Freezing the Mary Rose

We are always on the look-out for interesting chemistry and for new tools to perform tasks not previously possible.  Often, these new tools give us the means of unlocking historical mysteries and giving us a fresh insight into events long since past that were previously unfathomable.

One such tool is a bench top freeze dryer!  This very economical piece of apparatus has a surprisingly large capacity (8kg total, 6L/day max) is available with chamber for vials/raw materials and/or 8/12/16-port manifolds for flasks and can be used in many applications but I wonder if any of our customers are doing something as unusual with theirs as the team who are preserving the sunken tall ship, the Mary Rose?

The Mary Rose was the flagship of Henry VIII’s navy when it faced an invading French fleet at the mouth of Portsmouth Harbour in July 1545. After a recent re-fit the ship was launched, heavily laden with many guns and working well.  When she began to manoeuvre out at sea it was quickly realised that the gun ports had been left open though.  Unfortunately this realisation came too late to change their fate and the ship and it’s crew of over 350 men were lost to the ocean depths.

In 1982 what remained of the hull was raised from the seabed where it had lain, protected by the deep sediment surrounding it, and was carefully moved to its new home on Portsmouth dockyard.  The ship and its artefacts immediately began to be attacked by bacteria and fungi and react with the oxygen in the air.

With so much of this historical find made of wood the team had to take action fast!  To dry out these articles without causing shrinking and further damage they needed to freeze dry them!

These bacteria and fungi ate ate away much of the glucose-rich cellulose and hemicellulose in the cells making up these objects, leaving behind the harder-to-digest lignin.
The end result was that the cells were hollowed out. Only the lignin in the outer cell wall remained, forming a network outlining where the cells used to be, while seawater quickly entered and filled up the gaps in the hollowed-out cells. ‘What replaces the cell wall layers then is water, so that supports the shape of the cell,’ says Mark Jones, research director at the Mary Rose Trust.

Fortunately, this only happened in the surface layers of the wooden objects, because the microbes didn’t have enough time to work their way any deeper before the covering of sediment created an anaerobic environment. ‘The first 10mm is very decayed, very soft, basically lignin, really. And that structure is just supported by water,’ reveals Jones. ‘Beneath that you’re then entering into very, very sound wood; the cell walls are saturated with water but are well preserved.’ If all this water is removed by drying, the wooden objects would shrink dramatically, by up to 50%, causing them to warp and crack.
So when the hull and other wooden objects were first raised, they were simply kept wet, while Jones and his colleagues worked out a way to dry them without causing any shrinkage. Smaller wooden objects were simply immersed in water-filled tanks and containers, while the hull itself was constantly sprayed with filtered, sterilised water, which continued until 1994.

The scheme they eventually came up with was inspired by the approach used on the Vasa, a Swedish warship that sank in Stockholm harbour in 1628 and was raised in 1961. This involves replacing the water in the wooden objects with the polymer polyethylene glycol (PEG), which prevents the objects from shrinking as they dry.
Wooden artefacts such as plates, arrows and gun-carriage wheels were immersed in a tank containing liquid solutions of PEG. The artefacts were immersed for up to six months, giving sufficient time for the PEG to penetrate deep into the wood and displace the water in the cell walls, where it bound with what remains of the cellulose via hydrogen bonds.
Next, the artefacts were freeze-dried to drive off the majority of the remaining water. This involves placing the artefacts in a sealed chamber, where they are first frozen to temperatures of around –35°C and then dried by reducing the pressure and gradually raising the temperature. The advantage of this process is that any ice formed during the freezing process sublimes directly into vapour without going through a liquid stage, reducing drying stresses and preventing damage to the wooden objects. It’s also much faster than air drying, taking two to three weeks rather than up to a year.
Treating the wooden objects with PEG and then freeze-drying them has been going on for almost 20 years now, and has been a massive undertaking. The Mary Rose Trust owns several freeze-drying chambers of different sizes, with the largest able to dry entire gun-carriage wheels and wooden beams. ‘We have probably the largest number of vacuum freeze driers for archaeological material in the UK,’ says Jones, who was awarded an MBE in the 2014 New Year honours for his work on the Mary Rose.

For the small to medium sized pieces from the Mary Rose or more likely, from your lab, a LyoQuest -55 would be perfect for replacing water with PEG (polyethylene glycol) and water then removed by freeze drying @ -35 degC but if you have acetone, ethanol, chloroform then we have the -85 version too!

With all-in-one large capacity in a bench top sized unit, the Telstar freeze dryer offers a simple cost-effective package for all your lyophilisation needs.

If you freeze dry any tall ships though, we’d love to see photos!

(images and text taken from an original article within the Royal Society of Chemistry website)