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Glass changes little over time and is not affected by corrosive materials or even most acids. Weather affects glass windows, by causing the lead (in stained glass windows) or wooden frames to expand and contract. The temperature between the inside and outside of the building causes the strips of lead or wood to expand and contract by different amounts on the two sides of the window. After many seasons, the lead or wood weakens, and the glass begins to bulge. 

Lance Watsky
Preservation & Media Specialist
The Georgia Archives
5800 Jonesboro Road
Morrow, GA 30260
678-364-3764 (phone)
678-364-3860 (fax)
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www.GeorgiaArchives.org




-----Original Message-----
From: Association for Recorded Sound Discussion List
[mailto:[log in to unmask]]On Behalf Of Art Shifrin
Sent: Friday, January 30, 2004 10:46 AM
To: [log in to unmask]
Subject: [ARSCLIST] a pane in the glass.....


> Okay...how do we account for window glass that has become thicker at the
> bottom of the pane than at the top?  Apparently glass 'flows' over time -
> years not weeks - and window panes change shape!
>
> Nigel
> British Library

Well Nigel, according to the old wives and those who might decline to
check the website of the Corning Glass Museum, this can't occur!

The stuff has long been regarded as a very slowly
flowing liquid that is moderately elastic at temperatures normally
considered to be tolerable to we humans, and other similarly consituted
creatures.

As for 2000 year old bottles apparently not having changed shape
since they were blown, how can anyone possibly know this?

Assuming that an ancient metal mould might have survived and not have
changed shape
(I wonder if any glass vessels were moulded as such in antiquity), and that
an
object molded from it could be measured (accounting for shrinkage due to
cooling),
proving that it could not occur would be quite a challenge.

Perhaps with the exception of something that's been sealed hermetically,
(even that's possibly not an assurance of precluding physical changes of
solids) it is
highly questionable to  ASSUME that any old substance is in precisely the
same
condition as it was when created by any means, natural, man made, or
otherwise (whatever that might be!)

Shiffy