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 numberOrton Cones
Final temp in F
Color of FireWhat Happens to ClayTypes of Ware and Glaze
122435Porcelain maturesPorcelain
92336Stoneware clays matureStoneware
42161Red clays meltChina Glazes
022052Buff clays matureEarthenware
061855Red clays mature 
071809Orange Low fire earthenware
091706 Low fire lead glazes
0101679 Low fire lead glazes
0111641Cherry Red  
0121620 Lustre glazes
0141540Dull Red  
0151504Organic matter burns outChrome red glazes
0181353 Overglaze colors
0191283 enamels
0221094Dehydration begins 


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