Towards safe and effective femtosecond laser cleaning for the preservation of historic monuments

We explore femtosecond laser cleaning of materials used in the construction of historic monuments, such as stone and steel covered in typical contaminants caused by harsh environments that may be found in urban areas. We address the cleaning of these materials from a conservation perspective, taking as examples the preservation and cleaning of iconic structures such as the steel and the granite of the Sydney Harbour Bridge, Hawkesbury sandstone, a popular building material of a variety of monuments in Sydney (Australia), Makrana marble taken from the Soami Bagh Samadh temple of Agra in India, and also graffiti removal. We demonstrate that femtosecond laser pulses can clean a range of different contaminants such as biofilm, environmental soiling, rust, and spray paints, while preserving the integrity of the underlying substrates. Femtosecond laser cleaning is a fast and effective method and a safer alternative to lasers with longer pulse durations for the preservation of historic monuments.