A study on the heat distribution and oxidative modification of aged dammar films upon Er:YAG laser irradiation

Abstract

This paper aims to evaluate the Er:YAG laser efficacy to safely thin a varnish on a modern 19 th 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/cm 2 and 100 sec 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.

Er:YAG Laser Cleaning of Painted Surfaces: Functional Considerations to Improve Efficacy and Reduce Side Effects

“…Only area covered by water was well protected, while other areas were as cleaning in dry conditions, which became a layer with micro-bubbles (marked by a white arrow in Figure 5d). Similar morphological changes were also found by Chillè et al during the cleaning of aged dammar films by using the Er:YAG laser, as they described "By increasing the fluence, the possibility of identifying differences in appearance of the spots was reduced, due to partial melting of the varnish surface and to the increase in micro-pits and bubbles" [27].…”

“…Although the Er:YAG laser was successfully applied for varnish removal [23][24][25][26][27][28][29], to our best knowledge, there is limited study on the situation when pigments in pictorial layers are also sensitive to this laser. Besides, laser cleaning is thought to be intuitive, and controlled cleaning can be performed by simply changing the fluence or pulse duration of the laser beam based on the experience or skill of the operator.…”

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

“…5 III and Fig. 6 III), as also discussed elsewhere [30,31,36]. The tests showed that dry irradiation resulted in the collection of more resin fragments on the cover slips than the pre-wet tests at the same laser settings (Fig.…”

“…An over-treatment, identified as the generation of a brown hue of the irradiated area, was visually assessed after one laser scan at 2.1 J/cm 2 , as well as at two laser scans of the surface at 1 J/cm 2 using the stereomicroscope. This visually assessed colour variation can be related to laser-induced photothermal and photomechanical effects [6,29,31,36]. Satisfying results in terms of a gradual thinning of the varnish without a brownish discolouration were obtained in the presence of DIW+TW20 at one laser scan at 2.4 J/cm 2 , two laser scans applications at 2.3 J/cm 2 and three laser scans applications at 2.1 J/cm 2 , respectively (Fig.…”

“…Laser irradiation run both on dry surface and with wetting agents, ranging from auxiliary OH-containing solvents (such as DIW and IMS) to a non-OH solvent (WS). Since the efficiency of the Er:YAG laser is directly proportional to the number of OH groups present in the surface [29], hydroxides were introduced by adding hydroxylated liquids, which aided to increase the laser interaction with the varnish layer [11,30], therefore reducing the heat diffusion of the incident 2.94 μm beam in the bulk [5,31,32]. Laser irradiation resulted in circular spots that were identified on the dammar varnish surface.…”