In 1545, the Mary Rose, a flagship of the British Naval fleet and a favorite of King Henry VIII, headed out of Portsmouth Harbor off the southern coast of England to do battle with the French Navy. The vessel made too sharp a turn, listed badly, and began taking on seawater through gun ports that were left open. Thus, the Tudor warship, which had withstood 33 years of battle, quietly sank to the bottom of the harbor.
But that was just the beginning of the Mary Rose's long and fascinating history as a maritime artifact. From its excavation and raising in 1982, to its installation behind glass in an eponymous museum in Portsmouth, to a lab at Stanford's SLAC National Accelerator Laboratory where researchers are using X-ray science to stop the ravages of time from destroying it altogether, the voyage of the Mary Rose continues.
SUNKEN TREASURE: The remains of the Mary Rose are a time-capsule of the Tudor era. (Photo: Courtesy Mary Rose Trust) Today, the greatest threat to the 600-ton ship is a yellow salt-like deposit that first appeared on the wood about a decade ago and started turning it to mush. Preservation scientists were desperate to know what the destructive deposit was and turned to Stanford chemist Ritimukta Sarangi for help. Sarangi, PhD '07, is an expert on spectroscopy, the science of studying the composition of matter by analyzing the light and other forms of radiation it emits. She works at SLAC's Synchotron Radiation Lightsource, a multimillion-dollar machine that uses magnets to accelerate electrons to generate extremely powerful X-rays. The X-ray beams interact with material—such as small samples of wood from the Mary Rose—creating a specific emission pattern.
"All objects in the world have a very specific light spectrum," Sarangi says. "It's like a fingerprint. If you then measure the spectrum, we can know the specific chemical composition of that object."
Sarangi suspected that sulfur chemistry was behind the yellow deposits. To understand why, it's necessary to return to the ship's resting place at the bottom of the Portsmouth Harbor. When the Mary Rose sank in 1545, it landed starboard side down on the ocean floor, where there was little oxygen to support wood-digesting microbes. When it was raised four centuries later, the ship appeared to be in surprisingly good condition; little rotting damage was evident. But, in fact, rotting had occurred underwater.
As Sarangi explains, the wood had deteriorated down to nothing but a scaffolding of thick rods of cellulose held together by thin filaments of lignin—with the water providing structural support. Once the ship was back on dry land, it was necessary to spray the wood regularly with cool water to maintain this support. Then, in 1994, preservation scientists switched to spraying the ship with polyethylene glycol, a chemical that penetrates the wood and forms a more robust supporting structure for the rotting wood. The spraying continues today.
But that's not all that happened down in the depths. The oxygen-poor environment favored bacteria that converted sulfate ions in seawater to hydrogen sulfide, which permeated the wood. Then, when the ship was exposed to the air, a chemical reaction occurred that transformed the hydrogen sulfide into oxidized sulfur compounds—the most caustic of which is sulfuric acid. (This reaction was catalyzed by iron present in the wood as a result of corrosion of the ship's metal bolts.)
The sulfuric acid was eating away at the cellulose and lignin in the wood, leaving behind those telltale yellow deposits. The same thing happened to another warship, the Swedish Vasa, which was previously studied at SSRL. Sarangi joined her colleagues in the UK to confirm that the same process was happening to the Mary Rose. "What we observed showed that the same sulfur products were being formed," she says. "And that's what was forming all these salts."
MYSTERY SOLVED: The yellow deposits were the result of sulfuric acid eating away at the wood. (Photo: Courtesy Mary Rose Trust)
With the culprit identified, they faced the next challenge: What could be done to neutralize the sulfuric acid to save the Mary Rose?
"We looked into the literature and found one compound that had been tried previously by preservation archeologists, strontium carbonate," Sarangi says. Tiny dust-like particles of strontium carbonate can be dissolved in water, sprayed on the ship and—in theory—penetrate the wood. The strontium carbonate would form an insoluble compound with the sulfuric acid that could simply be washed off.
"When you treat the wood with strontium and look at the strontium spectra, it shows that it has penetrated the wood," says Sarangi. "The strontium has gone in and sucked the acid out." Still, she cautions, "taking the study from the lab and applying it to the Mary Rose is a big leap." But there's reason enough to be optimistic that the fabled ship's story is not yet at its end.
Tracie White is science writer based in Aptos, Calif.
