What Are Pyrometric Cones?
Usually simply referred to as cones, pyrometric cones are used to measure the effect of the kiln's atmosphere on the glazes being fired. Cones are made up of refractories, such as silica, and melting agents; each type of cone is carefully formulated and manufactured for accuracy.
Rather than talking about temperature, potters almost always refer to the cone a pot is fired to. Cone numbers go from cone 022 to cone 14, with numbers beginning with a zero being of lower temperature than those without.
Cones and Temperature
Although the word "pyrometric" means "measuring heat", cones do not really measure the amount of heat in the kiln. Rather, cones measure how much heat-energy the ceramic materials in the kiln have absorbed. In addition, glazes react to more than temperature, and so do cones. For example, differences between reduction and oxidation atmospheres can change glaze, and cone, behavior.
- Cone chart on WingedBlue Arts, giving heat data, including approximate temperatures for cones.
Soaking
Cones also help in monitoring soaking. Soaking a kiln means to keep it at the same temperature for a period of time. This gives the ceramic materials time to absorb more heat, and the glazes time to smooth out.
Generally, a twenty minute soak at the end of the glaze firing is appropriate. Soaking usually begins when the target cone has begun to bend. The potter will adjust the kiln burners to slow or arrest the temperature rise. By the end of the soak, the target cone should be fully bent to a ninety-degree angle.
Cone Sizes
Photo copyright 2008 Beth E Peterson
Cones come in two basic sizes, small and large; there are also pyrometric rods and self-supporting cones (as seen in the photo).
Small cones and pyrometric rods are made for use in electric kilns. They are placed in the kiln sitter which automatically shuts the kiln off as the cone bends when the kiln has reached temperature.
Large and self-supporting cones are used in fuel-burning kilns, where they are most often placed directly in line-of-sight of a spyhole or view port. These cones can also be placed in various areas throughout the kiln (electric or fuel-fired) to give the potter more information about how heat is distributed within the kiln during firing, but they cannot be read until the kiln has cooled and been opened.
Cone Packs
Large cones are used mainly in fuel-burning kilns. They are arranged in cone packs, a cylinder of clay and grog in which, generally, three cones are rooted.
The cones are set in ascending order. For example, if you were firing the kiln to cone five, your cone pack would have a cone four, a cone five, and a cone six. Cones are set into the pack at an eight degree angle, leaning toward the front of the pack.
Packing the cones in this manner ensures the cones fall in the right direction (rather than onto ware or the kiln shelves) and also keeps the lower melting cones from interfering with the higher temperature ones.
Reading Cones
Cone packs are placed in the kiln so that they are clearly visible from the spy hole. Large kilns should have two cone packs: one for the upper spy hole and one for the lower spy hole.
Cones register temperature and kiln conditions by bending. The more bend there is, the higher the kiln temperature is. When the cone you are firing to bends at a ninety degree angle, the temperature has been reached. Less bend than that and the glazes are under-fired. More bend than that, and your ware is over-fired.
When the first cone in the cone pack begins to bend, it is time for the potter to pay more attention to the kiln. Generally speaking, the kiln should be checked at fifteen minute intervals from that point until the end of the firing.