Feature Story | 5-Aug-2002

A great warship fights on

DOE/SLAC National Accelerator Laboratory

The Vasa on display in the Vasa Museum, Stockholm, Sweden. Dimensions: length 61 m (69 m including bowsprit), maximum width 11.7 m, stern castle 19.3 m high, displacement 1210 tons (photo by Hans Hammarskiöld, Vasa Museum, Stockholm).

August 5, 2002--As part of a desperate battle to save the 374-year old warship Vasa from the depredations of time, Farideh Jalilehvand (see profile) and Patrick Frank focused the power of the SLAC's X-ray spectroscopic techniques on small cores of wood taken from various locations on the ship. The results--a detailed analysis of the amount and types of sulfur and sulfur compounds in the wood--both defined the problem and indicated possible solutions.

"About 6 months after I started my work as Research Associate in SLAC, when I was thinking about different ways to apply XANES (X-ray absorption near edge structure), I learned of the problems with the Vasa," says Dr. Jalilehvand. "I asked Magnus Sandström (Professor of Chemistry at Stockholm University) to bring me some wood cores for analysis. This was the very first time that the sulfur x-ray absorption was measured on wood." The XANES work was conducted at the Stanford Synchrotron Radiation Laboratory (SSRL), a division of SLAC.

A Noble Ship Founders, Rises, then Founders Again
On her maiden voyage in 1628, the exuberantly overdesigned Vasa sailed less than a mile before sinking about 100 m from a small island in the Stockholm harbor. Her innovative design, with the extra deck of cannons, caused the top-heavy ship to heel over. Water flooded through the open cannon ports, 30 passengers and crew drowned, and the pride of King Gustavus Adolphus' navy sank under the waves. The incident has since become a classic study in project management gone wrong.

The sewage-laden waters of the harbor preserved the Vasa in excellent condition. The low oxygen content of the polluted waters prevented wood-consuming fungi and bacteria from growing, and the cold, low-saline Baltic sea is inhospitable to the nefarious wood-boring shipworm. Marine archeologists lovingly raised a nearly intact Vasa in 1961, but as soon as she came out of the water, she began to disintegrate. After developing and applying the best conservation techniques of the time, including 17 years of spray treatment, the situation seemed to be under control, and Vasa was put on display in 1990 at the Vasa Museum in Stockholm.

New and unforeseen problems emerged in the summer of 2000. The weather was unusually wet, and the museum became increasingly humid as Vasa enthusiasts carried their wet raingear inside. Curators noticed small powdery patches of salts on the wooden surfaces of the Vasa, and a general softening of the wood in the holds. Changes in the air-conditioning systems to reduce humidity slowed, but did not prevent, the disintegration.

Noting that several other shipwrecks raised from anoxic waters seemed to be facing the same problem, the Vasa conservators called upon international experts in chemistry and marine archeology to analyze and address the problem. Among them was Dr. Magnus Sandström, who collected the crystalline powders from Vasa and analyzed them with x-ray powder diffraction (XRD). Sandström found mostly sulphate salts, but detected only one incidence of crystals of elemental sulfur. Sulfur was clearly the issue; the warship as a whole was emitting and being consumed by sulfuric acid--thousands of kilograms of it.

A Sensitive Beamline Reveals the Truth
Every element, compound, or material will absorb x-rays of a specific and unique energy or range of energies. The x-ray beams generated at SLAC are extremely intense and highly focused, making it possible to "tune" XANES to be so sensitive that it can distinguish not only between different elements, but also between different species and compounds of one element.

The "near edge" term in "X-ray near edge spectroscopy" comes from the appearance of the results readouts. "The X-ray spectra look like a cliff face. There's a flat approach, then a steep rise at the 'near edge'--which is the energy where absorption for that particular element increases sharply--and then a plateau." says Frank, who taught Jalilehvand how to analyze the measured XANES spectra at SSRL.

The sulfur content of native wood is very low, measurable in parts per million. The highly sensitive XANES beamline 6.2, which is dedicated to the lighter elements, is sensitive to all the oxidative states of sulfur and was able to tell a surprising story about the Vasa timbers. When turned on a number of core samples taken from Vasa's hull, holds, and aftercastle, XANES showed not only high concentrations of elemental sulfur (several mass percent, for areas beneath some surfaces), but different species and compounds of the element (see the diagram of sample positions and sulfur concentration).

"When we ran the spectra of finely filed core samples at different depth, we found not only elemental sulfur and sulfates, which are the beginning and end products of oxidation, but also the intermediates of the sulfur-oxidizing reaction," says Jalilehvand. "By knowing the intermediates, we can find out what types of reactions are occurring and what the probable catalysts are."

Pollution Can Be a Mixed Blessing
The results of the XANES analysis indicate that the sulfur and its related compounds reside throughout the wood cores, not simply on the surface. Furthermore, the spectra indicated different states of oxidation along the length of the core, with elemental sulfur deep in the sample and oxidized sulfur intermediaries closer to the surface.

The low-oxygen conditions that protected the ship from the depredations of oxygen-based bacteria had also provided a favorable habitat for anaerobic bacteria, which do no harm to the submerged ship but which do produce hydrogen sulfide as a waste product. Researchers speculate that the hydrogen sulfide was absorbed from the polluted water penetrating the waterlogged wood and then transformed, either by a chemical reaction or through the agency of sulfide-oxidizing bacteria, into elemental sulfur or pyrite (a compound of iron and sulfur). The highest sulfur concentrations are found in exposed parts of the hull.

When the Vasa was brought to the oxygen-rich surface, the sulfur began an oxidizing reaction with the oxygen and water in the air, catalyzed by iron from the rusted iron bolts of the ship. The end product of full sulfur oxidation is highly corrosive sulfuric acid--in the case of Vasa, potentially 5,000 kilograms (more than 5 tons) of it.

"It's why we're here," says Frank. "We figure out the mechanisms of how things work. It's one thing to know what happened; it's quite another to know exactly how it happened. When you know that, you can start finding ways to deal with it."

Small Samples, Big Results
"The core samples taken from Vasa were generally 100 mm x 5 mm (about the size of a mini-golf pencil)," Jalilehvand says. "The samples were stored in an inert atmosphere to prevent any oxidation. I tried to file about 1-2 mm of the sample core--about the thickness of a lentil--at different intervals to have about 10 samples from this 100 mm core." She placed the powder from each filing on an x-ray tape, and then analyzed it using the XANES beamline.

The findings from the XANES analysis had an immediate effect on marine archeology. There is now a highly directed effort to find solutions to the problems afflicting recovered wooden ships, and a strong current of feeling that no more submerged vessels should be recovered until a preservation solution is found. For ships that have already been recovered, new conservation guidelines are being established. In particular, there is new guidance for removing sulfur and iron compounds from waterlogged marine-archaeological wood. One immediate beneficiary is the Mary Rose, King Henry VIII's flagship, which is undergoing conservation treatment in Portsmouth, UK.

"Before we had this technology, it would have required a fairly large team of people working for months and using wet-chemistry techniques on large amounts of wood from the ship," says Frank. "Even then, we would never have been able to determine exactly what elements, compounds, and intermediates there were, or where exactly they lay in the core sample."

For Jalilehvand, who took her second PhD in Stockholm, the clarity and decisiveness of the results are particularly sweet. "Scientists have an important role in keeping our historical treasures, which are our connection to the old days," she says.-- by Michaela B. Mann

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Media contact: Neil Calder, Director of the SLAC Office for Communications, (650) 926-8707, neil.calder@slac.stanford.edu
Technical contacts: Patrick Frank, Stanford Linear Accelerator Center, (650) 723-2479, Frank@ssrl.slac.stanford.edu
Farideh Jalilehvand, now at the University of Calgary, Canada, faridehj@ucalgary.edu

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