Lars Andersen Helps Me Understand Freeze Drying – AIC Blog Archives: Conservators Converse

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Lars Andersen Tries to Help Me Understand Freeze Drying

Lars Andersen has apparently written the most marvelous book on freezer drying, but it is only in Danish. Upon encouragement from a colleague and boldness from a beer on an empty stomach, I walked up to him and told him I had heard of this book, and was wondering if it would be easier for him to publish in English or for me to learn Danish. He laughed, and it was clear he was quite proud of the book and how accessible it was for many audiences because it did not involve lots of mathematical equations. But he is very busy with many projects and the book might not happen in English. He was kind enough to come up to me after my presentation and explain more about freeze drying to me.

Here is what I think I now understand about freeze-drying and PEG: at low temperature, ice forms, and because of the special qualities of water, ice is very pure. PEG molecules cannot be between the crystals of water, and so the PEG forms sheetlike shapes in the voids of the wood (let’s say we are talking about a big PEG molecule, like 3350.) These sheets go in various directions and make a nice supportive framework of struts. Then at low temperature, the ice sublimates directly to gas, which allows us to get around that pesky problem of capillary action. As I understand it, the problem with waterlogged wood collapsing on drying is largely about

2) the violent forces that water exerts upon the weakened wood when it evaporates (“evaporate” as in liquid changing to gas, and that is nicely handled by the solid ice skipping the liquid phase and going right to gas in sublimation.)

So to get rid of the water it needs to be in a freezer. Now, that process goes FASTER in a vacuum freezer dryer, because there is a vacuum and a heating element (it seems freezer drying gets slower as temperatures get colder…I never realized that before.) But the only big advantage of the vacuum freezer dryer is that it is faster than a non-vacuum freezer. Roughly twice as fast for small things like my basketry. Maybe up to 10X faster for big things like ship timbers. But beware, for lightweight little things sometimes the vacuum freezer dryer can toss things around in the chamber if you’re not careful. Oooops! And then there is that whole pesky eutectic point thing. The literature says you should avoid the PEG eutectic point of 55%. If I got this right, at a concentration of around 55%, the thermodynamics of the water and PEG system are such that water comes to a kind of happy balanced place in terms of energy and no more water wants to come off the PEG and make ice. So there is PEG with liquid water on its surface, not freezing into ice, and instead of making the nice sheetlike structures, it makes these massive weird gobs. Lars Andersen’s book has some great SEM images of these two very different looking PEG formations (the nice struts and the wired gobs at the eutectic) and you can see where the ice that does form in those gobs would have a hard time sublimating. The vapor would have a very crooked path to follow in order to get out.

Now I missed a bit of a point here, I think it had something to do with solving that by either making the temperature very very cold, or by keeping it in the vacuum freeze drier for a really really long time, so you are missing out on the benefit of the vacuum freeze dryer. And it is NOT AN ISSUE if you are using a non-vacuum freeze dryer. I also realize that my nice IPM freezer going down to -35C is not an advantage for me. The sublimation would actually happen faster if my freezer were not so cold. I could make this go faster by using a fan and some silica gel. But for me at this point, waiting a few weeks for a basket to dry is no big deal.

For a review of all the papers and posters at the 2010 WOAM conference, please see my weblog at http://ellencarrlee.wordpress.comhttp://ellencarrlee.wordpress.com.