Conservation and collection care: Using X-rays to preserve the Mary Rose Museum’s Cannonballs

By Adrian Murphy. Main Image: The Mary Rose Museum is using bright x-rays to better understand the corrosion of some of its 1,200 strong cannonball collection

The conservation and care of collections is a vital element to the success of a museum and its future, as its visitors will attest. Here Advisor discovers the lengths that the Mary Rose Museum has gone to to preserve its collection of 1,200 cannonballs and we also have a case study from Ecclesiatical Insurance on its work to save the Surrey Infantry Museum’s medal collection from the fire at Clandon Park and quiz Meaco on the merits of controlling the environment

The Mary Rose Museum is currently three years into a four year ground-breaking conservation project to preserve and ultimately display its collection of cannonballs found on Henry VIII’s flagship that was sunk in 1545 and raised in 1982.

When the new Mary Rose Museum opened to the public in 2013 among its objects on display were 300 cannonballs, part of a 1,200 haul of round shots recovered with the famous Tudor ship. However, as they were being marvelled at by visitors the conservation team noticed that they were showing signs of cracking and immediately took them off display.

This was when Eleanor Schofield, head of conservation and collections care at the Mary Rose, decided to set up a research project with the ultimate aim of finding suitable treatments to preserve the shots and also put them back on display.

Having a background in material science, Schofield realised that it was necessary to further investigate the structure and properties of the iron shots in order to understand what the best treatments would be to preserve them.

She sought the help of the doctoral training centre at UCL, which was specifically funded for students working on science and engineering in arts heritage and archaeology, and approached Diamond Light Source, who she had worked with previously on the wood from the ship, to fund the project and provide the cutting-edge facilities.

What’s unique about the Mary Rose’s collection, she says, and especially the cannonballs is that they were made around the same time, they were lost at the same time and they were excavated at the same time. Since excavation, however, they have been treated by various conservation methods and at Diamond’s facilities bright x-rays make it possible ‘to visualise differences in the corrosion profiles and trace them to the treatments applied, allowing unprecedented insight into conservation on a molecular scale’.

A synchrotron, a facility that uses very bright x-rays, is used at Diamond Light Source in Oxfordshire. Copyright: Diamond Light Source

“You need to understand what they are made of, which actually can feed into the interpretation,” she says. “With this project it would be the use of techniques at a synchrotron, a facility that uses very bright x-rays, which are used for all kinds of different material science. The one we use is Diamond Light Source in Oxfordshire and these facilities get used for all kinds of science with many different experiments taking place simultaneously such as cancer research and DNA. Using x-rays is a very powerful technique to understand materials at small length scales.”

In 2016 it was also decided to cut segments from six of the 12 cannonballs being tested at Diamond, which were showing signs of damage, so that it could be determined how best to save the rest.

“We also decided to do some destructive testing, which we would not normally do, but the problem is we have more than 1,200 cannonballs of which 900 are still in a passive solution and in quite good condition,” says Schofield. The 900 are stored in tanks in the museum’s conservation facility and have been kept in water with a solution added to slow down corrosion. The remaining shot have been through various different treatments and some show the signs of erosion and through taking a very small sample of these the research team can try to find the best way to treat the rest.

The risk of corrosion of the iron shots is very real as can be seen in these close ups. Copyright The Mary Rose Trust)

“This was justified as we took less than one per cent of the collection. In reality half of those were already shot that were in pieces so they wouldn’t have been going on display anyway. For me personally I thought it was more negligent to just stand by, not do anything and watch them all crumble rather than take a sample and get a much better understanding of what’s happening.”

The problem with marine archaeological iron, she says, is that chlorine from salt water gets into the structure and when dried the chlorine left inside rapidly forms corrosive products. If there are any kind of cracks, or even casting faults, from when the shots were made then they will start corroding from the inside out. The way this manifests itself, she says, is that the corrosion that is formed takes up a larger volume than the original metal and eventually bursts open at the surface. Previous to the current research project, conservators could measure the chlorine that came out of the cannonballs – and stop the treatment when they were not getting any more chlorine off – but that wouldn’t necessarily tell them whether there was still chlorine inside the shot.

“We have carried out many experiments and are learning all the time and there is still a lot more we can do. The idea is to build up a picture of what’s happened and develop our understanding of the mechanisms and use that to inform what is the best protocol when dealing with these materials. We can then look at new treatments tailored to some of the issues we are uncovering.”

Dr Eleanor Schofield, Dr Giannantonio Cibin and Hayley Simon with iron shot and samples. Photographer by Sean Dillow. Copyright: Diamond Light Source

The essential factor with the Mary Rose collection, Schofield says, is that the collection has essentially been marinating in sea water for hundreds of years and this research should give a clear understanding of what’s happening within to start developing treatments.

“Most of the time other conservators don’t have the unique set that we have with all these similarities where we can pinpoint that one shot is only different to another because of the treatment that has happened to it or because of how it was stored or dried. So, not only will it inform us, it will also inform other conservators dealing with this material because it is notoriously very hard to conserve marine archaeological iron.”

H mark on an iron shot. Copyright The Mary Rose Trust

She says if they did nothing they would still have 900 cannonballs in solution and not know what to do with them. They couldn’t apply treatments that had gone before in all good faith because they could not be sure how effective they would be and they know some have been massively damaging. That’s not to criticise what’s been done before, she says, as they did the best they could at the time.

“One of my pet hates is having all these artefacts behind the scenes and nobody gets to see. You are spending money storing them and it would be so much better if they were on display for visitors to see. One of the powerful elements of our story is how much of the Mary Rose’s objects we have and how serious they were about going into battle.”

Schofield is also keen to build bridges between science, history and archaeology. When she started working at the Mary Rose in 2012 it was a year before the museum opened and she say she was completely involved with all the designers and exhibition staff.

“My team works closely with the learning department to organise activities and I have some students developing activities and online tools that showcase the collection and the importance of science and research to it. People will often think of the history and archaeology and don’t always think of the physical sciences of engineering and technology, which were absolutely critical throughout the whole Mary Rose project. From raising the ship to conserving it to creating the new museum it has been critical.”

Last weekend she took part in the New Science Live festival and in March hosted a session at the British Science Week to showcase the research project and continues to take part in sessions in the museum where artefacts are laid out and she can talk to visitors about the ongoing conservation. Schofield now hopes to extend the research project and find the ultimate treatment for the cannonballs that would see them once again displayed at the Mary Rose Museum.