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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.
At first it seemed simple: The cone bends and the firing is done. I regarded that little cone as the one thing that was straight forward, fool proof. I could just stick them into a blob of clay without being overly concerned about different depths and angles, throw them in the kiln, and they bent over time after time. Soon, however, I had to wonder. My firings were not consistent! I blamed the clay, the glaze, the weather; everything but the way I used the cones. Well, it turns out there were instructions I failed to read. Yes, there is a lot more to using and interpreting these devices than you might have thought.
Hermann Seger studied the melt dynamics of oxide compounds and made the first cones to guide him in firing. Edward Orton followed and laid the ground work for the cones we use today. Shown here 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, in my case many things happened which began to muddy the water a bit. First, as already mentioned 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. The book cited 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.
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.[conenorm.gif||] 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. [coneslow.gif||]
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. The main 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.
If you must continue with cones, you are advised to follow the manufacturers instructions (set three cones in each firing and set them properly). Self-supporting cones and cone setters are so convenient, you will kick yourself for not having used them sooner. 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.
One more note: You can download software from the Resources section of the Orton website to calculate the temperature and bend angle for all cones. It is a useful tool for intrepreting cones distributed throughout the kiln.
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By Tony Hansen