One step-PEG Treatment Kopenhagen
The cleaned objects are placed into a 10% w/w solution of PEG 2000 in tapwater. The concentration of the solution is gradually raised up to 40% PEG, three times by steps in 10%. The treatment is done by room temperature No heated tanks are necessary. The concentration of the solution is assessed with a refractometer or by evaporation of a sample of the solution. It can be raised to 50% to accelerate the diffusion, but this is normally not done as standard method because it may cause fragmentation of the PEG molecule. The tempo is depending on the wood species, the size, the shape and the degree of degradation of the objects. We do not use any biocide any longer, cause biocides can be health hasardous or problematic regarding environment particularly because of the huge tanks hence huge amounts of solutions we have to deal with.
The impregnated objects are picked up, rinsed if necessary with a clean solution of PEG and placed into the freeze-dryer, protected by a plastic sheet to prevent from evaporation under the freezing phase. The plastic is removed when the objects are completely frozen and before starting the vacuum. The freezing phase takes place at about - 30 Celsius degrees. The temperature can be slightly raised after a couple of weeks, but must always be kept below the eutectic point to avoid melting of the solution inside the objects, which would lead to a risk of collapse under drying. The temperature of the chamber and the objects is monitored as well as the pressure. The temperature is controlled with thermocouple and the pressure is measured with a Pirani device. The length of the freeze-drying process depends mostly on the size and the porousness of the objects.
the PEG in excess on the surface, appearing as a white deposit, has so be cleaned off, using soft brushes, swabs with ethanol or hot air blower. Fragments can then be glued. We are using Acrylcopolymere Paraloid B72 in ethanol or acetone or melting glue Jetmelt 3792 fra 3M. If the wood needs more strength, this can be achieved by applying on the surface a solution, 5-20% PEG 2000 in ethanol, or another consolidant. Consolidation after drying: could be 5-10% Paraloid B72 in ethanol. We almost never need to do it. This is normally not necessary after an impregnation at 40% prior to freeze-drying.