Preparing materials for freeze-drying

I agree with Soren Ibsen's views on a simple approach to preparation
of materials for freezing. Cardboard boxes are cheap, strong and
most important, vapour permeable. There are though a few points to
bear in mind.

If the materials are to be treated individually direct from the
frozen state, wrapping might be unnecessary. If, however, a large
number of items are frozen and then dried in a low pressure chamber,
distortion can occur. This is the case in particular if volumes are
not restrained. The contraction of parchment and vellum bindings is,
as one might expect, considerable and can tear joints badly, but
this can happen with any material combination. In many cases the
contamination from flood waters and the migration of solubles in the
materials themselves will make a complete dismantling, washing,
stabilisation and rebinding necessary, but damage in drying chambers
as a result of dimensional changes should be minimised anyway.

One of the effects of low pressure drying is the production of a
characteristic line of concentrated(dried)solutes a couple of
centimetres in from the head, tail and fore edge. This is caused by
the same physical phenomenon as is chromatography. Even should the
water affecting the material have been pure, the effect of
deposited, migrated colour will be the same.

In conclusion:  treatment from the frozen state might be the best
option for some categories, but where very large quantities are
involved, low pressure drying might be the best. When making the
decision, the problems in controlling dimensional stability must be
considered and appropriate measures taken.

Mark Hingley, Conservation Section,
Norfolk Record Office,
Norwich, Norfolk, England


                                  ***
                  Conservation DistList Instance 16:13
                  Distributed: Friday, August 23, 2002
                       Message Id: cdl-16-13-001
                                  ***