Orton cones have become much less relevant in recent years. That being said they are still commonly used to calibrate kiln controllers. Most people just use one cone now (rather than the guard, firing and guide cones of the past). The problem illustrated in this diagram is our reason for doing that. At our typical firing rates, we found that bending the middle cone to 5 o'clock pretty well always started the guard cone. Orton said when that happens "you have exceeded the best time-temperature relationship". Thus, over time, we got into the habit of bending the firing cone to 3 o'clock (as shown in #1). Notice the situation in #2, that is not supposed to happen but often does. The first guide cone should be slumping, not just touching. And the guard cone should not be starting. Now we just use one cone bent to 5 o'clock to callibrate our electronic controllers.
Here is an example of our lab firing schedule for cone 10 oxidation (which the cone-fire mode does not do correctly). To actually go to cone 10 we need to manually create a program that fires higher than the built in cone-fire one. Determining how high to go is a matter repeated firings verified using a self-supporting cone (regular cones are not accurate). In our lab we keep notes in the schedule record in our account at insight-live.com. And we have a chart on the wall showing the latest temperature for each of the cones we fire to. What about cone 6? Controllers fire it to 2235, we put down a cone at 2200!
The blue line on this graph is from the Orton cone chart for 108F/hr. But those are not the target temperatures for our cone firings. We use the red line to program our controllers to produce accurate firings for each cone. Where is that red from? We made it. The self supporting cones we put in every firing verify that red line, time after time, for thousands of firings (we routinely fire at every cone shown, manually maintaining programs for them all our kiln controllers). One caveat: As pyrometers degrade and are replaced there is need to adjust the final temperature a few degrees. Should you use that red line? No. Use self supporting cones to establish your target temperatures.
Notice that from cone 06 to 04, the temperature difference between cones is far greater than at any other range. But this situation changes approaching cone 3, where the difference from one cone to the next drops and accelerates (thus the curved line). Firing a kiln accurate to cone 2, by cones, is difficult since the cone 1 and 3 guide and guard cones fall in a similar fashion. From cone 4 and up cones prove to be a much more stable indication of temperature and heat-work. Not surprisingly, it makes more sense to trust a pyrometer in the cone 02-3 range. Low fire terra cotta bodies vitrify in a similar manner.
People refer to the extent of cone-fall as numbers-on-the-clock or degrees. This cone is at 5 oclock or 80 degrees. Notice that from start-to-finish is 35 degrees C (not all cones have this same 35 degree fall). As you can read on the temperature scale, 25+ degrees happen before it reaches 2 o'clock! From 5 to 6 o'clock is only 1 degree! This is a standard cone that requires a plaque, notice that the down-touching position is when it hits the top of the plaque. It follows from this that one can convert cone-bend to equivalent temperature. That being said, remember that cones measure heat-work, so the conversion is only valid for a 60C/hr rate-of-rise.
Orton says “90 angular degrees is considered the endpoint of cone bending”. First, let's assume the normal: Examination of cones on kiln-opening to verify controller operation. Consider the cone on the left: The tip is touching. But it is also beginning to buckle, which means it was touching for a while before the firing ended. Who knows how long! The second one is not touching but has still fallen a little too far. Why do we say that? The third one, positioned on the Orton guide, has reached the recommended 90 degrees. This demonstrates a good reason why self-supporting cones are much better than standard ones: They are not touching when considered done. And standard cones, when sent in a 3/4" plaque, have a less consistent bending behaviour.
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.