Evaluation of femtosecond laser pulse irradiation of ancient parchment Malgorzata Walczak a , Mohamed Oujja a , Luis Crespo-Arca ´ b , Ana Garcı ´a c , Cruz Me ´ ndez c , Pablo Moreno c , Concepcio ´ n Domingo d , Marta Castillejo a, * a Instituto de Quı´mica Fı´sica Rocasolano, CSIC, Serrano 119, 28006 Madrid, Spain b Archivo Histo ´rico Nacional, Serrano 115, 28006 Madrid, Spain c Servicio La ´ser, Universidad de Salamanca, Plaza de la Merced s/n, 37008 Salamanca, Spain d Instituto de Estructura de la Materia, CSIC, Serrano 123, 28006 Madrid, Spain 1. Introduction In the last decades lasers have been incorporated in the preservation of cultural heritage, first to determine the composi- tion and the internal structure of objects and secondly for restoration, including cleaning [1,2]. Techniques such as laser induced fluorescence (LIF), laser induced breakdown spectroscopy (LIBS), Raman spectroscopies, optical coherent tomography (OCT), interferometries, holography, laser scanning, etc., have been incorporated in various extents into the practice of conservation, preferably when they are non-destructive, can be used in situ or developed as portable systems. Laser systems and techniques are also used for the restoration of objects or artistic surfaces in cleaning applications. In those, laser irradiation of the surface of the object aims at removing unwanted or degraded material with precision and control while preserving the texture and physico- chemical properties of the original substrate. The laser parameters required and the mechanisms involved in the interaction with the material are dependent on the type of substrate. Laser cleaning of architectural surfaces and statues is a well developed procedure, while laser cleaning of photosensitive molecular based materials is still a subject of investigation in various laboratories. Successful implementation of laser based techniques in the field of cultural heritage relies in understanding the interaction between laser light and the materials which constitute the object or artwork. Molecular substrates are encountered in art and historical objects such as paintings, ancient parchments, paper documents and textiles. Laser cleaning of paintings [3] consists in the elimination of the outermost oxidised varnish layers or overpaints through a laser ablation process in which thermal, photochemical and photomechanical mechanisms contribute in various extents. On the other hand, laser cleaning of fibrous substrates such as parchment, documents and textiles involves the selective removal of impurities via essentially a thermal evaporation process [4–9]. Parchment is a writing material made from the skin of sheep or calf, used as far back as 200 B.C., commonly employed for books, codex or manuscripts and also for painting, printing and book- binding [10]. The chemical composition of parchment is collagen, a natural biopolymer with relative molecular mass of 350 kDa. Restoration procedures for ancient parchment include elimination Applied Surface Science 255 (2008) 3179–3183 ARTICLE INFO Article history: Received 13 May 2008 Received in revised form 5 September 2008 Accepted 6 September 2008 Available online 12 September 2008 PACS: 42.62.b 61.80.Ba 61.82.Pv Keywords: Femtosecond laser irradiation Laser cleaning Polymers Parchment ABSTRACT In this work, femtosecond (fs) laser irradiation of ancient parchment is evaluated as a novel cleaning technique that, in comparison with nanosecond (ns) irradiation, could considerably reduce the thermal and chemical damage to the substrate. To investigate the interaction of ultrashort laser pulses with parchment, two historical specimens were exposed to Ti:Sapphire laser using the fundamental and second harmonic emission (795 and 398 nm, 120 fs pulses). The ablation threshold fluences were determined resulting in values of 1.1 and 0.15 J/cm 2 at 795 and 398 nm respectively. A study of the effects of the fs laser–parchment interaction as a function of fluence and wavelength was carried out and the results were compared with those obtained for Nd:YAG laser irradiation (532 and 355 nm, pulse duration 6 ns). FT-Raman spectra show the disappearance of the amorphous carbon bands upon fs treatment, indicative of the removal of the carbonaceous contamination of the parchment. At the same time the parchment structural bands remain unaffected. Spectrofluorimetric measurements reveal an enhancement of photodegradation products in the substrate. Results are discussed in terms of the comparison between expected mechanisms operating in the fs and ns irradiation regimes. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +34 91 5619400; fax: +34 91 5642431. E-mail address: marta.castillejo@iqfr.csic.es (M. Castillejo). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.09.011