Jim Pendley on mon 7 mar 05
Hi All,
maybe some recall my shorted digital controller
problem from before. Well that problem was resolved
but
the consequences were odd to me.
I have been experimenting with the chinese red
formula of Tichane's (1983) at cone 8-9 and was
getting
decent reds with that formula firing in the gas kiln
with reduction until the last hour and then oxidation
as it was described in his book.
A load of 3 small variations in the % of copper in
the stock glaze and a control of the original formula
that had given me the red before were in the gas kiln
when the controller and the kiln's fire went out at
approx. 1150F. The kiln remained closed and had almost
cooled down when the repair was completed and the
instructor "tested" it back to approx 1400 F. before
he
left, turning the kiln off.
The next afternoon, since he knew I was returning
in the evening he fired it again and when I came in,
it
was at 1100F and I stayed and finished it off at cone
8
using the red/ox time as above and actual cones. So it
was fired and cooled and refired and cooled and
refired.
When I opened it, a mess!!! all of the test pots
looked the same, even the control. The glaze inside
each pot had puddled in the bottom and was blue rather
than any shade of red, and the outside looked as if
almost all of the glaze had flowed off. Some spots of
glaze were inches from the nearest pot on the shelf :(
as if they had been so liquid the flame blew them off
of the pot.I had taken precautions and had bisqued
pads under the pots to catch the expected small amount
of flow, but this looked very overfired.
It wasn't by choice that this 3 firings took
place,
but I would like to learn why the effects occurred.
Were the cones affected by the first firings that they
didn't reflect the heat load of the last firing and
they looked like higher cones? I had thought they
would
have dropped sooner.
Anyway I would like the thoughts of
others with more experience than my 2 years. Thanks
Jim Pendley
Snail Scott on tue 8 mar 05
At 11:34 AM 3/7/2005 -0800, you wrote:
>Were the cones affected by the first firings that they
>didn't reflect the heat load of the last firing and
>they looked like higher cones? I had thought they
>would
>have dropped sooner.
This can happen when cones are fired high enough
to start sintering, then get cooled or at least
held at that temperature for a while. I don't
understand the mechanism, since I would have assumed
that any crystal growth would simply melt when
fired higher, and that the previous heat-work done
to the cones would merely reflect that which was
done to the clay. Cones melt at temperatures where
clay merely vitrifies, though, so they aren't truly
having the same experience. Whatever the process,
my experience with 'cone freeze' confirms your
observations: cones can only be reused if they never
got above very low temperatures the first time.
-Snail
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