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Varnishes made of natural terpenoid resins are vulnerable components of paintings since they can undergo complex and differentiated chemical and structural changes over time depending on the type of varnish and the conservation conditions. The present work aims at studying the in-depth effects of natural and artificial aging on terpenoid varnish layers using nonlinear optical microscopy (NLOM) in its modality of multiphoton excitation fluorescence (MPEF) complemented with laser-induced fluorescence (LIF) and optical microscopy measurements. To this end, solvent-based terpenoid varnishes with different thicknesses and degrees of aging were considered. A homemade nonlinear optical microscope, based on a tightly focused pulsed femtosecond laser emitting at 800 nm, was used for the MPEF investigation. Single-photon LIF measurements served to determine the degree of surface aging and the optimum NLOM-MPEF operating conditions and helped to interpret the results obtained by applying the latter.
The LIF results showed an increase of fluorescence intensity and a red shift of the maximum of emission for varnishes subjected to longer aging periods and of thinner layers. The purely non-invasive NLOM-MPEF approach serves well to assess the in-depth-dependent degradation gradients across the thickness of the layers and its dependence on the aging time and the thickness of the varnish layer.
Acknowledgments
This research has been funded by the Spanish State Research Agency (AEI) through project PID2019-104124RB-I00/AEI/1013039/501100011033, by the H2020 European project IPERION HS (Integrated Platform for the European Research Infrastructure ON Heritage Science, GA 871034), and by the Community of Madrid project Top Heritage-CM (Tecnologías en Ciencias del Patrimonio, S2018/NMT_4372). Support by CSIC Interdisciplinary Platform “Open Heritage: Research and Society” (PTI-PAIS) is acknowledged.
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