Cold storage

Karen Potje <kpotje<-a t->cca< . >qc< . >ca> writes

>At the Canadian Centre for Architecture we are considering buying a
>frost-free refrigerator for the storage of colour photographs, as
>described in Henry Wilhelm's The Permanence and Care of Color
>Photographs, 1993.

The first thing that you should know before you follow Henry's
recommendations is that Henry is no longer making that
recommendation. Back in October I learned that a few months
previously, Henry had discovered an error in his data regarding the
humidity in the frost free refrigerators that he had been working
with and in fact, the actual humidity was running around 15% or so
which he feels (and others of us agree) is too low. At this low
humidity you start running the risk of causing permanent deformation
of rolled film, for example. He now says

    a.  you will need some way of controlling the RH in the package
        and therefore will need to either go with the packaging
        created by Mark McCormick-Goodhart or condition and bag in
        foil laminate bags and

    b.  if you're going to that much trouble anyway, then you might
        as well buy a freezer instead.

The frost-free freezers that we've been playing with have been
running around 55%-65% RH and it appears that the excursions into
higher RH during the defrost cycle don't affect materials in the
packages that we've tested in there. (Humidity sensors put into
typical plastic microfilm boxes, cans and plastic bags hardly showed
a change during the defrost cycle. Also at freezer temperatures (we
were running at -16 C), the diffusion rate of water in and out of
photographic materials can be pretty slow.)

>Our Security Department, for example, is concerned about the
>possibility of a motor burning out one day and causing our Halon
>system to go off.

While I've never heard of mechanical problems setting off a Halon
dump, you do have to remember that the photographs are in an
insulated container. This means that if you have a compressor
failure and still have the defrost cycle, the refrigerator or
freezer soon becomes a good oven. We had a compressor failure twice
with a frost-free refrigerator about ten years ago (our fault) and
the freezer compartment was running around 130 F by the time we came
in in the morning. We don't know when the failure occurred and
therefore how long there was no compressor (maximum of probably 14
hours.) Since we fixed our error this unit hasn't failed in 11
years. However, it is important that there be 24 hour monitoring
(and notification in the event of a failure) as you would need with
a "real" cold storage vault.

One important thing that to note about the use of frost-free
freezers is that moisture control packaging is necessary for
long-term storage. You may chose between using a barrier film or
inhibiting film (such as the PE freezer bag) with moisture buffers
(a la Mark's design.) The reason that this is important is that the
equilibrium moisture content of a material in equilibrium with a
constant RH at low temperature is much higher than it would be at a
higher temperature. For example, if a typical motion picture film is
in equilibrium with a 50% RH environment at -16 C and it was warmed
up to 20 C, the film will have the same emc (equilibrium moisture
content) as it would have had if it had been in equilibrium with a
20 C/63% RH environment. This is because temperature equilibration
is so much faster than moisture equilibration. In either case, the
emc is about 3.5%.  The emc for that film at 20C/50% RH is about
2.8%.

The problem is that the emc of the gelatin emulsion may be high
enough at the freezer condition (given enough time) that when the
object is warmed up, you cross the glass transition temperature of
the emulsion. The result may be blocking, ferrotyping, or out-right
bonding of the gelatin to whatever is in contact with it. I've seen
a number of examples in which a few hundred sheet negatives, or
prints form a "brick".

-Douglas Nishimura
Image Permanence Institute

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                  Conservation DistList Instance 10:67
                 Distributed: Monday, January 27, 1997
                       Message Id: cdl-10-67-001
                                  ***