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Thermal glass problems

>> for a strong bend in standard glass, your glass should continue to move for
quite a long time after the bend is done.  I can't describe how long, for while
I've showed this to people countless times, I've never actually timed it. <<

This will obviously vary with the type of glass. I find that for lead glass this
would be around 4 or 5 seconds after you make the final adjustments in the bend.
Faster benders might have longer times after bending but also might not heat
quite as long. I always tell people to bend cool and splice hot. Basically, you
don't want to over heat the glass and you certainly want to spread that flame
over a broader area. A lot of benders and particularly beginners will heat too
small an area too hot and then stretch to make up the difference. The less you
distort the glass the better and so I say to heat larger areas and not so hot.
If you are only doing glass without powder then it doesn't matter as much
because overheating mostly affects the phosphors as far as thinning, crazing,
and over baking.

>> If your glass seizes up during the bend or directly 
after, you are looking at problems down the line.>>

This is because you are not heating the glass long enough or not enough area
(like the bottom will be very hot but the top of the bend will be stiffening).
The reason you will have problems in this situation is because you are applying
pressure to the glass and that pressure is being "frozen" in the "memory" of the
glass molecular matrix. This will set up cooling stresses that will make the
glass very succeptable to breakage when further stress is placed on the glass
from movement or even expansion and contraction from temperature changes caused
by the sun or snow or turn on/off cycles.

>> The wood of the table directly under the paper can chill the glass. <<

This can be good and bad depending on your philosophy. While it is not the best
thing in general to set up stresses caused by rapidly cooling and area of glass
vs. another in the same piece, it is sometimes done for strengthening purposes.
This is how tempered glass is made on purpose and I have seen cases where the
glass bends were actually stronger because of this rapid cooling. Some benders
will roll the glass on a screen when they do a splice in a long straight tube.
Have you ever tried to cut one of those? Besides dulling files (due to the extra
hardness) it is very hard to get a straight cut. On ribbon burner curves, it is
especially important to avoid the chilling effects Bettina is talking about. I
am sure everyone has seen those cracks on the bottom or picked up part of a
piece that was supposed to be a whole piece. Annealing glass is a complete
science in an of itself. You gotta gauge for this one, Morgan <g>.


  E-mail from: Tom Biebel, 17-Feb-1996

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