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

Abstract

The restoration of paintings always involves the removal of darkened superficial layers, which are mainly due to dust deposition and aged varnishes. As cleaning is an irreversible and invasive treatment, physical methods (i.e., laser cleaning) instead of chemical ones are frequently suggested to reduce side effects on pictorial layers. Among the most employed laser systems, the free-running Er:YAG laser is considered very suitable for fine arts cleaning. This laser works at 2.94 μm, at which only –OH and –NH bonds in molecules are excited. This character can become a disadvantage when pigments with these functional groups are present. To understand the potential of the Er:YAG laser in such situations or in the presence of degradable pigments, the effectiveness of varnish removal from paintings prepared with egg yolk as the binder and cinnabar and lead white as the pigments were systematically investigated. Different cleaning conditions were used, and a hyperspectral sensor was innovatively used as a rapid, in situ and non-destructive technique to assess the effects of laser ablation, besides microscopic analysis. Though results obtained show all these pigments are sensitive to this laser radiation, satisfactory cleaning can be achieved without damaging the pictorial layer. The best cleaning conditions were 0.5 W of power (50 mJ and 10 Hz for energy and frequency), with 2-propanol as the wetting agent.

Comparison of the Use of Traditional Solvents and Nanosecond 213 nm Nd:YAG Laser in Thinning Naturally Aged Varnish on a Contemporary Oil Easel Painting

“…Accordingly, excimer laser irradiation emission in the UV region (e.g., KrF laser at 248 nm), which is highly absorbed by most varnishes and coatings, has been used to thin down artificially and naturally aged varnish [8,9] and/or to remove overpaints [10]. Er:YAG laser has proven to be effective in removing artificially aged and/or fresh varnishes from mock-ups of easel oil [11], tempera [6,12], and mural paintings [13,14] without damaging the underlying paint layers. Naturally aged varnishes from tempera [5] to oil easel and panel paintings [6,7,15,16] and tempera overpaint [6] have been also successfully treated with this type of laser.…”

“…However, Er:YAG laser works at 2.94 µm, a wavelength that can only be absorbed by OH and NH functional groups. Consequently, when these groups are not present in the unwanted materials of the overlayers, wetting agents [6,12] need to be added for photothermal disaggregation to ensure efficient removal.…”