Deterioration of glass negatives

In response to Andrea Schwarz's posting on the deterioration of
glass negatives (Conservation DistList Instance: 14:29 Monday,
November 20, 2000), it is not very accurate to conclude that glasses
have unstable compositions from  "qualitative analysis". "Unstable"
compositions usually involve a deficiency in lime (CaO) and a
correspondingly high percentage of alkali (soda or potash).  These
can not be clearly defined through qualitative analysis.

Andrea is correct however, in that there is evidence of
deterioration of vessel *and* plate glass from environmental organic
acids.  That has been documented and is being seen more and more in
glass collections that have been in prolonged storage in closed wood
cabinets.  One thing that hasn't been mentioned is the fact that
stable glasses can also deteriorate under high humidity and/or
aggressive conditions.  This includes the relatively stable
silica-soda-lime glasses that are best known as "plate" or "window"
glass.  Some good examples include crizzling on the interior sides
of thermopanes, stacked crates of windows awaiting shipment, and the
recent discovery of the crizzling in the inner glass of the
enclosures used to house the Charters of Freedom (Declaration of
Independence and Bill of rights) at the National Archives.  None of
these examples involved organic acids, just a moderately high
humidity (around 55%), and the lack of air movement.  It now appears
that crizzling/weeping can be accelerated when the above conditions
are present, combined with organic acid vapors.

Stephen Koob
Conservator
The Corning Museum of Glass
One Museum Way
Corning, NY 14830
607-974-8228
Fax: 607-974-8470


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                  Conservation DistList Instance 14:30
                Distributed: Tuesday, November 21, 2000
                       Message Id: cdl-14-30-001
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