Microbes for Archaeological Wood Conservation

This project focuses on innovative biological methods of extraction for the preservation of waterlogged wood suffering from salt precipitation and acidification. The principal investigator and her team proposed to exploit biomineralization capacities of some bacteria for anticipating the extraction of iron and sulfur compounds when wood is still wet. A comprehensive assessment of the extraction performances achieved on wood objects from lake and marine environments will allow a versatile extraction method to be proposed to end-users

Assessing protective treatment performance on outdoor sculptures:to be or not to be a representative model sample?

Protective treatments to be applied in the field of cultural heritage need to fulfil several constraints that have gradually been better outlined over the years. Along with the impact on aesthetic appearance and reversibility, it is nowadays well recognized that patinas and/or original historical coatings and finishings can play a relevant role in protective treatments’behavior[1,2]. Nonetheless, no standard characterization method has been drafted to test new coatings and their application on real artefacts

The Potential of Microorganisms for the Conservation- Restoration of Iron Artworks

Archaeological iron artefacts encounter serious post-excavation problems when contaminated with salts. Once excavated, exposure to a higher oxygen concentration and lower relative humidity renders the corrosion crust formed during burial no longer stable. In particular, the process is induced by chloride ions and leads to flaking, cracking and the final loss of shape of the object. The MAIA project (Microbes for Archaeological Iron Artefacts) studied microbial metabolisms to explore their potential for the development of innovative and sustainable methods for the stabilisation of corroded iron archaeological objects. Two different approaches were investigated. First, bacterial reduction of iron solid-phases and biogenic mineral formation were studied as a way to replace unstable corrosion products. Several bacterial strains were compared. Spectroscopic investigations with Raman and Fourier transform infrared spectroscopy on iron coupons, nail surfaces and cross sections demonstrated the conversion of the outermost part of the corrosion layer into more stable biogenic minerals, such as vivianite and siderite. The second approach was to study fungi and their metabolic ability with iron. In particular, alkaliphile fungi that tolerate chlorine were studied for their ability to produce biogenic minerals and to adsorb metals in their biomass. Colorimetric investigation and evaluation of the thickness of the corrosion layer demonstrated that fungi are good candidates for developing bio-cleaning methods for corroded iron, permitting the selective removal of the unstable and powdery corrosion layer without damaging the original metal surface. This study details these approaches and explores the possibilities of their exploitation for development of innovative and more sustainable treatments for the conservation-restoration of corroded iron

Evaluation of bio-based extraction methods by spectroscopic methods

New technologies are in development regarding the preservation of waterlogged archaeological wood items contaminated with Fe/S species. To this purpose, a bio-based treatment to extract these harmful species before further damages occur is presented. Thiobacillus denitrificans and desferoxamine were employed based on their specific properties to solubilize iron sulfides and uptake iron. The biological treatment was compared with oxidizing and complexing agents (sodium persulfate and ethylene diamine tetraacetate) traditionally used in conservation-restoration. Mock-ups of fresh balsa as well as fresh and archeological oak and pinewood were prepared to simulate degraded waterlogged wood by immersion in corrosive Fe/S solutions. The efficiency of both biological and chemical extraction methods was evaluated through ATR-FTIR and Raman spectroscopies and validated by statistical approach. Results showed that treatments did not affect the wood composition, meaning that no wood degradation was induced. However, the chemical method tended to bleach the samples and after treatment, reduced sulfur species were still identified by Raman analyses. Finally, statistical approaches allowed validating ATR-FTIR result

Characterization of model samples simulating degradation processes induced by iron and sulfur species on waterlogged wood

Reduced iron and sulfur species accumulated within waterlogged archaeological wood artefacts during their burial time. Oxygen exposure of the artefacts during recovery leads to acidification and salts precipitation, which causes irreversible physical and chemical damages. Prior to accurately evaluating novel extraction methods, the procedures for creating analogous samples were evaluated for efficacy. Waterlogged wood analogues provide access to a whole set of homogeneous and sacrificial samples that replicate characteristics of waterlogged archaeological wood in terms of content degradation and the presence of reduced iron and sulfur species. In this study, we evaluated the preparation of model samples from fresh balsa wood artificially contaminated with reduced iron and sulfur species. Wood degradation and the formation of reduced iron and sulfur species were assessed by Fourier Transformed Infrared (FTIR) and Raman spectroscopies and validated through statistic methods, such as Principal Component Analysis (PCA). Among the three impregnation protocols investigated, one method appeared to be the most effective in term of iron sulfide formation, especially partially oxidized mackinawite Fe1-xS. The selected protocol proved reproducible and efficient on both fresh balsa and Neolithic oak samples. From these observations confirmed by the PCA analyses on spectroscopic dataset, a suitable method to model waterlogged archaeological wood was established

Biological oxidation of sulfur compounds in artificially degraded wood

Marine and lacustrine archaeological aterlogged wood encounters serious problems after xcavation due to the accumulation of sulfur and iron compounds during burial. Exposure of these compounds to oxygen results in precipitation of salts and acidification, which can lead to serious structural damage, and ultimately the loss of important cultural heritage. In this study, we evaluated the capacity of the bacterium hiobacillus denitrificans to transform sulfur compounds commonly found in waterlogged wooden objects, to more readily extractable compounds thereby eliminating the threat of degradation. Oak samples, impregnated with a solution containing iron(II) and sulfides, were used to assess the efficiency of the bacterial treatment. The model wood samples were characterized before and after treatment using different techniques such as ESEM-EDS, micro-Raman spectroscopy, XRD and Sy-XRF mapping. Before treatment, mackinawite (FeS) and mineral sulfur (α-S8) were detected in the impregnated wood. After treatment with T. denitrificans, even though some mineral sulfur remained in the samples, the predominant phase corresponded to oxidized sulfur. This demonstrates that T. denitrificans was able to use the reduced sulfur compounds present in the wood samples as an energy source, thereby producing more soluble oxidized sulfur compounds. In addition, non-invasive techniques such as Fourier transform infrared (FTIR) spectroscopy, were carried out to assess the consequences of the biological treatment on the wood structure. No negative effect on the wood was detected after the treatment in comparison with the referenceimpregnated wood. This study demonstrates the feasibility of a biotechnological procedure for the preventive extraction of sulfur species from archaeological waterlogged wood

Assessment of Various Iron Extraction Treatments on Waterlogged Archaeological Oak

Treatments of organic objects to extract ferrous compounds from waterlogged archaeological wood are well documented. For several years, numerous laboratories have been seeking to determine suitable conservation treatments for such organic objects. For chemical treatments, complexing agents such as EDTA and DTPA, along with acids such as citric and oxalic acids, were selected. In addition, oxidants and reducing agents were tested as pre-treatments to improve extraction rates. In fact, chemicals produce a selective reaction on ferrous compounds, which may improve or be inhibited by complexation or dissolution reactions. Their action depends on the type of compound to be extracted and those present inside the wood. The objective of this study was to make a comparative assessment of the various chemical conservation treatments identified and complement the evaluation of their extraction efficiency with a study of their impact on organic matter by adding criteria such as their visual aspect (using a spectrocolorimeter) and physicochemical actions by means of infrared spectroscopy and micromorphological observations. The effectiveness of EDTA was confirmed, as was that of citric acid, despite some questions arising concerning the presence of wood fibers in the treatment solution, even after rinsing. On the other hand, the extraction rate of oxalic acid, which has a very acidic pH, was unsurprisingly lower, but its visual and anatomical results raised the possibility of using it over a short period of time in view of the effectiveness observed on the wood surface. Pre-treatments improved extraction rates, except in the case of EDTA, which independently had a high extraction rate. It was observed that pre-treatments did not appear to cause any significant chemical degradation of the organic matter. This study provides a tool to assist conservators in selecting a chemical treatment that is in line with the state of decay of the wood, the characteristics of the ferrous compounds to be extracted, and the conservation objectives

Evaluation of an alternative biotreatment for the extraction of harmful iron and sulfur species from waterlogged wood

Aninnovative bioextractionmethodwas tested and compared to common chemical extraction for the preservation of waterlogged archeological wood (WAW) artifacts. During burial, WAW artifacts accumulate iron and sulfur species forming iron sulfides. These compounds are harmless in the burial environment, where the oxygen content is low. But upon excavation, the WAW undergoes the oxidation of these compounds, and thus, irreversible physical and chemical damages occur. Fresh and archeological oak and pine samples were selected as representative species of WAW artifacts. Fresh samples were previously artificially contaminated to ascertain the presence of iron and sulfur. Thiobacillus denitrificans and natural iron chelators, called siderophores, were investigated to extract iron and sulfur as a 2-step biological treatment (BT) and compared to sodium persulfate–EDTA as chemical treatment (CT). Consolidation and freeze-drying were performed on the samples after BT and CT as traditional conservation protocols. BT and CT efficiency was evaluated through Raman, inductively coupled plasma–optical emission (ICP-OES), and Fourier transformed infrared (FTIR) spectroscopies. Raman and ICP showed that most of the iron and sulfur was extracted after BT, while some sulfur species remained present on CT samples. None of the extraction methods resulted in a degradation of the wood, as ascertained by FTIR analyses. Yet, all samples presented visual modifications after conservation. Pine samples treated with BT illustrated the oxidation of the species. Present principal component analysis (PCA) and analysis of variance (ANOVA) which were selected as statistical approaches and validated

Assessing the versatility of bioextraction to preserve waterlogged wood

An innovative bio method was investigated to extract harmful iron and sulfur species from waterlogged wood samples. The method was compared with a chemical treatment. Both approaches were applied on lacustrine and marine samples, from different wood genera, to evaluate the versatility of the proposed bio method. Non-invasive and non-destructive methods were carried out to investigate both bio-based and chemical treatments. The result was that some wood genera were more affected by the bio approach, with a clear distinction between lacustrine beech and pine against oak and lime wood species. The chemical approach showed potential harm for the wooden structure, with acidic pH values and an increase of maximum water content, both implying degradation of the wood structure. In terms of extraction, no iron or sulfur products were detected by Raman spectroscopy on biologically treated samples, in agreement with extraction rates calculated. It was also suggested that iron bonded to wood was extracted with the chemical approach, and calcium content affected by both approaches