Extraction of iron compounds from wood from the Vasa

In 2000, salt precipitates were found on surfaces of the Swedish warship Vasa, accompanied by low pH values, partly as a result of oxidation of accumulated sulfur compounds. One hypothesis is that oxidation of the sulfur compounds was catalysed by the large amounts of diverse iron compounds present in the wood. It is therefore of interest to develop a method to extract the iron compounds and simultaneously neutralise the acids. The iron compounds could be extracted using an aqueous solution of ethylenediimino-bis(2-hydroxy-4-methylphenyl)acetic acid (EDDHMA) or diethylenetriamine penta-acetic acid (DTPA) at alkaline pH, leaving only small amounts of iron compounds. The polyethylene glycol (PEG) used as the conservation agent, salts and other water-soluble compounds were co-extracted. The extraction rate was enhanced by stirring and by higher concentrations of the chelator, but diffusion of compounds through the wood was the most important factor for the overall extraction efficiency. Extraction of iron compounds from deep inside the wood is time-consuming and may take years. The results from this study imply that aqueous extraction with strong chelators at relatively high pH, 9–11, effectively removes iron compounds and neutralises the acids present. Although the results are promising, it is still too early to start major re-conservation of Vasa wood using extraction, as all the effects on the wood are not known, but such studies are in progress.