PEG treatment of wet wood on the base of the PEGcon calculation followed by vacuum freeze drying

Text: Stephan Brather

The stabilization of wet wood in mixed solutions of lower and higher molecular weight polyethylene glycols (PEG) following the PEGcon calculation developed by Cook and Grattan is in use since the late 1990th at the Brandenburgisches Landesamt für Denkmalpflege und Archäologisches Landesmuseum.

After cleaning and documentation the fully immersed objects are weighted on air and under water. The species of wood is also determined. The calculation of the necessary PEG concentration is possible with this informations.

The wooden objects are sorted in batches where the variation of the necessary concentration is lower than 5%. The maximum concentration needed in the batch is used.

The stabilization bath starts with a 5% solution of the lower molecular weight PEG 400 in tap water. The concentration is raised step by step by 5% till the maxim concentration is reached. The concentration of PEG 3350 is also increased in % steps while the percentage of PEG 400 remains constant. Each step needs between 4 and 8 weeks depending on the size of the objects. The refractive index is used to measure the PEG concentration in the bath. Biocides are not used.

The objects are removed out of the PEG solution, their surface is cleaned with water and they are frozen in the vacuum chamber of the freeze drying unit or external in a deep freezing chamber at -30°C.

The freeze drying follows with a pressure of 0,37 mbar at -30°C. The duration of the freeze drying process depends on the size and porosity of the objects - normally about 3 - 4 month. It is possible to increase the sublimation rate by increasing the temperature during the freeze drying after a couple of weeks. The process is controlled by measurements of the chamber temperature, the pressure and the mass of the objects in the chamber. If there is a mass constant over a longer period the freeze drying is ended.

After freeze drying an excess of PEG 3350 as a white deposit on the surface can be removed with soft brushes, swaps with water or ethanol or hot air blower. Fragments can be glued with polyvinylbutyral (PVB) in ethanol or an acrylcopolymer (Paraloid B72). Larger fragments are assembled mostly without pivots (stainless steel).