Freeze drying and vacuum drying

Does anyone have first-hand experience (as opposed to theory) of the
relative merits and demerits of vacuum-drying versus freeze-drying of
archive material?

I would also like to know if there are any recent scientific studies
of the effects of these treatments on the aging properties of
already degraded material.

    **** Moderator's comments:   Because the nomenclature of drying is,
    to say the least, unstandardized and inconsistent, I asked the
    poster for a clarification of the terms used here and he sent the
    following.

As I understand it, the distinction between freeze-drying and
vacuum-drying is one of vacuum pressure and therefore of drying time.
The sublimation of vacuum drying involves a liquid water phase (albeit
brief) and gradually approaches the ambient water content of the
material, where it stops.  Hence there is no need for a period of
equilibration afterwards.

Freeze-drying on the other hand, would be carried out at a higher vacuum
pressure ( say 20-30 MTorr ),  together with the application of heat to
bypass the free water state completely. The over-dry material then needs
to be re-acclimatized over a period of time before it can be handled.

So much for the theory. What I would like to hear is other peoples'
experiences of the two processes, especially those who have had them
carried out by commercial recovery firms (without mentioning names, of
course!).

Can one draw any firm conclusions about the problems of distortion,
blocking and dimensional change of various library and archive
materials?

Any offers?

Ylva Player-Dahnsjo
Conservator
University of Dundee

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                  Conservation DistList Instance 6:49
                  Distributed: Friday, March 12, 1993
                        Message Id: cdl-6-49-011
                                  ***