Today I went on a Nautical Archaeology Society (Part III) run course covering the conservation methods of waterlogged wood. This was held at the Historic Dockyard in Portsmouth, where the skeleton of the Mary Rose and 19,000 recovered artefacts are to go on show to the public later this year, in a new £23 million museum.
The first exercise was about wood recognition - I was shown how to identify a wood sample by cutting a thin section and comparing the cell characteristics under a microscope.
This revealed the differing cell structures between hardwoods and softwoods. I was then given the opportunity to try and identify a piece of archaeological wood in the same way, this was more difficult as the preserved wood was softer so cutting a perfectly horizontal section was challenging. Eventually I was able to cut a good section and identify the preserved wood as oak. I was also shown how preserved wood is prepared for storage before conservation using plastic and foil film to make heat-seal vacuum bags.
Next we were given the opportunity to visit the conservation labs at Portsmouth and see large wooden pieces in tanks being conserved using various chemicals such as Polyethylene Glycol or PEG (which has a rather strong smell!) or freeze drying. Wet or green wood generally splits because as the fibres dry out they also shrink. The cells vary in size and orientation, so they can deform the wood as they pull against one another. Rather than sealing wood with something like varnish, PEG works by displacing water so that the cells dry, but without shrinking.
There was a variety of different types of material being preserved such as wood, metal and leather from many different wreck sites. I was also able to see some of the longbows and arrows (above left) recovered from the Mary Rose mid-way through conservation, as thousands were found on the wreck and many are still being conserved by freeze drying.
A number of cannonballs were being stored in climate controlled rooms, possibly early examples of armour-piercing rounds that used cheaper iron inside the lead balls. These would have worked like a modern-day armour-piercing round; on impact the soft lead outer material would deform, throwing the solid iron core through the armour plating.
The day was very enjoyable from a practical point of view and I learnt about an area of archaeology I had little first-hand experience of. It also means I am a little closer to attaining the NAS Part III: Advanced Certificate in Foreshore and Underwater Archaeology.