|Monthly Tech-Tip |
The tip of the firing cone 03 on the left has just touched and it is beginning to deform. Yet the guard cone 02 is not moving at all and the cone 04 is practically melting. However the tip of the cone 7 firing cone on the right has not quite touched. But the cone 8 is already well on the way and the cone 6 touched not long ago. Yet cones separate by about 30 degrees in both ranges. Why the difference here? At low fire the kiln can climb quicker so less heat-work is done (that is what bends cones). Also, the iron-based low fire cones are more volatile and begin and complete their fall through a narrower range. So at low fire cones can be an absolute measuring device. But at high temperature their use is more about comparing behavior firing-after-firing and adjusting procedure by that experience.
Ware is not turning out as expected and a potter needs to verify the temperature in the kiln. The standard cones on the upper right are misleading. The cone 7 is telling one story but the cone 6 and 5 another. On the lower right is a better way: Self supporting cones. They are always at the right angle and this set of three is bending as expected. To be a full cone 6 the middle one needs to bend just a little more until the tip is even with the top of the base (maybe 2 or 3 degrees). On the top set, the cone 6 is clearly totally flattened and the 5 is a pool of glass, this firing went way beyond cone 6.
Cones are ceramic and bend through a narrow temperature range. They used to be actively used to determine when firings were completed but now are used to calibrate electronic devices.