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Sulfur within Recovered Shipwreck Threatens Its Preservation

It might seem that raising a 1,334-ton warship from the bottom of the ocean would be the most difficult part of preserving the relic for future museumgoers. But in the case of the Vasa, a 17th-century Swedish shipwreck, a report published today in Nature indicates that the greatest threat to the ship's longevity comes from within¿in the form of sulfuric acid.

In 1628 the Vasa sank on her maiden voyage. The ship was salvaged from the bottom of Stockholm harbor 333 years later and has been on display in Sweden's Vasa museum since 1990. Scientists had spent 26 years preparing the ship for viewing by treating it with polyethylene glycol, which replaced internal water in the ship's structure and allowed it to dry without shrinking. Following up on previous reports of salt crystals forming on wood surfaces within the Vasa, Magnus Sandstrom of the University of Stockholm and his colleagues analyzed the wreck. The team discovered large amounts of sulfur within the vessel's beams¿enough to produce 5,000 kilograms of sulfuric acid.

In explanation, the researchers suggest that great amounts of hydrogen sulfide gas¿a byproduct of bacterial decomposition in oxygen-deprived waters¿entered the ship's beams as it lay on the bottom of Stockholm harbor. Over the course of more than three centuries, the gas slowly transformed into elemental sulfur. According to the report, iron ions produced from the breakdown of more than 8,000 bolts in the wreck can catalyze the reaction that turns sulfur atoms into sulfuric acid. "The most immediate threat," the authors write, "is acid-catalyzed hydrolysis of the cellulose which would reduce the stability of the wood."

Conservation techniques that either oxidize or remove the sulfur without degrading the cellulose of the wooden structures should be developed, the authors propose. In addition, iron or copper parts could be treated to stop them from catalyzing destructive reactions. The Vasa represents an extreme case that contains more sulfur than other comparable wrecks. The findings may, however, provide additional support for a recently launched project that supports leaving sunken ships at sea and using technology to let the public visit them.

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