The conservation of a national treasure

As the seven-and-a-half-year conservation study of one of Sweden’s most prized maritime treasures comes to an end, Ingmar Persson, Professor of inorganic and physical chemistry at the Swedish University of Agricultural Sciences tells projects explains just how the Vasa has been preserved for ever.

On June 20 , Ingmar Persson, Professor of inorganic and physical chemistry at the Swedish University of Agricultural Sciences (SLU), concludes a seven-and-a half- year conservation study of Sweden’s premier tourist attraction, the 17th century Swedish Royal warship Vasa. Persson’s project is part of a comprehensive research effort by the Swedish Maritime Museums engaging a large number of laboratories in Sweden and abroad, supported financially by several Swedish research councils and foundations. It aims to tackle the chemical reactions that threaten the long-term preservation of this and other priceless maritime artifacts.

In 1628, Sweden’s most famous warship, the Vasa, made one of the shortest – and most ignominious – maiden voyages in history. Less than a nautical mile into the journey from Stockholm harbour, the topheavy ship, bristling with cannons and other heavy weaponry, keeled over and sank, ending the career of one of Sweden’s mightiest warships before it began.

Recovered in 1961, the Vasa was found to be largely intact. To preserve her wooden structure, the ship was sprayed with boric acid to kill off any micro-orgamisms and algae within the timbers, and polyethylene glycol (PEG) to stabilise the wood’s structure as it dried out from 333 years under water. Stockholm’s Vasa Museum, opened in 1990, displayed the ship to an eager public, today pulling in more than a million visitors a year.

By the turn of the millennium, however, something strange was happening to Sweden’s prize attraction. “2000 was an unusually wet year,” says Persson, “and there were a lot of visitors coming to the museum in wet clothing, which increased the humidity around the ship. In the autumn when it became drier and the number of visitors decreased the ship began to dry out again, the conservators at the Vasa museum noticed glittering crystals appearing on the wood, along with white and yellow precipitations. They wondered, “what are those and what are they doing to the wood of the Vasa?”

Initial X-ray analysis by Professor Magnus Sandström of Stockholm University revealed them to be sulphates, iron compounds, gypsum and elemental sulphur. “Our first thoughts were therefore that it was sulphuric acid attacking the wood” says Persson. Persson and Sandström conducted further studies starting in 2001, using synchrotron light analysis. Synchrotron light is extremely intense X-ray radiation formed in storage ring for electrons moving at the speed of light, this technique showed the presence of a range of sulphur compounds in the outer 2-3 cm of wood. They published these initial findings in the journal Nature in 2002.

Based on these results, and the supposed sulphuric acid attack on the wood, the Swedish Maritime Museums secured funding for the international research project “Preserve the Vasa” in October 2003, followed by the “A Future for the Vasa” in 2008. The challenge was to identify the distribution and speciation of various chemicals, in particular acids, and sulphur and iron compounds, in the timbers and elucidating possible degradation reactions and their influence on the mechanical stability of the wooden structure.

“In our project, we observed that the acidity of the wood increased the deeper in we went,” Persson says. “It could not therefore be sulphuric acid at the greater depths, so something else had to be causing the acidity. The reasons to this surprising observation are at present being further investigated by us and some other research groups.”

The research project confirmed the presence of iron, which was distributed uniformly throughout the ship’s timbers, both as iron(II) and the more acidic iron(III). The iron originated from the iron bolts holding the ship together, and from other corroded objects such as cannonballs. Iron compounds are known to catalyse deterioration reactions of different kinds, including the decomposition of wood components and PEG. To remove the iron compounds from the timbers would mean minimum risk of deterioration of wood and PEG.

Persson and his research team set about finding a safe method of extracting the iron from the timbers. The team developed a wet chemical extraction method using a ‘ligand’ called EDDHMA – a molecule that combines strongly with iron compounds to form a new compound - intensely red in colour - which allowed them to see when the iron had combined with the EDDHMA.

After soaking pieces of Vasa wood containing relatively large amounts of iron for more than a year, most of the iron was extracted. The red colouring disappeared after being rinsed with water, which left the wood looking natural. The state of the PEG sprayed into the wood as a stabilizer was also analysed in Persson’s research project. “The PEG was more or less intact at the surface of the wood but at some places at larger depths there are strong indications of severe PEG degradation” says Persson. “We concluded that iron compounds were most likely responsible for this degradation, also causing the PEG to decompose into fairly strong ‘weak acids’ such as formic, glycolic and oxalic acid, in relatively high concentrations, deep inside the wood. ” Left untreated, such iron-catalysed degradation reactions might, over hundreds of years weaken the structure of the wood sufficiently to cause the ship to collapse under its own weight, without a supporting structure.

The identification and extraction methods developed and applied by Persson’s team have made very important contributions to the overall research efforts described in the Swedish Maritime Museums research projects. The inter-disciplinary research conducted in these projects has made it world leading in this kind of conservation. “We are now sharing our research findings with other conservation teams around the world, including that of the Mary Rose in the UK,” Persson says. The Mary Rose, an English Tudor warship, built by Henry VIII, suffered a similar fate to the Vasa, when, overladen with heavy guns and supplies, it too capsized and sank. Its remains were recovered in 1982.

So what are the priorities for the final six months of the research project? “Now we know what happens to the wood, we need to find out when it happens and how exactly the wood deteriorates,” says Persson. “That’s what we’ll be focusing on for the remainder of the project. Within the Vasa research team there are also other groups looking at other aspects of timber conservation, such as the mechanical strength of the wood and how fast it decays under different conditions – with and without iron, with and without O2, how much CO2 is formed and so on.”

Persson is pleased with the progress and sees important findings not just for the future of the Vasa but for other maritime artifacts as well. “The Vasa is unique – the most complete ship of its age in the world and is therefore of huge cultural and economic importance. But I also hope our findings will be taken up by other conservation teams, to safeguard similar historical artifacts for the future.” He also sees the potential for a new kind of conservationist too; “I hope our work will educate conservationists and the wider public about how chemistry can be a force for good. Too often it is associated with destructive industrial processes but I hope our findings will begin to put chemistry in a very positive light.”

To find out more, please contact Ingmar Persson.

Published: Wednesday, 20th July 2011 by Clive Somerville