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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. [ABSTRACT FROM AUTHOR]

[Display omitted] • Femtosecond pulse laser cleaning of Makrana marble. • Comparison of laser wavelength from infrared, green and ultraviolet irradiations. • Laser cleaning with ultraviolet irradiation presents high risk of melting the stone. • Infrared and green wavelengths are safer for laser cleaning. • Successful cleaning of contaminants and dirt using femtosecond pulse laser. We studied femtosecond pulse lasers for cleaning Makrana marble, used in the construction of Soami Bagh Samadh, Agra, India. We investigated laser irradiation at different wavelengths (1029 nm, 515 nm, and 343 nm) and found that ultraviolet radiation had the lowest ablation threshold at 0.23 J cm−2, followed by green at 0.34 J·cm−2, and infrared at 1.3 J·cm−2. Green irradiation was the most efficient, ablating marble at 0.9 mm3·(min W)–1 at 4 J·cm−2 compared to 0.73 mm3·(min W)–1 at 12 J·cm−2 for 343 and 1029 nm. We evaluated the wavelength's effect on the stone. Infrared irradiation, having a higher ablation threshold, allowed for a broader range of laser fluence for the safe removal of contaminants, without risking inducing any chemical or morphological changes to the stone, and thus was selected for cleaning. We explored the cleaning effectiveness of infrared femtosecond laser pulses on a sculpted Makrana marble covered by environmental soiling. Optical and scanning electron microscopy, optical profilometry, Raman spectroscopy, and colorimetry demonstrated that satisfactory cleaning was achieved, with the removal of the unwanted layers without causing damage or chemical alteration to the marble structure. This study demonstrates the potential of femtosecond lasers for safe and effective heritage marble cleaning. [ABSTRACT FROM AUTHOR]