13 A Comprehensive Study of the Coloration Effect Associated with Laser Cleaning of Pollution Encrustations from Stonework P. Pouli, G. Totou, V. Zafiropulos, C. Fotakis, M. Oujja, E. Rebollar, M. Castillejo, C. Domingo, and A. Laborde Summary. The application of infrared laser wavelength (at 1064 nm) for the removal of pollution encrustations from stonework is limited in many cases by discol- oration effects, which are commonly known as “yellowing”. The understanding of this effect is crucial for the investigation of cleaning methodologies. Several methodolo- gies have been proposed aiming to avoid or rectify such unpleasant colorations. The most promising ones are based on the combination of a cleaning beam at 1064 nm with an ultraviolet beam at 355 nm, either in spatial and temporal overlapping and in variable fluence ratios (wavelength blending approach ), or in sequential irradiation conditions (sequential approach ). To evaluate the two approaches, a comparative study on two different types of encrustations and substrates was carried out. Thick dendritic crust on limestone and homogeneous thin encrustation on marble have been studied on the basis of their composition and origin, together with the conditions that may induce yellowing effects when laser cleaned with infrared wavelength. The analytical methods employed to characterize the crusts and the effects of laser irradi- ation include colorimetric and optical microscopic observations of surface and cross sections, Confocal Micro-Raman Spectroscopy and FT-Raman. The obtained results have provided a comprehensive approach for understanding the reasons responsible for the discoloration effect, while allowing the establishment of well-defined laser- cleaning methodologies that ensure optimum results without any colour or structural alterations of the original stonework surface. 13.1 Introduction In the last years, the number of monuments affected by air and water pollution has greatly increased [1–3]. The most noticeable examples of stone suffering from the ravaging action of urban pollutants are marble and limestone, in general the main materials of many statues and decorative elements in mon- umental buildings of high artistic and historical value. Air pollution causes black soiling and loss of structural integrity due to attack by atmospheric sulphur in various forms, most noticeable sulphur dioxide and the subsequent formation of gypsum. Sulphur pollution stems from a variety of combustion