All-In-One Treatment For Conserving Archaeological Wood Artifacts

Conservators trying to preserve historic sunken ships have their work cut out for them.

Not only have the vessels been damaged by water, but marine organisms—microbes, fungi, and other critters—have chowed down on the wood. Oftentimes, iron trapped in the wood has also catalyzed the formation of acid that’s eaten at the ship from the inside.

But help might be on the way: A new preservative material fights all those types of damage simultaneously.

One historic ship that’s gotten a lot of attention from conservators is the Mary Rose,an English warship that sank in battle in 1545. It was discovered in 1971 and raised from the seafloor in 1982.

The conservation process for the Mary Rose has been similar to that used for other historic wooden vessels, such as the Vasa in Stockholm. For nearly 20 years, the Mary Rose was sprayed with solutions of polyethylene glycol containing a broad-spectrum biocide. And it’s been treated separately with chelating agents to remove iron.

“The main problem with archaeological ships like the Vasa and the Mary Rose is it’s hard to avoid wood degradation after excavation,” says Lars Berglund, a polymer scientist at the Royal Institute of Technology who has been involved with conservation of the Vasa. “With the Vasa, iron present in the wood leads to significant and continuing chemical degradation as the ship is standing in the museum,” Berglund says. “At present, there are no practical preservation methods that can solve the problem.”

A new preservative could be the material conservators have been looking for (Proc. Natl. Acad. Sci. USA 2014, DOI: 10.1073/pnas.1406037111). It was designed by Zarah Walsh and Oren A. Scherman of England’s University of Cambridge and coworkers in collaboration with Mark Jones, head conservator at the Mary Rose Trust, in Portsmouth, England.

The new material is a supramolecular polymer network made from four components, each of which has a specific job to perform. The biopolymer chitosan, which has antibacterial properties, is functionalized with both naphthol and catechol. A second biopolymer, guar, is decorated with a viologen derivative, which also has antibacterial properties. A macrocyclic host molecule, cucurbit[8]uril, links the polymer chains together. The fourth component—iron—comes from the wood itself and also helps connect the network.

To form the network, the viologen moiety acts as the first guest in a ternary host-guest complex with cucurbituril. Naphthol or catechol can act as the second guest. If iron is present in its +3 oxidation state, catechol lets go of the cucurbituril and binds iron instead.

“We made a two-tier system,” Walsh says. “We add enough naphthol that the structural stability of the material will always be at a certain minimum level. But it can get stronger if iron is present.” That extra strength is important because iron often signals the presence of acid, which weakens the wood.

So far, the material has been tested as a surface treatment with small pieces of wood from the Mary Rose. “The next stage is to scale up and use these new polymers on large ship timbers,” Jones says.

One hoped-for benefit is that it will speed up conservation, “so it won’t take 20-plus years, like it’s taken to conserve the Mary Rose,” he says. “It will speed things up, and it will solve a number of problems, all with one form of treatment.”