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.
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.
The rear cones are Orton 5 and 6. The front ones are the L4532F recipe, it is bending too much at six and not quite enough at cone 5. The L4532F recipe employs ball clay instead of kaolin, which is making for better casting properties and better dry strength. It has also greatly reduced the cost, removing the need for Veegum. The difference in bending for this one-cone range is also looking similar to what an Orton cone would do.
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.
Make Your Own Pyrometric Cones