Conservation of Ancient Timbers from the Sea 285
3.2 Properties and Condition of Waterlogged
Archaeological Wood
The result of microbial decay in the marine environment is a progressive struc-
tural weakening of the wood due to the sequential loss of important polymers.
Chemical analysis of waterlogged archaeological wood shows a significant loss
of hemicellulose and cellulose, whereas the lignin component remains more
or less unaffected. Small wooden artefacts appear to deteriorate almost uni-
formly through their entire thickness. Larger artefacts are frequently found to
have a heavily degraded surface layer, surrounding a core, which is sound.
These important differences in decay pattern are reflected in a classification
scheme for waterlogged archaeological wood. This scheme gives a simple
description of a wood sample in terms of loss of solid component. The most
heavily degraded wood is considered to be class I (Figure 12), and the least
degraded is class III, as shown in Figure 13. Surprisingly, it is the class III
timbers where some of the biggest conservation problems occur.
Chemical hydrolysis can also cause loss of hemicellulose and cellulose,
but this type of degradation is not usually significant in comparison to bio-
logical attack. These processes have been implicated in the loss of carbohy-
drates from wood preserved in cold anoxic environments. The fastest chemical
change to archaeological wood is that of solvent swelling that is caused by
the cellulose molecules becoming surrounded by a sheath of hydrogen-bonded
seawater. This results in increases in size of about 12% tangentially, 5% radially
and 2% longitudinally. Chemical changes also occur as a result of mineral
Figure 12An SEM of class I condition of waterlogged archaeological oak. Note the heavily
degraded cell wall layers