A cone is a small pyramid of ceramic material designed to melt when a specific ratio of temperature and time is reached during a kiln firing. This displays the ‘heat work’ on the clay, so you know when the clay is vitrified and the glaze has matured. Cones come in a range of sensitivity from 022 up to 14.
In a glaze firing I want to reach cone 10, which on my pyrometer may read at a temperature of 1260 to 1300 C. The temperature reading is a guide, but the cones are a more accurate measure for what is happening to the clay in the kiln.
I put 3 cones in a glaze firing (cone 8, 9 and 10) and I peep through the hole in the door of the kiln, after I remove the bung, to see if the cones are beginning to melt. Potters use quaint terms like “tipping” and “touching its toes” to describe how much a cone has melted.
‘Heat work’ is dependent on the rate of temperature rise and the length of time the kiln has been firing and strangely enough is also affected by the humidity and barometric pressure. It is a bit like slow cooking meat, the meat will cook at a lower temperature if the cooking time is increased.