On Fri, 6 Feb 2009 07:55:23 -0800 (PST), Mel
wrote:

I have a new Paragon Electric Kiln with a digital controler. I program
it for cone #5 with a 10 minute hold for my glazing. What slower
cooling rate than the normal cool down should I use to allow crystal
glazes to work and help keep Greenware from cracking?

I can't give specifics on crystal glazes since
I've never done these. But I do know that it is a
*very* finicky process to get nice crystal growth.
There are books devoted to the subject, and Web
sites. This is probably not an area to dive right
into if you are just starting out... get
comfortable with conventional glazes first.

It's not just because it's hard to get good
crystals, it's because crystal glazes have to be
very runny... they almost always flow down the pot
and run off the bottom. Those who work with
crystal glazes have to put little saucers under
their pots to keep the run-off from flowing onto
the kiln shelves. Afterward they have to break
off the glazed-on saucer and grind the foot of the
pot smooth.

I'm not sure what you mean by greenware cracking.
Greenware is unfired ware. Some people use this
term to refer to undried ware (Hamer and Hamer, in
"The Potter's Dictionary"), but I think it's more
commonly applied to dried but unfired ware. So
the cool-down curve clearly doesn't apply here.

As far as simple cracking from thermal shock, this
will never be an issue. You don't need to program
anything for it, because the kiln will naturally
cool *way* slower than needed to prevent thermal
cracking. (Assuming you allow things to cool
naturally until you can touch the ware... never
try to remove hot ware from the kiln unless you
are doing raku or something like that, and know
what you are doing.)

That doesn't mean you'll never get cracking, just
that any cracking you get will have other causes
like stress left over from the making, or too much
thickness difference between parts of the ware, or
bad glaze fit, etc.

Best regards,


Bob Masta

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