Comparing the bioremoval of black crusts on colored artistic lithotypes of the Cathedral of Florence with chemical and laser treatment

The external walls of the Cathedral of Florence are made of green serpentine, red marlstone and Carrara white marble, and intensive air pollution attack has led to their weathering, which caused black crust formation. A study was performed to evaluate the most appropriate cleaning treatment for black crust removal, adopting chemical (ammonium carbonate poultice), laser (1064 nm, Nd:YAG laser), and microbial (poultice embedding sulfate-reducing bacteria) cleaning. The effects of the different procedures on the original surfaces were evaluated by scanning electron microscopy coupled with energy dispersive X-ray (SEM/EDS) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and color measurements. One year later further color measurements were made. It was found that chemical cleaning led to non-homogeneous crust removal and that for the extremely powdery and incoherent red substratum the preferred treatment was laser cleaning. Overall, the most satisfactory treatment was the microbial cleaning process. It was the most controllable process and the most efficient for sulfate removal. Its main drawback appears to be the time needed to remove thick black crusts since numerous applications were necessary.