Laser cleaning of graffiti from building stone using high power diode laser

Regrettably, graffiti are a frequent aggression to historic buildings. Particularly, Morelias historic centre (Mexico) (appointed World Heritage Site by UNESCO) suffers, for some years, a high number of graffiti. A local igneous rock called Pink Morelia Quarry is the building stone in most of this architectural heritage. The cleaning methods conventionally employed to remove these graffiti lead to serious damaging to the appearance of these monuments. In this paper, we present the results of a study on the feasibility to remove the graffiti from this natural stone using a high power diode laser treatment. An extensive experimental analysis of the operating conditions has been carried out leading to successful results. The optimal parameters to achieve a total removal of graffiti without side effects in the stone have been determined. We concluded that continuous wave regime leads to better results than modulated wave regime. Finally, a two laser passes process demonstrated a high performance improvement with regard to any combination of parameters in the single pass process.