Wood conservation can be a lengthy process. It certainly does not take place overnight. Below Ian Panter, Principal Conservator at the York Archaeological Trust, explains the process.
What is conservation?
Conservation aims to stabilise materials to prevent or slow down further decay. The overriding principal of conservation is to change as much as necessary out of as little as possible so as to retain the material’s cultural significance without imposing too many modern elements.
Four stages of conservation
The wooden artefacts and timbers that York Archaeological Trust will be conserving, will undergo four stages of treatment: cleaning and desalination, baths of ammonium citrate, baths of Polyethylene Glycol (PEG) and finally freeze drying. Currently the carvings, whose conservation MAST is funding, are being desalinated.
How long this takes depends on the thickness and state of decay of the wood. The amount of chloride coming out of the wood is monitored and the water is changed quite frequently to speed up the process.
Once the wood is desalinated and cleaned scientists examine the amount of iron and sulphur present. If there are no iron fittings and provided it has not been adjacent to iron such as a cannon, then the wood should be ok. They will likely extract a core sample to test the iron and sulphur. The sulphur presence is due to the anaerobic, anoxic environment - decaying matter – the timber was in. The sulphur can cause problems when the iron reacts with the sulphates in the wood producing sulphuric acid which hastens the decay of the wood.
It’s a lengthy process, depending on the size of the timber.
Scientists have not yet got to the bottom of the sulphur problem. Both the Mary Rose in Portsmouth Naval dockyard and the Vasa in Stockholm have a problem with sulphur. What they are looking at is trying to control the display environment and playing with the levels of relative humidity, or trying to deal with the problem whilst the wood is in the treatment tank.
Those timbers with a sulphur content are put into baths of 2% ammonium citrate solution. The chemical reacts with the iron in the wood and removes it. The solution in the tanks is changed four or five times over 12 months. By removing the iron staining the natural colour of the wood will also be restored making it easier to see the wood’s natural features.
Scientists have realized they can never remove all the iron or sulphur but they can sterilise the wood with chemical biocides which helps to keep the sulphur at bay. This treatment is done whilst the timber is being impregnated with PEG. This is a waxy substance. As it dries it acts as a scaffold to support the collapsed cell structure in the wood.
PEG is a byproduct of the oil refining process so conservation of timber is becoming increasingly expensive. Experiments with sucrose, as a replacement a few years ago, were largely unsuccessful as the process needed a more “hands on” approach than was originally anticipated. The combination of the warm, humid environment and the sugar was a magnet to insects, particularly wasps.
After the PEG process is complete, a process that can take up to 18 months, the timbers are freeze dried. They are placed in a container that resembles a decompression chamber used by divers. A vacuum pump is attached alongside a condenser unit or cold trap. The water, removed from the wood as a vapour, then collects in the condenser and reforms as ice. It’s a closed-circuit system that requires regular monitoring.
The carvings should take about 12 months . They are quite soft and so should need plenty of wax but they will absorb it quickly.