Freezing glass plate negatives

This is in regards to Martha Little's query on the freezing of glass
plate negatives. All responses I could find were opposed, but it
actually can be done quite safely.  I apologize if I missed some
responses to this but I think it's important to get the facts
straight.

Freezing glass plate negatives is not only OK, it is recommended for
long-term preservation.  Though the additional benefits of freezing
over say 40 deg. F are generally considered to be marginal, below
freezing is a recommended storage option.

There was concern in the past regarding the possibility of
delamination of the image binder (typically gelatin or collodion)
from the glass support.  However, these concerns were unfounded and
the ISO standards now clearly specify that freezing glass plate
negatives is safe.  "ISO 18918 Imaging materials--Processed
photographic plates--Storage practices" deals with these issues, but
the results are summarized in the Image Permanence Institute's
publications, including their Media Storage Quick Reference, 2nd
edition available as a PDF here:

    <URL:https://www.imagepermanenceinstitute.org/resources/publications>

As with most museum and archival objects that are frozen, the boxes
should be sealed in plastic so that when they are brought back to
room temperature the condensation occurs on the plastic rather than
on the glass plate negatives.  This is most safely done over the
course of 24 hours by placing the box in a cooler to slow down
thermal equilibration.  This is done to prevent condensation within
the box due to thermal gradients within the stack of photographic
materials.

Freezing glass plates after a disaster can also be useful in some
situations as noted by Nicole Christie, but this is a completely
different situation.

Gawain Weaver
Photograph Conservator
San Francisco Bay Area
415-446-9138


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                  Conservation DistList Instance 28:26
                Distributed: Saturday, November 29, 2014
                       Message Id: cdl-28-26-002
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