Water in archaeological wood : a critical appraisal of some diagnostic tools for degradation assessment

Abstract

Waterlogged wood conservation is ultimately directed towards effective drying of artefacts, since through this procedure is the best hope for their stabilisation. This is a much more risky activity for the conservator than it is for the modern timber technologist, because of the extreme chemical and physical changes the material has undergone during its burial. Nevertheless, we already have a number of adequately successful methods for the conservation of archaeological wood. What we don't have is all the information we might want for the modification of these techniques to provide fully predictable results for the widely variable material which comes in for treatment or for the economical treatment of large structures and bulk assemblages. The conservator's primary concerns are with permeability, diffusion rates, drying behaviour,and internal surface calculation, all of which depend on the wood's chemical preservation, and are mirrored in its sorption characteristics. Wood-water relations of archaeological material have not as yet received much attention from researchers, and the chemistry of this material only slightly more. The findings of wood science can not be assumed to apply to archaeological material where radical chemical change has significantly changed its properties. This thesis addresses this problem by providing a comprehensive and critical synthesis of past and current research from the fields of waterlogged wood conservation, wood technology, chemistry, and microbiology, wood degradation, and wood-water relations. One of the outcomes of this work was the bringing to light of evidence for a much greater role for anaerobic decay vectors than has up to now been acknowledged. This only serves to emphasise the very real need for more research in the water relations and degradation chemistry of archaeological waterlogged wood, and perhaps even more for research into the development of assessment techniques specifically designed for this material, and easy and inexpensive to apply for the practising conservator. This thesis investigates the development of such techniques and compiles a critical review of a number of the techniques already in common use by conservators and wood scientists. The designs for a simple, accurate sorption measuring apparatus are given, and the results from a large group of archaeological wood samples reported. From these results it is determined that the controlling factor in the water relations of this material appears to be bulk mass losses rather than changes to chemical constituent ratios. Alternative diagnostic tools which have the potential of obtaining indircct information about wood-water relations indirectly are also critically appraised. The results from bulk chemical analyses are compared to those from physical tests of density, moisture content and strength measurements. Results from Sibert resistance drill trials and polarising microscopy are appraised. Certain findings from these contradict some of the assumptions which underpin our models for calculating internal void space in archaeological waterlogged wood which are used in defining our conservation treatment approaches. Three techniques of instrumental analysis are also examined. The results from FFIR, Py-GC/MS, and elemental analysis reveal a number of useful and casily-recognisable markers for degradation. A series of important new markers for syringyl lignin degradation arise from this study. In the final appraisal, preference lies with these latter diagnostic tools, and arguments are provided for their greater accessibility to the conservator