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What is Cone?
As a potter, you are guaranteed to hear and use the term "cone" on approximately a regular basis.
Generally, potters use the term in three ways:
- To demote to the definite pyrometric cones, from which we obtain the term.
- To illustrate a property of glaze or clay. ex. "this is a beautiful cone 6 glaze" or, "I wish we had real, cone 10 porcelain." This is the most common use of the word.
- To consign to temperature, which is seen in the chart below.
History of Kiln Thermometers
In the past time, prior to pyrometers and thermocouples, the solitary way to gage temperature was with a mercury thermometer, but mercury thermometers would burst out and thaw out when heated to 2000 degrees Fahrenheit. So in order to gauge temperature in something really hot, like a kiln, some intellectual fellow produced 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" despite of 2381 degrees Fahrenheit? That’s because cone is not as simple as temperature.
Cone as a Measure of Heat
Cone is a calculating of "heat-work," AKA heat over time. Just like setting an ice cube into a pot of boiling water that won’t dissolve right away, it will take some time for the ceramic to heat up. Similarly, when you chuck a raw steak onto a hot grill, it doesn’t revolve medium rare immediately, it consumes time for the heat to work on the steak to make it appetizing. Your pottery is your steak, but at the place 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 gauge of energy. It takes energy to convert green clay into bisqueware, or bisqueware into vitrified ceramic. A cone will tell you when a piece in the kiln has engrossed the essential amount of energy needed to make those transformations occur. There are wide varieties 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 a signal of the energy output of the kiln then you are focusing on the most crucial thing - the impact of the fire on the contents of the kiln.
So when you apply the word “cone,” you must be recitation a glaze or clay, 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|