AutoFire® Kiln Controller User’s Guide
34
Appendix B – Pyrometric Cones and Their Use
Using Orton Pyrometric Cones
Pyrometric cones have been used since the late 1800's to monitor kiln behavior. They are useful in determining
when a firing is complete, if the kiln provided enough heat, if there was a temperature difference in the kiln or if
a problem occurred during the firing.
Cones are made from carefully controlled compositions. Each cone bends in a repeatable manner over a
relatively small temperature range (less than 40
o
F). The final bending position is an indication of how much
heat was absorbed.
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 difference in
the kiln or if a problem occurred during the firing.
Temperatures are given for three series of cones. The soft series covers Cone 022 to 011. The low temperature
series covers Cone 010 to 01 and the intermediate temperature series starts at Cone 1 up to 12. Cones are made
to Cone 42.
Temperature equivalent charts are periodically updated. The temperatures shown inside represent the most
accurate data available for cones shipped today. It will be noted that the temperature spacing between cone
numbers varies. As a result, some cone numbers deform closer to their neighbors than others. In some cases,
Orton produces a half cone.
Cones located on a kiln shelf near the ware are referred to as witness or shelf cones. Small (junior) cones/bars
are used in a KilnSitter®. Small cones are sometimes used as witness cones, but this must be done with care
because they bend at substantially higher temperatures. Cones bend when sufficient glass forms and the cone
becomes soft. The composition of the cone and the amount of heat determine when the cone bends.
Kiln-Sitter® cones/bars bend due to the weight of the sensing rod. Witness cones bend due to gravity pulling the
cone over. Mounting heights 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. Temperatures are given for different cone types and mounting heights, all mounted 8°
from the vertical.
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