Later this year the famous English warship Mary Rose will be unveiled in its new permanent home, the Mary Rose Museum at Portsmouth Historic Dockyard. The museum will exhibit the Mary Rose dry and unobstructed by water sprays for the first time.
Getting the ship to this state has required cutting-edge research using the world’s most advanced x-ray facilities, including the Diamond Light Source. It’s also required some old-fashioned inventiveness, as the rarity of the Mary Rose has meant its conservation has little precedent.
The Mary Rose has acquired world fame as the only recovered 16th century warship. Back in 1545, the Mary Rose was the flagship vessel in Henry VIII’s fleet when it sank in a battle with the French.
By the time the Mary Rose was pulled up in 1982 it had spent over 400 years on the sea floor off the south coast of England. Since it was covered with sediment, the artefacts within the ship were exceptionally well-preserved.
Preserving for the long term
The immediate preservation priorities of the Mary Rose Trust were focused simply on keeping the recovered ship intact. The ship was sprayed constantly with water to prevent the wood from drying and cracking.
Then, over many years the hull was coated with a water-soluble wax called polyethylene glycol. The wax penetrated and replaced the water in the timbers, preventing the wood from distorting or cracking.
While that work went on, the team also had to remove some of the chemicals that had seeped into the wood over the four centuries it had been under water. By the time the ship was sealed with wax, acidic compounds in the wood had been neutralised.
With that work done, however, the researchers began looking to the future of the ship. And one of the most significant longer-term risks for recovered marine wood is formation of sulphuric acid.
The chemistry of decay
Iron components in the Mary Rose, such as nails and bolts, have long since corroded, infusing the nearby wood with iron.
Iron components, such as guns, have corroded and infused the ship's wood with iron.
The wood is also impregnated with sulphur. In seawater, bacteria feeding on organic matter give off the compound hydrogen sulphide. The ship has therefore been bathed in this sulphur-containing compound.
Iron and sulphur weren’t problems when the ship was submerged, because there’s not much oxygen in seawater. When exposed to air, sulphur combines with oxygen to produce sulphuric acid, a reaction that’s sped along by the presence of iron. Sulphuric acid - the same kind that’s in acid rain - destroys the cellulose in wood, causing it to weaken and crumble.
These threats were recognised thanks to the earlier restoration of the 17th century Swedish warship Vasa.
Research published in 2002 found that the Vasa harboured a surprisingly large concentration of sulphur. Further measurements revealed that both ships contained about 2 tons of sulphur. (The Mary Rose, for reference, weighs only 280 tons in total.)
In addition, the Vasa research showed that thousands of new iron bolts installed as part of the restoration work had added to the problem by spurring the formation of sulphuric acid. Luckily, the Mary Rose restorers had opted for non-reactive titanium bolts.
Still, if the Mary Rose is to last for future decades and centuries, researchers needed to find the quantities and locations of the sulphur and iron compounds in the wood. So, scientists from the Mary Rose Trust approached the Diamond Light Source in 2008 to scrutinise samples of the ship at a molecular level.
Molecular measurements
The combined research team analysed both the baseline chemistry of the ship as well as how that chemistry changed when treatments were applied to the wood.
Fred Mosselmans, principal beamline scientist on the experiments, analyses a timber of the Mary Rose.
The first technique that interested the team was a chemical reaction called “chelation”. This type of reaction removes iron by adding a chemical that bonds strongly with it.
The second technique they tested focused not on the iron but on the sulphur. The researchers applied nanoparticles of strontium carbonate – more colloquially, a powder. This powder is a weak alkali, so will neutralise acids, and is particularly powerful against sulphur.
Tiny splinters of the Mary Rose were soaked in one or the other treatment and then placed in Diamond’s Microfocus Spectroscopy beamline. The resulting chemical analysis showed where these treatments went into the wood and how effective they were at eliminating iron and acids.
Remain neutral
The chelation experiments removed between 50 and 90 per cent of iron atoms in the timbers tested, according to a 2009 publication (pdf). The nanoparticles also showed promise as a future treatment. Research published last year reported that the nanoparticles not only penetrated the wood but reacted with sulphur present to make it a less destructive variety.
Fred Mosselmans, chief beamline scientist on the experiments, says that the results have helped to preserve especially important bits of the ship. “To chemically treat the whole ship with anything is almost prohibitive unless it’s absolutely dirt cheap. So most of the focus is learning lessons and looking at preserving special objects – handles of swords, wooden bowls, et cetera.”
The Mary Rose Trust has used some of the chelating treatments on parts of the ship that were in bad shape, Mosselmans says.
By the time the Mary Rose is unveiled anew later this year, the elaborate chemical choreography that has gone on for years will be largely unseen - and that’s just as well. For the first time, the ship’s preservation will become invisible, and visitors will be able to see the Mary Rose in all her historic majesty.
For more, see Elements’ special report on Diamond Light Source.
Top image by James Stringer via Flickr; cannon by tsuki_chama via Flickr; research photo courtesy of Diamond Light Source