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

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