High efficiencies for laser cleaning of glassware irradiated from the back: application to glassware historical objects

We present a systematic study of laser cleaning of black paint deposited on both standard and frosted glasses. We performed laser cleaning of black paint layers of different thicknesses in both front- and backside laser irradiation geometries. Using laser ablation induced photoacoustics (LAIP), we determined the ablation threshold of the paint that turns out to be independent of the paint thickness and substrate’s properties. To characterize the efficiency of the cleaning process as a function of the number of laser shots, we measured the transmission of the glass in the ablated region and simultaneously the amplitude of the acoustic signal generated by the ablation process. We show that laser cleaning is much more effective when the glass sample is irradiated from the back. To explain this effect, we propose a phenomenological model. This model also predicts the existence of a critical thickness, above which backside cleaning is no longer efficient. The method of back incidence laser cleaning was successfully applied to two real objects, namely a piece of advertising glass covered with black paint and an antique glass bottle with black dirt inside, both archeological objects founded in excavations made in the city of Buenos Aires.