Characterization of ablation conditions with the grayscale representation for Nd:YAG laser to decolor the blue‐ink on marble stone surface: Decoloration dependency on the laser flux power, wavelength, and the water spraying

Abstract

Aiming at artwork restoration application, to decolor the blue‐ink stained marble stone surface was studied for finding out efficient ablation conditions with the use of Q‐switched Nd:YAG laser as the function of laser flux power, wavelength, and its water spray effect. The blue ink mostly consists of indigo dye which absorbs light in the visible region. The decoloration, or decolor efficiency, was evaluated by using the grayscale representation instead of the RGB method. Our experimental results show that the 532 and 355 nm wavelengths are more suitable for blue‐ink decoloration. No decoloration was observed at the 1,064 nm ablation, which is usually regarded as effective on black‐crust stained surfaces. The decolor effect of the water‐sprayed surface was found to be effective at the higher laser flux power. A photo‐absorption‐assisted laser ablation mechanism is proposed in addition to the plasma‐assisted laser ablation mechanism.