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Date: | Fri, 2 Sep 2005 10:03:02 -0400 |
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Carl,
The difference between freezing and freeze-drying is that one inactivates the water in an object and the other aids in the removal of the water. Water from flooding etc.. can cause both chemical and physical damage to materials. Additionally, its presence can catalyze other forms of decay. However, rapid drying of wet artifacts can lead to damage in the form of dimensional changes. Water also has a high surface energy that leads to a high surface tension (think of how a thin film of water can hold two panes of glass together). This high energy hold water in micropores and capillaries with the result that air drying will remove some but not all of the water in a wet object. Also as the water evaporates from larger capillaries, the receding water front pulls the surfaces of the capillary together. If the walls are frail, collapse can follow. In the case of wet paper this can also lead to pages becoming stuck together.
Unfortunately in flood situations the number of books or documents affected is typically very high. Freezing these materials allows the deterioration caused by the water to be halted and the materials held until disaster responders and conservators have time to assess the condition of the material and deal with them on an individual basis. However it does not remove the need to deal with the materials. Although the water has been removed the object has not been truly stabilized as removing it from the freezer or power failures could render it wet again.
Freeze-drying is often used to permanently dry such materials as it is a relatively gentle and slow form of drying organic materials and it allows the issues of surface tension to be eliminated. In freeze-drying the object is frozen and then a vacuum is pulled which causes the water to sublime off (meaning that it passes from the solid ice form to the gaseous vapor state without passing through the liquid state). The vapor is given off to the surrounding chamber and is then trapped by freezing it on a much colder external condenser. During the process the internal temperature or weight of the object may be monitored to determine when an end point has been reached.
Emily Williams
Conservator of Archaeological Materials
The Colonial Williamsburg Foundation
Date: Thu, 1 Sep 2005 12:25:55 -0600
From: [log in to unmask]
Subject: Re: Hurricane damage
Forgive my ignorance, but what is the difference between freezing and
freeze drying?
Doesn't plain o'l freezing arrest (?) the influence of moisture?
Inquiring minds want know.
Carl Barna
Lakewood, CO
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