Janet H Walker on tue 5 may 98
Hi Lori.
...and what is the accepted temperature for each cone...
When someone says "fire to cone thus and so" they mean the large
witness cones on the shelf. With the cones placed at an 8 degree
angle to the vertical with the right amount sticking above the cone
pack. When I realized that setting the cones differently from this
could affect your "readings" by a huge amount (a whole cone off for
example), I decided to invest in the "self-supporting" kind that
just stand there and then droop. Take out one source of variation.
The accepted "temperatures" vary, both between large and small and
within the same cone. That is because the bending of a cone happens
NOT at a specific temperature but after a certain amount of heating
over a certain amount of time.
That is, it matters not just how hot it got but how fast it got there.
This is crucial to understand. This is why you can't use the same
number on the little jnior cone in the sitter as on the one standing
on the shelf. It is a constant source of confusion but that's just
the way it is.
The "temperature end points" for the big cones are reported for two
different rates of heating, 60C and 100C (roughly I think). This
rate applies to the heating that goes on during the final two hours
of the firing. If you have a digital pyrometer you can measure how
fast your kiln can climb in an hour when you are already way up there
near the end of the firing. Then you'll see whether your kiln rate
of climb is closer to one or the other of the rates shown in the charts.
The little junior cones are calibrated for a heating rate about triple
that for the large cones. This means that if you fire them more slowly
they will droop earlier because they "soak up" more heat in that time.
So that is why a junior 8 cone in the sitter can drop before a large 6
cone on the shelf. That's just the way they're built.
One reason that it is nice to have a controller is that the controller
handles the rate of increase. When you put all the switches on high
with new elements, the rate of increase is going to be faster then
when those elements get older. So you have to either learn to adjust
or watch your cones and expect longer firings.
Again, the one you care about is the one on the shelf beside the pots.
Cheers,
Jan Walker
Cambridge MA USA
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