9
A pyrometric cone is a slender pyramid composed of materials compounded in selected
combinations to make a cone that will react to time and temperature in the same way as
the ware you place in your kiln. Because different clays and glazes require different heat
treatments to mature properly, pyrometric cones are made in a series of cone numbers
so that one is available to properly control the firing of the clay or glaze you are using.
Even though small cones are always used on the KILN-SITTER, large size cones are
recommended as witness cones when test firing your kiln and KILN-SITTER, and
periodically during normal firing. This will help you determine whether firing shut-off is
just right … too soon (temperature too low) … or too late (temperature too high.) During
the test firing you will also learn your kiln's "personality" or its heat distribution.
For this test firing you will need a plaque,
which is a porous clay base designed to
hold three large size cones at an angle o
8-degrees from vertical. See Figure 12.
(Note: self-supporting cones are also
available.)
A cone, one number cooler than the firing
cone, is place on one side. This is called
the GUIDE CONE. It matures at a
temperature approximately 30-degrees
lower than the firing cone. When it bends,
it serves warning that shut-off time is near.
See Figure 13.
The cone number we wish to fire to is
placed in the middle. It is called the
FIRING CONE. It is the same cone
number as the cone on your KILN-
SITTER. When it bends over with the tip
close to, but not quite touching, the base,
you know that the desired heat treatmen
has been reached. See Figure 14.
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