Cold storage for glass plate negatives

William L. Hopkins <whopkins [at] uwyo__edu> writes

>We are currently considering our options regarding placing our glass
>plate negatives into cold storage.  Our unit is set at 32 degrees F.
>with a relative humidity of 40%.  We are planning to place those
>plates that are in the worst condition into storage first, using a
>24 hour cool down period in a beverage cooler to slowly equalize
>conditions.  My question is will the cold conditions exacerbate any
>peeling of the emulsion from the glass plate or otherwise weaken the
>adhesion between the glass and the emulsion? ...

The single most important preservation factor for glass plate
negatives, whether collodion or gelatin, is relative humidity. If
you store your negatives at a constant level of about 35-40% RH,
even at moderate temperatures, the negatives should last for a very
long time with no significant change. I do not see the benefit of
freezing glass-supported negatives, but keeping them cool (room
temperature or slightly lower) would be recommended.  It is
important to choose temperature and humidity levels that allow you
to bring the plates from cool into room temperature without crossing
dew point. For example, if your normal room environment is
approximately 72 degrees F +/- 5 degrees and 50% RH +/- 5%, good set
points for your store room would be 62 degrees and 35 - 40% RH. If
you cannot have a separate storage room for your plates, the
important thing is to keep the negatives at a constant humidity
level (approximately 35-40% RH).

The deterioration seen in glass plate negatives is generally caused
by 1) fluctuation in humidity or low humidity (below 25% RH) causing
the binder to separate form the support and/or the varnish to crack,
and 2) the by-products of glass deterioration, which damage the
binder and varnish. Glass deterioration is driven by fluctuations in
humidity, and further driven by elevated temperature and humidity .
By controlling the humidity and keeping it at a moderate level, the
glass negatives will remain in good condition for many years to
come.

Regarding freezing: I have seen and heard accounts of failure in
some cold vaults in which humidity could not be maintained,
resulting in, among other things, mold growth. Mold is a serious
problem that I would take major precautions to avoid.

Regarding duplication of your negatives: be sure to use a negative
duplication firm with years of experience, and plan to expend plenty
of time and resources for quality control inspection and test
printing to compare prints from original and duplicate negatives.

Constance McCabe
Senior Photograph Conservator
National Gallery of Art


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                  Conservation DistList Instance 15:69
                  Distributed: Tuesday, April 9, 2002
                       Message Id: cdl-15-69-005
                                  ***