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NEON- More on annealing
Tom-
>bending and on what surface you bend (assuming you don't do it in mid-air) it
>is difficult to speculate on the actual cause of the stress. Obviously, there
It was really amazing to watch the guys at Neon Lewa in Holland. Their
glass almost never touches the table. They do in fact bend in mid-air;
even very large curves. The glass is held just a couple of inches above
the table. If they should need to flatten out a curve, they place it on
their bench with a large cutout in it and heat it with a large flame with
the handtorch and let gravity do the work.
>be due to the way that you are heating in the ribbon burner itself. Some
>people teach to heat the bend and let it sort of dangle in the flame with the
>bottom getting more heat and that part would become the outside of the bend as
>it naturally bends that way. I find that that will set up stresses as one part
>of the tube cross section is moving so much more on cooling. It is also hard
Just wondering out loud; does that brief period really cause a significant
uneven heating of the inside of the bend to cause problems? Obviously the
heat is more intense on the bottom of the bend, but I'd guess a whole lot
of heat is still getting to the inside. I guess it depends on how long you
do it. I might give gravity only a few seconds in the flame to do the job.
>developing. Some shops teach to "pat" the glass down flat with a tool of some
>sort like a wood paddle or screen covered wood piece (which I sometimes use).
>This will also set up stresses in the glass on both sides. I NEVER do this
>with any of the colored glasses.
I don't know if this agrees with the laws of heating/cooling, but my
observation is that sometimes that gentle, very brief "pat" helps a more
even cooling. If my RB bend is going onto the pattern briefly and I pat it
on top w/ a piece of non-bestos covered wood, it seems like at least the
top and bottom get the same treatment.
>percentage thing unless you do it like Dirk and anneal each piece in the oven
>after the work is made. Of course, to be sure you would have to anneal each
>bend before going onto the next which would be impractical. So we settle for
>what is an acceptable amount of stress and try to eliminate as much as
>possible by using common sense heating and cooling techniques and knowing the
>idiosyncrasies of each glass type.
>
> E-mail from: Tom Biebel, 06-Sep-1996
>
When I watched the guys at Neon Lewa, they anneal every single bend. And
they do it again and again every time that bend might be exposed to some
heating. For example, if they're making a 8" letter "E" and start with the
center part, before and after each bend, the entire piece is annealed in a
large flame. Even when they've worked their way to make the last, top and
bottom right angles, the whole center part is reheated. And that's all in
addition to the oven annealing.
So, I guess I'm curious, in your knowledge, is this kind of care
specifically required for the characteristics of soda glass or would lead
glass benefit from this kind of treatment as well? I'd also add that the
time they spend is very different than the time we spend here in the States
and their torches really make it quite easy to anneal in this way.
Tom U.