Summary and conclusions

The purpose of this section is to pull together the various strands of the work described. The use of DSC to measure the UFW of modem and waterlogged archaeological woods

suggested that there was no variation in the fibre saturation point of modem wood samples with wood density. This was confirmed by inverse chromatography, using sugars, dextrans, ethylene glycol and diethylene glycol as probe molecules. For waterlogged archaeological wood, DSC found higher fibre saturation points than for modem wood species, and the reasons why this might be so have been discussed. It was not possible to confirm this with inverse chromatography because of the difficulty experienced in packing columns with waterlogged degraded wood. The precision of the method is not high, but could be improved by using longer columns, and better packing. The major limitation is the small pore volume of the wood, which limits resolution. The elution of strongly adsorbed species cannot be

observed in isocratic elution, as the peaks become too broad and cannot be distinguished.

The elution of PEGs fi*om modem woods showed a range of elution behaviours, fi’om slight adsorption for molecular weights of 1000 and above, to adsorption beginning at a molecular weight of 194. This was thought to be caused by adsorption on the lignin content of the wood, and the result of hydrophobic interaction. The adsorption increased as the temperature was raised, which again could be the result of hydrophobic interaction, or the result of the

glass transition temperature of the lignin being reached. It seems at least possible that the reduced diffiision coefficients of PEGs and similar molecules in wood are caused by this adsorption: this requires further investigation.

The elution of salts was found to be very concentration dependent, and was interpreted as being the result of a Donnan equilibrium, caused by the weak charge of the wood polymers interacting with the salt ions. This effect should also apply to ionic polymers.

The variation in fibre saturation point of oak of different degrees of degradation was measured by DSC. A simple model suggested that the observed variation would require different solution concentrations of PEG or other bulking agent to achieve adequate cell wall bulking for degraded outer shell of oak, and the sound inner core. However, the difficulties found in packing columns of waterlogged archaeological wood meant that it was not possible to measure partition coefficients of PEGs in these woods, and the simple model was not taken any further. In any case, the partition coefficients that would have been measured would have been only the dilute solution partition coefficients. Higher concentrations would need to be studied to complete the investigation. This was not achieved, as a great deal of time was required to unravel the unexpected complexities of the elution behaviour of PEGs fi*om the various wood samples.

5.2 Suggestions for future work

The methods used here could, and I believe, should be applied to other waterlogged organic materials, such as horn and leather. There is a vast literature on the properties and

degradation of wood, and on wood conservation, but little fundamental work has been carried out on other waterlogged archaeological materials.

The present work should be developed. New methods of packing, and longer columns, would give more precise results for the inverse chromatography. The effect of concentration on the elution of size excluded and adsorbed molecules should be investigated by frontal analysis. The relationship between adsorption and diflhision should be studied. The fibre saturation points found by inverse chromatography should be confirmed by non-solvent water

determinations using dextrans as probe molecules. Finally, a model should be developed to predict the anti-shrink efficiency of bulking agents from the results obtained, and tested against bulking experiments.

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