G’day: I mentioned in my earlier input on the subject that I
experienced discolouring of the softer (lower softening point)
glasses when making glass animals. The glasses I had most
problems with were the opaque and semi-opaque reds , greens,
yellows, orange, pale blue, white, etc. I was using a fixed
glassblower’s bench torch with coal gas and compressed air at the
time (yes a long time ago - around 40-45 years ago in fact!) A
recent Orchid beading correspondent mentioned that it was taking
a good while to heat the glasses properly without causing
discolouration. Which probably indicates that the very tip of
the flame was used, giving the cooler, though strongly oxidising
conditions necessary to avoid blackening. It was because of
this difficulty that I switched to a coal-gas/ oxygen torch. It
was taking me far too long to make the glass animals (I couldn’t
make any money at it!) Using oxy/coalgas flames, I could make a
daschund in 4 1/2 minutes, whereas it took 7 1/2 with
air/coalgas. The big bonus was that the oxy flame gave me the
right conditions to avoid the discolouration, only slightly
offset by the fact that the warm-up in the flame had to be more
careful to avoid violent shattering. In those days natural gas
, propane, etc wasn’t available. (Rural school laboratories had
to make do with those dreadful petrol-air machines for their
Bunsen burners.) In those days too, only the softer soda
glasses were available in colours, though I used harder,
borosilicate (Pyrex, Jena, Schott) glass almost exclusively for
scientific glassware used in research establishments.
Incidentally, I worked pure quartz tubing too, but used
oxy/hydrogen torches for that. Another point that strikes me
is that the correspondent mentioned using copper tube as the base
for the beads. Whilst I never did beading, I did have to make
glass-to-metal seals at times for high vacuum systems.
Borosilicate to copper seals were out of the question - they
cracked at once due to incompatibility with expansions. One had
to use layers of several special glasses of slightly differing
expansion coefficients, and this could be quite tricky. (They
were called Housekeeper seals after the bloke who invented them)
The point I make here is that if using copper tube as a base for
beads, one would have to have very thin walled tube to avoid the
expansion/contraction incompatibility problems and in this case,
the lead and cadmium based glasses would give the best
compatibility. I used clear, colourless lead glass for making
glass-to-metal seals with 28SWG specially coated copper wires.
These were called ‘pinch-seals’ as the softened glass tube was
pinched to the wires, and even the clear lead glass blackened if
care wasn’t used even with a powerfully oxidising flame . These
pinch seals were - and still are - used in making light-bulbs and
neon tubes. As each lead glass/copper seal was completed, it
was placed on a tray of dry sand, heated by a Bunsen burner, and
when the tray was filled it was placed in a LEHR (or small
furnace) kept at about 100C below the softening point of the
glass. At the end of the working day, the lehr was switched off
and allowed to cool slowly overnight. The leaded pinch seals were
then sealed into soda-glass bulbs or tubes. A similar
annealing process was used when scientific glassware was
completed, and occasionally we would examine a particularly
tricky piece in polarised light (the work was examined between
two pieces of Polaroid) to enable us to detect any strains and
thus potential faults in the job. So you think all that might be
irrelevant? Well, I submit it because I believe the problems I
encountered with scientific glassblowing and with making coloured
glass animals aren’t all that different from those to be
encountered when lamp-working beads, and someone might find the
comments helpful in some way…Cheers for now,
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/ /| \ @John_Burgess2
(____)
At sunny Nelson NZ