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 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. When used passively, the state of the cone is used as a guide by which to edit the firing schedule for the next time or calibrate the electronic controller.
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 below is a fragment of an Orton cone chart. 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 slow enough rate of rise that the kiln fires evenly and all ware is 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 a bit. I needed to be able to interpret a cone to derive a number that could be recorded with shrinkage and absorption data. As a result there were questions to answer. For example, at what point in a cone's fall do you judge it to be complete? At what point is it half-complete. I made an arbitrary decision that when the tip touches it is complete, the expected heat input is achieved. Thus, I record cone 6 just touching as "6.0".
Orton Cone Chart (Partial) ----Temp. Increase Per Hour----- Cone ---Large Cones---- ---Small Cones----- Number 60C 108F 150C 270F 300C 540F Color --------------------------------------------------------- 020 625 1157 635 1175 666 1231 Dull Red 018 696 1285 717 1323 752 1386 016 764 1407 792 1458 825 1517 014 834 1533 838 1540 870 1598 012 866 1591 884 1623 900 1652 010 887 1629 894 1641 919 1686 08 945 1733 955 1751 983 1801 Orange 06 991 1816 999 1830 1023 1873 04 1050 1922 1060 1940 1098 2008 03 1086 1987 1101 2014 1131 2068 02 1101 2014 1120 2048 1148 2098 Yellow 01 1117 2043 1137 2079 1178 2152 1 1136 2077 1154 2109 1179 2154 2 1142 2088 1162 2124 1179 2154 3 1152 2106 1168 2134 1196 2185 4 1168 2134 1186 2167 1209 2208 5 1177 2151 1196 2185 1221 2230 6 1201 2194 1222 2232 1255 2291 7 1215 2219 1240 2264 1264 2307 8 1236 2257 1263 2305 1300 2372 9 1260 2300 1280 2336 1317 2403 10 1285 2345 1305 2381 1330 2426 11 1294 2361 1315 2399 1336 2437 White 12 1306 2383 1326 2419 1355 2471 *Note that Orton changes the formulations on some cones from time to time and these numbers can vary.
A booklet from the Edward Orton Jr. Ceramic Foundation that details the pyrotechnical theories of how cones operate. It cites the amount of variation possible when cones are improperly set and contained several other revelations. For example, it had a small diagram which parallels cone-bend to degrees of temperature increase. This diagram showed that 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 meant that a cone 6 at 3:00 should be 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.
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But now new questions became obvious: 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. 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.
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Consider the first set of cones shown here. The cone 6 is at 3:00, so I interpret that as 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.
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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 which I would read as 6.7. So did I get cone 5.0, 5.8, or 6.7? To answer this, the booklet from Orton said the 'deformation of the guard cone means that heat treatment has been exceeded'. I called the Orton office, and they confirmed I should go by the highest cone to show deformation. Okay, that means I had 6.7.
But this is still somewhat disturbing. If I would have shut off the kiln when the cone 7 started to bend then the cone 6 would have been at 1:00, which is only 5.5, which is 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. However, this explanation is not adequate in my experience. The strength of cones is supposed to be their ability to measure heat work, so they should be most useful when firing speed varies.
I called Orton again and they sent charts and testing data on cones 5, 6, and 7. It was very intimidating information, to say the least. All of a sudden cones were not looking so simple; so I went to plan B.
I 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. The disadvantage is that you can't tell when to shut the kiln off. So cones tell you the shut off time, rings tell you what heat-work you achieved at shut off. I began to put these into firings each day beside the set of cones and recorded the value measured with a device intended for this purpose (see diagram). For example, cone 10 yields a Buller's ring value of 40 on the scale. Cone 6 produces values from 27 to 29. One can thus conclude that even with a combination of rings and cones, we again are faced with device limitations and must apply skill and experience to achieve consistent results.
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 measures temperature, so cones are better. But now we use controllers that run on temperature! However, this is not true. Controllers determine and replicate firing schedule also, that is the heat-work. Industry likes to have numbers for testing records and audit trails, working with temperature and schedules provides that. Potters are increasingly adopting this view, thinking much more about schedules and temperatures, not just heat-work of cones.
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 is critical to consistent ware. Industry thus has much motivation to maintain its thermocouple systems. To calibrate your controller it is definitely best to use self-supporting cones. Only one cone is generally needed. After years of experience and thousands of firings we have been able to build a chart that cross references cone-bend with temperature. Here it is.
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.
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.
Four o'clock. These are self-supporting cones, use these. I was consistently getting the cone on the left 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, not the bottom. So I adjusted the program to finish at 2200F and got the cone on the right. But note: This applies to that kiln with that pyrometer, our other test kiln puts cone 6 at 4 o'clock at 2195F. Of course, if you want the kiln to hold at cone 6 for longer the cone will bend further, so the top temperature would need to be reduced to compensate for that. If you are using the automatic programs (e.g. cone 6 schedules go to around 2230!) your kiln is almost certainly over firing.
The tip of the first cone is touching. As soon as that happens it is no longer giving a reading so how can you know how much higher the temperature has gone? The second one has fallen a little too far. I have the third one positioned on the Orton guide (with the bold line lined up with the bend point). It is just about right. Just another reason why self supporting cones are much better than standard ones.
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 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).
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.
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.
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.
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.
Electric Hobby Kilns: What You Need to Know
Electric hobby kilns are certainly not up to the quality and capability of small industrial electric kilns, but if you are aware of the limitations and take precautions they are workable.
Interpreting Orton Cones
Interpreting how high a kiln fired based on the look of the cones can be a much more complicated matter than it might first appear.
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.
Orton Ceramic Website