Optimisation and on-line acoustic monitoring of laser cleaning of soiled paper |
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Authors: | M Strlič VS Šelih J Kolar D Kočar B Pihlar R Ostrowski J Marczak M Strzelec M Marinček T Vuorinen LS Johansson |
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Institution: | (1) Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5, 1000 Ljubljana, Slovenia;(2) National and University Library, Turjaška 1, 1000 Ljubljana, Slovenia;(3) Institute of Optoelectronics, Military University of Technology, 2 Kaliskiego Street, 00-908 Warsaw, Poland;(4) Fotona d.d., Stegne 7, 1000 Ljubljana, Slovenia;(5) Helsinki University of Technology, P.O. Box 6300, Helsinki, 02015 HUT, Finland |
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Abstract: | Laser cleaning is an attractive and advanced technique of cleaning cultural heritage materials. However, in the case of paper-based substrates, it may lead to surface discoloration and long-term destabilisation of the cellulosic component. The origin of discoloration during pulsed laser removal (Nd:YAG laser, 532 and 1064 nm) of carbonaceous soiling was studied using UV resonance Raman spectroscopy and X-ray photoelectron spectroscopy. It was concluded that macromolecular conjugated systems are the most probable cause of discoloration, formed as a consequence of thermolytic reactions. With chemiluminometry, we analysed the processes following irradiation. Using photo-ageing and thermally accelerated ageing, it was indicated that, even at fluences lower than the ablation threshold of cellulose, long-term destabilisation of the material should be taken into account. By using much lower laser shot repetition rates than usual, i.e. 1 s-1, we can allow the substrate to cool between consecutive laser shots, thus minimising the adverse effects of laser treatment. Acoustic monitoring was shown to be an affordable possibility for on-line process control. The amplitude of shock waves was shown not to be wavelength dependent (532 or 1064 nm); however, the parameter can be used to monitor the progress of cleaning. PACS 42.62.-b; 61.80.Ba; 83.80.Mc; 78.20.Hp; 68.37.Xy |
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