bulking agent (PEG 200–600, 30–40% w v^1 ) and a lumen-filling treatment
(PEG 2000–4000, 70–100% w v^1 ).
Recent investigations at the Mary RoseTrust have shown that no advantage
would be gained from adopting a complex schedule for the drying of PEG-
treated archaeological wood. Use of high initial relative humidities is likely
to result in slightly elevated dimensional change and may cause unwanted
migration of low molecular weight PEG from the core to the outer surface.
Drying temperatures above 20°C will also cause small increases in wood
shrinkage and collapse. Current data suggests that the most suitable drying
schedule would be initiated at 55% relative humidity and this level uniformly
distributed around the drying object. Drying directly to 55% RH at 18°C is
also likely to have additional and financial benefits.
Recent studies have also highlighted the importance of tightly controlling
environmental conditions after drying. Humidity levels higher than the rec-
ommended 55% RH are thought to have contributed to the formation of sulfuric
acid within the timbers of the Swedish warship, the Vasa. The air-conditioning
system in the display hall has now been upgraded to cope with this problem.
Vacuum Freeze Drying. One way of eliminating surface tension forcesduring
drying is to freeze the object, and then remove the frozen water within the
wood by sublimation. This is the most commonly-used drying technique for
treatment of pre-treated waterlogged archaeological wood. However, this tech-
niquehas the disadvantage of requiring the water within the object to undergo
a two-phase change as shown in Figure 20.
As water freezes it expands by approximately 9%, so a cryoprotectant such
as PEG 200–600 is required to protect the object during freezing. As small
amounts of low molecular weight PEGs are required, the process of freeze
drying is very much faster than the consolidation process required by air dry-
ing. Within a frozen PEG-treated object, a complex mixture of phases can
exist. As the temperature is lowered, there is a tendency for the material with
the highest freezing point (water) to freeze first, possibly with some cryopro-
tectant as a eutectic. In theory, a whole range of different compositions could
freeze out, one after another; more usually most material will freeze as a
glass. As water is subsequently sublimated out from the wooden object, the
cellulose matrix of the wood cell walls will come to favour PEG. The ideal
of freeze drying is not, therefore, attained, as the drying front will be in the
liquid phase. When all of the pure ice has sublimated, the vapour pressure of
the sample will fall, and there is some risk that the remaining eutectic will
warm up above its melting point and this would be detrimental to the sample.
Despite these theoretical problems, freeze drying is more reliable and much
faster than air drying. The freeze drying facility at the Mary RoseTrust has a
capacity of 3 m^3 and has been used with excellent results.
306 Chapter 11