Dissertation for the Degree of Doctor of Philosophy
By:
Supervisor:
External mentor:
Birgitta Håfors, M.Sc.
Professor Emeritus Jan Rosvall, Ph.D.
Professor Emeritus Ingvar Johansson
CONSERVATION OF THE WOOD OF THE SWEDISH WARSHIP VASA OF A.D. 1628
Evaluation of Polyethylene Glycol Conservation Programmes
ABSTRACT
The hull of the Swedish warship Vasa from 1628 was salvaged in April 1961. A year later, two other
archaeological wet site projects were started, namely the excavation of the Danish Viking ships at
Skuldelev, and the salvage of the medieval Bremen cog. Thus, in the early 1960’s conservation of
waterlogged archaeological wood became an important topic in the museum world.
Polyethylene glycol (PEG) had early been brought to the attention of the management of the Vasa project
by Rolf Morén who was in charge of manufacturing PEG at the Mo&Domsjö paper mill industry at
Örnsköldsvik (Sweden). He and conservator Bertil Centerwall at the Historical museum of Lund
University had developed a method for preserving with PEG small wooden objects from archaeological
wet sites. Before the hull of the Vasa was raised, some small objects - that had been salvaged from the
site - were treated with PEG 4000, using this method. After some investigation about other possibilities
PEG 4000 was chosen for the conservation of all of the wooden material. Since PEG earlier had been
used only for wooden material of small dimensions there was no knowledge available about how to
proceed, neither with the large-scale immersion conservation treatment, that would be needed for Vasa’s
wooden sculptures and loose construction timbers, nor with the treatment of the hull as a whole. There
had been no laboratory research done to prepare for the conservation treatment of the Vasa wooden
material. Depending on the judgement that it was necessary to start the conservation procedure
immediately after the salvage operation was finished, the research and the conservation procedure were
started almost simultaneously.
The principal aim of this dissertation is to investigate whether or not PEG has acted as a dimension
stabilizing agent of the Vasa wooden material, e.g. whether or not the PEG molecules have penetrated
into the secondary cell wall, and therein have been able to preserve the capillary system through the
drying and continuing maintenance periods.
This dissertation deals mainly with the experimental work made during the conservation period,
intended, and performed aimed to gain reasonable knowledge about the conservation parameters to
proceed with the treatments, and to adjust the methods chosen. The experimental work was mainly
performed as immersion experiments, where various parameters of the PEG solution and PEG of various
molecular weights were tested. At most of the experimental work oak removed from the Vasa hull was
used as test material. The results were used both at surface application on the Vasa hull and at
immersion treatment of large loose timbers and wooden objects from the Vasa, that were treated in two
large steel tanks, each of a volume of c. 40 m3. Some results, presented in this dissertation, of the
experiments performed may be of interest to those who intend to use PEG as a dimension stabilizing
substance for archaeological wood from wet sites.
Information from the monitoring of the full-scale immersion treatments has also been used to evaluate
conservation parameters. The aim was to enable decisions about how to handle the conservation
parameters to achieve the best penetration of PEG with a minimum of shrinking and distortion of the
wooden objects during the conservation procedure.
The achievement of an equilibrium between the PEG-concentration of the conservation solution and the
PEG absorbed by the wood was agreed upon as a basis for the conservation procedure, but it soon
became obvious that the equilibrium criteria were difficult to define. This situation aroused some internal
disagreements within the Vasa project.
The construction of a permanent museum building was part of the Vasa project from the beginning.
However, the timing of this process was not in the power of the Vasa museum management to influence,
and political decisions about the financing of the construction of a permanent museum became an
important factor that forced the conservation procedure towards its completion.
This dissertation is based on the results from the pioneering work of conservation of the Vasa hull and of
large wooden objects belonging to the Vasa find, to develop and generalize conservation methods using
PEG as a dimension-stabilizing substance for archaeological wood from wet sites.
2010 05 13
Birgitta Håfors