Stockholm's famous preserved warship attraction, the 17th century Vasa, is reported to be suffering from a condition that poses a risk of decay.
The problem stems from sulphurs in its timbers rising to the surface to leave an acidic residue which, if left, will eat away at the wood.
The problem is said to have been caused by the ship's iron fittings, which produce the sulphur; and by higher-than-ideal levels of humidity in the ship hall encouraging the sulphur to wick out to the wood surfaces and react with air to form sulphuric acid.
The humidity is said to have been caused in large part by the thousands of visitors who flock to Stockholm's Vasa Museum each year.
Now a team led by Magnus Sandstrom, Head of Structural Chemistry at Stockholm Museum, has examined timbers on the ship's interior and a number of cabinet-housed artefacts.
It found that some 80% of the samples were affected, and is now working out how to treat the timbers and reduce the humidity to check the problem.
In the longer term, thousands of modern iron bolts used to rebuild the ship might have to be replaced, as they could have contributed to the production of sulphur.
The 64-gun warship, some 65m long, sank in Stockholm Harbour on its maiden voyage in 1628. It was raised, almost intact, in 1961 and, over 30 years, its timbers were flushed and dried. Polyethyleneglycol was used to replace its water content to prevent shrinkage.
The Vasa's problem has prompted questions about whether it is sensible to raise vessels and artefacts in view of the difficulties of stabilising preserved materials. But ship conservationists maintain that continuing improvements in conservation technologies justify such work.
"We can't stop now, with all the expertise we've built up and the ever-improving results we obtain," Mark Jones, Head of Collections at Britain's Mary Rose Trust, told Diver.
The Mary Rose, housed with artefacts in a museum at Portsmouth's Historic Dockyard, is being preserved along similar lines to the Vasa Ð but with crucial differences.
"We share research and information with the Vasa team, and you need only to compare the two ships' treatments to understand how things move forward," said Jones.
The Mary Rose began a similar flushing and drying process to the Vasa in 1994, after being sprayed for 12 years with chilled cold water.
However, differences in temperature control and spray concentrations, and polyethyleneglycol of lower molecular weight, mean that the flushing of impurities is now more efficient. "I am confident that what has happened with the Vasa will not occur with the Rose," said Jones. He felt sure that the Vasa's predicament would be overcome.
The Rose will remain in its wet-spray state until 2008, when air-drying will begin.
The biggest challenge to effective conservation lies in obtaining enough funding.
"We make about £700,000 a year from visitor receipts, but our operations budget is some £1.3 million a year," said Jacquie Shaw, The Mary Rose Trust's PR officer. "Recent grants that have helped us include £488,000 from English Heritage, and £300,000 from the DCMS."
The Vasa Museum takes "millions" a year from visitors, but it goes on upkeep, said a spokesman. "Now the sulphur problem will cost extra to put right. We will have to turn to the government for help."
The preserved vessel's lower gun deck
the upper deck towards the stern