Laponite micro-packs for the selective cleaning of multiple coherent deposits on wall paintings: The case study of Casina Farnese on the Palatine Hill (Rome-Italy)

• Coherent deposits insoluble with usual methods were treated with bacteria.
• Organic deposits were degraded by Stenotrophomonas maltophilia UI3E.
• Gypsum, carbonates and phosphates were solubilised by Cellulosimicrobium cellulans TBF11E and Pseudomonas koreensis UT30E.
• Laponite micro-packs containing individual strains were applied singly or in succession.
• The procedure was selective and safe for operators, wall paintings and the environment.

As a result of the research described in this article a safe, bio-based procedure has been established to treat hard-to-remove coherent deposits composed of gypsum, weddellite, calcium carbonate, apatite, nitrate, and aged proteinaceous matter from the wall paintings of the lower loggia of the Casina Farnese (Palatine Hill, Rome). Following a laboratory screening, three bacterial non-spore-forming strains were selected from our laboratory collection to solubilise calcium sulphate and carbonate (Cellulosimicrobium cellulans TBF11E), to degrade proteins (Stenotrophomonas maltophilia UI3E), to solubilise inorganic compounds and to degrade protein material (Pseudomonas koreensis UT30E). The living bacterial cells were suspended in a Laponite gel, which is highly compatible with the survival of the cells and is also easily applied, and removed from, vertical walls. Compresses containing microorganisms (micro-packs) were used in a series of biocleaning tests carried out in situ, from July 2012 to February 2013, in temperatures ranging from 6 °C to 37 °C. Each micro-pack contained a single bacterial strain (TBF11E, UI3E or UT30E). The micro-packs containing the different bacteria were applied, individually or in succession, depending on the nature and layers of the deposits to be removed. Contact times of between 24 and 48 h were established in accordance with the advice of the restorers. The strain TBF11E removed the inorganic darker layer, UI3E dissolved the brownish layer (probably aged casein) and UT30E removed the mixed deposits. No residues were left after the treatment and the restorers successfully completed the restoration.