An analytical evaluation of Er:YAG laser cleaning tests on a nineteenth century varnished painting

•Dry and pre-wetted Er:YAG laser irradiation tests on the varnish layer of a painting.•Evaluation via gloss-related factors generated from colourimetric data.•Evaluation via spectral clusterisation maps generated from multispectral imaging data.•Py-GC/MS and ATR/FT-IR analysis of the varnish before and after Er:YAG irradiation. This paper aims to evaluate the Er:YAG laser efficacy to safely thin a varnish on a modern 19th century oil painting. Tests were carried out under single and multiple laser scans directly on the surface (dry) or after pre-wetting with deionised water (DIW) and a non-ionic surfactant (Tween 20), fluence ranges of 0.56–2.40 J/cm2 and 100 μsec pulse duration. Microscope glass coverslips were placed on the painting surface during irradiation to collect the condensed resin fragments that were extracted from the varnished surfaces. Spectral clusterisation maps of Multispectral Imaging (MSI) data of the irradiated surface supported the evaluation of the procedure. Further evaluation was performed by stereomicroscopy and colourimetry. Fourier Transform Infrared spectroscopy (FT-IR) and Pyrolysis Gas Chromatography / Mass Spectrometry (Py-GC/MS) analysis indicated that the varnish resin was dammar. The collected resin fragments were analysed by FT-IR. The results showed that the resin did not degrade even at the highest level of fluence employed, thereby allowing a subsequent analytical evaluation.