Graphic vandalism: preliminary cleaning tests on stone materials by (Yb:YAG) active fiber laser system

Architectural surfaces, façades and monuments made of stone are subject to alteration/decay phenomena due to natural or anthropic agents. Salts, black crust formations, biodegradation and graphic vandalism (writing, drawing and inscription) are responsible for detrimental effects as they can accelerate the stone decay leading to considerable loss in value and significance. Cleaning approaches are usually based on mechanical (e.g. sandblasting) or chemical methods, but they are potentially harmful for operators and the environment. In this paper, the feasibility of a new laser system for the removal of spray paints and felt-tip permanent markers from marble and sandstone is investigated. The laser system INFINITO (by El.En. SpA) is an active fiber laser operating at 1064 nm (Yd:YAG source) consisting of a lightweight and compact instrument which lends a high portability and adaptability to site work. INFINITO permits fine tuning of the laser parameters and allows an automatic scanning of the surface thanks to an effective mirror handling system (i.e. the scanned area size is up to 40x40 mm2 with a lens focal length of 250 mm). This study has been devoted to demonstrate the capability of the system to control the level of cleaning ranging from thinning of the paint to complete cleaning of the surfaces. Consequently, the ablation and thermal effects on the materials were investigated and the evaluation of the damage threshold of the stone substrate to ensure a safe and sensitive cleaning method has been carried out. Laser parameters such as power, fluence, repetition frequency, spot size and scanned area size were varied in a systematic analysis in order to achieve the desired cleaning performance.