Edith Joseph

New Advances in the Application of FTIR Microscopy and Spectroscopy for the Characterization of Artistic Materials

Fourier transform infrared (FTIR) spectroscopy is one of the most widely applied techniques for the investigation of cultural heritage materials. FTIR microscopy is well established as an essential tool in the microdestructive analysis of small samples, and the recent introduction of mapping and imaging equipment allows the collection of a large number of FTIR spectra on a surface, providing a distribution map of identified compounds. In this Account, we report recent advances in FTIR spectroscopy and microscopy in our research group. Our laboratory develops, tests, and refines new and less-studied IR spectroscopy and microscopy methods, with the goal of their adoption as routine analytical techniques in conservation laboratories. We discuss (i) the analysis of inorganic materials inactive in the mid-IR region by means of far-IR spectroscopy, (ii) the development of new methods for preparing cross sections, (iii) the characterization and spatial location of thin layers and small particles, and (iv) the evaluation of protective treatments. FTIR spectroscopy and microscopy have been mostly used in the mid-IR region of 4000-600 cm(-1). Some inorganic pigments, however, are inactive in this region, so other spectroscopic techniques have been applied, such as Raman spectroscopy. We suggest an alternative: harnessing the far-IR (600-50 cm(-1)). Our initial results show that far-IR spectroscopy is exceptionally useful with mural paintings or with corrosion products from which larger sample quantities can generally be collected. Moreover, the inorganic composition of a sample can be characterized by the presence of several compounds that are inactive in the mid-IR range (such as sulfides, oxides, and so forth). Stratigraphical analyses by FTIR microscopy can be hindered by the process of cross section preparation, which often involves an embedding organic polymer penetrating the samples porous structure. Here, the polymer bands may completely cover the bands of organic compounds in the sample. However, a correct methodological approach can prevent such limitations. For example, it is always advisable to analyze the sample surface before preparing the cross section in order to characterize the preparation layers and the varnish layers, which are generally applied to the surface of a painting both to protect it and improve the color saturation. Furthermore, the innovative use of IR-transparent salts as embedding material for cross sections can prevent contamination of the embedding resin and improve detection of organic substances. Another key point in the use of FTIR microscopy in artwork analysis is spatial resolution. The high-energy output of a new integrated FTIR microscope enhances the ability to characterize and spatially locate small particles and thin layers. Moreover, the new configuration proves extremely useful in the evaluation of protective treatments, because larger areas may be analyzed in less time in comparison to traditional systems, allowing the collection of more statistical data.

Protection of Metal Artifacts with the Formation of Metal–Oxalates Complexes by Beauveria bassiana

Several fungi present high tolerance to toxic metals and some are able to transform metals into metal–oxalate complexes. In this study, the ability of Beauveria bassiana to produce copper oxalates was evaluated. Growth performance was tested on various copper-containing media. B. bassiana proved highly resistant to copper, tolerating concentrations of up to 20 g L−1, and precipitating copper oxalates on all media tested. Chromatographic analyses showed that this species produced oxalic acid as sole metal chelator. The production of metal–oxalates can be used in the restoration and conservation of archeological and modern metal artifacts. The production of copper oxalates was confirmed directly using metallic pieces (both archeological and modern). The conversion of corrosion products into copper oxalates was demonstrated as well. In order to assess whether the capability of B. bassiana to produce metal–oxalates could be applied to other metals, iron and silver were tested as well. Iron appears to be directly sequestered in the wall of the fungal hyphae forming oxalates. However, the formation of a homogeneous layer on the object is not yet optimal. On silver, a co-precipitation of copper and silver oxalates occurred. As this greenish patina would not be acceptable on silver objects, silver reduction was explored as a tarnishing remediation. First experiments showed the transformation of silver nitrate into nanoparticles of elemental silver by an unknown extracellular mechanism. The production of copper oxalates is immediately applicable for the conservation of copper-based artifacts. For iron and silver this is not yet the case. However, the vast ability of B. bassiana to transform toxic metals using different immobilization mechanisms seems to offer considerable possibilities for industrial applications, such as the bioremediation of contaminated soils or the green synthesis of chemicals.

Development of an analytical procedure for evaluation of the protective behaviour of innovative fungal patinas on archaeological and artistic metal artefacts

In the literature, the ability to transform metal compounds into metal oxalates has been reported for different species of fungi. This could be an innovative conservation method for archaeological and artistic metal artefacts. In fact, with a high degree of insolubility and chemical stability even in acid atmospheres (pH 3), metal oxalates provide the surface with good protection. Within the framework of the EU-ARTECH project, different fungal strains have been used to transform existing corrosion patinas on outdoor bronze monuments into copper oxalates, while preserving the physical appearance of these artefacts. Given the promising results obtained with this first attempt, the same approach is now applied within the BAHAMAS (Marie Curie Intra European Fellowship action) project, but extended to other metal substrates, for example iron and silver, which are frequently found in cultural heritage artworks and also encounter several problems of active corrosion. The research is investigating the formation mechanisms and adhesion properties of the newly formed metal oxalates by means of complementary analytical techniques (X-ray diffraction (XRD), FTIR microscopy, Raman microscopy, scanning electron microscopy (SEM-EDS), electrochemical impedance spectroscopy (EIS), colorimetry). For each metal substrate, the most appropriate fungal strain is going to be identified and applied to corroded sheets and the novel fungal treatment compared with those used so far. Treated metal sheets will be monitored during 1-year exposure to different cycles of artificial ageing, to evaluate the corrosion resistance of the fungal patinas obtained. The objective of this contribution is to present the first results achieved so far on naturally corroded bronze sheets during the EU-ARTECH project and the analytical procedure used for the testing of the proposed treatment performances during the BAHAMAS project.

Attenuated total reflectance microspectroscopy mapping for the characterisation of bronze corrosion products

Samples collected from an archaeological object and an outdoor bronze monument as well as a bronze coupon naturally aged in an outdoor marine environment have been analysed in order to determine the composition of the corrosion and deposition compounds and map their stratigraphic distribution. Cross-sectioned samples and the bronze coupon have been placed onto the x-y-z motorized stage under the objective of an FTIR microscope and both punctual analysis and mapping of small areas was performed in ATR mode. The combination of visible light microscopy, which allows the microphotograph documentation of chemically heterogeneous samples, with FTIR-ATR mapping, is of the outmost importance in the characterisation of the corrosion features and behaviour of a wide variety of corroded bronze samples streamlining the selection of the most appropriate conservation materials and cleaning procedures to be adopted by conservators. Limitations, such as the spatial resolution of about 20 μm and silicon crystal opacity, are compared with the great advantage of obtaining distribution false colour plots of the different compounds. As non-destructive of the sample analytical technique ATR mapping can further be combined with other analytical techniques such as metallography, X-ray EDS and Raman spectroscopy. The limitation of the ATR technique in identifying important corrosion products, which do not absorb in the mid infrared, and the need to be combined with other type of analysis is also highlighted.

Use of bacteria to stabilize archaeological iron

ABSTRACT Iron artifacts are common among the findings of archaeological excavations. The corrosion layer formed on these objects requires stabilization after their recovery, without which the destruction of the item due to physicochemical damage is likely. Current technologies for stabilizing the corrosion layer are lengthy and generate hazardous waste products. Therefore, there is a pressing need for an alternative method for stabilizing the corrosion layer on iron objects. The aim of this study was to evaluate an alternative conservation-restoration method using bacteria. For this, anaerobic iron reduction leading to the formation of stable iron minerals in the presence of chlorine was investigated for two strains of Desulfitobacterium hafniense (strains TCE1 and LBE). Iron reduction was observed for soluble Fe(III) phases as well as for akaganeite, the most troublesome iron compound in the corrosion layer of archaeological iron objects. In terms of biogenic mineral production, differential efficiencies were observed in assays performed on corroded iron coupons. Strain TCE1 produced a homogeneous layer of vivianite covering 80% of the corroded surface, while on the coupons treated with strain LBE, only 10% of the surface was covered by the same mineral. Finally, an attempt to reduce iron on archaeological objects was performed with strain TCE1, which led to the formation of both biogenic vivianite and magnetite on the surface of the artifacts. These results demonstrate the potential of this biological treatment for stabilizing archaeological iron as a promising alternative to traditional conservation-restoration methods. IMPORTANCE Since the Iron Age, iron has been a fundamental material for the building of objects used in everyday life. However, due to its reactivity, iron can be easily corroded, and the physical stability of the object built is at risk. This is particularly true for archaeological objects on which a potentially unstable corrosion layer is formed during the time the object is buried. After excavation, changes in environmental conditions (e.g., higher oxygen concentration or lower humidity) alter the stability of the corrosion layer and can lead to the total destruction of the object. In this study, we demonstrate the feasibility of an innovative treatment based on bacterial iron reduction and biogenic mineral formation to stabilize the corrosion layer and protect these objects.

Bacterial iron reduction and biogenic mineral formation for the stabilisation of corroded iron objects

Exploiting bacterial metabolism for the stabilisation of corroded iron artefacts is a promising alternative to conventional conservation-restoration methods. Bacterial iron reduction coupled to biogenic mineral formation has been shown to promote the conversion of reactive into stable corrosion products that are integrated into the natural corrosion layer of the object. However, in order to stabilise iron corrosion, the formation of specific biogenic minerals is essential. In this study, we used the facultative anaerobe Shewanella loihica for the production of stable biogenic iron minerals under controlled chemical conditions. The biogenic formation of crystalline iron phosphates was observed after iron reduction in a solution containing Fe(III) citrate. When the same biological treatment was applied on corroded iron plates, a layer composed of iron phosphates and iron carbonates was formed. Surface and cross-section analyses demonstrated that these two stable corrosion products replaced 81% of the reactive corrosion layer after two weeks of treatment. Such results demonstrate the potential of a biological treatment in the development of a stabilisation method to preserve corroded iron objects.

Microbial biotechnology approaches to mitigating the deterioration of construction and heritage materials

Microorganisms are the main engines of elemental cycling in this planet and therefore have a profound impact on both organic and mineral substrates. As such, past and present human‐made structures and cultural heritage can be negatively affected by microbial activity. Processes such as bioweathering (rocks and minerals), biodeterioration (organic substrates) or biocorrosion (metals) participate to the degradation or structural damage of construction and heritage materials. This structural damage can cause major economic losses (e.g. replacement of cast‐iron pipes in water distribution networks), and in the case of heritage materials, the entire loss of invaluable objects or monuments. Even though one can regard the influence of microbial activity on construction and heritage materials as negative, remarkably, the same metabolic pathways involved in degradation can be exploited to increase the stability of these materials.

Evaluation of the performances of a biological treatment on tin-enriched bronze.

Recently, research gives emphasis to eco-friendly and sustainable approaches for the preservation of cultural heritage that could offer advantages in terms of compatibility, durability and safety. Hence, a biological treatment, based on a specific fungal strain of Beauveria bassiana, is exploited for the stabilization of soluble and/or active bronze corrosion products, converting them into copper oxalates. The chemical stability of the latter represents a real improvement for the long-term preservation of bronze, especially in case of exposure to acid rain. However, the corrosion behaviour of bronze differs from that of pure copper due to the presence of additional alloying elements. In natural environments, the selective dissolution of copper leads to a relative tin-enrichment within the corrosion layers, mostly in unsheltered areas exposed to rainwater runoff. To understand the influence of tin-enrichment on the formation of oxalates, pure tin and artificially tin-enriched bronze coupons were treated with this novel biological system and, in the case of bronze coupons, exposed to accelerated ageing. Tin enrichment and accelerated ageing were performed through runoff tests. Before and after treatment and ageing, the sample surface was characterized through Fourier transform infrared (FTIR) and Raman spectroscopies, scanning electron microscopy coupled to energy dispersive spectroscopy (SEM-EDS). Metals released in the ageing solutions were analysed through atomic absorption spectrometry (AAS). The analytical results allowed to better understand the response of unsheltered areas from outdoor bronze monuments to the biological treatment proposed.