Anne K. Wellings on sun 5 may 02
I use two ^5 self-supporting witness cones in my 3-ring electric kiln,
one on top and one on the bottom. I put a ^6 in the sitter but don't
allow it to go all the way. Usually, the bottom reaches temperature
faster, so I end up turning the bottom switch off first and leave the top
2 on high until the top cone goes. ( I don't need to soak or fire down
with the glazes I use.)
My last firing, the scenario was reversed, and the bottom cone was
behind, so I turned the top ring off first and left the bottom 2 on. Not
too long after, the bottom cone suddenly dropped to about 10:00, which is
my stopping point, so I turned off the kiln.
It turned out that the bottom shelf was quite overfired (still sellable,
but very toasty; honey jar lids were stuck on tight and I broke one while
loosening them). The bottom cone looked very strange. It had bent down at
the base but the top part was still more or less straight. The top part
had not arched and did not look shiny
and vitrified like the bottom part and like the whole other cone.
I am convinced that this was a defective cone, that the top part of it
was of a different composition than the bottom part and did not soften
normally. I think the heatwork on the pots took place at the usual rate
but the cone did not indicate it by bending properly.
The middle ring was a bit overfired, too, having been left on longer than
usual, and the small ^6 in the sitter was more bent than usual.
Has anyone else had a problem with a defective cone which does not bend
at the right temperature, or where the bottom part appears vitrified but
the top part doesn't? Not that we could do anything about it, just
curious. I have no other explanation for why my pots in the bottom were
overfired.
Anne
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