Conservation of Archaeological Material
Conservation of Artifacts by Supercritical Drying
by David Cole-Hamilton
These pages highlight research carried out into new techniques for conserving archaeological material from wetland and marine sites. This research has led to the development of a new technique - Supercritical Drying, which shows great promise for conserving waterlogged wood and more complex composite artifacts.
Wood can be preserved for long periods of time under anoxic, waterlogged conditions. Whilst the absence of oxygen reduces the rate of decomposition, wood is always modified in the burial environment.
Prior to the oxygen being used up, fungal and microbial action results in the loss of cellulose, an important structural polymer in the wood. As a result the wood is very much weakened -under extreme conditions, only a lignin skeleton is left - and whilst this may retain all of the detail of the original wooden artefact, its strength is less than 1% of that of modern wood.
After excavation the wood is exposed to air again, and the decay process can re-start. Most conservation processes rely on drying the wood - so the absence of water now becomes the limiting factor for decay. In its weakened state, however, wood shrinks excessively, and tends to break up.
The conservation problem is to dry the wood without it being destroyed by this process.
Supercritical drying, a new technique developed at St Andrews avoids the shrinkage problem by replacing the liquid in the wood with a high-density gas or supercritical fluid. The supercritical fluid can be removed from the wood by decompression without forming a liquid phase. As shrinkage is due to surface tension forces at a liquid surface, supercritical drying does not damage the artefact.
The supercritical fluid we use is carbon dioxide, which is cheap and safe, but will not mix with water. In consequence the water in the artefact must first be replaced with an organic solvent - methanol. At high pressure the carbon dioxide's density is similar to that of common liquids, and it readily dissolves the methanol.
Below are some of the samples conserved by supercritical drying, which demonstrate some of the advantages of this technique.
Supercritical drying, allows wood in any condition to be dried without modifying the methodology. So that heavily degraded material, with no inter-fibre adhesion, (Mesolithic timber above), and sound timber, such as that from the Dartmouth, both responded well.
Other methods rely on replacing water within the wood structure with a polymer, which cannot normally be introduced into cork, and some other dense woods, which remain impermeable. Further, polymers and other foreign material accelerate the corrosion of metal, by dissolving the protective surface oxide, or simply by keeping it wet.
Under some circumstances supercritical drying offers advantages over other methods. Firstly in helping to stabilize the material, for example, bone. In this instance the carbon dioxide used in the technique converts any organic calcium salts into calcium carbonate.
If left in place, these salts can result in the formation of 'Bynes disease', a white efflorescence which can result in loss of surface detail. Secondly, the absence of any foreign material means that the internal structure of the wood is not obscured.