A pyramid-shaped ceramic device used to quantify the amount of heat delivered by a kiln. These devices are formulated from different mineral mixtures and numbered accordingly. They are placed in a kiln so they can be viewed during firing and when a cone begins to bend it is closely monitored and the firing is terminated when it reaches a specific position.
What temperature do Orton cones actually go to in my kiln?
The blue line on this graph represents numbers from the Orton cone chart for 108F/hr. It is not as straight as what I expected. The red line is the temperature measurements that we have recorded after many test firings at each temperature. We use large cones in these firings and finish the firings manually to shut the kiln off just before the firing cone touches. These are now target temperatures that we use for automatically firing each temperature.
The way cones bend
An unfired cone (right) with others at various stages of bending. It can take 20 or 30 degrees to go from straight until bent as the first one. But the more a cone bends the faster it goes down (between the next two may only be 5 degrees). If the tip touches (as has happened with the front one) then it no longer indicates temperature change accurately. It is wise to have a cone in all glaze firings to verify the electronic readings.
How to get more accurate firings time after time
When I fire our two small lab test kilns I always include cones (I fire a dozen temperatures). I want the firing to finish when the cone is around 5-6 oclock. To make that happen I record observations on which to base the temperature I will program for the final step the next time. Where do I record these? In the schedules I maintain in our Insight-live.com group account. I use this every day, it is very important because we need accurate firings.
Why is the clay blistering on this figurine?
This is an admirable first effort by a budding artist. They used a built-in cone 6 program on an electronic controller equipped electric kiln. But it is over fired. How do we know that? To the right are fired test bars of this clay, they go from cone 4 (top) to cone 8 (bottom). The data sheet of this clay says do not fire over cone 6. Why? Notice the cone 7 bar has turned to a solid grey and started blistering and the cone 8 one is blistering much more. That cone 8 bar is the same color as the figurine (although the colors do not match on the photo). The solution: Put a large cone 6 in the kiln and program the schedule manually so you can compensate the top temperature with what the cone tells you.
What cones do at low fire is different than what they do at high fire
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.
Bullers ring vs. cones for measuring kiln temperature
This is a Veritas measuring device. It was used to measure the size of Bullers rings. The system was set up so that an unfired ring would measure close to zero (the difference from zero was added or subtracted from the final measure). These rings provided a measure of what temperature the kiln was (as opposed to cones which say what it is). Actually, many companies placed many rings in a firing and extracted them, one-at-a-time (using a metal rod), cooled them quickly and measured them; this gave an accurate indication of the current temperature. Some companies still use these today to verify electronic measuring devices. The Orton TempCHEK system is based on this same principles, but is much more refined (and much more accurate).
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