Pyrometric cones are essential tools for obtaining reproducible results during firings. Cones enable you to determine when your kiln has reached the desired temperature, if the kiln was evenly heated and whether a problem arose during the firing. Because cones are made from carefully formulated ceramic compositions, they bend reliably at specific temperature ranges.
Each cone type is assigned a number (for example, cone 06), and this number denotes a specific – and fairly narrow -- temperature range. For example, cone 06 indicates a temperature range of approximately 1800 - 1850F.
During firings, cones are placed within the kiln, usually in a cone holder or a block of clay, and as the temperature increases, cones absorb heat. As the cone reaches the temperature at which it is formulated to melt, the tip of the cone begins to bend. As heat continues to be absorbed by the cone, it bends even more and may even start to melt. This is why your kiln has spy holes -- so you can look inside and see what your pyrometric cones are doing.
It is recommended that you use three cones in your firing: one cone that will mature at the temperature you wish to achieve (say, cone 6), one guide cone right below this temperature (cone 5) and one guard cone above (cone 7). This will help you achieve accuracy in your temperature measurements. Because different cones require specific temperatures to mature, they will bend at different temperatures from each other, enabling you to determine how hot your kiln has gotten and whether you have reached your desired temperature. If you are firing a relatively large kiln or a kiln that you know has a temperature differential (such as a kiln that tends to stay cooler on the bottom and hotter on the top), it may be useful to place cones in different spots within the kiln, to determine whether you kiln was evenly heated.
If you are using an electric kiln with a digital controller, it is not required that you use cones. However, we do recommend it, as it will allow you to confirm that your kiln is properly calibrated. Placing cones in each and every firing will allow you to better trace wear and tear of your elements and thermocouple over time and ensure that everything in your kiln is working as it should.