Vasa (ship) - Conservation

Conservation

Although Vasa was in surprisingly good condition after 333 years at the bottom of the sea, it would have quickly deteriorated if the hull had been simply allowed to dry. The large bulk of Vasa, over 900 cubic meters (32,000 cu ft) of oak timber, constituted an unprecedented conservation problem. After some debate on how to best preserve the ship, conservation was done using polyethylene glycol (PEG), a method that was also used years later in the conservation process of the 16th century English ship Mary Rose. Vasa was sprayed with this glycol for 17 years, followed by 9 years of slow drying.

The reason that Vasa was so well-preserved was not just that the shipworm that normally devours wooden ships was absent but also that the water of Stockholms ström was heavily polluted until the late 20th century. The highly toxic and hostile environment meant that even the toughest microorganisms that break down wood had difficulty surviving. This, along with the fact that Vasa had been newly built when it sank, contributed to its conservation. Unfortunately, the toxicity of the water also had a negative effect. The sulfides present in the porewater of the sediments around Vasa had penetrated the wood, and when the ship was salvaged, and exposed to air after about 300 years of immersion in oxygen-depleted water, it began reacting with atmospheric oxygen. After exhumation in 1961 from the protective anoxic water, sulfide oxidation produced sulfuric acid. In the autumn of 2000, spots of white residue from only a few centimeters to half a meter (c. 3 to 20 in) were noticed on Vasa. These turned out to be sulfate-containing salts that had formed on the surface of the wood when the sulfides reacted with atmospheric oxygen. The stains had a very low pH and were the first indications that the ship contained considerable amounts of sulfuric acid. The salts on the surface of Vasa and objects found in and around it are not a threat themselves (even if the discoloring may be distracting), but if they are from inside the wood, they may expand and crack the planking from inside. This would cause particularly serious damage if it happened to objects made by skilled craftsmen, such as household items or some of the hundreds of carved sculptures. As of 2002, the amount of sulfuric acid in Vasa's hull was estimated to be more than 2 tonnes, and more is continually being created. Enough sulfides are present in the ship to produce another 5,000 kilograms (11,000 lb) of acid at a rate of about 100 kilograms (220 lb) per year; this might eventually destroy the ship almost entirely.

While most of the scientific community considers that the destructive substance responsible for Vasa's long-term decay is sulfuric acid, Ulla Westermark, professor of wood technology at Luleå University of Technology, has proposed another mechanism with her colleague Börje Stenberg. Experiments done by Japanese researchers show that treating wood with PEG in an acidic environment can generate formic acid and eventually liquify the wood. Vasa was exposed to acidic water for more than three centuries, and therefore has a relatively low pH. Samples taken from the ship indicate that formic acid is present, and that it could be one of the multiple causes of a suddenly accelerated rate of decomposition.

The museum is constantly monitoring the ship for damage caused by decay or warping of the wood. Ongoing research seeks the best way to preserve the ship for future generations and to analyze the existing material as closely as possible. A current problem is that the old oak of which the ship is built is starting to give way, and the braces that support it are pressed deeper into the hull every year. "The amount of movement in the hull is worrying. If nothing is done, the ship will most likely capsize again", states Magnus Olofson from the Vasa Museum. An effort to secure Vasa for the future is under way, in cooperation with the Royal Institute of Technology, Texas A&M University and other institutions around the globe.

To deal with the problem of the inevitable deterioration of the ship, the main hall of the Vasa Museum is kept at a temperature of 18–20 °C (64–68 °F) and a humidity level of 55%. To slow the destruction by sulfuric acid, different methods have been tried. Small objects have been sealed in plastic containers filled with an inert atmosphere of nitrogen gas, for halting further reactions between sulfides and oxygen. The ship itself has been treated with cloth saturated in a basic liquid to neutralize the low pH, but this is only a temporary solution as acid is continuously produced. The original bolts rusted away after the ship sank but were replaced with modern ones that were galvanized and covered with epoxy resin. Despite this, the new bolts have also started to rust and are releasing iron into the wood, which accelerates the deterioration. Plans call for new bolts made from materials that are non-reactive, such as titanium, carbon fiber or fiberglass.