Warship's New Foe: Wet Anoraks

Nearly four centuries after it sank on its maiden voyage, and 40 years after it was first raised, one of the most magnificent men o' war ever built faces a second tragedy: slowly but surely, it is turning to dust.

The Stockholm museum that houses the mighty Vasa, the world's largest warship when it was launched in the 17th century, has started an urgent appeal to save the vessel after scientists discovered its timbers contained up to five tonnes of sulphuric acid.

"Scientists are working on a possible solution," said Ingrid Hall-Roth of the Vasa museum, which has been visited by 9m people since it opened in 1990. "But as things stand we don't have enough money to rescue the ship. It's not going to be easy."

The saga of the Vasa began when it cast off for its maiden voyage on August 10 1628. More than 61m (200ft) long, armed with 64 guns on two decks, the ship was the pride of Sweden's growing Baltic empire and of its ambitious king, Gustavus Adolphus.

But within minutes of the ship setting sail, to the horror of the tens of thousands of onlookers gathered on Stockholm's beaches, it began heeling dangerously. Water rushed in through the open gun decks and the Vasa sank, killing 30 of the 150 sailors on board.

At the time, shipbuilders had no reliable way to test their designs and the Vasa was top-heavy, its stone ballast unable to offset the weight of its massive superstructure and guns. In the rush to launch the vessel during Sweden's war with Poland, early indications that it might be fatally unstable were ignored.

When the Vasa was finally raised in April 1961, after 333 years in the brackish water of Stockholm harbour, Sweden stood still. Schools and factories closed for the day and the country sold out of television sets. Miraculously, the vessel was almost completely intact.

In a mammoth 30-year operation, the water in its timbers was replaced with a chemical, polyethylenglycol, to prevent shrinkage as the vessel slowly dried out.

Just over a decade ago the extraordinary Vasa museum - built entirely around the newly restored ship - was opened to immense critical and popular acclaim. But during last year's wet summer, strange, damaging deposits began appearing on the Vasa's hull and beams.

Recently completed analysis showed that the sulphur slowly absorbed by the wood while it lay on the seabed had been drawn to the surface by the moisture from hundreds of thousands of rain-soaked visitors. As soon as it came into contact with the air, it formed sulphuric acid - and began eating away at the wood.

Swedish and American scientists, headed by Magnus Sandstrom, professor of structural chemistry at Stockholm University, have now discovered that 80% of the ship and the wooden objects it contains have suffered some degree of acid attack.

In an article for Nature magazine, they say the Vasa's interior has been treated with a solution to temporarily reduce the impact of the acid. A longer-term plan has also been drawn up to replace the 8,500 iron bolts used to rebuild the ship, which release more oxygen as they rust and further feed the fatal chemical reaction.

But ultimately, experts believe, the Vasa experience may convince scientists and marine archaeologists to leave valuable sunken treasures alone, an approach recommended by a recent Unesco report on the world's underwater heritage. Computer technology could then allow the public to make "virtual tours" of the find, while scientists preserve and study it strictly in situ.