To Vacuum Freeze Dry or Not?

Vacuum freeze drying is a common method for final drying of waterlogged materials. For those who haven't worked with this method here is the basic process: once the waterlogged material has been impregnated with PEG it is rinsed and patted dry and then frozen, once fully frozen the piece is placed in the vacuum freeze dryer to dry the piece through sublimating the water from the structure. One of the major reasons to use this method is to greatly speed up the drying process of treatment.

I have never freeze dried my waterlogged basketry treatments since I don't have access to this equipment. I have dried samples in the freezer with silica gel and I have slow air dried pieces in controlled humidity environments. These methods have worked very well for the basketry materials I have worked with.

Recent conversations with conservators with greater experience working with a range of waterlogged materials regarding vacuum freeze drying and lightweight, delicate materials has been interesting. In all it seems that many people avoid vacuum freeze drying for basketry and other delicate artifacts. Some feel that this protects the lightweight materials from blowing around in the freezer and unintentional damage.

A few archaeologists have mentioned that they prefer the appearance of air-dried materials to freeze dried materials. It would be interesting to compare some samples from the same basket, treated with the same impregnation treatment, in various drying methods just to see how the macro, and possibly micro, physical characteristics vary.

High vs Low Molecular Weight PEG

I recently had the opportunity to attend the 11^th 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.

Older treatment solutions

In the midst of examining treatment records for the collections of basketry material found, treated and stored in the Northwest coast region I ran into the mention of two treatment materials I have never worked with or even heard of... Firewater and Houston #3 Preservative. The Firewater was used to dilute the Plymul adhesive used to treat the Biderbost basketry and is described as a heavy detergent from the Firewater Company of Los Altos. I have no information about Houston #3. Help!! Does anyone have any knowledge of these materials, I would love to understand more about these materials.

Firewater was used in combination with Houston #3 Preservative on the Beach Grove materials. My notes indicate that the 'excavators preferred the Firewater because it hardened the punky wood' (Bernick 1991, pg 113). I think this is an interesting comment and I would like to understand this a bit.

Opening post

For several years I have been gathering information from published and unpublished sources regarding past treatment of waterlogged basketry. After a recent opportunity to present this research to colleagues I heard some wonderful feedback and new ideas to pursue. This blog is an attempt to organize some of this information and continue to gather information from those experienced with the treatment of basketry removed from water saturated terrestrial sites. I look forward to hearing from other folks working with these materials.