Ceramic Firing & Kilns

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 glaze or clay. ex. "this is a beautiful cone 6 glaze" or, "I wish we had real, cone 10 porcelain." This is the most frequent use of the word.

To refer to the actual pyrometric cones, from which we derive the term.

To refer to temperature, which is seen in the chart below.

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 melt and explode 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," AKA 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 glaze or clay, a kiln firing temperature or an actual pyrometric cone. Keep this in mind, and happy firing!

Cone number	Temp in F	Color of Fire	What Happens to Clay	Types 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

A barbecue grill will reach 1300 degrees or Cone 018, which is fine for firing low-fire earthenware. A grill will bisque fire greenware before reaching ember stage, but it's best to use raku clay, groggy stoneware or paper clay that can resist thermal shock.

A trash can raku kiln will reach 1832+ degrees or Cone 06, which is fine for decorative pieces.

An open fire will reach 2000+ degrees or Cone 03-02, which is fine for firing earthenware.

  

A sawdust pit fire will also reach 2000+ degrees or Cone 03-02, which is also fine for firing earthenware.

  

A cob or brick oven kiln will reach 2400+ degrees or Cone 9-12, which is perfect for firing stoneware, flameware and porcelain.

If you prefer to work with a commercial kiln, I recommend a crystalline glaze kiln that will fire at Cone 12. You might as well get a kiln that will fire anything. The L&L E23S is the smallest and most affordable option I found. It's a Phase 1, 240 volt. The base price is $1,883.