304 Chapter 11
Figure 19Spraying the hull timbers with a solution of PEG
use of PEG, four methods of application can be considered: (i) tank method
for complete structures (Bremen Cogg – Bremen, Germany), (ii) tank method
for individual units or components, (iii) spraying of large structures (Vasa–
Stockholm, Sweden and Mary Rose – Portsmouth, UK), and (iv) surface
treatment (dry objects only).
PEG: Two Stage Method (Case Study – The Mary Rose Hull). Research by
Trust scientists explored in great detail the physical and chemical characteris-
tics of the hull timbers and quickly discovered that they were differentially
decayed. The outer surface to a depth of 10 mm was soft and heavily
degraded, while wood cells beneath this region were found to be well preserved.
According to shrinkage data, there can be as much as 25% shrinkage to the
wood structure as the timbers dries out. Drying the hull without some form of
stabilisation was considered unsuitable and the use of PEG was investigated.
The wide variety of properties of PEGs has complicated conservation efforts
in the past. For example, low molecular weight PEGs could move swiftly and
efficiently into the wood cell walls, helping to stabilise them and prevent cell wall
shrinkage. But they are hygroscopic and give a tacky feeling. Thus, until recently,
conservators would only use molecular weight of 1500–4000; they might not
have a rapid uptake of PEG but they would not be left with a tacky surface.
To overcome this problem, conservators and scientists began to experiment
with a two-stage treatment process involving low and high molecular PEGs.
This research programme indicated that a lower weight PEG (mw 200) to
start with, followed by a higher molecular weight (mw 2000) was the best
combination for the differentially decayed timbers of the Mary Rose. The treat-
mentprogramme began in 1994 (Figure 19) with the entire hull surface