Pyrometric cones are designed to deform at certain moments during a firing. This deforming action allows us to either shut the kiln off at the proper point (as in the case of the mini bar and Kiln Sitter) or simply to record what happened during the firing (as in the case of the witness cone).  The exact moment the cone deforms depends primarily upon two factors: time and temperature. The temperature present at the moment "cone" is achieved depends upon the rate of temperature increase. With this simple concept in mind it's easy to understand why "cone" and temperature are not the same thing. For a proper cone firing, time and temperature must always be considered.

The chart below attempts to do the impossible by assigning a temperature to a cone number. The values given are valid only when the rate of temperature increase is controlled as per the chart.

Pyrometer Users:
Because of the time/temperature relationship of the cone, it's not
recommended that you attempt to calibrate a pyrometer using a cone.

Cone Chart

Please note that the Cone Chart gives 3 different temperatures for each cone number. As discussed above, the equivalent cone temperature depends upon how fast one gets to it. Generally speaking, the temperatures in the 108°F - 60°C column are used when discussing ceramics, porcelain, china and pottery.