|Monthly Tech-Tip |
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.
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.
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.
Yes. This kiln has a Dawson LT-3 kiln sitter (new ones have electronic controllers). These kilns are most useful at low temperatures (cone 06-04) where a little error can be tolerated. The LT-3 is a mechanical device with safety timer and triggered latch on the external housing. A ceramic tube extends into the chamber where a pyrometric cone sits on supports with a rod on the top center. As the cone softens and bends the rod drops with it and eventually releases the shut-off latch. The utility of this depends on sitter condition and adjustment. People also placed cones on each shelf (self supporting cones are recommended, show are 06, 05, 04 - 1860, 1920, 1966°F). Although not shown, these kilns had a pyrometer and one or more thermocouples (temperature gauge and heat sensing probes). Learning to use a kiln like this involved "babysitting" the switch settings (low, medium, high) during the firing. People maintained a paper log (and graph) to track the kiln's temperature against time and used the fall of a standard cone to calibrate the sitter shutoff. Over many firings they created a schedule of switch-setting, from low-to-medium-to-high, to get the desired climb and even heat distribution. All of this being said, an Orton Autofire controller can be used to convert these kilns to programmed firing (see link below).
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.