Sulphuric acid is slowly eating away at the wood on the Vasa, which lay on the seabed for 333 years until it was located by a shipwreck hunter on August 25, 1956. It is now one of Stockholm’s most popular tourist attractions.
“We have installed a new climate system in the museum (to stabilise the humidity level) and now the rotting is being observed at a much slower rate,” museum head Klas Helmerson told AFP.
“We are also experimenting with other ways of reducing the sulphur … to try to halt this process,” he said.
The warship stands in a special museum built over an old dry dock near the port.
In 2000, the museum discovered white salt stains on the ship’s hull, indicating that sulphuric acid was eating away at the wood.
Sulphur-reducing bacteria is naturally present in the seabed where the Vasa lay for hundreds of years.
Now that the ship is out of the water, the sulphur has combined with traces of iron on the hull left by rusted iron bolts to turn into iron sulphate.
When the iron sulphate oxidises in the the high humidity levels caused in part from the many visitors in the museum, sulphuric acid is created.
At the time the rot was discovered, museum officials warned that if nothing was done the boat could be destroyed in 10 to 20 years.
The Vasa was commissioned by Swedish King Gustav II Adolf in 1625 and was designed to be the mightiest warship in the world.
It was armed with 64 guns on two gun decks, but did not have enough ballast to counteract the weight and sank just minutes after setting sail on its maiden voyage. The warship lay on the seabed for more than three centuries, until a Swedish shipwreck enthusiast, Anders Franzen, located it in 1956 at a depth of 32 metres.
“He used a plummet with a jagged edge that he dragged behind his boat, and on August 25, 1956 it got caught on something. He pulled up a tiny piece of black oak measuring about one centimeter … and he knew he had found some old wood,” Helmerson said.
“He sent divers down to see what it was, and it was the Vasa.”
The ship was raised in 1961, and the museum built around the ship opened in 1990. It is one of the most popular tourist sites in Scandinavia, recently recording its 25 millionth visitor.
On Friday, the museum was to celebrate the 50th anniversary of Franzen’s find with a reception for specially-invited guests.