Er:YAG laser interactions with natural and synthetic varnishes for paintings

Chille, Chiara (2021) Er:YAG laser interactions with natural and synthetic varnishes for paintings. Doctoral thesis, Northumbria University.

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Abstract

This PhD study aims to deepen the comprehension of the Er:YAG (2940 nm) laserinduced physicochemical effects on aged varnishes by a systematic examination of the side effects caused by the temperature rise in the irradiated surfaces, the heat propagation in the bulk and examines if the laser beam could reach the underlying layer during the laser-material interaction. It also focused on whether Er:YAG lasers can be used to safely thin varnishes applied to works of art, therefore, enhancing the use of laser as an alternative to traditional cleaning methods for paintings. A series of dammar, Ketone N, MS2A, and Paraloid B67 varnish mockups on glass slides were light and hydrothermally aged for a comprehensive study. Artists and conservators commonly used these resins in paintings of the late nineteenth and twentieth centuries. Changes in temperature upon laser irradiation were determined by i) recording live thermal imaging with an IR thermal camera and ii) obtaining the optical properties of the varnishes to estimate the absorption coefficients and the temperature increases. A mathematical model and a twodimensional simulation of the laser heat diffusion through each irradiated varnish supported the second approach. They also provided an additional possibility to study the heat propagation in the bulk of the irradiated varnishes. It was found that, in this case, the IR thermal camera was unable to provide accurate maximum peak temperature data. Nonetheless, the pre-wetted varnishes using an aqueous solution showed ΔTmean values higher than on the dry varnishes due to the superheating of the aqueous film on the surface (Nahen and Vogel, 2002). Transmission Infrared Spectroscopy and Differential Scanning Calorimetry determined the absorption coefficients and the specific heat capacities of films. The mathematical model and the 2D simulations showed that the maximum temperature was obtained using a short pulse duration (VSP) mode, and the heat distribution was less than the one obtained with a long pulse duration (SP) mode. The temperature at the interface between the coating films and the substrate remained at room temperature, thereby protecting the underlying paint surface. Transmission studies were carried out on the aged varnishes in real-time upon laser irradiation, showing that the energy transmitted upon a single laser pulse in VSP and SP modes increased almost linearly with fluence. By pre-wetting the varnishes’ surface with an aqueous solution, the laser energy propagation into the selected varnishes was reduced because of the maximum absorption coefficient of water at the Er:YAG laser wavelength (Shori et al., 2001). Since Paraloid B67 does not contain O-H groups in its chemical structure, the transmission infrared spectroscopy showed almost all the Er:YAG laser radiation passed through the film, reaching the underlying layer. Chemical changes were monitored with Attenuated Total Reflection/Fourier Transform Infra-Red Spectroscopy. This analysis registered a reduction of hydroxides and carbonyls relative to hydrocarbon bonds compared to the films before irradiation. The decrease in hydroxides confirmed that the dominating mechanism of Er:YAG laser is directly related to the maximum absorption of the 2940 nm laser wavelength from the hydroxides in the irradiated surface. The resulted laser spots were observed with Scanning Electron Microscopy. A comparison between dry and pre wetted irradiation revealed morphological differences between the dry irradiated surfaces and the pre-wetted ones with less marked laser spots in the latter. After the tests, an Er:YAG laser was used to thin an aged varnish on a nineteenth-century oil painting. Dry and pre-wetted laser tests were conducted using single and multiple laser scans. The efficacy of the cleaning procedure was evaluated using stereomicroscopy and colourimetry, and spectral clusterisation maps derived from multispectral imaging (MSI). MSI data, used herein for the first time, were particularly useful in evaluating and monitoring the laser effects to the surface. Fourier Transform Infra-Red Spectroscopy and Pyrolysis Gas Chromatography/Mass Spectrometry 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.

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