Press Release: Terror of the Tudor Seas Suffers from Sulfur
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Date Issued: September 27, 2005
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Henry VIII's warship, the Mary Rose, wreaked havoc on the French navy for 34 years until she was wrecked in 1545. Salvaged from the sea in 1982, she now rests in the Mary Rose Museum in Portsmouth, England. Pieces of her helm, however, recently travelled to Menlo Park, California and Grenoble, France where intense x-rays pierced the wood to analyze the sulfur and iron within.
A research team, led by University of Stockholm Professor Magnus Sandström, and including Mary Rose chief scientist Mark Jones, used synchrotron x-rays from the Department of Energy's Stanford Synchrotron Radiation Laboratory (SSRL) at the Stanford Linear Accelerator Center and the European Synchrotron Radiation Facility (ESRF). The prevalence of sulfur and iron in the oak timbers poses long-term preservation challenges, but the new information can also help to overcome the threats.
The results, published Sept. 26 in Proceedings of the National Academy of Sciences, indicate the surviving wood contains two tons of sulfur in different forms, uniformly distributed within the 280-ton hull. Over time, sulfur can convert to sulfuric acid, which could slowly degrade the wood until its stability is lost. Sandström's earlier work, also done at SSRL, on the Swedish warship Vasa (under water for 333 years) showed that the accumulation of sulfur within shipwrecks in seawater is common.
Complementary experiments at SSRL and ESRF revealed the quantities, locations, and chemical state of sulfur and iron in the Mary Rose wood, zooming in on their structures on an atomic scale. Researchers found the highest concentrations of sulfur in areas between the wood cells, areas primarily made of lignin that acts like glue to hold cells together. The lignin reacted with hydrogen sulfide produced by marine bacteria, accumulating sulfur in surprising amounts. Other marine bacteria chewed up the cellulose cell walls, leaving a hollow "house of cards" once the ship was pulled from the water. The sulfur may have helped preserve the ship while it was still submerged.
"The amount of accumulated sulfur in the wood was indeed unexpected, and especially the formation of organosulfur compounds. We are trying to find out more precisely what the reactions are," Sandström said. "These compounds are also of interest for understanding how sulfur accumulates in marine sediments, and eventually ends up in fossil fuels such as oil."
Once exposed to air, the ship faces dry perils. The Mary Rose contains a great deal of iron from corroded iron bolts, nails and other ship objects. Exposed to the oxygen in air, the iron catalyzes the oxidation of sulfur into sulfuric acid. Studies also show that atmospheres with high, varying humidity accelerate this process.
The ship is in no immediate danger because the acid gets washed away during conservation. A spray treatment that replaces the water in degraded wood with waxy polyethylene glycol, so the wood does not shrink or crack as it dries out, also washes out acid. However, in the Vasa, two tons of acid has gradually built up in the 26 years after its spray treatment ended.
The researchers suggest that long-term preservation requires chemical treatments to remove or stabilize the remaining iron and sulfur compounds, and reducing humidity and access to oxygen.
At the Mary Rose Trust, conservation scientists are already investigating new treatments to prevent new acid formation in this British treasure. Trust scientists are currently using the synchrotron facilities of the Council for the Central Laboratory of the Research Councils (CCLRC) in the U.K. to test the efficacy of new methods. To slow down the oxidation reaction and prevent new acid formation, wood samples from the Mary Rose are being treated with antioxidants in combination with low- and high-grade polyethylene glycol. Another conservation approach is to maintain the archaeological wood in a stable climate, with a constant temperature and a constant low humidity of 55 percent. To maintain a stable microclimate within the wood structure, a surface coating offers a possible solution, although the effectiveness of this approach has yet to be tested.
"The method used in these studies—synchrotron radiation-based x-ray absorption spectroscopy—is a particularly powerful tool to study samples taken directly from these ships," said SSRL Professor and Assistant Director Britt Hedman. "It can be used directly on the samples without any pretreatment that could change them. It provides not only information on which chemical element, such as sulfur, is present, but also the type of chemical form, or species, it has, and therefore can directly provide the chemical pointer towards the proper preservation approach."
Researchers also employed an additional approach not used on the Vasa: imaging the wood samples with a very small x-ray beam to map where the chemical species were located.
"This leads to a better level of understanding," Hedman said. "We see this as an increasingly important tool for many areas of archaeometry."
Conservationist Jones agrees. "This ongoing research is an important step forward in devising improvements to the current Mary Rose hull treatment program," he said.
Along with the hull, some 19,000 artifacts were recovered after 437 years underwater. Certain remaining sections of the bow and anchor of the once-mighty Tudor warship will be raised to the surface on October 11.