Seger cone

Pyrometric cones are pyrometric devices that are used to gauge heatwork during the firing of ceramic materials. The cones, often used in sets of three as shown in the illustration, are positioned in a kiln with the wares to be fired and provide a visual indication of when the wares have reached a required state of maturity, a combination of time and temperature. Thus, pyrometric cones give a temperature equivalent; they are not simple temperature-measuring devices.

The pyrometric cone is "A pyramid with a triangular base and of a defined shape and size; the "cone" is shaped from a carefully proportioned and uniformly mixed batch of ceramic materials so that when it is heated under stated conditions, it will bend due to softening, the tip of the cone becoming level with the base at a definitive temperature. Pyrometric cones are made in series, the temperature interval between the successive cones usually being 20 degrees Celsius. The best known series are Seger Cones (Germany), Orton Cones (USA) and Staffordshire Cones (UK)."

For some products, such as porcelain and lead-free glazes, it can be advantageous to fire within a two-cone range. The three-cone system can be used to determine temperature uniformity and to check the performance of an electronic controller. The three-cone system consists of three consecutively numbered cones:

Additionally, most kilns have temperature differences from top to bottom. The amount of difference depends on the design of the kiln, the age of the heating elements, the load distribution in the kiln, and the cone number to which the kiln is fired. Usually, kilns have a greater temperature difference at cooler cone numbers. Cones should be used on the lower, middle and top shelves to determine how much difference exists during firing. This will aid in the way the kiln is loaded and fired to reduce the difference. Downdraft venting will also even out temperatures variance.

Both temperature and time and sometimes atmosphere affect the final bending position of a cone. Temperature is the predominant variable. The temperature is referred to as an equivalent temperature, since actual firing conditions may vary somewhat from those in which the cones were originally standardized. Observation of cone bending is used to determine when a kiln has reached a desired state. Additionally, small cones or bars can be arranged to mechanically trigger kiln controls when the temperature rises enough for them to deform. Precise, consistent placement of large and small cones must be followed to ensure the proper temperature equivalent is being reached. Every effort needs to be made to always have the cone inclined at 8° from the vertical. Large cones must be mounted 2 inches above the plaque and small cones mounted 15/16 inches. With the cones having their own base, "self-supporting cones" eliminate errors with their mounting.

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Wikipedia