Both iron and sulphur have diffused into Vasa’s wood since its time on the seabed, iron in the form of corrosion products from rusted bolts and cannon balls, and sulphur in the form of hydrogen sulphide. Iron can, in a number of ways, speed up deterioration of both wood cellulose and the conservation agent, PEG. It is therefore important to determine whether and 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 in Uppsala and been studied in a number of trials since 2002 is the extraction of iron using so-called complexing agents or chelators. Extraction is a process in which a substance is removed from a material with the help of a solvent. In this case the iron is removed with the help of the complexing agent EDDHMA (ethylenediimino-bis(2-hydroxy-4-methylphenyl)acetic acid)or DTPA (diethylenetriamine pentaacetic acid) in solution. These have the ability to strongly bond with iron ions. Experiments have shown that treatment in these solutions can extract a large amounts of iron from  Vasa wood, over 80% in some cases. 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. How much iron is present in the wood and how easily accessible it is are also factors, as are concentration of complexing agent, solution pH and how often the solution is changed. Even during ideal conditions, the extraction of iron is a very slow process that requires years of treatment.

During the extraction treatment, the conservation agent PEG is also removed. This means that the object must be re-conserved afterwards. This has principally been done through new PEG-impregnation followed by freeze-drying, the primary method used today in the conservation of newly excavated waterlogged wood. The results so far have been good. Iron has largely been removed from the objects. We anticipate that conditions for their future preservation have been improved. The objects have even returned to their natural wood character after the re-conservation.

Other methods of iron extraction and re-conservation 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.