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Glassworking heat


G’day again; I did a lot of scientific equipment glassblowing
in Pyrex (boron) glass, and ordinary (soda) glass - and even
quartz. I also made glass animals in soda and coloured glasses.
For the ordinary and coloured glasses, we used coal gas and
compressed air which was quite adequate, but when Pyrex glass or
quartz was used oxygen was essential. Quartz softens at around
2000C. I have since used propane/oxygen flames for pyrex and
soda, and found the propane flame hotter than coal gas, and
perfectly satisfactory even when used in torches without
compressed air for soda and coloured glass. I also worked with
lead glass, which has a much lower softening temperature, but if
one tries to work lead glass with an ordinary torch it will
quickly blacken: a special oxidising flame must be used, with a
torch sold especially for working lead glass. Many of the
coloured glasses, such as the yellows, orange and red (cadmium)
and the white and turquoise blue behave much like lead glass,
blackening easily, so in the absence of the special torch I
worked these glasses with oxy-coal gas or oxy-propane using a
high oxygen flame, despite their lower softening point. One can
also work them using a propane-air flame, but one has to be very
careful to use the very tip of the flame only; it is cooler
there, hence the use of oxygen with an ordinary torch.
Finally, whilst soda glass, lead glass, and the coloured glasses
are mostly compatible with each other, having similar
coefficients of expansion, one cannot use any of these with Pyrex
or other higher softening point glasses; the join will crack and
fall apart before the work is even cool enough to handle.

And here endeth the lesson. Cheers,

   / /    John Burgess, 
  / /
 / //\    @John_Burgess2
/ / \ \

/ (___)

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