In 1628, the Swedish warship Vasa sank a mere 4,265 feet (1,300 meters) into her maiden voyage. Whereas merchant vessels of the day were stabilized by the weight of cargo in their holds, warships carried their cargo - heavy guns - higher up. Though 122 tons of stone had been stored low in the Vasa as ballast, it was not enough to counter the weight of the ship's upper hull, three masts, 10 sails and 64 guns. The ship leaned too far, and water poured in through her open gun ports. She capsized and sank to the bottom of Stockholm harbor, where she remained for 333 years.
In 1961, the Vasa was raised in good condition from her brackish grave. After extensive treatment to stabilize and dry her timbers, in 1990 she was put on display at the Vasa Museum in Stockholm. Ten years later, museum conservators noticed that powdery salts were rapidly forming on her surfaces and that the wood in her holds was growing soft and acidic. In short, the Vasa was disintegrating.
The conservators called an emergency meeting with colleagues and chemists from Sweden, Denmark and Australia to seek solutions to a problem that threatens the Vasa and other famous finds including the Skuldelev Viking ships, the Bremen Cog, the Mary Rose and the Batavia. Collaborators in Sweden and at Stanford have analyzed the chemistry of wood decay in the Vasa using a new technique - x-ray absorption spectroscopy. It employs high-energy synchrotron beams, produced when electrons accelerate around rings, to make chemical "fingerprints" that identify different oxidation states in a sample. Their findings, published in the Feb. 21 issue of Nature, may help conservators worldwide preserve wooden artifacts retrieved from the deep, many of which are displayed in museums. The findings explain "the important role that scientists can have in keeping our historical treasures, which are indeed our connection to the old days," said researcher Farideh Jalilehvand of the Stanford Synchrotron Radiation Laboratory (SSRL). "We are now analyzing the samples from other historical ships, and it seems that the Vasa problem is in fact a general problem for all of them." That problem is formation of highly corrosive sulfuric acid inside wood beams. The Vasa had sunk to a depth of 105 feet (32 meters), and the dearth of oxygen there inhibited wood-metabolizing microbes. But this environment favors bacteria that convert sulfate ions in seawater to hydrogen sulfide. In the hundreds of years that the Vasa was submerged, hydrogen sulfide penetrated into the deepest layers of the wood. Under the sea, chemical reactions turn hydrogen sulfide into elemental sulfur or pyrite, depending on the amount of available iron ions.
The Swedish and Stanford researchers found evidence that sulfur had accumulated within the beams. The presence of many sulfur intermediates indicated stepwise oxidation toward a corrosive end product of sulfuric acid. In the air of the museum, iron species in the wood were catalyzing oxidation of the accumulated sulfur.
Almost half of the sulfur has already been oxidized, according to the researchers. Full oxidation of the remaining elemental sulfur could produce up to 11,000 pounds (5,000 kilograms) of sulfuric acid. Oxidation was facilitated by rust released from the completely corroded original bolts, as well as from new iron bolts inserted after salvage. "The work is important because it draws attention to a previously unknown danger to recovered wooden marine-archaeological artifacts that are on display or in storage in museums all around the world - that is, the destruction of the wood by way of sulfuric acid produced when sulfur embedded in the wood is oxidized in air," says SSRL chemist Patrick Frank. "Museum curators world-wide will have to pay attention to the threat revealed by this work." Saving the past for the future To learn the anatomy of a shipwreck, scientists applied present technologies to past relics. They used x-ray spectroscopic techniques to bombard samples with high-energy x-rays that produce energy "fingerprints" identifying a sample's chemical composition. One technique, x-ray powder diffraction (XRD), was used to chemically identify products of oxidation on wooden surfaces. Another technique, x-ray absorption near-edge structure (XANES), allowed identification of sulfur compounds inside the wood by analysis of ground-up core samples. Yet another technique, x-ray photoelectron spectroscopy (XPS), allowed scientists to determine the amounts of all significant chemical elements, except hydrogen, in the wood.
Based on their findings, the researchers recommend strict regulation of a wooden marine artifact's environment. A relative humidity of 55 percent and a temperature of less than 68 degrees Fahrenheit (20 degrees Celsius) will slow migration of water and oxygen into wood and ensuing mechanical and chemical damage. The researchers also recommended replacing the 8,500 iron bolts holding the boat together - inserted to replace the completely corroded original bolts - with bolts made of an inert material. Eventually the infused iron in the wood might be rendered electrochemically inert with a chelating agent, and even extractable in an alkaline solution, but more work is needed to investigate this option.
The lead author on the Nature paper was Professor Magnus Sandstrom of Stockholm University. The other Swedish collaborators were Professor Ulrik Gelius of Uppsala University; Professor Ingmar Persson of the Swedish University of Agricultural Sciences and Chief Conservator Ingrid Hall-Roth of the Vasa Museum.
Jalilehvand has worked at the SSRL, an arm of the Stanford Linear Accelerator Center, since August 2000. Before that, she studied with Sandstrom at the Royal Institute of Technology in Stockholm, where she had the chance to visit the Vasa Museum many times.
"Just six months after I arrived here at Stanford, I learned about the acid problem in Vasa, and I asked Professor Sandstrom to send me a core sample of Vasa if possible," Jalilehvand said. "Last year on Feb. 16, he brought a 10-centimeter-long core sample, and I ran the X-ray absorption spectrum of the core sample at different depths in SSRL. This was the first time that the sulfur x-ray absorption spectrum was measured on wood."
Said Frank, who taught Jalilehvand how to analyze the spectra she collected: "X-ray spectroscopy is almost uniquely suited to evaluating the chemistry of elements in situ in these artifacts. There is no doubt that the information we obtained would not have been available by any other method."
This research was supported by grants from the Knut and Alice Wallenberg Foundation and the U.S. Department of Energy.
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