Mercy Langford on wed 26 jun 02
Hi- I have a question for you guys. I'm trying to softfire(018) my porcelain
pieces and they are blowing up in pieces within the first half hour. My kiln
is infinite switch and this second time it happened I left in off position
just so it would heat up very slowly. I was then going to turn it on low,med,
high but never got too. can somebody please tell me the causes to this? I've
come to the point where I'm afraid to turn it on. Never happened before only
the last two. The kiln did feel pretty hot forhaving it off but if it's
heating on it's own even if I have it on low-I just do not understand.
Thanks for your input in advance- Mercy
Dave Gayman on wed 26 jun 02
Kilns that are off should not warm up at all. Infinite switches work by a
contact on the end of a bimetal piece -- click to low, and a tiny amount of
electricity is fed through the bimetal piece and heats it up, causing the
contact to open. As you turn the switch up, you add to the "on" side of
the equation (on "low" it barely wants to make contact and backs off with
only a little heat -- on "high," it's permanently closed and the circuit
that causes the bimetal to heat is turned off entirely.)
It sounds like one or more of your switches is permanently on, either
because the circuit that is supposed to heat the bimetal contact is open
(causing the switch to stay on), or because the contacts have welded
themselves together.
Also, doublecheck the thickness and dryness of your porcelain
pieces. Porcelain without grog (chamotte) can be very dense and slow to
release moisture while green, especially if bentonite or other plasticizers
have been added to make it more plastic.
I assume you know about going slowly up to 400 deg. F / 200 deg. C. during
early firing so that the "chemically combined water" (okay, adsorbed water
for you chemistry purists) can be safely driven off...
Dave
At 12:59 PM 6/26/2002 -0400, you wrote:
>. The kiln did feel pretty hot forhaving it off but if it's
>heating on it's own even if I have it on low-I just do not understand.
iandol on thu 27 jun 02
Dear Mercy Langford,
You say ""I'm trying to softfire(018) my porcelain pieces and they are =
blowing up in pieces within the first half hour."" =20
From what you say, this is the first firing of this clay, not a post =
maturity firing to add "On Glaze" decoration.
This sounds like free moisture and steam pressure, but, assuming this =
clay is fully dry your pots may be suffering from Low Temperature =
Dunting. The temperature is sufficiently high to remove the chemical =
water but not high enough to develop mechanical strength through =
sintering. Thermal stresses created when it cools are sufficient to =
destroy its mechanical integrity.
I cannot see the advantage of trying to work at such a low temperature =
with a material designed to be translucent when fired at cone 8 and =
above. Your pieces will be so fragile, if you can overcome the problem, =
that they will be unusable as other than decoration in a collectors show =
case.
Perhaps you should seek out a white firing earthenware clay for your =
work.
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