An excellent way to check the temperature on any shelf in any location in the kiln is to use a cone
plaque containing three large cones. One is for the desired maximum firing temperature. The
other two should be numbered above and below the firing cone. These are often referred to as the
guide cone, firing cone, and guard cone. The lower temperature (guide) cone bends as a warning
that the desired temperature will soon be reached, the firing cone should bend until the tip is level
with the base of the cone, and guard cone, if bent too far, signals overfiring; Seeillustration fig. 7-
2 for proper degree of deformation.
Small size cones are intended specifically for use with the Kiln Sitter
® (automatic shutoff)
mechanism (see section on Kiln Sitter
® operation). It may be that there will be a slight difference
in the reaction of the cone in the sitter relative to the large cone of the same number in a holder,
due to the fact that the large cone is heavier, more upright, and bends under its own weight, while
the small cone in the sitter is encouraged to bend slightly sooner by gravity and the weight of the
Kiln Sitter
® rod on top of it. If you find this to be true, it may be necessary to use a small cone
one number hotter in the Kiln Sitter
® than the correct large cone number to achieve the desired
results. Normally if firing to cone 04 or colder you will need one cone hotter in the Kiln Sitter
®.
If firing to cone 5 or hotter you will normally use the same cone number. When you get to know
your kiln and the typical cone reactions, you will be able to achieve consistent firing results.
Be sure to keep pyrometric cones bone dry so that they will not crack in the kiln. Once
dropped, roughly handled or exposed to moisture, cones develop cracks which tend to make
them bend prematurely and give an inaccurate temperature indication.
The cones generally used in firing clays and glazes in pottery work are numbered ranging from 07
to 04. A very common one which works satisfactorily in most cases is cone 06. Cones used to fire
china paint, gold, lusters and decals range from 015 to 019. Cone 4, 5, & 6 are used for most
porcelain. Some stoneware may be fired to cone 8. All clay and glaze manufacturers recommend
the correct heat treatment for their products. Clays and glazes do vary, so check labels or ask your
local ceramics supply dealer for advice on the proper cones to use, since they know the
characteristics of the clays and glazes they handle.
The Pyrometer (Optional)
The second commonly used device for reading the temperature in the kiln is the pyrometer. This
consists of a temperature sensing device (thermocouple) connected to a meter which indicates the
temperature inside the kiln. Its principal value to the hobbyist is a monitoring device which gives
warning when the maximum firing temperature is neared. It is also very valuable in working with
glass, which is extremely sensitive to differences in heat of only a few degrees. In firing ceramic
materials, the ideal situation is to fire with both pyrometric cones and a pyrometer, since cones
reveal what is happening to the ceramic in the kiln only at the final firing stage and the pyrometer
shows the actual air temperature continuously through the firing. The pyrometer does not take
into account the effects of time on the firing process as does the cone.
When using a pyrometer, be sure that the meter is hung on a wall or shelf near the kiln (not
on the kiln itself) away from the radiated heat of the kiln and away from drafts to obtain a
more accurate reading.
Another way of gauging the approximate temperature is by the color produced by different heats.
The temperature of the lowest visible red heat (observing the entire kiln atmosphere not just the
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