In 1545, freshly refitted to carry a greater number of heavy cannon, the warship Mary Rose sailed into battle against a French fleet north of the Isle of Wight. The debate over what happened next is still heated, but the most accepted version is that the added weight of the cannons made the Mary Rose sit almost a meter lower in the water than before. When the ship made a sharp turn—or perhaps when a sudden gust of wind caught the sails—water poured into open gunports, flooding the ship. Nets in place over the deck, meant to repel enemy boarders, ended up trapping more than 500 sailors aboard as the ship went down.

The Mary Rose, and many members of her crew, spent the next 437 years getting buried under several meters of silt at the bottom of the Solent, the strait north of the Isle of Wight. That silt helped preserve about 40 percent of her hull and about 19,000 artifacts and pieces of timber, which archaeologists recovered in 1982. That’s incredible luck for a 500-year-old shipwreck, especially one as historically significant as the Mary Rose, but the waterlogged wood of Henry VIII’s prize warship still faces a threat.

Eating away at the wood

Some of the marine bacteria that move in when a ship sinks munch on sulfur and release a compound called hydrogen sulfide. And when iron fittings, cannons, and other artifacts corrode, they release ions that react with the hydrogen sulfide to produce iron sulfides. That doesn’t matter much in an environment without much oxygen—and there's not much in several meters of silt at the bottom of the Solent, for instance. But when exposed to air again, the iron sulfides react with oxygen to produce sulfate salts and sulfuric acid, which eat away at the already fragile timbers and artifacts where wood is in contact with iron.

To help preserve badly waterlogged wood, artifact conservators use a compound called polyethylene glycol (PEG to its friends) to fill and strengthen the wood’s cells, which keeps wood from shrinking, warping, and cracking as it dries. The Mary Rose started PEG treatment in 1994, and the soaking spray finally got shut off to allow the ship to dry in 2013. That’s when the acid became a real problem.

“By exposing the timbers to air, we risked the promotion of oxidation of the sulfur present in the wood,” University of Glasgow nanomaterial chemist Serena Corr told Ars Technica. It was a worrying problem for Eleanor Schofield, head of conservation at the Mary Rose Trust, but, over a glass of wine with Corr, she found a likely solution.

Corr had previously worked with magnetic nanoparticles, guided to a precise destination by magnetic fields, for medical imaging and targeted drug delivery. She and Schofield thought a similar approach could work for getting the iron out of the Mary Rose’s hull and wooden artifacts. When the PEG spray shut off in 2013, conservators sent samples of the wood to Diamond Light Source UK to be monitored with X-ray absorption spectroscopy. That would let researchers measure which compounds were present and when they formed in different areas of the wood as it dried.

“An initial increase in the amount of oxidized sulfur was observed on the surface of the wood and developing further into the timbers,” Corr told Ars Technica. Those detailed measurements helped Corr and her team custom design nanoparticles for the task.

Getting the iron out

The core of each particle is a compound called magnetite, which in much larger chunks is a commonly mined iron ore. Most magnetic materials have their own magnetic fields with their own polarity. But at such a small scale—each particle is about 10nm across—the particles’ magnetic fields are easily influenced by an external one, making it easier to apply a magnetic field to the material and direct the particles where you want them to go.

For conservation, each particle is bound to molecules of porphyrin, which will in turn bind to free iron ions in the wood, catching them before they can react to form damaging acid. That whole package is coated in a polymer that responds to changes in temperature; at around 22⁰C, the composite is a thick gel. With a slight dip in temperature, the polymer becomes a liquid, which soaks into the wood and carries the nanoparticles with it. Conservators can use magnetic fields to steer the composite to the right area of the wood and then draw it out again with the captured iron ions in tow. And when the whole process is done, they can heat the composite to a gel and peel it off the surface of the wood.

Schofield, Corr, and their colleagues tested the nanocomposite on samples of fresh oak, which they first soaked in iron sulfates, and they say it removed nearly all the iron without damaging the wood.

“With the porphyrin, we were getting about 85 percent of the iron out of the solution, and that was within about 10 hours. I think if we'd gone for longer, we could have potentially taken more out,” said Schofield during a presentation at the recent 256th National Meeting and Exposition of the American Chemical Society. And that’s a very good sign for upcoming work on centuries-old waterlogged wood from the shipwreck, because wood that has been waterlogged for centuries has a more open, porous structure that will make it even easier for the nanocomposite to soak and seep its way in and out.

Soon, the nanocomposite will have a chance to prove itself on core samples taken from the Mary Rose’s hull—mostly the same ones, in fact, that went to Diamond Light Source to help the researchers understand what happened to the wood as it dried after PEG treatment. If that goes well, conservators will feel confident enough to use the nanocomposite on parts of the ship’s hull and other wooden artifacts where they see signs of iron corrosion.

Spot treatment

But the Mary Rose won’t be bathed in a shower of magnetic nanocomposite the way she was bathed in PEG. “We envisage that these magnetic nanocomposites could be used as a spot treatment for the Mary Rose hull to remove iron ions,” Corr told Ars.

The chemical reactions that produce the acid are going to happen in different spots at different times, so conservators will monitor the ship and artifacts for trouble and then apply magnetic nanocomposite as needed. And if all goes well, Schofield and Corr want to try a similar approach for other organic materials from the shipwreck, like leather and textiles (the artifacts recovered from the seafloor include pieces of velvet, sailcloth, and other fabrics).

“Once we show that the technology can work, we can then tailor it to some of the other materials,” said Schofield. “One of the big problems for us with everything is these artifacts have been soaking and marinating in the seawater for years and years and years. There's all kinds of things in there, so once we get the technology working, we can then try and look at different agents to sequester the particles.”