On-line monitoring of laser cleaning of limestone by laser-induced breakdown spectroscopy and laser-induced fluorescence

The application of laser-induced breakdown spectroscopy (LIBS) to monitor the laser cleaning process of polluted limestone from a historic building is examined. The combination of a Q-switched Nd:YAG pulsed laser with on-line diagnostics by the LIBS technique is shown to be very useful for controlling and characterizing the cleaning process in order to avoid overcleaning. In addition, the coupling of this spectroscopic technique to the cleaning process provides important information about the optimal experimental conditions to be selected for achieving an adequate cleaning procedure. Furthermore, the spectroscopic study of the plasma emission can be used to determine the elemental composition of both the black crust and the underlying stone. The application of LIBS as a diagnostic technique to monitor and control the laser cleaning process of limestone is based on the different elemental composition of the black encrustations covering the stone surface and the underlying stone. On the other hand, a different experimental setup for probing the ablation products by laser-induced fluorescence (LIF), in order to achieve a signal amplification of some atomic emission lines with weak intensity in the LIBS spectrum, is described.