The PEG In Sweden’s Vasa Warship
The vessel took four years to build and was armed with the highest tech weaponry available in 17th-century Sweden, but the four-story, top-heavy Vasa warship sunk before it managed to sail a nautical mile out of Stockholm’s harbor. That was 1628.
When the ship was pulled out of the water 333 years later in 1961, archeologists found all sorts of well-preserved goodies on board, as well as a hull in excellent shape. The wood had mostly managed to avoid two major evils: Degradation via wood worms (probably because the ship had sunk when it was still brand new) and degradation via microbes (the quirky bacteria that could survive in Stockholm’s particularly polluted harbor weren’t much interested in snacking on wood).
Letting the boat dry out would have been a death sentence for the gigantic artifact, since water-logged wood tends to shrink and warp as the water evaporates away, explains Martin Nordvig Mortensen, a researcher at the National Museum of Denmark, who is studying the degradation of the Vasa’s wood. (The vessel is located in Stockholm at the Vasa Museum).
Instead of letting the boat dry out, conservators sprayed the Vasa with a polymer called polyethylene glycol (PEG) until the ship was entirely saturated. (This took 17 years of spraying!) Since PEG doesn’t evaporate away upon drying, the wood is thus stabilized against warping. (Incidentally PEG has a curious spectrum of other applications, such as in theater smoke, toothpaste, antifreeze and personal lubricant.)
Given that PEG is a polymer and polymers aren’t known for having long museum shelf lives, one might wonder whether impregnating the Vasa with it was a particularly wise conservation choice. But the polymer seems to be holding up. A 2006 study found that “the PEG after 30 years is still in a reasonably good condition.” This is good news since conservators decided to spray the stuff on England’s famous Mary Rose ship too.
However researchers worried that the trace amounts of problematic formic acid detected in the Vasa ship’s wood was coming from PEG break-down. But it turns out that formic acid can also originate when wood breaks down. So how can one tell whether the formic acid is coming naturally from the wood or from the PEG?
One answer is isotopes, Mortensen says. Carbon can exist in three isotopic forms: carbon-12 (the most common), carbon-13 and carbon-14 (which is radioactive). The relative ratio of these carbon isotopes present in formic acid can provide clues to where the acid originated. For example, if the formic acid was coming from PEG degradation, it should have very little carbon-14, since the PEG is made from petrochemicals, which have a low or no carbon-14. The presence of higher amounts of carbon-14 would suggest the formic acid was from the wood.
Mortensen told me he’s found that the formic acid in the Vasa is probably primarily coming from wood degradation although it’s still possible a small amount was coming from PEG. But the point is that his studies also suggest that the PEG doesn’t seem to be seriously falling apart inside the Vasa.
More recently, Mortensen has been studying how oxygen percolates in to the Vasa’s wooden structure, where the gas can oxidize the artifact, and lead to its degradation. He’s found that PEG seems to get oxidized at a rate 100 times slower than the pieces of Vasa wood (impregnated with PEG). This would suggest that the PEG in the ship is having its intended protective effect. (As an aside, Mortensen is also trying to figure out all the problematic chemistry oxygen might be doing inside the Vasa wood, which will be the subject of an upcoming article. And while we’re on a tangent, the ship does face other conservation problems, such as from the sulfides that percolated into the wood when it was submerged in Stockholm’s polluted harbor.)
But back to PEG. If the PEG ever did turn out to be a problem, it would be massively time-consuming to get the stuff out of the boat, Mortensen says. Although PEG is water-soluble, you’d have to spray some serious water on that ship to pull out all the PEG. The onslaught could potentially harm the boat’s surface, he says. Let’s hope it never comes to this.