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Subject: Designing freeze-drier

Designing freeze-drier

From: Celine Bonnot <bonnot>
Date: Wednesday, December 18, 1996
This message is posted on behalf of G. Chaumat who is not on the DistList

    Howard Wellman <106000.3617 [at] compuserve__com> writes

    >As you might guess, freeze-driers are not common appliances in
    >the Middle East.  Our lab is working on ways to acquire one, and
    >building it ourself (or finding appropriate engineering to build
    >it for us), would probably be much cheaper than paying import
    >duties on a complete system from European or US suppliers.

    If you intend to build a freeze-dryer by yourself, we would like
    to give you some general advice concerning the choice of
    operating parameters to conduct freeze-drying and avoid
    irreversible damages of organic artefacts.

        *   First ensure that the artifacts are completely frozen
            before launching the drying with the vacuum. Indeed, it
            is very important to establish a sublimation drying
            instead of an evaporation mode (to avoid collapse of
            brittle hollow structure). If you use pre-treatment with
            polyethylene glycol (PEG) the freezing point is close to
            -26, -27 deg. C (so it is very low, be careful). If you
            used no impregnation steps, -15 deg. C is coarsely
            satisfactory to permit a good freezing of most of
            organic artefacts.

        *   The quality of the results after freeze-drying depends
            strongly on the level of the considered decay of the
            parts, i.e. the pre-treatment of these parts. For
            instance, for very damaged  waterlogged woods, PEG 1000
            up to PEG 6000 can be used to avoid collapse of the
            cells (but it is not a necessity). Although, for sound
            archaeological woods, a PEG 200/400 pre-permeation is
            needed to limit shrinkage under the fibres saturation

        *   Concerning vacuum, a [0.05-0.1 mbar] range around the
            artifact is sufficient to allow efficient drying. This
            factor does not depend only on pumping but depends also
            on the water traps of the apparatus (capacity of the
            water traps; length between water traps and artifact;
            temperature gradient between water traps and artifacts)
            and on the energy given to the system.

        *   If necessary, a power source (a resistor) can be added
            to accelerate the drying if the vacuum group is
            well-sized. Indeed, with more energy, drying efficiency
            increases and if the system is unable to collect water
            molecule quickly, the pressure around the artefact can
            increase and the temperature may reach the freeze point;
            consequently an evaporation mode occurring.

    Do not hesitate to contact us for further information.

    Sincerely yours,

    G.Chaumat, ingeneer
    Q.K.Tran, ingeneer
    17, rue des Martyrs
    38054 Grenoble cedex 9
    chaumat [at] chartreuse__cea__fr

Celine Bonnot

                  Conservation DistList Instance 10:59
                 Distributed: Monday, December 30, 1996
                       Message Id: cdl-10-59-002
Received on Wednesday, 18 December, 1996

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