Femtosecond Laser Cleaning of Painted Artefacts; Is this the Way Forward?

Abstract

The laser cleaning of painted artifacts relies on the synergy of thermal, photochemical, and photomechanical effects that are involved in laser ablation. A crucial issue, however, for a successful cleaning intervention is the spatial confinement and control of these effects for safeguarding the original surface from potential damage. Extensive studies have shown that in many cases there is an optimum interplay of laser and material parameters, which result in successful laser cleaning applications. The laser pulse duration is an important parameter in this context. The scope of this work has been the exploration of any advantages that may be offered by using ultrafast UV (248 nm) lasers for the cleaning of sensitive painted artworks. To achieve this goal, comparative study on the ablation rate and threshold of femto- and nanosecond laser pulses of typical varnishes (dammar, mastic, etc.) have been performed. Femtosecond pulses appear to be superior in terms of the spatial resolution and etching resolution, and this fact has been demonstrated for both technical samples and original objects. Additionally, possible induced photochemical modifications have been investigated by monitoring the photoproduct laser-induced fluorescence of varnish-systems doped with low concentrations of well-defined photosensitive dopants (e.g., PhenI). It is established that irradiation with fs UV laser pulses results in minimal photochemical modifications. Importantly, the amount of photochemical products is largely independent from the optical properties (i.e., absorptivity) of the varnish. Considering the recent advances in ultrafast laser technology, the use of such lasers appears to provide a viable approach.