Synchronous Use of IR and UV Laser Pulses in the Removal of Encrustation: Mechanistic Aspects, Discoloration Phenomena and Benefits

Abstract

The removal of encrustation from sculptures and monuments has been by far the most pronounced application of lasers in LACONA field. Considerable effort has been put in recognizing the operative ablation mechanisms when using different laser wavelengths and power levels, as well as in understanding the associated “yellowing” phenomenon. The deeper insight into the laser-matter interactions for different laser parameters as well as the understanding of side discoloration phenomena has led us in combining IR and UV laser pulses with excellent results. More specifically, the fundamental and the third harmonic of a Q-Switched Nd:YAG laser has been spatially and temporally combined to create a double pulse. The power levels must be fixed at a certain ratio for the two wavelengths in such a way, that the different operative removal mechanisms (mainly selective vaporization for IR- and spallation for UV-pulses) take action at the same depth from the surface of encrustation. The resulted surface after the final removal of encrustation lacks the yellowish hue, which is present when only IR pulses are used. A theoretical model has been applied to account for the different phenomena observed with the synchronous use of laser pulses as well as the absence of yellowing. This novel methodology has been applied to Parthenon West Frieze with success (see parallel case study contribution by Pouli et al.).