induced fluorescence (one-photon excitation) measurements. These provide a full characterization of<br> the lateral and in-depth chemical and morphological changes following laser removal of the varnish<br> protective layer.</p>