The Conservation of the Mary Rose
Though many artefacts were discovered in relatively good condition, excavation and subsequent exposure to air can threaten their stability, making it essential to develop relevant conservation treatments. The Mary Rose project led the way in techniques to conserve archaeological finds.
Right from the start of the diving, archaeologists and conservators worked together to ensure that the artefacts found would not deteriorate. After recovery, artefacts were placed in ‘passive’ storage, to stop any immediate deterioration before the team actively conserved them.
The passive storage used depended on what an object was made of. For instance, small wooden objects could simply be sealed in polyethylene bags to preserve their moisture, while timbers were stored in unsealed water tanks.
The team prevented any growth of damaging fungi or microbes with a number of techniques, including low-temperature storage, chemicals and even the use of common pond snails, which eat wood-degrading organisms but leave the wood untouched.
Leather, skin and textiles were also put in passive storage, kept moist in tanks or sealed plastic containers. Bone and ivory were desalinated to stop any damage from salt crystallisation, as was glass, ceramic and stone.
During passive conservation, the ship structure itself couldn’t be completely sealed for obvious reasons. Instead it was regularly sprayed with filtered, recycled water kept at a low temperature, which stopped it drying out, and prevented microbial activity. Without spraying, the wood would have shrunk by anything up to 50%, warping and cracking as the water evaporated from its cellular structure.
The conservation team then sprayed the hull with polyethylene glycol (PEG) to replace the water in the cellular structure of the wood. There were three distinct phases, with a low molecular weight PEG to begin with, then a higher weight to strengthen the outer layer of wood. Finally the hull will be carefully air dried.
The hull and many of the artefacts are susceptible to the development of acids which can destroy the wood structure. The acid originates from sulphur species incorporating into the wood whilst on the seabed which eventually can transform to acid when exposed to oxygen. This is particularly a problem in artefacts which originally contained iron, as the corroded iron has migrated into the wood and drives the sulphur reaction to form acid.
Therefore, a way to inhibit acid production is to remove the iron by reacting the wood with a suitable complexing agent. This method has been used extensively at the Trust as a surface treatment and it is now being explored as a submersion treatment for entire artefacts.
Application of Strontium carbonate treatments to artefacts which are showing the presence of sulphate salts.
Many of the waterlogged archaeological wooden artefacts have the potential to form acids due to the presence of iron and sulphur. The sulphur originates from biological reactions which occurred on the seabed and the iron is present due to the corrosion of original fixtures. While they were exposed to limited oxygen on the seabed these posed little threat but can form acid when exposed to oxygen for extended periods of time. Removal of iron is one way of treating this problem and is explored in another section. Another route is to neutralize the products. At the Mary Rose, a treatment of Strontium carbonate is being pioneered to neutralise acids and elements which have the potential to form acids if left unattended. A stable product is formed which can remain in the wood.