杏吧原创

IT WAS one of the great finds of the 20th century. The Vasa, the pride of the Swedish navy, heeled over and sank on its maiden voyage in 1628, drowning about a third of the 150 crew 鈥� and the ship鈥檚 cat 鈥� in the catastrophe. Yet when the warship was discovered over 300 years later it was almost completely intact. Conditions at the bottom of Stockholm鈥檚 harbour where it sank were perfect for preserving the ship鈥檚 timber. The Vasa was salvaged in 1961 and has been carefully preserved in its own museum. Today this stunning ship 鈥� complete with masts, over 700 wooden sculptures and the world鈥檚 oldest sail 鈥� is Stockholm鈥檚 biggest tourist attraction.

But despite years of careful preservation, archaeologists have suddenly discovered that the warship is under attack from an enemy just as destructive as those it was originally designed to face. The very chemicals being used to preserve the ship have helped trigger a reaction in its timbers that is generating sulphuric acid on a vast scale. The ship鈥檚 oak planks and beams may already have 2 tonnes of acid in them, and if the reaction continues unchecked, the vessel could eventually begin to crumble away. And this problem isn鈥檛 unique to the Vasa. Recent studies show that other wooden ships salvaged from the seabed, including the Mary Rose at Portsmouth and the Batavia in Fremantle, Western Australia, are suffering the same problem.

The discovery raises fundamental questions about the conservation of waterlogged wood. How can we hope to save the ships around the world that are threatened by acid attack? Is there a better way of conserving waterlogged wood so it can be exhibited in museums? Or would it be better to excavate a wreck, examine it where it lies and then simply rebury it with silt and sand?

These are hugely important questions. According to the United Nations Educational, Scientific and Cultural Organization (UNESCO) there are 3 million undiscovered wrecks on the ocean floor. Most of them are wooden ships and all of them contain valuable wooden artefacts. This is an enormous slice of human history, the rotting remains of thousands of swashbuckling yarns. The Queen Anne鈥檚 Revenge, for example, thought to be the flagship of the notorious pirate Blackbeard, was found off North Carolina in 1996. The Spanish Main is littered with similar hulks. About 850 ships have foundered around the Azores alone since 1522. And there are ancient vessels dotted all over the Mediterranean.

While many of these wrecks have been smashed by storms or eaten away by shipworm 鈥� the creature that rapidly destroys submerged wood 鈥� the Vasa was more fortunate. It had settled gently into the sediment at the bottom of the harbour and the Baltic鈥檚 brackish waters aren鈥檛 salty enough to sustain shipworm. The only real damage was caused by motorised anchors which had clawed away some of the ship鈥檚 timbers.

The Vasa was also the first major shipwreck to be treated with what was then a new technique for conserving wood. Wood that has been submerged for centuries is very fragile because the cellulose in the cell walls of the timber is eaten away by bacteria. It is also saturated with water: when the Vasa was raised every kilogram of wood contained 1.5 kilograms of water. However, the structure could have collapsed had it been allowed to dry out, so the water was replaced with polyethylene glycol (PEG), which takes the place of the missing cellulose and helps to strengthen the wood. Small artefacts can be simply soaked in a bath of the stuff. Larger objects need a different approach 鈥� a slow drenching that gently removes all traces of water. So the Vasa was sprayed with a solution of PEG for 17 years. The treatment proved so successful that it has now become the standard way of conserving waterlogged wood.

The first signs of a problem appeared in 2000. After a humid summer, museum staff noticed powdery deposits on the surface of some of the ship鈥檚 planks. So they called in Magnus Sandstr枚m, professor of structural chemistry at Stockholm University. He discovered that an unexpected reaction in the timbers was generating sulphuric acid (see 鈥淭he acid test鈥�). From samples of the ship鈥檚 timber, Sandstr枚m estimated that the timbers already contained 2 tonnes of acid. And if the reaction continues unchecked, he adds, it will eventually generate a further 6 tonnes of sulphuric acid, which would eat away at the wood. We don鈥檛 know how fast it鈥檚 breaking the wood down, says Sandstr枚m. 鈥淚t may take 5, or who knows, 50 years before the damage is too severe for the wood to be treated.鈥�

Sandstr枚m鈥檚 immediate concern is to neutralise the sulphuric acid that has built up in the Vasa and to halt the reaction so that no more is formed. Wood does not naturally contain high levels of sulphur. The problem began on the seabed when bacteria in the sediment ran short of oxygen. 鈥淚f they do not have oxygen,鈥� says Sandstr枚m, 鈥渢hey take it from sulphate ions in seawater.鈥� The bacteria reduced the sulphate ions to hydrogen sulphide, which worked its way into the timber and ended up as sulphur. In the Vasa鈥檚 case this was compounded because the growing city of Stockholm treated the harbour as a sewer.

In common with other wooden ships the Vasa contained a lot of iron, from bolts and nails to the metal fittings of muskets and cannon balls. Most of the metal corroded when the ship was on the seabed, leaving iron deposits that turned the ship鈥檚 oak planking black. When the vessel was restored, the 5500 one-metre-long bolts that held it together were replaced with new iron bolts. But PEG corrodes iron and the continuous spray treatment helped carry the iron deeper into the timber, where it catalysed a reaction between sulphur and water (from moisture in the air), forming sulphuric acid. 鈥淚t was an unfortunate combination,鈥� says Sandstr枚m. 鈥淭hey didn鈥檛 know that at the time.鈥�

The result is that some parts of the Vasa are extremely acidic: tests in April this year found pHs between 1 and 3.5 at 850 points around the ship. Sandstr枚m has already tried spraying the timber to neutralise the sulphuric acid. However the problem is complicated because the museum does not want to close its main attraction, so any spray treatment cannot pose even the smallest threat to public health. 鈥淭echnically it can be done,鈥� says Sandstr枚m. 鈥淲e will do it in sections, one part at a time.鈥�

His priority is to find a way of halting the acid attack and preventing the rest of the sulphur in the ship from turning into acid. The museum plans to remove the new iron bolts from the ship, but they will only be able to take out about half of them, because as the ship settled down in its new environment the wood shrank and moved, locking many of the bolts in place. And removing even half the bolts will weaken the 60-metre-long vessel. 鈥淲e will have to make a cradle for the ship to support it,鈥� says Sandstr枚m. The bolts will be replaced, possibly with ones of titanium or carbon fibre. Sandstr枚m also hopes to stop the remaining iron compounds in the wood from catalysing the acid reaction by preventing oxygen reaching the sulphur. He is also considering using chelating agents 鈥� chemicals that lock up the iron as an inert complex. This should help stop the iron from catalysing the acid reaction.

The bad news for archaeologists is that the same problem almost certainly affects most wooden wrecks salvaged from the sea. Sandstr枚m has already discovered that the Mary Rose, Henry VIII鈥檚 flagship that sank in Portsmouth harbour in 1545, also contains high levels of sulphur. Its hull is still being sprayed with PEG in its Portsmouth museum.

Problems down under

High levels of sulphur have also been found in the wreck of the Dutch ship Batavia, which sank off the coast of Western Australia in 1629. Part of the wreck is now preserved in the Western Australia Maritime Museum in Fremantle. 鈥淭he Batavia has a lot of sulphur in it,鈥� says Sandstr枚m, who has analysed cores taken from the ship. The Batavia also has very high levels of iron. In all, Sandstr枚m has found signs of the problem in almost all the wrecks he has tested 鈥� including three still on the seabed. In fact the only one that he has found which is effectively free of sulphur is the Bremen Cog 鈥� a 14th century trader that sank in the freshwater of the river Weser.

Ironically, because only part of the Batavia survived, it will be easier to protect from sulphuric acid than the Vasa. The Batavia was salvaged plank by plank and only reassembled in the museum, while the huge hull of the Vasa is intact, and many parts of the ship鈥檚 structure are inaccessible.

Currently the Western Australia Maritime Museum is keeping the Batavia stable by controlling humidity levels around the ship. 鈥淏ut this is a very expensive thing to do,鈥� says Ian Godfrey of the museum. 鈥淔or the Batavia the running costs are A$100,000 a year.鈥�

Meanwhile small objects from the Vasa will be soaked in an alkali such as sodium bicarbonate to neutralise the acid. Then they will be treated with either more PEG, or with silicone oil. This latter process has a number of variations. Essentially the wood is soaked in ethyl alcohol to remove the water and then acetone to remove the alcohol. It is then soaked in silicone oil with a 鈥渃ross linker鈥�, methyl trimethoxysilane, to promote polymerisation.

According to Wayne Smith of Texas A&M University, who developed the use of silicone for conserving wood, the result is an artefact that looks like wood, handles like wood and doesn鈥檛 need any expensive environmental controls to ensure its stability. The polymer should act as a barrier to oxygen, and computer modelling of accelerated wear tests carried out on silicone-treated wood show that it will be 250 years before it needs treating again, he says.

However, the use of silicone oil on wooden artefacts is highly controversial. Many museum conservators dislike the technique because the silicone forms a permanent bond with the wood. Smith dismisses these concerns. He says that PEG also bonds with wood. 鈥淭here is no such thing as a totally reversible process. Reversibility is never the issue. Retreatability and long-term stability is. Polyethylene glycol is a loose cannon.鈥�

Unfortunately the silicone treatment isn鈥檛 cheap. It is roughly three-and-a-half times more expensive than the PEG treatment (which includes the cost of controlling the museum environment). And the size of the vat required to treat an object is also a limiting factor. The biggest thing that Smith has treated so far is a sea chest. Nevertheless he points to a success that would be difficult to emulate with any other technique. Researchers at the university treated a wicker basket full of cannon balls salvaged from the wreck of La Belle, a ship lost off the coast of Texas in 1686. The wicker was weak and the basket was full of silt as well as cannon balls. 鈥淚t was a mess,鈥� says Smith. So they soaked the lot in silicone and once it was stable, 鈥渨e just brushed off the dirt鈥�, says Smith, 鈥渁nd excavated the cannon balls.鈥�

In December 2001, UNESCO recommended that the best long-term solution for most wrecks was not to try and conserve them out of the water but simply to rebury them once archaeologists have explored the site and recorded the remains. This is partly because of the unresolved problems with conserving waterlogged timber, but also due to the sheer number of sites: conserving all these wrecks out of water would be prohibitively expensive. 鈥淚t鈥檚 a huge problem trying to raise the money to conserve ships,鈥� says David Gregory, who leads a team at the National Museum of Denmark in Copenhagen researching reburial techniques. The Western Australia Maritime Museum, for example, recently drew up a plan to salvage the James Miller, a former slave trader that sank outside Fremantle in 1841, but abandoned it when the project鈥檚 sponsors decided it was too expensive. 鈥淯ntil the problem of conservation is resolved, it is probably best left where it is,鈥� says Godfrey.

Yet leaving shipwrecks where they are has its drawbacks. Shipworm, as well as divers searching for buried treasure, can damage sites. Even attempts to clean up the environment can backfire: the heavily polluted waters of Stockholm鈥檚 harbour helped preserve the Vasa since the lack of oxygen killed the bacteria that would have eaten away its timbers. But since the 1940s the water quality of the harbour has improved dramatically. Salmon were reintroduced in 1970. If the Vasa had been left where it sank, the cleaner waters in the harbour would have accelerated its deterioration. Who knows how much of the ship and its precious contents would have been lost forever.