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What is Cone?
As a potter, you are guaranteed to hear and use the term "cone" on almost a daily basis.
Generally, potters use the term in three ways:
- To describe a property of clay or glaze. ex. "this is a beautiful cone 6 glaze" or, "I wish we had real, cone 10 porcelain." This is the most frequent use.
- To refer to temperature, which is seen in the chart below.
- To refer to the actual pyrometric cones, from which we derive the term.
History of Kiln Thermometers
In the old days, before thermocouples and pyrometers, the only way to gage temperature was with a mercury thermometer, but mercury thermometers would explode and melt when heated to 2000 degrees Fahrenheit. So in order to measure heat in something really hot, like a kiln, some clever fellow invented pyrometric cones (pyro: heat, metric: measure) to tell us how hot our kilns were. But now, you may be thinking, it isn't the old days anymore and we do have thermocouples and pyrometers with digital readouts, so why do we still say "cone 10" instead of 2381 degrees Fahrenheit? That’s because cone is not as simple as temperature.
Cone as a Measure of Heat
Cone is a measure of "heat-work" or heat over time. Just like putting an ice cube into a pot of boiling water that won’t melt right away, it will take time for the pottery to heat. Similarly, when you throw a raw steak onto a hot grill, it doesn’t turn medium rare instantaneously, it takes time for the heat to work on the steak to make it delicious. Your pottery is your steak, but instead of looking for cone medium rare, you are looking for cone 05 or cone 6 or cone 10.
Cone as a Measure of Energy
You can also look at cone as a measure of energy. It takes energy to transform green clay into bisqueware, or bisqueware into vitrified ceramic. A cone will tell you when a piece in the kiln has absorbed the necessary amount of energy needed to make those transformations happen. There are a lot of advantages to thinking about cone in terms of energy. For one, you avoid saying things like "cone isn't heat, it's heat-work," or "cone isn't temperature, it's temperature over time." For another, if you think of cone as an indication of the energy output of the kiln then you are focusing on the most important thing - the effect of the fire on the contents of the kiln.
So when you use the word “cone,” you must be describing a clay or glaze, a kiln firing temperature or an actual pyrometric cone. Keep this in mind, and happy firing!
|Cone number||Orton Cones |
Final temp in F
|Color of Fire||What Happens to Clay||Types of Ware and Glaze|
|9||2336||Stoneware clays mature||Stoneware|
|4||2161||Red clays melt||China Glazes|
|02||2052||Buff clays mature||Earthenware|
|06||1855||Red clays mature|
|07||1809||Orange||Low fire earthenware|
|09||1706||Low fire lead glazes|
|010||1679||Low fire lead glazes|
|015||1504||Organic matter burns out||Chrome red glazes|