Usage
For some products, such as porcelain and lead-free glazes, it can be advantageous to fire within a 2-cone range. The 3-cone system can be used to determine temperature uniformity and to check the performance of an electronic controller. The 3-cone system consists of three consecutively numbered cones:
- Firing Cone - cone recommended by manufacturer of glaze, slip, etc.
- Guide Cone - one cone number cooler than firing cone.
- Guard Cone - one cone number hotter than firing cone.
Additionally, most kilns have temperature differences from top to bottom. The amount of difference depends on the design of the kiln, age of the heating elements, 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. We refer to the temperature 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" eliminates errors with their mounting.
Reference tables for the temperature equivalents for a number of different Pyrometric cones can be found through links in the 'External Links' section.
Read more about this topic: Pyrometric Cone
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