Transfiguring biodegradation of frescoes in the "Beata Vergine del Pilone" Sanctuary in Polonghera (Italy): microbial analysis and minero-chemical aspects.

When Conservation scientists casted a glance at the inner cupola of the 'Beata Vergine del Pilone' Sanctuary in Polonghera (Italy), they scarcely could believe their eyes. Some 18th Century frescoes looked as if an unknown vandal covered the skin of religious characters with brown paint. This disquieting transfiguration was due to a viscous patina, grown on those areas painted in pink-to-reddish. To study this atypical phenomenon, both minero-chemical and DNA-sequencing techniques were adopted. The obtained outcomes allowed to reconstruct the biocolonization sequence and related chemical deterioration. H2S/SO2 vehiculated by dampness triggered formation of gypsum crusts on the painted surface. High S concentration, coupled with presence of Cinnabar, Zinc White and Barium White in pink/reddish areas, probably favoured settlement of a first-generation sulphate-cycling bacteria which caused Zn/Ba mobilization as sulphates/sulphides byproducts. These bacteria dead bodies supplied those nutrients necessary for the growth of a second-generation scavenger bacteria and saprophyte fungi, responsible for biofilm development. These frescoes suffered both an aesthetic and structural damage, due to the transfiguring patina growth and pigment alteration after metals mobilization in microbial metabolites, respectively. The scientific survey paved the way for a restoration intervention, which allowed biofilm extirpation preserving the residual pigments vividness.