By Dr. Ingrid Stelzner
Wood is one of the key materials in archaeological research. Besides the shape of the object, the wooden structure provides archaeological information. In our recently started project CuTAWAY (Conservation and wood analyses), computed tomography (CT) is used to analyse the structure of conserved archaeological wood non-destructively. The project aims to analyse the wood anatomy for dendrochronology of finds as well as the evaluation of the most common conservation methods.
Wood has excellent material properties and is readily available. Therefore, it was used frequently in prehistoric times. In most cases, the organic material has decomposed during burial, but wherever preserved, it plays a key role in archaeological research. The woody taxon used, the function of the object and information concerning the environment can be gained from the analysis of wooden objects. Moreover, precise dating is possible using dendrochronology. Therefore, the conservation of archaeological wooden objects, which are mainly preserved as waterlogged finds, is of great importance.
Like other organic materials, wood is easily decomposed mainly by insects or microorganisms under aerobic environments. In contrast, wood decays very slowly under anaerobic conditions and may survive up to several thousand years in such conditions. When wet archaeological wood is excavated, the decayed structure is soaked and stabilized in water. Archaeological wooden finds are therefore very fragile but heavy. Without conservation the finds disintegrate within a few hours after their recovery (Figure 1a-d).
Conservation measures are a prerequisite for the preservation of such found material. A variety of materials, like artificial resins (Kauramin, BASF), sugars (saccharose or lactitol and trehalose), silicon oil, polyethylene glycols or specific mixtures of oils and resins (alcohol-ether-method), are currently applied to conserve waterlogged wood.
In 2009 these conservation methods were compared in a research project, coordinated by the Römisch Germansiches Zentralmuseum in Mainz (RGZM) and funded by the Federal Cultural Foundation and the Cultural Foundation of German States with international participants, resulting in a comparative collection of wood samples that have been preserved using the methods mentioned. This reference collection is stored at the RGZM.
Ten years later, in 2019, we were able to start the binational project CuTAWAY (Conservation and Wood Analyses) with the RGZM and the Lucerne University of Applied Sciences and Arts (HSLU) to get more insights into conserved archaeological wood and its analyses. The comparative analysis will be carried out using non-destructive, three-dimensional methods, such as 3D-scanning, X-ray micro-computed tomography (µCT) and microscopic techniques. To analyse the conserved wooden samples with µCT imaging techniques, we will measure a selection from the extensive RGZM sample collection at HSLU facilities using diondo d2, an industrial µCT system from Hattingen, Germany (Fig. 2). An optimised setup and acquisition protocol for the CT measurements will be developed specifically for conserved archaeological wood.
Firstly, we will focus on the assessment of different conservation methods. The principal aim of conservation is to bring the artefact into a condition of long-term chemical and physical stability with minimal intervention into the structure of the archaeological wood. Ideally, it is possible to analyse the structure of the finds without causing changes even after they have been conserved. In CT data, both changes in the volume of the finds and in the structure—for example, cracks and signs of collapse—can be quantified (Fig. 4). Additionally, a special experimental design for CT will focus on simulated ageing tests to investigate the long-term stability of conserved wood. Moreover, microscopic techniques will be used to analyse the success of the conservation treatments.
Conventional analyses of the wooden genus and dendrochronology are destructive. Attempts at non-destructive dating of wood samples based on CT images have been pursued since the 1980s, but non-destructive analysis and successful dating of wood samples only became feasible with the development of industrial µCT. Until then, only a few objects could be dated with CT. An important factor affecting the validity of the results is the condition of the object; the conservation method has an impact on the quality of the CT imaging. Our own measurements confirm that visualisation of conserved wood can be difficult in cases where we are unable to achieve sufficient contrast to analyse the wooden structure. We found that three aspects play an important role: the type of wood, the state of preservation and whether the wood is either waterlogged or impregnated with a large amount of medium due to conservation. A major challenge in X-ray tomographic imaging of conserved archaeological wood is the minor difference in X-ray absorption between the different constituents.
Our preliminary results with conserved oak samples show the different contrast levels achieved with different conservation agents (Fig. 3a & b). In the next phase, we will analyse more challenging wood species with lower contrast, and we will compare the µCT results with images from complementary methods such as neutron and phase-contrast micro computed-tomography.
The results of our project will be documented in an open access database. This will serve as a reference collection for the community and will help in the decision-making process for conservation. In this way, it will guarantee that archaeological wooden artefacts can be better studied and conserved for future generations.
The cross-border research project is funded by the German Research Foundation (DFG - 416877131) and the Swiss National Science Foundation (SNF - 200021E_183684).
The article Stelzner et al. “Non-destructive assessment of conserved archaeological wood with computed tomography” will provide more detailed information and will be published through the ICOM-CC 19th Triennial Conference, Beijing 17.-21. May 2021, and will be available online here: https://www.icom-cc-publications-online.org/
More information about the project can be found here: www.rgzm.de/cutaway
The authors thank the German Research Foundation, the Swiss National Science Foundation, the Federal Cultural Foundation and the Cultural Foundation of German States and all participants for their work in conserving the wooden samples: Stephan Brather (BLDAM), Stephan Gebhard (LfA Sachsen), Verena Gemsjäger-Ziegaus and Susanne Klonk (ASS), Anne Le Boedec Moesgaard (Danish National Museum), Katharina Schmidt-Ott and Cedric André (SNM), Wayne Smith (Texas University).
Dr. Jörg Stelzner, Dr. Jorge Martinez-Garcia, Markus Wittköpper, Damian Gwerder, Waldemar Muskalla, Prof. Dr. Markus Egg and Prof. Dr. Philipp Schuetz
Dr. Ingrid Stelzner heads the project CuTAWAY together with Prof. Dr. Philipp Schuetz (HSLU) and Prof. Dr. Markus Egg at the RGZM. She was trained as a conservator and holds a PhD in conservation. Her special interest focuses on the restauration and conservation of archaeological organic objects with which she is involved through both practical conservation and research projects.
(Read the article and see all the images in the February-March 2021 "News in Conservation" Issue 82, p. 10-13)