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Devices that melt and bend in a ceramic kiln at specific temperatures. By viewing them through a peephole the operator can tell accurately what temperature the kiln has reached.


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.

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 set me straight on the value of cone plaques (setters) and self supporting cones. 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 the second set of cones here. 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.

In recent years, it appears that more and more potters and small plant operators are employing electronic kiln controllers. Most ceramic industry likewise, long ago adopted these devices in the pursuit of repeatable firings. As one might suspect, electronic controllers also have their own set of surprises and trade-offs. Although it is not obvious, using a kiln controller takes us back to simple measurement of temperature. While a potter will say that his kiln fires to cone 6, a company technician will say his ware fires at 1200°C. Industry likes to have numbers for testing records and leave little to interpretation or chance.

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 and this is easily done with high-tech hardware. Since thermocouples (the critical sensing element of industrial temperature control systems) measure only temperature, industry has little choice but to think in terms of temperature and be careful to maintain its measurement systems. In my experience, while cones can be used to determine the end point of a firing, the temperature based system seems to work very well also. If you have access to electronic controllers I recommend you go that route and use cones where they really shine: to verify the accuracy of the thermocouple, determine the degree of uniformity within the whole kiln, and act as a backup.

What can you do? Self-supporting cones and cone setters are so convenient, they are more expensive but worth the cost. Continue to fire to the center cone and when you examine the cones interpret the center one first, then look at the others and let them temper your initial interpretation to come up with a number that is a reasonable representation of the firing. If you can possibly build a database of cone interpretations and corresponding Buller's ring measures, you can use this additional information to further temper your judgment.

Related Information

Cones bending normally

Cones bending badly

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.

What position should the cone be for correct firing?

Four degrees F. These are self-supporting cones, use these. I was consistently getting the cone on the left using a custom-programmed firing schedule to 2204F. 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 changed the temperature to 2200F and got the cone on the right. But don't assume your kiln fires cone 6 at 2200F, it could be much higher or lower, depending on your pyrometer.

At what point is a cone bent to the correct degree?

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.

Program your firings manually, calibrate the final temperature

Here is an example of our lab firing schedule for cone 10 oxidation (which the cone-fire mode does not do correctly). We need it to actually go to cone 10, the only way to do that is verify with a cone (self supporting cones are the only accurate way). Then make a note in the record for that schedule in your account at insight-live.com.

What cones do at low fire is different than what they do at high fire

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.

Bullers ring vs. cones for measuring kiln temperature

Veritas kiln temperature bullers ring measuring device

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).

Why is the clay blistering on this figurine?

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.

How to get more accurate firings time after time

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.

The way cones bend

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.

What temperature do Orton cones actually go to in my kiln?

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.

Cone plaques and cones from a cone 10R firing at Plainsman Clays.

Cone bending from a slow firing

Low fire red cones inhabit their own volatile world

A chart of temperature vs cone number

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.


Articles 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.
Articles 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.
Glossary Kiln Controller
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.
URLs http://www.ortonceramic.com

By Tony Hansen

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