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Protection of cultural heritage

 

While often considered as harmful for cultural heritage, microorganisms can also be used for its safeguarding. Indeed, there is a growing interest for the development of biological technologies that are environmental friendly (close to ambient temperature and pressure, neutral pH) and do not require the use of toxic materials. A real progress could be expected in terms of durability, effectiveness and toxicity. Over the last decades, the development of biological methods and materials became a significant alternative for the conservation-restoration of cultural artefacts. Hence, biotechnology has been applied with success in different domains, such as bioremediation or corrosion control. In particular, different soil bacteria have already been reported as a conservation treatment for ornamental stone, demonstrating the great opportunities offered by using such alternative treatments. Possibilities offered by microorganisms for the conservation-restoration of metal artworks are evaluated in our laboratory. In particular, two research projects (BIOPATINAS and MAIA) are developed aiming to modify existing corrosion products into more stable compounds, while maintaining the surface’s original appearance.

 BIOPATINAS

 
The objective of the project is to propose an alternative biological treatment for the conservation-restoration of copper alloys artefacts. Taking advantage of unique properties of carefully selected fungal species, the project relies on the conversion of existing corrosion patinas into copper oxalates. In fact, thanks to their insolubility and stability even in acidic atmospheres, copper oxalates are expected to provide the treated objects with long-term protection and no 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 of a) culture of B. bassiana on a copper-enriched media with copper hydroxysulfates showing the embedded copper oxalates crystals. The same crystals are formed on corroded coupons with b) a copper hydroxysulfates patina and c) a copper hydrochlorides patina. Results achieved during the master thesis of A. Simon “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    (2010-2012), Biological patinA for arcHaeological and Artistic Metal ArtefactS . In particular, the newly formed copper oxalates were in-depth characterized in order to define their properties (formation mechanisms, adhesion...), either on copper-enriched media or on corroded coupons. Cross-section examination suggested that the first micrometers of the urban natural patina are completely converted into copper oxalates, as showed in figure below.
 
ATR-FTIR raster scanning: a) visible light photomicrograph of a cross section of corroded coupon with an urban natural patina after treatment with B. bassiana, embedded in polyester resin: the box indicated the selected area for the ATR raster scanning. FTIR false colors representing b) the original patina 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 (FP6-EU-ARTECH, 2004-2009 and FP7-BAHAMAS, 2010-2012), the efficacy of the fungal treatment is now improved and validated against ageing procedures. The environmentally-friendly use of microorganisms for restoration of metal artworks that is now proof-tested, could present a breakthrough innovation for the conservation industry. The project should provide conservation professionals with access to a simple-to-use, reliable, versatile, environment-friendly and low-cost technology, for art objects, archaeological objects, architectural components, etc.
One of the expected outcomes is the development of an easy-to-use kit dedicated to conservators-restorers. A feasibility study without implementation partner that deals with the ageing of coupons replicating the commonly found patinas in nature has been approved by the Swiss Commission for Technology and Innovation (CTI) for an 18-months funding and is now ongoing.
In parallel, a complementary project has been made possible thanks to Gebert Rüf Stiftung   , which is providing financing and monitoring progress. The treatment under development will be:
- proof-tested under ageing conditions through the exposure of real objects to aggressive weather conditions,
- proof-tested for the appearance, resistance to corrosion and cohesion of the biopatinas onto foundry object’s surface,
- standardized to become (and be perceived as) simple and efficient for all uses and consumers.
 
Three industrial companies also already expressed their interest in a future partnership:
- Actitec Sàrl, Confignon (GE), technologies and equipment for the restoration of the built heritage. Represents also the Amonit   company (France) that commercializes a bacterial treatment for stones.
- Kunstgiesserei   St Gallen AG (SG), realization of artworks and conservation-restoration interventions.
- CTS Suisse SA   , Taverne (TI), products, equipment and installations for conservation-restoration.

MAIA

Archaeological iron artefacts encounter serious post-excavation problems when contaminated with salts. In fact, once excavated, the exposure to a higher oxygen concentration and lower relative humidity renders the corrosion crust formed during burial not longer stable.
 
 
 
Iron nail (Gerechtigkeitsgasse, Bern) showing disastrous swelling and delamination of the surface following damp storage conditions (Photograph courtesy of the archaeological service of Canton Bern, Switzerland).
 
 
We propose here to exploit the unique properties of some microorganisms for the stabilization of archeological iron. To this purpose, three different strategies will be adopted either leading to the formation of stable compounds of low molar volume or using chloride-translocation properties.
Tests will be achieved with the precipitation of iron oxalates by Beauveria bassiana. The same approach will be exploited to precipitate magnetite (Fe3O4), another very stable compound of low molar volume, using microorganisms reported in the literature for the biosynthesis of magnetite nanoparticles: Fusarium oxysporum or Verticillium sp. or magnetotactic bacteria.
 
Beauveria bassiana cultures on malt-agar medium with an iron washer. a) Optical microscopy observations. b) Secondary electron image. c) Transmittance FTIR spectrum (4000–650cm−1) obtained from hyphae incrusted with red crystals. Results achieved during the master thesis of S. Cario “Etude et optimisation de la formation d’oxalates d’origine pour la conservation d’objets en cuivre, fer et argent".
 
In order to enhance the removal of chloride ions from the iron object, the possible translocation of those by fungi will be studied. In chlorine-rich environments, some species of fungi develop strategies for detoxify their surrounding from the toxic chlorine. These halophiles will be isolated and cultured from natural places such as salt lakes. In addition, white-rot fungi are also studied in bioremediation for allowing chlorine migration.
Based on the results achieved, we could contribute to the development of a synergetic microbial consortium specially designed for the removal of chloride ions and the simultaneously formation of stable iron compounds. Particular attention will be devoted to the efficiency and the impact on metallographic structure of the proposed treatment to overcome the problems associated with the treatments in use nowadays. Real samples will be also included in this study in order to validate the new methodology. This research issue presents innovative aspects in biogeochemistry of micro-organisms and conservation science.

PERSONS INVOLVED

This research line of conservation science about the use of microbes for conservation-restoration is truly developing in LAMUN: 1 ambizione fellowship, 2 PhD students over a 3-year period and a technician (50%) over an 18-month period are involved.
A first PhD thesis about the “Optimization and application of a fungal treatment for copper-based surfaces” has been proposed. Ms. Monica Albini   has already been hired as candidate and started on 01.05.2013. Together with Mrs. Lidia Mathys-Paganuzzi   , they are involved into the CTI project.
A second PhD thesis has been offered under the MAIA project, dealing with the “Evaluation of translocation and biomineralization properties of fungi for the removal of chloride species from iron artefacts”. Ms. Lucrezia Comensoli has already been hired as candidate and started on 01.05.2013. She is also involved in the Gebert-Rüf Stiftung project.

ECHO

The excellence of our research has been granted with an invitation to the conference Marie Curie Actions for an Innovative Europe in 2010, representing the 50.000th Marie Curie research fellows, and also with the participation as invited speakers to different conferences, such as the annual congress of the Swiss association of conservators-restorers Innovation in conservation practice in 2011 and 2013 or the triennial International Council of Museums Committee for Conservation (ICOM-CC) meeting of the metal working group METAL2013.
 

Contacts

Edith Joseph - Ambizione FNS

Pilar Junier  - Professeure

Wafa M. Kooli - Doctorante

Monica Albini - Doctorante

Lucrezia Comensoli - Doctorante

Lidia Mathys-Paganuzzi - Laborantine