Conservation science ADAM HART-DAVIS / SCIENCE PHOTO LIBRARY / SCIENCE PHOTO HART-DAVIS ADAM

Monsters from the deep preserved Finding and then raising historic ships wrecked centuries ago is challenging business – but it’s just the first part of the rescue process, says Elisabeth Jeffries. Preserving these giant artefacts for future generations is a mammoth task

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0609CW-ShipWreck.indd 52 18/05/2009 07:54:28 In February of this year, a discovery has lost a substantial part of its mass ‘Present day fact that it was held together by the was made at the bottom of the so is not as strong as normal wood. pegs, the starboard side of the ship, English Channel: the remains of Due to the loss of mass, air drying conservation which was buried in the seabed, the HMS Victory – predecessor to causes shrinkage and then collapse,’ methods are was lifted in one piece. The exposed the ship from which Nelson won explains Jensen. port side had been eaten away by the battle of Trafalgar. If, as many not optimal – in wood-boring shipworms and other experts believe, she had four tonnes Treating timbers some cases it creatures. of gold on board when she sank in During the first 12 years of the Mary Once exposed to oxygen, the 1744, the treasure seekers who found Rose preservation project, the ship might be better iron sulfides could have formed her will be keen to get their hands was sprayed almost constantly with to leave it for sulfuric acid – a problem that may be on the ship’s remains as soon as chilled fresh water while the team of exacerbated by sulfur-metabolising possible. archaeologists carried out cleaning the future’ bacteria – which would inevitably The find is not as rare as it and reconstruction work. The water rot the structure. The judicious use may seem. Experts are aware of kept the wood in its saturated state, of sodium carbonates to neutralise it thousands of undersea wreckages in while washing out some of the salts avoided this problem. northern European waters. Yet most and sediment. Chilling the water Following the water treatment, of the time they remain beyond the minimised bacterial and fungal the ship’s structure was stabilised reach not just of fortune hunters but growth. by spraying with polyethylene also of archaeologists. The expense However, the water also kept air glycol (PEG) for several years – a of lifting, treating and finding a away from the structure, helping now well-established technique for suitable home for these submerged the conservation team avoid a major preventing shipwrecks from falling historic treasures, and doubts as to problem encountered by another apart. This waxy, water soluble the exact whereabouts of the wreck, great ship, the Vasa in Sweden, polymer infiltrates cavities in the mean that most are left alone. But which is also undergoing long- wood, displacing the water and there is also a third, more surprising term treatment. Both suffer from steadying the structure. It would reason: the ships may be better the problems typical of wooden be very difficult to freeze- or air- preserved if left where they are. shipwrecks, with significant dry any wooden ship without prior The dilemma of whether to raise amounts of sulfur compounds in the treatment of this kind, says Jensen. the carcass or keep it in place for lignin of the wooden cell walls. The future, better equipped generations Mary Rose timbers contained iron Environmental regulation has been faced by many a museum sulfides, which had built up when Mary Rose scientists have so curator. ‘Present day conservation the ship was lying on its starboard far successfully managed the methods are not optimal – in some side in anaerobic sediment dating delicate, step-by-step process of cases it might be better to leave it from Tudor times. Their presence conserving the ship and succeeded for the future,’ says David Gregory, was probably due to the corrosion in avoiding the build up of sulfuric a scientist at Denmark’s National of guns and a few metallic nails – acid, introducing antioxidants Museum. For now, a major challenge though most of the ship was held to prevent further oxidation. But is to ensure that rescued ships do together by wooden pegs. this conservation work is full of not disintegrate. This is despite ‘If the ship had been held together unknowns, and it is difficult to the fact that the last few decades by iron bolts, they would have predict what challenges could come have seen the steady development corroded so there would have been The Mary Rose, which up in future. of a new science – marine heritage nothing to hold it in position,’ says sank in 1545, was said to Operating in a different conservation and archaeology Mark Jones, head of collections at be Henry VIII’s favourite environment, scientists working – which is going through the the Mary Rose Trust. Thanks to the warship on the Vasa in Sweden, which was painstaking process of learning how to deal with these artefacts on land. In the UK, the raising of the Mary Rose – Henry VIII’s warship, which sank off Portsmouth in 1545 while battling the French – is probably the most notable example of a ship’s resurrection. Yet more than 25 years after the event, curators will have to continue battling in the longer term if they are to succeed in displaying her for decades to come. By 2012 they want to house the Mary Rose in a new museum alongside the artefacts that were contained in it. And by 2016 they want the public to be able to view her in a dry, completely treated state. ‘The aim is to remove the water and transform the ship from an unstable to a dry, stable state,’ says Poul Jensen, an expert at Denmark’s National Museum. However, simply removing the water from the timbers

isn’t an option. ‘Waterlogged wood GEOGRAPHIC/GETTY IMAGES NATIONAL

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raised in the 1960s, came up against the Baltic means that fewer wood- The Vasa sank on her becoming the first submarine in an unexpected problem relating to boring organisms are present, and maiden voyage in 1628 history to sink an enemy ship – was acidity. The ship, built in the 1620s the ships in the region are more fully found in waters off South Carolina and sunk on her maiden voyage, preserved. in 1995 and salvaged in 2000. The started to ooze a yellow substance team have had to deal with a range in the early part of this decade while Submarine science of materials, including cast iron, on display at Stockholm’s Vasa In the US, scientists are tackling wood, brass, glass and rubber. As in museum. The scientists had not similar problems, though in this case Corrosion is the biggest the case of the Mary Rose and Vasa, allowed for the effect of humidity in with a metal structure. The Hunley threat to largely metal the vessel was immediately treated the museum’s climate, which was – a submarine used in the Civil War, wrecks such as the using fresh water once it was out of too high. Subsequent investigation which sank in 1864, shortly after Hunley the sea. It had been subjected both to revealed that the humidity had concretion (a cover of hard matter) triggered off the conversion of and corrosion (varying in extent in methyl sulfides into sulfuric acid. different parts of the submarine) ‘In the year 2000, humidity levels processes while under water. were unstable and, as a result, The most pressing concern is to chemicals were transported in remove chlorides from the artefact. and out of the wood. But now the Typically, scientists treat artefacts situation is stable. It doesn’t look using electrochemical methods in as though we’ve had any new salt a caustic solution and extraction in formations during the last two alkaline sulfite, a repetitive process years,’ says Magnus Olofsson, head which may take up to 10 years, in of the Vasa conservation unit. The order to minimise the corrosion team halted the progress of acid within the metal caused by these build up by installing a new climate salts. ‘However, once put on display, control system. corrosion may start up again after a They were, however, more few years and is sometimes severe,’ fortunate than the Mary Rose team says Michael Drews, director of in that they have not had to deal the Clemson Conservation Center with the conditions present in the where work is being carried out on English Channel. The low salinity of the Hunley. 54 | Chemistry World | June 2009 www.chemistryworld.org

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To counter this problem, Drews more destructive than keeping Collecting artifacts from The technique would involve developed a new method that could them underwater. ‘When you the sea floor demands an impregnating the artefacts with be used for treating the vessel, bring it out of the sea, the clock organised approach an aqueous solution of ‘functional which they have tested on smaller starts ticking straight away. The precursors to self-assembling artefacts. The scientists believe archaeological work is such that materials,’ says Scherman. These that akaganeite, a ferric mineral it disturbs an artefact that has will be designed to react with iron present in marine artefacts, may be reached an equilibrium,’ explains and sulfur species present within one of the most important factors Paul Mardikian of the Clemson the wood, leading to ‘a densely cross- responsible for the active corrosion Conservation Center. Hence there linked polymeric network’ which observed before and sometimes after is plenty of work ongoing to better would sequester the iron and sulfur treatment, and that it is certainly a preserve ships in situ. The advantage species and prevent them from potential source of future chloride of leaving a wreck in place is that accelerating degradation processes release. Any successful treatment microbiological activity reduces if in the wooden artefact. At the same involves minimising the chloride the ship is buried at least 10cm under time, it will also mechanically inside the artefact, they think. the sea bed, and so this may be a reinforce the wood. Scherman likens Experiments have shown that better short or medium term place the process to inserting ‘macaroni after iron samples from a marine to preserve it. Some ships have even noodles’ whose ends find each other environment have been dried in air, been reburied after being salvaged, and fall into line, rather than trying some of the chlorine can no longer to better preserve them. to insert long spaghetti strands that be removed through immersion in a only permeate the wood after years caustic soda, although all of it could Preservation evolution of spraying. be removed if the samples had not Of course, things may change All the scientists working been exposed to oxygen. They have in future and the technology in the area are dealing with concluded that a chlorinated phase will improve. Scientists at the complex problems and operate in is triggered off by exposing the metal University of Cambridge say they uncharted territory, since there to oxygen and humidity. Drews hope to develop an improvement is little experience in the field and his colleagues have therefore to the PEG treatment, using a compared to other, more established developed a technique that involves new technique that could have far chemical and archaeological fields. using subcritical water alkaline reaching implications in other fields ‘For example, is PEG the best solution at up to 230°C. They have too. Oren Scherman, alongside impregnation material?’ wonders found that after this treatment (in colleague Jonathan Nitschke, says ‘When you Gregory ‘And in the case of freeze smaller objects), very low levels he is developing self-assembling bring a vessel drying - are there other methods of chloride remain. However, they polymeric materials that he which are more cost effective? There cannot predict yet whether this hopes may offer a new method of out of the sea, is plenty that needs to be improved,’ treatment will be appropriate for the protecting waterlogged wooden the clock starts he concludes. submarine as a whole. artefacts excavated from marine Managing salvaged vessels is a environments – though the research ticking straight Elisabeth Jeffries is a freelance finicky business and potentially is in the early stages. away’ science writer based in London, UK www.chemistryworld.org Chemistry World | June 2009 | 55

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