AutoFire® Kiln Controller User’s Guide
35
Behavior of Pyrometric Cones
Typically, it 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.
Temperatures shown on the inside charts were determined under controlled firing conditions in electric kilns and
air atmospheres.
Cone bending may also be affected by reducing atmospheres or those containing sulfur oxides. Orton
recommends the use of Iron-Free cones for all reduction firings (cones 010-3). If a cone is heated too fast, the
cone surface fuses and binders used to make cones form gases that bloat the cone. If cones are to be fired
rapidly, they should be calcined (pre-fired) before use. Cones should be calcined to about 850°F (455°C) in an
air atmosphere.
Temperatures are shown for specific heating rates. These heating rates are for the last 90 to 120 minutes of the
firing. If the heating rate is different, so will be the equivalent temperature. The temperature will be higher for
faster heating rates and lower for slower heating rates.
If a cone is soaked at a temperature near its equivalent temperature, it will continue to mature, form glass and
bend. The time for the cone to bend depends on several factors and Orton does not provide such data for its
cones. However, as a general rule, a 1 to 2 hour soak will be sufficient to deform the next higher cones. A soak
of 4 to 6 hours will be required to deform two higher (hotter) cones.
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