|Monthly Tech-Tip |
Devices that melt and bend in a ceramic kiln at specific temperatures when subjected to specific up ramps. Today, cones are used to calibrate controllers.
A pyramid-shaped ceramic device made by Orton Ceramics, it is 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. When used actively they are closely monitored and the firing is terminated accordingly. Of course, the vast majority of manufacturers and potters use electronic controllers to fire nowadays, but cones are important to this also. They are used passively, the state of the cone being a guide to calibrate thermocouples and adjust controller programs for the next firing.
Hermann Seger studied the melt dynamics of oxide compounds and made the first cones. Edward Orton followed and laid the ground work for the cones we use today. Shown here is a fragment of an Orton cone chart (showing temperatures common traditional ceramics). As you can see, the cones bend at different temperatures according to rate of temperature rise. Hence, rather than measuring temperature, cones quantify the combined effects of time and temperature (their correct use, of course, depends on a kiln firing evenly and all ware being permeated by the heat).
While this sounds like the ideal measuring system for firing ceramics in periodic kilns, there are things that muddy the water. An example is interpreting the temperature that a cone indicates (according to its degree of bend). This is important in lab situations where data is being recorded for plotting on temperature vs property graphs (often there is discrepancy between the controller and cones). An important question is: At what point in a cone's fall do you judge it to have reached its "endpoint", as Orton calls it?
The booklet "Cones and Firing" from ortonceramic.com (link below) details the pyrotechnical theories of how cones operate. It cites a number of examples of the variation possible from the ways people commonly misset and misuse cones.
Note the above diagram: By the time a cone reaches three o'clock (3:00), it has travelled through 80% of the temperature range from start-of-bend to touching-down. This means a cone 6 at 3:00 is interpreted as 5.8. At 1:00, it is already 50% complete, that would be 5.5. Thus it is clear that a cone is most sensitive and best readable when it nears the end of its travel.
Consider these questions: Does the fall of all cones span the same number of degrees? If a cone falls over a 30 degree range, how much does this overlap with adjacent cones? How does one react when guide and guard cone behaviour are not as expected? This is relevant because Orton highly recommends the use of a set of three cones (guide, guard, and firing) in each firing. The first signals when the firing is nearing completion and the last warns of over firing. In addition three will provide information that will make looking at one cone seem a bit like wearing blinders.
Consider this first set of cones. The cone 6 is at 3:00, so that is cone 5.8. The cone 5 is well-melted and the cone 7 is not started. This is a textbook situation which presents no difficulty.
Now look at this second set of cones. Like the first set, the cone 6 is at 3:00, so I read it as 5.8. However, this time the cone 5 is not touching, so on its merits only I would assign the firing a value of 5.0. The cone 7 is at 2:00 or cone 6.7. The booklet from Orton states the 'deformation of the guard cone means that heat treatment has been exceeded'. Orton technician say to go by the highest cone to show deformation. That means I had 6.7.
This is somewhat difficult to accept. If the kiln was shut off when the cone 7 started to bend then the cone 6 would have been at 1:00, which is only 5.5, under-fired. There appears to be no way to achieve a cone 6 firing! Further, the logical explanation is that a slow firing should deform all three cones and a fast one should put the first cone down before the second even starts. Since cones measure heat work, they should be most useful when firing speed varies, but that is in question here.
To resolve this in our lab, we got a supply of Buller's rings. These ceramic devices are precision dust-pressed and designed to shrink in a linear fashion with temperature increase. The advantage of rings is that one device works across a wide temperature range and expresses a discrete number representing the degree of firing. They don't indicate when to shut the kiln off but what heat-work was achieved at shut off. For a number of years, we put these into firings each day beside the set of cones and recorded the value measured (using their measuring device, as shown below). For example, cone 10 yields a Buller's ring value of 40. Cone 6 produces values from 27 to 29. The combination of rings and cones, coupled with collected historical data, enabled us to better interpret the temperature achieved in each firing.
As noted, pretty well all potters and plant operators employ electronic kiln controllers now. Ceramic industry likewise, long ago adopted these devices in the pursuit of repeatable firings. We used to say that cones measure heat-work and thermocouples measure temperature, but now it is not so clear that is true. Controllers enable controlling the up and down ramp of firings, making them very repeatable. Industry likes to have numeric data (for testing records and audit trails), these controllers provide that. Potters are increasingly adopting this view also.
Continuous industrial kilns maintain a constant temperature in the middle of a long tunnel, and the speed of the ware-cars or conveyor through the kiln determines the firing curve each piece is subjected to. In this situation, the maintenance of a specific temperature at the hot zone is critical to consistent ware. This supplies plenty of motivation to maintain thermocouples and wiring well.
Self-supporting cones are much superior to standard cones in plaques. They automatically set at the right angle and they are easier to read (using their template the active zone starts from the top of the base). Only one is usually needed in a firing. Using our experience (as described above, from thousands of firings) we have created a chart that cross-references cone-bend with temperature. Here it is.
Orton says: “If the Guard Cone has bent, you have exceeded the best time-temperature relationship”. We have found that bending the firing cone down to 90 degrees most often starts the guard cone. So, if the quard cone is starting, we have gotten into the habit of judging the firing as accurate when the firing cone reaches the 3 o’clock position. If the guide cone is not started we judge a firing as accurate when the firing cone is at 5 o'clock, before touching.
This is not supposed to happen. The guide cone should be flattened. And the guard cone should not be starting. But what you see does happen often. this indicates that the real-world performance of cones does not always match the theoretical behaviour. This is because the bending range of many cones overlap that of their neighbours. And performance is affected when firings are slower and faster than the 60F/hr reference given by Orton.
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.
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, that 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.
I was consistently getting the cone on the left when using a custom-programmed firing schedule to 2204F (for cone 6 with ten minute hold). However Orton recommends that the tip of the self supporting cone should be even with the top of the base (they consider the indicating part of the cone to be the part above the base). So I adjusted the program to finish at 2200F and got the cone on the right. But note: This applies to that kiln at that point in time (with that pyrometer and that firing schedule). Our other test kiln bends the cone to 5 o'clock at 2195F. Since kiln controllers fire cone 6 at 2230 (for the built-in one-button firings) your kiln is almost certainly over firing!
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 F (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 60F/hr rate-of-rise.
When we fire our two small lab test kilns we always include cones (we 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.
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.
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!
We sometimes see customers doing this with cones: Putting them in the plaque backwards! Of course, they are not going to be accurate. Actually, self supporting cones are much better, they are idiot-proof because they enforce the 8 degree angle and bending direction.
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.
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).
Orton "Cones and Firing" book
Official PDF format guide from OrtonCeramic.com.
Orton Ceramic Website
In ceramics kilns the firing schedule of a kiln is typically managed automatically by an electronic device called a kiln controller. These are especially common on electric kilns.
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