Waterlogged archaeological wood - chemical changes by conservation and degradation

Conservation of waterlogged archaeological wooden artefacts from the Nydam Bog in the southern part of Denmark was investigated by Raman spectroscopy. Two different conservation methods were used: the cellosolve/petroleum method and the polyethylene glycol (PEG)/freeze-drying method. Conservation with cellosolve/petroleum at both room and elevated temperatures was used to preserve archaeological arrows made from pinewood and ash wood. The double bonds in lignin were affected by the warm treatment. Holocellulose in the archaeological wooden arrows had decomposed completely. A band around 180 cm(-1) in the R((v) over bar)-representation of the low-wavenumber Raman spectrum was used to monitor the presence of water in PEG2000 with a structure like the one in 'free' bulk water. The lignin/PEG2000 relative band intensities were used to make a quantitative estimate of the wood/PEG2000 ratio in a PEG2000 impregnated piece of hardwood. A decreasing degradation of holocellulose was observed from the surface to the centre of an oak piece from the Kolding Cog, built around 1200 A.D. A broad background was observed in the spectra recorded close to the surface of the plank. A similar, very intense, broad background was seen in untreated collapsed ash wood from the Nydam Bog. This broad background, most probably arising from fluorescence, was too intense to allow a Raman spectrum to be seen. In these cases, ATR-FTIR spectroscopy is a valuable complementary tool in studies of waterlogged archaeological wood, confirming that holocellulose degrades more rapidly than lignin. Copyright (C) 2006 John Wiley & Sons, Ltd.