Saturday, June 5, 2010
Sunday, May 30, 2010
I recently had the opportunity to attend the 11th ICOM WOAM conference, held in Greenville NC. In the midst of the papers there were a few discussions regarding the two step low and high molecular weight PEG and a single step of high molecular weight PEG. While the majority of the conversation was in context of vacuum freeze drying post impregnation, there were a few comments about strategies in preparation non freeze drying. When I started this blog I intended to keep my postings short- but am going to break this rule to share what I think I learned from this conversation:
- The conservators at the National Museum of Denmark have been working with a single step system for many years, impregnating objects with 35-40% PEG 2000, freezing the materials and then using the vacuum freeze drying process to sublimate the water out of the objects. They moved away from using the two step low molecular weight method because the freezing point of PEG 200 is to low for their vacuum freezer dryer and they risk greater cellular damage of the structure because the PEG isn’t frozen, the water cannot sublimate from the structure and is rapidly evaporating instead causing greater damage to the cell structure.
- The conservators and scientists from the CCI have used the two step system of PEG 400 and PEG 4000 for many years, using PEGcon as their guide to how much of each to put into the impregnation bath system. They feel confidant that the PEG 400 enters the cell wall and bonds with the structure, and that the PEGcon program helps them avoid putting too much PEG 400 in the system. Excess PEG 400 would stay in the lumina and would remain active with fluctuations of relative humidity.
- Other conservators have also had good successes with the two-step system.
- Artifact size makes a difference in selecting the appropriate PEG system for treatment. The thickness of a timber isn’t as effected by climate fluctuations and would protect the lower molecular weight PEG’s from moving around the system. Smaller artifacts – such as the woven elements of basketry!!—are much more reactive/susceptible to the environment- and lower molecular weights more likely to move and weep from the structure.
This conversation has pushed me towards continued testing of higher molecular weight PEG in basketry materials. I recently tested a few samples of spruce root with PEG 3350 (inspired by Ellen Carrlee’s research) with some positive results. I would be interested in examining treatments of both archaeological root and inner bark PEG 2000.