BIOPATINA

The objectives are to propose an alternative biological treatment for the protection of copper-alloyed archaeological objects or outdoor sculptures. The project relies on converting existing corrosion patinas into copper oxalates—taking advantage of the unique properties of carefully selected fungal species. In fact, thanks to their insolubility and stability even in acidic atmosphere, copper oxalates allow to provide the treated surfaces with long-term protection and low aesthetical alteration.

During the EU-ARTECH project (2004-2009), a strain of Beauveria bassiana isolated from vineyard soils highly contaminated with copper showed the best performance with almost 100% of conversion from copper hydroxysulphates and hydroxychlorides.

ESEM observations: a) culture of B. bassiana on a copper-enriched media showing copper oxalates as rosette aggregates. Copper oxalates formed on coupons corroded in b) urban environment and c) costal environment. MSc A. Simon (2007) “Etude et optimisation de la formation fongique d’oxalate de cuivre sur du vert-de-gris en vue de la protection de monuments".

The efficiency of the innovative treatment developed on bronze monuments was further investigated within the BAHAMAS project (Biological patina for ArcHaeological and Artistic Metal ArtefactS), 2010-2012. In particular, the newly formed copper oxalates were in-depth characterized in order to define their properties (ex., formation mechanisms, adhesion), either on copper-enriched media or on corroded coupons. Cross-section examination suggested that the first micrometres of the corrosion layer are completely converted into copper oxalates, as showed in the figure below.

ATR-FTIR imaging after treatment of a cross-sectioned coupon corroded in urban environment: a) visible light photomicrograph with the selected area for the ATR analyses indicated in red. Chemical images representing b) the original corrosion alyer composed of brochantite (region of interest 1105 - 1085 cm^-1), c) the newly formed copper oxalates (peak area 1320 cm^-1), and d) the embedding resin (region of interest 1735 - 1715 cm^-1).

After initial successful attempts, the efficacy of the fungal treatment is validated against ageing procedures in climatic chamber and natural exposure sites (Innosuisse-previously CTI in 2013-2014, Gebert Rüf Stiftung in 2013 - 2016 and Stiftung zur Förderung der Denkmalpflege in 2015-2018).

The environmentally friendly use of microorganisms to restore metal artworks is now proof-tested in the field of metal conservation. These research works allowed the development of an easy-to-use kit dedicated to conservators-restorers for the protective treatment of artefacts, archaeological objects, architectural components or scukptures.

Collaborations :

• Laboratory of microbiology (LAMUN)
• Haute Ecole Arc Conservation Restauration, HES-SO
• Institute for the study of antropic impact and sustainability in the marine environment, CNR-IAS (ancien. CNR-ISMAR), Genoa, Italy
• Swiss National Museum
• Laténium, musée et parc archéologique Hauterive-Neuchâtel