Experimental and theoretical study of the Nd: YAG laser removal of beeswax on Galician granite at 355 nm

Beeswax coatings applied to seal off the granitic surfaces of many monuments have resulted in detrimental effects with time passing. Conservation procedures must be carried out in a selective way, removing the beeswax without any degradation of the stone.

In this study we present an experimental analysis of the removal of beeswax using a Nd:YAG laser at a wavelength of 355 nm which is absorbed weakly by the beeswax. For this reason, an important percentage of the laser radiation reaches the granite substrate and the influence of the layer thickness must be studied. At each laser fluence, for single-pulse irradiation we find a maximum thickness for complete removal of the film. For thicker layers, the beeswax is not removed and additional effects such as color changes and thickness increase are observed. The experimental results suggest that a photomechanical mechanism is dominant and that granite absorption plays a fundamental role. Multiple-pulse irradiation is also performed to determine the optimal parameters for laser cleaning.

Finally, infrared thermography measurements allow for the determination of the granite surface temperature after laser irradiation and theoretical calculations allow one to estimate the absorption behavior of granite at 355 and to explain the results obtained.