Time-resolved imaging for investigating laser-material interactions during laser irradiation cleaning on murals

The surface quality of murals degraded seriously after long-term contamination. The existing cleaning methods are challenging to remove these contaminants without damaging the mural surface. The objective of this study was to eliminate the contaminants on the surface of murals by nanosecond pulsed laser irradiation. We adopted the apparatus of the Nd: YAG laser with a Q-switch (wavelengths lambda = 355 nm and 1064 nm) to study the cleaning characteristic of the mural. The time-resolved imaging of the laser interaction process between contaminants and mural surfaces was visualized through pump-probe detection technology to elucidate cleaning mechanisms. Results showed that laser cleaning was suitable for cleaning the soot and dust contaminants on the surface of white lime, but it generated irreversible damage to the surface when eliminating these contaminants on the pigment layer. The control method of damage suppression and the safe range of laser parameters for different surface materials was obtained by investigating acoustic wave propagation and statistics of ejecting particles. The element analysis was carried out by fourier transform infrared spectroscopy and X-ray diffraction, which indicated that laser irradiation did not affect the chemical characteristics of the mural surface. This investigation provides a comprehensive understanding of laser cleaning, which is significant to developing cultural heritage protection technology.