Iron from rusted bolts and cannon balls has diffused into Vasa's wood since its time on the seabed. Iron can speed up deterioration of both wood components and the conservation agent polyethylene glycol, PEG. It is therefore important to determine by what means iron can be removed from Vasa wood.
One method which has been developed in collaboration with the Department of Chemistry at the Swedish University of Agricultural Sciences is the extraction of iron using so-called complexing agents. Theory and practice are described in our brochure Iron Removal from Waterlogged Wood. Complexing agents have the ability to strongly bond with iron ions, and can in solution remove iron from wood. By treatment with these solutions, significant amounts of iron have been extracted from Vasa wood in a series of experiments. We are now applying the method to a group of loading equipment objects in our collection, work you can follow on The ship's blog.
The degree of extraction depends on a number of factors. It is harder to extract iron from oak, which is a very close-pored wood species, than from pine which is more porous. More degraded wood is also more porous, and therefore more easily extracted. The thickness of the object is decisive, as are type of iron compounds and their distribution in the material. Concentration of complexing agent and pH in the solution, and how often the solution is changed, also affect the outcome. Even during ideal conditions, the extraction of iron is a very slow process that requires long treatment times.
- Department of Chemistry at the Swedish University of Agricultural Sciences (external website)
- Iron Removal from Waterlogged Wood (pdf)
During the extraction treatment, the conservation agent PEG is also removed. This means that the object must be re-conserved afterwards. This is done through PEG-impregnation followed by freeze-drying, a gentle method used today in the conservation of archaeological waterlogged wood.
Results are good. Iron has largely been removed from the wooden objects that have been treated. In addition, the acidity of the wood is reduced in the process. This means a chemically more stable wood and improved conditions for the future preservation of the objects. Surface detail and natural wood character have also been restored to the objects after the re-conservation.
Parallel to continued work, studies on other methods of iron removal are planned in future projects. However, even if methods are improved, new treatments will always cause further stress to objects. Each case must therefore be assessed in terms of the benefits and risks involved. Much still remains to be done to establish both the positive and negative effects of iron extraction and re-conservation.