Pyrometric cones are devices used in kilns to monitor the heat level. Kilns are used in ceramics and manufacturing to produce goods which need to be exposed to very high heat levels in a process called firing. Small differences in the level of heat can result in damage or malformed products, which is why pyrometric cones are used. The cones monitor the firing process as well as the overall performance of the kiln.
The composition of pyrometric cones is carefully calculated so that they will perform predictably. The cones measure the amount of heat they have absorbed, which may or may not reflect the ambient temperature, since time is also a factor. As the cone absorbs heat, it starts to deform, ultimately bending over entirely. Some technicians use pyrometric rods, which operate on a similar principle, although they take the form of rods rather than cones.
Several companies manufacture pyrometric cones. They are made in numbered series. The higher the number, the higher the temperature must be for the cone to start to bend. The temperature difference between each cone is small, but important. For this reason, most pyrometric cones are employed in sets of three. The first cone is the firing cone, the cone which deforms at the desired temperature. A guide cone is one level lower, and is used to indicate that the kiln needs to be hotter. A guard cone is a level higher, and it suggests that the level of heat in the kiln may be dangerous.
Several things influence the determination of how hot a kiln should be. The type of material being used is important, as is the temperature at which the glaze will mature. Many glazes and clays have ratings, indicating which cone should be used. Small fluctuations in thermostat controls over time make pyrometric cones an important safeguard, even with electronically controlled kilns.
In a manually fired kiln, the pyrometric cones are placed near a peep hole, so that the operator can determine when the kiln needs to be shut off. Cones can also be used with a kiln-sitter, a device which turns the kiln off automatically when the firing cone deforms, using a weighted trigger system. The devices are also used to dissect the performance of the kiln after it has been unloaded, and are often installed on several shelves to measure firing differences in varying regions within the kiln.
Two common brands of pyrometric cones are Seger cones and Orton cones. Cones have been used historically for hundreds of years, particularly in China, but it was not until the 1700s that the design of pyrometric cones began to be refined, by Josiah Wedgewood. He was followed by Hermann Seger, who developed his cones in 1886, and was followed by Orton in 1896. The cones use a similar numbering system, with 022 being coolest cone and 42 being the hottest, but subtle manufacturing differences lead most people to use one type or the other.
