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 numberOrton Cones
Final temp in F
Color of FireWhat Happens to ClayTypes of Ware and Glaze
15 2615 White    
14 2552    
13 2462    
12 2435 Porcelain matures Porcelain
11 2417    
10 2381    
9 2336 Stoneware clays mature Stoneware
8 2320    
7 2295    
6 2269    
5 2205 Yellow    
4 2161 Red clays melt China Glazes
3 2138    
2 2127    
1 2109    
01 2080    
02 2052 Buff clays mature Earthenware
03 2019    
04 1971    
05 1911    
06 1855 Red clays mature  
07 1809 Orange   Low fire earthenware
08 1753    
09 1706   Low fire lead glazes
010 1679   Low fire lead glazes
011 1641 Cherry Red    
012 1620   Lustre glazes
013 1582    
014 1540 Dull Red    
015 1504 Organic matter burns out Chrome red glazes
016 1465    
017 1405    
018 1353   Overglaze colors
019 1283   enamels
020 1180    
021 1143    
022 1094 Dehydration begins  

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