Appl Phys A (2012) 106:447–464 DOI 10.1007/s00339-011-6696-2 INVITED PAPER Practical issues in laser cleaning of stone and painted artefacts: optimisation procedures and side effects Paraskevi Pouli · Mohamed Oujja · Marta Castillejo Received: 25 March 2011 / Accepted: 7 November 2011 / Published online: 1 December 2011 © Springer-Verlag 2011 Abstract In the last twenty years lasers have acquired an important role in the study and the preservation of Cul- tural Heritage (CH) objects and Monuments, as they have effectively illuminated a number of complex diagnostic and restoration problems. Their unique properties have enabled their use in a wide range of conservation applications, since they ensure interventions with precise control, material se- lectivity and immediate feedback. Surface cleaning, based on laser ablation, is a delicate, critical and irreversible process, which, given the multitude of materials that may be present on a CH object and the of- ten fragile or precarious condition of the original surfaces, is fraught with many potential complications. Therefore it is crucial to choose the best possible laser cleaning method- ology for each individual case, which involves optimising the laser parameters according to material properties, as well as the thorough knowledge of the ablation mechanisms in- volved. In this context the systematic investigation and elu- cidation of potential damage or side effects occurring upon cleaning is essential, as it delineates the possibilities and P. Pouli ( ) Institute of Electronic Structure and Laser (IESL), Foundation for Research and Technology-Hellas (FORTH), P.O. Box 1385, 71110 Heraklion, Crete, Greece e-mail: ppouli@iesl.forth.gr Fax: +30-2810-391318/05 M. Oujja · M. Castillejo Instituto de Química Física Rocasolano, CSIC, Serrano 119, 28006 Madrid, Spain M. Oujja e-mail: m.oujja@iqfr.csic.es M. Castillejo e-mail: marta.castillejo@iqfr.csic.es limitations of laser ablation and allows the fine-tuning of the operating parameters for a successful cleaning intervention. This paper is an overview of studies investigating the mechanisms which are responsible for the laser-induced dis- coloration effects. Emphasis is given on the yellowing col- oration observed on stonework upon infrared (IR) ablation of pollution encrustations, while the various theories intro- duced to approach the different physical and/or chemical processes and mechanisms responsible for such side effects are discussed. In this respect the different laser cleaning methodologies, which are based on the use of laser systems with different pulse durations and wavelength characteris- tics, introduced in order to rectify or prevent discoloration on stonework are presented. In parallel, the darkening phe- nomena which occur upon laser irradiation of painted sur- faces are also considered. Studies on series of model paints performed in order to understand the sensitivity of pigments to laser irradiation are critically reviewed. In this respect the importance of the optimal wavelength and pulse-duration se- lection for a safe and controlled laser cleaning intervention is also addressed. 1 Introduction Cultural Heritage (CH) objects and monuments (exhibited indoors and/or exposed outdoors) are subject to ageing and to changing environmental conditions so that they undergo different modifications which endanger their life and in- tegrity. Their preservation requires careful interventions, which may encompass surface cleaning (the removal of un- wanted and non-original material) and consolidation (sta- bilisation or fixing). Due to their nature and irreversibility, these interventions are exceptionally complex and delicate and thus call for particular control and selectivity. In fact,