When Conservation scientists casted a glance at the frescoes decorating the cupola of the ‘Beata Vergine del Pilone’ Sanctuary in Polonghera, near Cuneo (Italy), they scarcely could believe their eyes. Those paintings, dating back to the 18th Century, looked as if an unknown vandal purposely covered the skin of most religious characters with brown paint. A couple of cherubs even reminded of the exhortation in the famous “Angelitos Negros” song: “Painter, born in my land… though the Virgin may be white, paint me some little black angels, for they go to heaven too, as all good black people do.” (Fig. 1.a). This transfiguring effect was the result of a bio-chemical deterioration due to the growth of a viscous, brownish biofilm selectively covering those frescoes areas painted in pink. To study this phenomenon, a multi-disciplinary approach was adopted including both minero-chemical methods and DNA-sequencing techniques. The former allowed to characterize those materials used by the original artist(s) as well as the components responsible for their chemical deterioration, together with the by-products of microbial activity; the latter brought to identify those microbial species responsible for the sequential colonization steps. Water played a key-role in both chemical and biological aspects. Dampness percolating from outside, due to cracks in the cupola walls or capillarity, brought pollutants such as H2S and SO2 on the frescoes surface thus triggering formation of gypsum-sulphurous crusts. This fact, coupled to the particular composition of these frescoes pinkish pigments – a mixture of Cinnabar (HgS) and Zinc White (ZnO2) – caused significant S concentrations to occur in selected areas which, in presence of catalysts (i.e. Zn and other metals), favoured colonization of a first-generation sulphate-cycling bacteria. The dead bodies of these bacteria and abundant moisture, in turn, supplied those nutrients necessary to allow the settlement of a second-generation microbial community, represented by scavenger bacteria and saprophyte fungi. These late colonizers were responsible for the biofilm development, selectively covering the flesh-coloured areas. These frescoes suffered therefore both an aesthetic and a structural damage, the first related to the biofilm growth and the second to pigments alteration caused by mobilization of certain elements recycled in the by-products of microbial activity (i.e. sulphates). The performed survey paved way for a restoration intervention, achieved by delicately dry-cleaning the frescoes surface and applying a proper biocide which allowed the effective biofilm extirpation while possibly preserving the vividness of the residual pigmented layers.

Biodegradation of frescoes in the “Beata Vergine del Pilone” Sanctuary, Polonghera (Italy)

GIUSTETTO, Roberto;BIANCIOTTO, VALERIA;LUMINI, ERICA;VOYRON, Samuele;COSTA, Emanuele;DIANA, Eliano;
2014

Abstract

When Conservation scientists casted a glance at the frescoes decorating the cupola of the ‘Beata Vergine del Pilone’ Sanctuary in Polonghera, near Cuneo (Italy), they scarcely could believe their eyes. Those paintings, dating back to the 18th Century, looked as if an unknown vandal purposely covered the skin of most religious characters with brown paint. A couple of cherubs even reminded of the exhortation in the famous “Angelitos Negros” song: “Painter, born in my land… though the Virgin may be white, paint me some little black angels, for they go to heaven too, as all good black people do.” (Fig. 1.a). This transfiguring effect was the result of a bio-chemical deterioration due to the growth of a viscous, brownish biofilm selectively covering those frescoes areas painted in pink. To study this phenomenon, a multi-disciplinary approach was adopted including both minero-chemical methods and DNA-sequencing techniques. The former allowed to characterize those materials used by the original artist(s) as well as the components responsible for their chemical deterioration, together with the by-products of microbial activity; the latter brought to identify those microbial species responsible for the sequential colonization steps. Water played a key-role in both chemical and biological aspects. Dampness percolating from outside, due to cracks in the cupola walls or capillarity, brought pollutants such as H2S and SO2 on the frescoes surface thus triggering formation of gypsum-sulphurous crusts. This fact, coupled to the particular composition of these frescoes pinkish pigments – a mixture of Cinnabar (HgS) and Zinc White (ZnO2) – caused significant S concentrations to occur in selected areas which, in presence of catalysts (i.e. Zn and other metals), favoured colonization of a first-generation sulphate-cycling bacteria. The dead bodies of these bacteria and abundant moisture, in turn, supplied those nutrients necessary to allow the settlement of a second-generation microbial community, represented by scavenger bacteria and saprophyte fungi. These late colonizers were responsible for the biofilm development, selectively covering the flesh-coloured areas. These frescoes suffered therefore both an aesthetic and a structural damage, the first related to the biofilm growth and the second to pigments alteration caused by mobilization of certain elements recycled in the by-products of microbial activity (i.e. sulphates). The performed survey paved way for a restoration intervention, achieved by delicately dry-cleaning the frescoes surface and applying a proper biocide which allowed the effective biofilm extirpation while possibly preserving the vividness of the residual pigmented layers.
International Symposium on Archaeometry (ISA 2014)
Los Angeles USA
19-23 May 2014
Book of Abstracts - International Symposium on Archaeometry (ISA 2014)
ScientiCommittee of International Symposium on Archaeometry (ISA )
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R. GIUSTETTO; D. GONELLA; V. BIANCIOTTO; E. LUMINI; S. VOYRON; E. COSTA; E. DIANA; S. CONTE
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2318/157298
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