Femtosecond pulse laser cleaning of biofilm and dirt: Preserving the Sydney Harbour Bridge

We investigated the cleaning of biofilms and general dirt soiling from the granite of the pylons of the iconic Sydney Harbour Bridge using a femtosecond pulse laser. We demonstrated that this laser emitting pulses at 1029 nm was successful to restore the surface of the stone without damaging the sensitive minerals. The effectiveness of the treatment and the effects on the substrate were evaluated using optical microscopy, infrared spectroscopy, and colorimetry. We showed that satisfactory cleaning results can be achieved at a laser energy of 1.0 J·cm^-2, below the damage threshold of the stone. Biofilms and environmental soiling were completely removed from the stone surface after five to seven laser scans, and no residues were found after treatment, even at cracks and grain boundaries. This study illustrates the ability of femtosecond pulse lasers to clean heritage stonework without damaging the underlying substrate.