When you first start looking at ordering clay bodies, you’ll see ‘cone rating’ as one of the characteristics that can affect your decision. What is a cone rating? Cone ratings specify what temperature the clay body is intended to be fired at.
The term cone comes from pyrometric cones that are placed in the kiln before firing. These cones will melt and curl over to indicate that the temperature has been reached inside the kiln, giving a good visual indication of the temperature inside the kiln if you don’t have a digital thermometer installed. Cones will also tell you that the appropriate amount of time has been spent at that temperature. A cone that has bent at a 90 degree angle indicates a perfect firing. Less than 90 degrees shows it was under fired, over 90 degrees indicates overfiring.
Cones range from Cone 10 at the highest to cone 022. at the lowest. Cones 1 through 10 cover the high fire temperature range, from 1155 degrees celcius to 1300 degrees celcius. Below Cone 1, the cones start counting at 01 and continue down to 022. These represent the low fire temperature ranges. Ceramics fired in the low fire range are more porous and are typically not food safe, but they can often have more brightly colored glazes.
When you are choosing a clay body, keep in mind the cone that it is rated at and what you have available for a kiln. Kilns also come rated at different cone levels, so don’t intend to fire at a cone 10 when your kiln can only reach cone 8 reliably.