Acoustic monitoring for the laser cleaning of sandstone

Abstract

For the laser ablation of crust on historical sandstone samples contaminated due to prolonged interaction with the environment, and also for specimens covered by artificial crust layers, the potential of acoustic monitoring is examined. Measurements of the snapping sound amplitude vs. the deposited laser energy carried out for dry, moistened and wet samples at laser fluences in the range of 0.1-3 J cm-2 (Nd:YAG, 6 ns, 1064 and 532 nm) confirm the advantages of wet cleaning. The exponential decay of the signal corresponds to a similar decrease of the crust thickness, characterized by an average rate of about 10-14 μm per pulse, and the data of original samples reveal better reproducibility compared to those of the model crust. From data analysis, a narrow band of the reference signal of 8.5-11% of the maximal one follows, which corresponds to the crust-free surface, and for parabolic dependences of both sound amplitude and cleaning speed vs. laser fluence, the clear maxima agree with optimal processing parameters. The strong correlation observed between the acoustic signal and the ablation progress supports the conclusion of the usefulness of acoustic monitoring for laser cleaning of stone artefacts. © 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.