ABSTRACT
The soiling of material in the environment implies changes in the surface appearance (colour, transparency, roughness, etc.) that can induce a serious problem for both esthetical and economical point of view. Indeed, once soiling reaches a critical level, objects have to be cleaned or replaced and the economical cost supported is often very high (Rabl 1999, Watt & Hamilton 2003). This assessment is not only valid for historical works of art (monuments, statues, wall paintings, mosaics, stained-glass windows, etc.) but also for modern buildings either private or public.
