Ingrid Stelzner
Abstract
Plants adapt to environmental conditions throughout their lives and store this information in their tissue. Prehistoric objects that were crafted from wood are preserved in the temperate zone mainly under waterlogged and anoxic conditions. Their analyses allow the reconstruction of the living conditions of the trees. Therefore, archaeological wood offers an archive for interdisciplinary research. Ecological, economic, vegetation-historical, climatic and cultural-historical questions can be addressed. The evaluation of tree ring widths permits dendrochronological dating making wood to a key material for archaeological research.
Without conservation, the finds disintegrate within a few hours after their recovery. Conservation measures are a prerequisite for the preservation of waterlogged organic materials. Written reports about conservation measures are proven already in the 19th century. Until today a plethora of methods with different conservation agents were applied to preserve archaeological waterlogged wood.
Several comparative studies aimed to compare the efficiency of the treatment methods. For the conservation of wooden archaeological objects, the first priority is a long-term stabilize. Invasive or destructive analyses, such as dendrochronological methods, are contrary to this principle. The establishment of non-destructive examination methods with micro computed tomography (µCT) will make them more readily available in archaeology. Due the difficulty of analysing wet archaeological wood the question arises whether non-destructive examination of already conserved objects is still possible. Therefore, another requirement for conservation can be added: The structural information of the historical sources should be still accessible and evaluable.
This study aimed to clear which common conservation method allows non-destructive dating with µCT. Therefore, conserved oak samples from the roman period were available from the scientific reference collection of the Römisch Germanisches Zenralmuseum Mainz, Germany. The collection was built up by collecting large archaeological samples of different types of wood and degrees of degradation. These were then divided into several equal sized subsamples and impregnated using the following methods: Kauramin 800, Saccharose, Lactitol/Trehalose, Silicone oil, Alcohol-Ether-Resin, polyethylene glycol (PEG) 2000 and freeze-drying, PEGcon, PEG 400, 1500, 4000 and freeze-drying.
The selected oak samples were analysed at the Lucerne School of Engineering and Architecture with a diondo d2 µ-CT system. The µCT data were compared by their quality, which was assessed by using appropriated image processing methods. Decisive criteria for dendrochronological dating are the visibility if the wood anatomy and the possibility to measure the ring widths. The measurements were evaluated with both analogue measurements and an automatic image processing approach which was recently developed. In addition, the identification of the wood species was assessed. Therefore, core samples were taken from the sub-samples and these were measured with high resolution µ-XCT.
First results confirm that visualisation of the wooden structure could be difficult in several cases: Influences are the µCT technique used, the size of the object, the conservation method and the condition of the wood. A major challenge in X-ray tomographic imaging of conserved archaeological wood is the quality of the data, which is determined by the contrast and the geometrical resolution of the measurement.