Freezing botanical specimens

Vicky Purewal <vicky.purewal [at] nmgw__ac__uk> writes

> ... If the bags are sealed
>properly before freezing then the RH fluctuations will be greatly
>reduced, but there will still be some stress between the mount
>sheet, the specimen and the method of securing that specimen.

This is entirely dependant upon the atmosphere in which you seal the
bags, not the moisture content of the specimen. We have also
discovered that the rate at which you freeze the specimen also has
an effect. If your freezer unit is slow, any excess moisture in the
bag penetrates the specimen before freezing thus causing potential
harm to the specimen. If your freezing system is fast on the other
hand, at worst, you get frosting inside the bag and at best, no
freezing-out of excess moisture.

We have also found that the greatest increase in stress, in a
specimen, (due to the differing contraction rates of the specimen
and the paper it is mounted on), is most noticeable when the
temperature reduction is too slow, passing the 4 deg. C mark.

In the Preservation Unit, when we have to freeze specimens, we
ensure that the atmospheric RH when bagging is around 45-55%, and
use a freezer which is capable of reducing the temperature  as
quickly as possible. The Vestfrost freezers that we use here are
capable of freezing an individual specimen to < -30 deg. C in as
little as five minutes. Bundles require about 30 minutes. Any larger
quantities than this (which we have had on occasions, but not for a
good five years) and we use a spare blast freezing system we have at
our remote site.

If we do encounter frosting, which is rare, then de-frosting is
conducted within a reduced air pressure environment to ensure there
is no melt damage; not quite vacuum freeze-drying, but good enough!

Mary-Lou Florian <mflorian [at] telus__net> also writes

>A problem may occur with specimen sheets  in
>which an acrylic glue has been used. The adhesive may
>became tackier and cause the specimen sheets to
>slightly stick together. There is a great range of tackiness of
>acrylic adhesives, its a matter of checking with the
>manufacturer and obtaining one with low tack and works for
>you.

I too have witnessed this with two acrylic adhesives that have been
used here on occasions: Bostick Clear and HMG. Again, in both cases
I found that fast-freezing obviated the problem, whereas slow
freezing showed up an increase in tackiness. From the temperature 7
deg.c down to -3 deg.c, tackiness increased, following which a
general reduction in flexibility began. Full solidification for both
of these adhesives occurred at around -11 deg.. As a consequence, we
try to get through this temperature range as quickly as possible,
both when freezing and de-frosting. I would think that the tack-
ranges of most other acrylic adhesives are similarly discernible and
thus similarly avoidable.

Jonathan S Farley MA ACR MIPC
Senior Conservator
Royal Botanic Gardens
Kew
+44 208-332-5419
Fax: +44 208-332-5430


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                  Conservation DistList Instance 18:39
                 Distributed: Friday, February 18, 2005
                       Message Id: cdl-18-39-006
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