A witness cone is a pyrometric cone placed in the kiln where you can see it, or find it after the firing, to show how much heatwork the load received. Potters often set three cones together as a cone pack.
What Is Witness Cone?
Kiln controllers measure temperature at one point with a thermocouple, but they can drift out of calibration, and different parts of a kiln fire differently. Witness cones show what actually happened to the ware, which is what matters for glazes and clay.
The usual arrangement is a guide cone one cone cooler than your target, the target cone, and a guard cone one cone hotter, set in a small pad of clay at the angle the maker specifies. After firing, the target should be bent over so its tip is level with its base, the guide bent further, and the guard nearly upright.
Placing cone packs on different shelves shows hot and cool spots, which helps with loading. If cones show a kiln is firing hot or cold, the controller’s offset can be adjusted, or the schedule changed.
Self-supporting cones are easiest to use, because they stand at the correct angle without needing a cone plaque.
This pottery glossary entry from Le Studio Art Space is general educational content. Always follow the clay, glaze and kiln makers’ data for your own materials and equipment.