There have been countless approached to the preservation of waterlogged organic material, with an emphasis on leather and wood. These materials are the most extensively excavated and most commonly found in marine or waterlogged environments.
While at Cardiff I have treated a treenail from the Newport medieval ship and leather Mill Leat from a leatherwork, excavated next to Cardiff Castle. Through both these treatments I became familiar two of the most popular treatments and their working mechanics.
Newport Ship Treenail: Polyethylene glycol (PEG)
After surface cleaning, our first step was to use dETDA (disodium ethylenediaminetetraacetic acid) to pull out any iron ion present in the wood and limit the final staining. PEG has been widely used in the treatment of waterlogged wood for several decades. The two-step process utilised in the lab was taken from Pegcon, a program created by the Canadian Conservation Institute. This process allows for PEG of a smaller molecular weight, for example PEG 200, to penetrate the cell walls themselves, bulking any collapsed spaces. At a later stage PEG of a high molecular weight is added, for example PEG 4000, this supports the cells themselves through the middle lamella.
Freeze-drying is common practice in the treatment of waterlogged organics. The process causes the water within the object to sublime to a gas, this way the surface is not impacted by surface tension during air drying. Each of the treenails was monitored individually for weight change during the process, however it was found they behaved as a batch. All were taken out after 57 hours of freeze-drying. We had some problems with slime mould or fungus growing. On two occasions during the treatment we added biocide (n-alkyl dimethyl benzyl ammonium chloride), however it was not effective. Luckily, this has no adverse affect on the results.
The aim of this treatment was for the treenails to be prepared for handling. Therefore, the feel and aesthetics needed to remain faithful to the material. PEG has a tendency to leave wood dark and waxy to the touch, however this was not a much of a problem. The grain and texture of the original treenail is still clear. Small bore holes from previous insect damage can even be seen, damage that possibly came about during ships days at sea.
Mill Leat Leather: Glycerol
The use of glycerol was introduced in the 1980s by Helen Geniaris, of the Museum of London, and quickly became the recommended treatment for waterlogged leather.
After surface cleaning with a soft brush, dETDA treatment was the first step once again. This was partly to remove iron ions within the leather, however it was also reported to improve flexibility of the final product. After a couple of days of rinsing the leather was moved into a 25% glycerol and deionised water solution and left for a further three days.
As we treated a large amount of leather we separated it into batches according to weight. This way we could more accurately monitor the weight change. They ranged from 2 to 7 hours in the freezedryer. We didn’t take any precautions in regard to distortion during the process, and some of the leather did curl during the process. This, I feel, was our largest area of improvement during this treatment.
Going through these treatments was a brilliant learning experience, and I feel confident about approaching this kind of objects in the future.