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My GUESS:
The resistance wire is probably pretty soft and ductile and is planned to operate at
low current and fairly low temperature. When turned on it will be at the temperature of
the glass to which it is bonded and it will warm up relatively slowly, bringing the glass with it.
Most kinds of glass conduct heat fairly fast and have a fairly LOW coefficient of thermal
expansion. (You can water-quench quartz glass from 1000°F and not break it.)
Also the core of tempered glass is under pretty great tension because of the heat treatment.
Warming it does NOT make it expand, it just reduces the tension.
--
George Downs Bartlesville, Oklahoma
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