Laser cleaning is a gift to the physics community to help preserve historic buildings. Air pollution, specifically sulfuric acid aerosols, interacted with the calcium carbonate statue, forming a layer of plaster composed of calcium sulfate and water. Laser cleaning can be used to remove film deposits and contaminants such as sulfates and nitrates from marble structures. Figure 1 below shows a statue with black deposits on it.
When comparing laser cleaning to other stone cleaning methods, the former stands out for its accuracy, touchless implementation, and versatility. Widespread use of laser cleaning was initially inhibited because of suspicions that it was a complex method and that there were limitations in accessing original monuments for testing.
However, improvements in models of laser pulse interaction with stone and technological advances in laser systems have overcome these initial doubts, and laser cleaning has become a front-runner in cleaning historic monuments.
Therefore, to address the first two questions posed at the beginning of this article, we can answer that pulsed laser systems, such as the 1064 nm Nd:YAG laser, and their harmonic wavelengths are ready for stone cleaning.
Other types of pulsed lasers with adjustable pulse widths can also be used.
As for the advantages of laser cleaning over sandblasting, scrubbing, chemical methods and ultrasonics, it can be said that it is the non-contact method and the accuracy of removing deposited films.
To answer all of the related third, fourth, and fifth questions, one must consider the nature of the laser pulse's interaction with the stone. The interaction of laser pulses with stone can be divided into self-limiting and non-self-limiting laser ablation processes. In the case of self-limiting laser ablation processes, the damage threshold of the deposited contamination film is much lower than that of the underlying substrate.
On the other hand, for non-self-limiting laser ablation processes, the damage thresholds of the substrate and film are very close to each other.
Therefore, in the former case, the cleaning process will be much easier, because if appropriate laser pulse energy and duration are used (the term laser fluence is widely used, which means energy per unit area, in Joules/cm2), Then the pollutant film will be removed. Evaporates before causing damage to the underlying substrate. For non-self-limiting cases, the likelihood of damage is much higher.
Many workers in the field of laser cleaning for art restoration utilize fiber lasers because of their flexibility and ability to shine the beam into hard-to-reach locations on statues. The act of laser cleaning is a work of art in itself, requiring a skilled operator and fiber laser to achieve flexibility. Lasers are mostly of the industrial type, and laser systems have no moving parts. If you use a MOPA (Master Oscillator Power Amplifier) laser, you get tunable laser pulses and no crystal aging like Nd:YAG lasers. Figure 3 shows an operator using a fiber laser to clean artwork.
One could also point out that using lasers for stone cleaning is healthier for the environment because the process doesn't produce as much particulate matter as sandblasting. The operator will of course need to wear laser protective glasses.
