Pyrometric cones are slender pyramids made from about 100 carefully controlled compositions and have been used since the late 1800’s to monitor kiln performance. Originally thought to be temperature indicators, studies demonstrated that time and atmosphere also affects cone bending behavior. Each cone number is unique in measuring temperature within a small temperature range (less than 30¡C)
Cones measure the amount of heat absorbed. As the cone nears its maturing range, it softens and the tip begins to bend, drawn down by the influence of gravity or the weight of the sensing rod for cones or bars used in the Kiln-Sitter¨. It typically takes about 20 minutes for the cone to bend fully. Each higher cone number requires more heat to bend. Faster heating rates require the cone to be heated to a higher temperature.
Pyrometeric cones are designed to mature (bend) at precise moments during the firing, by either shutting the kiln off at the proper point (as in the case of the Kiln Sitter and sitter cones) or used to simply indicate that a certain point was reached, as in the case of the witness cones. The exact moment the cone matures depends primarily upon two factors: time and temperature. The temperature present at the moment “cone” is achieved depends upon the rate of temperature increase. With this simple concept in mind it’s easy to understand why “cone” and temperature are not the same thing. For a proper cone firing, time and temperature must always be considered.
Large and Self-supporting cones are used as witness cones to monitor the firing and often are used to decide when the firing is complete.Use Self-Supporting or Large Cones whenever possible. Self-supporting Cones give the most consistent, repeatable performance due to the fixed mounting height and angle.
Small cones and Pyrometric bars are used in a Kiln-Sitter, a mechanical switch that cuts off power in electric kilns. Small cones are occasionally used as witness cones, but caution is recommended, since these shorter cones require slightly higher temperatures to bend, as compared with Large or Self-supporting cones.
Cones are still needed when a kiln is equipped with an electronic controller. Electronic controllers allow the kiln to be heated to a temperature. Controllers use a thermocouple to sense temperature. These change with time, causing the kiln to fire hotter than desired. Witness cones will check whether the controller is firing accurately.
Heating rate is important in the cone selection process. The temperatures shown in the temperature equivalent chart for each cone number are based upon specific heating rates. For faster heating rates, the cone must be heated to higher temperatures before it will bend.
Height of the firing chamber or the space available in the furnace can also be a factor in deciding which cone to use.
Consider the temperature range. While cones deform through a combination of time and temperature, the expected temperature range can determine the cone number and even the type of cone to use. For very high temperatures, a Large Cone or a PCE Cone may be needed. Self-supporting Cones are available for all low or moderate temperature use. Only small cones or bars should be used in the Kiln Sitter