"Michael Daly" wrote in message
news

On 6-Feb-2006, Chris Jarshant wrote:

One more question. If I could heat up tempered glass slowly to a
ridiculous temperature, what would eventually happen? Would the
glass liquify, or shatter first?

It would anneal. Well, actually, it would reduce the stresses in the
tempered glass and would end up annealed if you then cooled it slowly.
If you just kept heating it, it would melt.

It will only shatter if it has a good reason. There has to be a reason
for a
crack to form and/or propagate. If it already has a crack (or scratch) as
a
stress riser _and_ the heating causes sufficient differential expansion
that
the stress riser goes into tension, the crack will grow rapidly and the
glass
will shatter. If the tempering process has created sufficiently high
internal
stresses that heating one side causes the expansion (or even annealing) on
that side to result in serious tension stresses, it might crack as a
result of
the high stresses without a precursor stress riser.

Personally, I take claims of tempered glass shattering "with no reason"
with
the same enthusiasm as claims of people spontaneously combusting.

Mike

sorry, it wouldn't anneal.

it would become untempered once you get it over bending temperature, which
for ordinary and normally found clear float glass, is around 1030F. on
cooling, it would no longer be tempered but regular window glass.

annealing is the process of reducing temperatures slowly enough so that
stress is evened out across the entire sheet of glass. if the temperature on
one part of the glass is different enough than another, when cooled, it
would set up uneven stresses, leading to eventual cracking and shattering.

in order to temper glass, it is heated up in special tempering ovens to
around 1200F (which takes about 5 minutes), then cooled to around 700F in
another 5 minutes. the tempering oven i'm familiar with has a 60 hp blower
to remove that amount of heat quickly enough. the equipment to do so starts
around $2million. it's not possible to temper glass without use of this type
of equipment.

there are other glasses, like pyrex or borosilicate, which can be tempered
the same way, but just at different temperatures. pyrex usually isn't
tempered at all.

regards,
charlie
http://glassartists.org/chaniarts