Cellulose nitrate and cellulose acetate

Katrin Pietsch <katrin.pietsch [at] gmx__de> writes

>I am a German student in photographic conservation and working on
>some cellulose nitrate and acetate roll films (1940-50s) that I try
>to relax and flatten with different procedures.
>...
>Can any one have experience or ideas about softening, redimensioning
>of aged plastics or can offer citations to literature on the
>subject?

I worked with some material like this in flat sheet negatives, and I
would caution you that aged cellulose nitrates/acetates are very
unpredictable. As you know, some are susceptible to spectacular
deterioration with eventual catastrophic results.  This is easy to
detect if it is on-going, but there appears to be a time period
during which the base material is beginning to deteriorate, while it
still looks and acts ok to the naked eye.  As the material begins to
deteriorate, it releases acid vapours which cause the gelatin image
to become very soluble in water.  Immersion in water or exposure to
humidity could cause massive loss of the image layer.  And the
deterioration can be very random throughout the material.  For
instance, the beginning of a roll may be just fine while the inner
part of the roll is beginning to deteriorate, so testing may not be
conclusive.

Before I stopped working on this problem, I was experimenting with a
method to determine if cellulose nitrate was becoming unstable, and
that was to enclose the material with an acid-indicating paper for a
length of time. This method seemed to work but was not perfected.
Perhaps a similar method could be developed to try to determine the
chemical stability of each roll before treatment.

As for actual relaxing, I did try once to restore flexibility to
brittle cellulose nitrate with camphor vapours.  This appeared not
to work at all, though I may have gone about it the wrong way or
given up too soon.

Whatever you try, however, be cautioned that unstable cellulose
nitrate itself can be completely de-polymerized in the presence of
moisture.  If the films are curled, brittle, discolored or have any
sort of odor at all, chances are deterioration has already begun and
there is probably little that can be safely done to help.

An increase of a few degrees in temperature may help increase
flexibility long enough to copy the film, but will cause enhanced
deterioration in the long run, and any heat more than a few degrees
should probably not be used on cellulose nitrate at all.

Sorry to be so negative (no pun intended), and I hope you hear from
some others who have found some safe methods for doing what you are
trying to do.

Linda S. Roundhill
Art and Antiquities Conservation, L.L.C.
18121 157th Ave NE
Woodinville, WA 98072
425-481-0720


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                  Conservation DistList Instance 20:35
                 Distributed: Monday, January 15, 2007
                       Message Id: cdl-20-35-003
                                  ***