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PYROMETRIC CONES
Pyrometric cones have been used to monitor ceramic firings for more than
100 years. They are useful in determining when a firing is complete, if
the kiln provided enough heat, if there was a temperature imbalance in
the kiln, or if a problem occurred during the firing. Cones are made from
carefully controlled compositions. They bend in a reliable manner (over
a relatively small temperature range - less than 40°F). It should
be noted that the temperature spacing between cone numbers varies. As
a result, some cone numbers deform (bend) closer to their neighbors than
others. Cones located on a kiln shelf near the ware are referred to as
witness or shelf cones. Small (junior) cones are used in a Kiln Sitter.
Small cones should not be used as witness cones because they have been
formulated to bend with the weight and pressure of the sensing rod (on
the Kiln Sitter). Witness cones bend due to gravity pulling the cone over.
Mounting height and angle affect the bending. If the cone is mounted higher
than recommended or if it leans over more than 8°, it will bend earlier.
Orton developed Self-Supporting witness cones to fix mounting height and
angle. These provide for more repeatable performance. Temperature are
given for different cone types and mounting heights, all mounted 8°
from the vertical. It typically takes 15 to 25 minutes for a cone to bend
once it starts. This depends on the cone number. The cone bends slowly
at first but once it reaches the half way point (3 o’clock), it bends
quickly. When the cone tip reaches a point level with the base, it is
considered properly fired. This is the point for which equivalent temperatures
are determined. Differences between a cone touching the shelf and a cone
at the 4 o’clock position are small, usually a few °C or °F.
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