Bio-Removal of Black Crust from Marble Surface: Comparison with Traditional Methodologies and Application on a Sculpture from the Florence's English Cemetery

In the present study we investigate the use of sulphate-reducing bacteria during the black crust's bio-removal process. The aim of our work is to compare microbial cleaning with more traditional cleaning methods (chemical and laser cleaning) and to discuss the pros and cons of this new technique. The research builds in two steps: a first experimental phase examining the process on stone materials coming from the Cathedral of Florence, on a Carrara marble column taken from the Baccio d’Agnolo Balcony under the Dome by Brunelleschi (treated in our laboratory) and on an external pilaster (treated in situ) on the left side of the Cathedral made out three different stone typologies: green serpentine, red marlstone and white Carrara marble; a second application phase on a marble sculpture from 19th century representing the Allegory of Death from Florence's English Cemetery. The effects of the different procedures on the original surfaces were evaluated by a scanning electron microscopy coupled with energy dispersive X-ray (SEM/EDS) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and colour measurements. Results prove that microbial cleaning is the most controllable process and it has a great efficiency in sulphate removal. These techniques have also a great ecological importance as bacteria reproduce the same biological process achieved in natural contests.