ATM - Kiln Design

[Richard Schwartz <richas@idt.net>]

Design of a kiln is a non-trivial exercise.   A good starting point is
to determine how much power your house wiring can supply.   Design the
kiln around that limitation.

There is a big difference between a slumping/fuzing kiln and one that
can melt and cast pyrex.   My experience with melting and casting has
been pretty negative.   Problems include:
- what to use for a mold that will not dissolve or react with glass?
- how to get rid of bubbles?  Molten pyrex is more like honey than like
water.
- how to mix molten glass to assure homogeneous composition

As for annealing, you do not need to worry about the accuracy of the
thermocouple if you bracket the annealing range.   As long as the
thermocouple has a continuous and monotonic response curve, you will be
OK.   Pyrometric cones are worthless for glass slumping, fuzing, and
annealing.   You need a thermocouple-driven ramping controller so that
you can program in the desired temperature curve.  (I have an anneal job
running right now, as I type this, and it is fully automatic.)

As for the shape of the kiln, since we are dealing with round mirror
blanks, the kiln should be round.   (OK, mine is a seven sided
polygon.)   With a round kiln, any thermal gradients will at least be
symmetric and you won't end up with McBride's problem where one side of
the glass cooled faster than the other.   Of course, you need to
position the glass in the center of the kiln.

On the way home from the Optics Workshop, I visited my old middle school
buddy, Steve Cress.   He manufactures kilns in Carson, Nevada.   He will
help me with design of my next kiln.   I believe that I can build a much
larger kiln if I accept a top temperature of 750 degC instead of 1250
degC, and if I have a thicker layer of firebrick to insulate it.  My
power limit is 30 amps at 220 VAC.

Heat flow software, such as SINDA, can be pretty pricey.  However, you
might look into the offerings of the makers of refractory materials.  I
have an ancient program (that I have not yet tried) from Thermal
Ceramics, Augusta, Georgia, PC Steady State Heat Flow version 2.5.
This is so ancient that it is on low density 5" floppies!   There must
be something better by now; we need to search the web to find it.

There is much to do here.   If you have a kiln, you can be a real
developer of new technology that will liberate us from the obnoxious
suppliers of thick pyrex blanks.

. . . Richard