Application of semiconductor continuous and Nd:YAG pulsed laser processing for nondestructive cleaning of the historical paper

Preservation of cultural legacy is essential for ensuring its availability for future generations. Long-term indoor storage can sometimes cause the papers to adhere, which can result in the transfer of ink stamps or prints from paper to paper, and subsequently, there emerges the need to remove these types of stains. Laser cleaning emerged as a promising technique for paper surface cleaning. This work presents the laser cleaning of a historical paper dated to the end of the 19th century using the Nd:YAG laser in a nanosecond pulsed regime, at two wavelengths, 532 and 1064 nm, and a semiconductor laser in a continuous regime, at a 552 nm wavelength. The cleaned areas are observed by optical microscopy, scanning electron microscopy, energy-dispersive spectroscopy, and spectrophotometry. The obtained results are discussed in terms of the laser cleaning efficiency characterized by several aspects. The results obtained by green and near-IR laser irradiation in pulsed and continuous regime are compared, showing favorable effects regardless of the laser wavelength. However, in some cases, laser cleaning at 1064 nm wavelength damaged the surface layer of the paper. Cleaning at 532 nm wavelength did not completely remove the ink, and the ink traces were visible under the microscope or spectrometer. The proposed efficient cleaning method for stain, dust, and stamp ink removal from historical papers includes a semiconductor laser at 552 nm, owing to the continuous action of its beam.