Glazing bisqueware (clay object after the first firing) is very different from painting an object. You can’t see which texture and color (effects) you are applying during glazing. Applying different glazes, slips or oxide washes, the ceramicist must compose the colors in his or her mind.
In doing so, he/she must take into account that overlaps of glazes do not always lead to a mixed color, the thickness of the glaze effects color and/or texture and the application method (spraying, pouring over, brushing, etc.) may or may not lead to desired effects. Just to name a few variables. So the disappointment is great when the thought experiment does not match the final result.
I haven’t even mentioned one of the most important “wildcards”: the glaze firing. To tame this beast, many digital tools have been developed over the past 20 years. The most important of course is the (computerized) kiln control. however, this does not make the Seger cones superfluous.
Keep as many variables the same as possible
It is always important for the ceramicist to keep as many variables the same as possible in order to achieve the desired result. Firing it is one of the most important aspects of glazing. The modern kiln control is a great tool for accurately determining the firing curve in advance. Yet this is not a replacement for Seger cones.
How do Seger cones work
Seger cones (also called “Pyrometric Cones” and “Orton Cones” in the US) were invented by the German chemist Hermann Seger (1839–1893) at the end of the 19th century. He developed a system to accurately determine the firing speed and temperature of ceramics. He made cones of ceramic material that bend at a certain temperature and duration.
A true revolution, for the first time ceramicists could determine during firing whether the desired temperature had been reached by seeing if the cone was “bending over” (so the top of the cone is bent to the bottom).
In practice, 3 cones are placed of increasing temperature in front of the peephole: The “guide” the “fire” and the “guard” cone. When the temperature advancing in the firing, the cones are looked at once in a while.
As soon as the guide cone, the cone with the lowest temperature, bends, the kiln must be closely monitored. Until the firing cone, the cone with the temperature at which the ceramist wants to fire his/her glaze, has bend. The guard cone serves as a “safeguard”, if it is bend, the fire has been too high, so that must be prevented (by lowering or turning off the kiln).
Cone numbers
When firing ceramics, not only the temperature at which it is fired applies. Glazes are fired on a certain “cone”, which is good for time and temperature. Think of it as heat absorption and not just as the temperature of the fire.
Cones are made in different numbers, each of which corresponds to a certain temperature and firing speed combination. At the start of firing the cone is at an angle of 8 degrees. A perfectly fired cone will be bent to an angle of 90 degrees. So when the cone is less curved, the firing temperature has been lower than indicated by the cone number. If the cone is more bend, then the firing temperature has been above it (overfired).
Note the way the number of the cones. Seger numbered the cones from 10 (the highest temperature) to 1 (the lowest temperature). Later it turned out to be possible to make cones that bend at an even lower temperature.
As a result, the numbers 01, 02 etc. have been added to the sequence, where 010 is the lowest temperature (slightly below 900 oC). So there is a big difference between cone 5 and 05 (see also table)!
Current use of cones
Nowadays, most (Western) ceramists use a pyrometer (thermometer for high temperatures) whether or not linked to an kiln control. I usually use my Bentrup operating system (see photo). This automatically keeps a log on my PC, so I can check how the firing went after the end of the firing. So my firing is automatic and yet I always use Seger cones.
Orton Cones
I do not use these cones to check the temperature during firing, but to verify afterwards whether the desired temperature has been achieved. After all, the pyrometer and/or the kiln control may have had a malfunction or the calibration of the pyrometer not correct (due to wear or breakage, the pyrometer may indicate an incorrect temperature over time).
A glaze mystery solved
At one point, I took a number of jugs and cups out of the kiln that had a completely different color, while I had glazed them all with the same glaze. I knew that the glaze often came out of the oven “differently”, but I had never had it in one and the same fire. Everything was the same, wasn’t it?
The glaze, the application (thickness) and the firing. Eventually I realized that the jugs were placed at the top of the kiln and the cups at the bottom. The next fire I placed both cones at the bottom and at the top of the oven. After firing, it turned out that cone 6 (ca 1200 oc) at the top of the oven was bend, but cone 8 (ca 1235 oC) at the bottom also . So there was a temperature difference of about 35oC in my kiln. Nowadays I take that into account and get the desired result.
Cones still not redundant
The cone technology has not become obsolete with the advent of computer-controlled kilns. I recommend anyone who fires ceramics also use cones. Especially if you are firing in a (for you) new kiln, check with cones whether temperature differences occur during firing and how your glazes react to this.
Good luck with your next fire!
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