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Marine archaeology

Acid attack

Credit: HANS HAMMARSKIÖLD/VASA MUSEUM, STOCKHOLM

If you were planning to visit the impressive restoration of the Vasa, now is a good time. The Vasa was a 61-metre,1,210-tonne warship, which sank in Stockholm harbour on its maiden voyage in 1628, but was raised in 1961 and restored for display in a museum in Stockholm (see pictures). A multidisciplinary group of researchers, however, has discovered that the ship's timbers are in danger of disintegrating. As Magnus Sandström and colleagues report in this issue (Nature 415, 893–897; 2002), and describe in an exhibition that opens this week in the Vasa Museum, sulphuric acid is being produced within the beams of the ship. The acid attacks the wood both chemically, by acid hydrolysis of cellulose, and physically, as the sulphate minerals expand during crystallization.

So where is the sulphuric acid coming from? Alerted by sulphate crystals forming on the surface of the Vasa's timbers, Sandström et al. went on to find large quantities of elemental sulphur inside the wood. They believe that hydrogen sulphide — a common product of bacterial decomposition in anoxic waters — permeated the ship's timbers and was gradually transformed to elemental sulphur during the 333 years that the ship lay at the bottom of Stockholm harbour. This created a reservoir of sulphur, which, if fully oxidized, could produce as much as five tonnes of sulphuric acid.

Credit: HANS HAMMARSKIÖLD/VASA MUSEUM, STOCKHOLM

A complicating factor comes from the legacy of the ship's 9,000 original iron bolts that have largely rusted away. Iron (iii) ions are effectively a catalyst here, gradually oxidizing elemental sulphur to sulphuric acid. The reduced iron is then itself re-oxidized by oxygen from the air, ready to go through the cycle once more.

Museum curators are well aware of the threat of oxidation to waterlogged wooden artefacts after salvage. As well as stabilizing the fragile lattice of remaining wood by replacing the water with non-volatile preservatives, conservation methods routinely involve limiting further biological and chemical oxidation with sterilizing solutions, and carefully controlling humidity and temperature. But the newly observed sulphur threat calls for different measures. Neutralizing five tonnes of sulphuric acid is not really feasible, and the most promising solution lies in tackling the iron catalyst. One proposal of Sandström et al. is to identify an agent that will form a complex with the iron solutes, making them inert, and possibly even extractable by rinsing the ship with alkali.

What about other wrecks? Fortunately, the Vasa seems to be an extreme case. The decision in the sixteenth century to close off two inlets into Stockholm harbour, to hinder attack from the Russians, along with centuries of sewage disposal in the harbour, helped to create an especially stagnant and sulphurous resting place. Moreover, the threat of sulphur acidification in wrecks that were completely buried in sediments, such as the Mary Rose, now on display in Portsmouth, UK, should be smaller. Indeed, Sandström et al. found that sulphur accumulation was greatest in the exposed timbers of the Vasa. But in ships that were not buried — the Batavia of the Dutch East India Company, for instance, which was wrecked off Western Australia in 1629 — initial analyses confirm that the sulphur problem is more common, if not as severe as in the Vasa.

These new investigations support recent moves to let sunken ships lie, and preserve and study them where they sank (Nature 415, 460; 2002) — virtual technology would still allow the public to view the wrecks and their sites. After all, why not capitalize on the preserving features of marine sediments, rather than let them express their acidic side later on?

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Gillon, J. Acid attack. Nature 415, 847 (2002). https://doi.org/10.1038/415847a

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