This study compares two cleaning methods, one involving an ammonium carbonate-EDTA mixture and the other involving the sulfate-reducing bacterium Desulfovibrio vulgaris subsp. vulgaris ATCC 29579, for the removal of black crust (containing gypsum) on marble of the Milan Cathedral (Italy). In contrast to the chemical cleaning method, the biological procedure resulted in more homogeneous removal of the surface deposits and preserved the patina noble under the black crust. Whereas both of the treatments converted gypsum to calcite, allowing consolidation, the chemical treatment also formed undesirable sodium sulfate.Cleaning is part of the conservation and maintenance program of historical buildings and under current guidelines (1, 3, 11) is described as the removal of harmful deposits from the building fabric to help prevent future decay. Until now, the lack of a shared protocol for in situ evaluation of the cleaning process has often led to the use of cleaning procedures inconsistent with the goals of preservation of monuments. Vergès-Belmin (23) proposed the following criteria for the selection and assessment of a cleaning method: (i) conservation of the patina noble (which imparts an aged character to a surface and has a preservative function); (ii) physical and chemical harm; (iii) homogeneity of removal of the surface deposits; (iv) persistence of the cleaned state; (v) efficiency of cleaning; (vi) color; and (vii) esthetics.Black crust is a deteriorated surface layer of stone material spontaneously formed from the interaction between a calcareous substrate and the polluted atmosphere in a humid environment and in areas sheltered from rainfall (7,14,19). The chemical transformation of the substrate (calcite) into gypsum, together with the deposition of mineral and smog particles, can be assumed to be the mechanism of formation (10). Traditionally, to carefully control the removal of black crust and its effects, only chemical treatment was considered (20). In recent decades, an alternative cleaning technology employing sulfatereducing bacteria (SRB), in particular Desulfovibrio desulfuricans, has been proposed by Atlas et al. (2) and Gauri and Chowdhury (9). These authors reported that D. desulfuricans converted gypsum to calcite, suggesting that this was both a cleaning treatment and a conservation treatment. Recently, this biotechnology has been optimized to avoid sulfide precipitation by employing an improved delivery system and using the aerotolerant strain D. vulgaris subsp. vulgaris ATCC 29579 (4, 16). Although in their relevant review Webster and May (24) claimed that bioremediation of sulfates is very promising and has received much attention, they also asserted that the risks posed by this technology are still to be addressed and that its advantages and limitations over other cleaning procedures have not yet been fully elucidated.The aim of this research was to compare the efficacy of and harm caused by a traditional chemical cleaning procedure with the efficacy of and harm caused by a recently improv...

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