Salt on glass

I have been away for the past month, but wish to add a few comments
concerning the recent messages (Cons DistList Inst. 17:12, 17:13)
concerning "salt on glass".  First, thanks to Bill Wiebold, Niccolo
Caldararo, Alan Derbyshire and Loren Pigniolo for their observations
and excellent comments.

This phenomenon is more common than one thinks, and can occur on
glass whenever the one side (or part) of a glass is subjected to
high humidity for a prolonged period of time.  The problem becomes
worse if the glass is sealed in some manner and the moisture is
trapped against one surface.  In these instances, we see the
hydration of glass, and leaching of alkali, as a result of the high
humidity.  Not only does this happen to the cover glasses on
daguerreotypes, miniatures, prints and drawings, but it also occurs
on the interiors of glass vases, decanters, cover glasses for
biological specimens, cover glasses or "crystals" of clocks and
watches, underneath labels applied to glass, on the insides of
thermopane windows, and even the original 1950's glass pane housings
of the Declaration of Independence and Bill of Rights, recently
redone for this very reason.

The primary cause is the high humidity, which over time, leaches the
alkali to the surface of the glass.  If the humidity is maintained
over a prolonged period, the alkali can be seen as droplets, and
eventually will pool or drip, thus creating the "weeping"
phenomenon.  If the alkali is not removed from the surface it will
eventually attack and dissolve the silica, thus freeing up more
alkali, and causing the glass to develop cracks, or "crizzling".
Fluctuations in humidity can result in the glass cracking further,
and/or the droplets drying out and forming "salt crystals".

As stated in previous postings, there is considerable documentation
of this phenomenon, and two other references to note are:

    Brill, R.H.
    "Crizzling--A problem in Glass Conservation
    Conservation in Archaeology and the Applied Arts, Stockholm
    Congress, London, IIC, 1975, pp. 121-134

and

    Brill, R.H., Hanson, B., and Fenn, P.M.
    "Some Miscellaneous Thoughts on Crizzling"
    Proceedings of the XVIIIth International Congress on Glass, San
    Francisco, July 1998.

This deterioration phenomenon can occur in stable glasses, owing to
the microclimate created, but we see it much more commonly in
16th-19th century glasses that have unstable compositions (usually
low lime, high alkali).  Current research also indicates that the
problem can be exacerbated by airborne pollutants, such as those
found in wooden storage cabinets. Interestingly, the glass, or side
of the glass (as in a cover glass or frame) that is NOT exposed to
high humidity generally does not show this problem, simply because
air movement and occasional cleaning prevent it from happening.
Moderately low humidity, around 40-45% virtually stops the
weeping/crizzling, but this is difficult to achieve in private
collections. The humidity should never be dropped below 30%, as
glasses that have already begun to hydrate and crack, will develop
even worse cracking.

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


                                  ***
                  Conservation DistList Instance 17:15
                   Distributed: Monday, July 28, 2003
                       Message Id: cdl-17-15-002
                                  ***