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Posted by Joe on February 3, 2010, 9:17 am
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On Wed, 3 Feb 2010 01:46:51 -0600, "William Wixon"
>hi,
> wondering if i could ask a question, worried it's been asked many times
>before, apologies if it has.
> i got a few 4 foot long broken lengths of neon tube. i want to cut them
>into approximately 16 inch long pieces and heat the cut ends to round over
>and smooth off the sharp broken edges. i was told i could use a
>("bernzomatic") propane torch to heat the glass enough so it melts (and
>rounds over the sharp edge). i tried it and much to my surprise it worked.
>i tried it on a few more pieces and i got erratic results. on some it
>worked fine, on others it caused a crack to run down the length of the tube,
>about 3 or 4 inches. this is what i don't want to happen. can anyone give
>me tips or pointers on how to heat the end to round off the sharp edge
>without causing the tubes to crack? i am so much hoping i'm not going to
>need an annealing oven (cause i don't have one!).
>
>b.w.
>
Welcome to the wonderful world of glass! The best you can hope for is
to lightly play the flame over a good bit of the length (4 - 6"?) to
prevent just the end from getting hot, causing the sharp transition
from hot to cold, and the resulting thermal shock. If you have an OA
rig (and since you post to rcm, there's a chance you do), try using it
(careful - it's a lot hotter!) gently, and "insulate" the warmed areas
by turning down the O2, to coat the glass with soot (after you warm
it); this will help the glass to cool more slowly & evenly. Don't get
soot on the end when it's molten, though, or it might get embedded
into the lip.
Generally, the thicker the glass, the more problems you will see with
trying to do this sort of operation. The soot coating is a great way
to anneal thin pieces - regardless of their overall size. For somewhat
more mass, dropping the finished piece into a container of vermiculite
will do much the same. Above that, you need an annealing oven.
What are you gonna do with the tubing? Is it real neon tubing (clear
glass) or does it have the phosphor coating? Just curious.
Joe
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