Clay Arts: FAQ: What is a ceramic CONE and how is it used?

A) What's a CONE?

A pyrometric cone is made of a combination of ceramic materials shaped like a cone or a bar. The cone will "bend" when the ware in the kiln has reached the cone's state of "maturity" (a combination of temperature and time). The coolest cone is 022 (often used for firing overglazes and lusters). The hottest cone used for stoneware and porcelain is usually cone 10 or 11.

Different types of cones have slightly different temperature ranges, and there may also be slight differences in the ranges between cone manufacturers. The table below lists the temperature equivalents for Orton Standard Pyrometric Cones, when heated at 150 degrees Centigrade / 300 degrees Farenheit per hour.

Cone Temperature Table:

Cone 022:   (Centigrade - 600, Farenheit -1112)
Cone 021:   (Centigrade - 614, Farenheit -1137)
Cone 020:   (Centigrade - 635, Farenheit -1175)
Cone 019:   (Centigrade - 683, Farenheit -1261)
Cone 018:   (Centigrade - 717, Farenheit -1322)
Cone 017:   (Centigrade - 747, Farenheit -1376)
Cone 016:   (Centigrade - 792, Farenheit -1457)
Cone 015:   (Centigrade - 804, Farenheit -1479)
Cone 014:   (Centigrade - 838, Farenheit -1540)
Cone 013:   (Centigrade - 852, Farenheit -1565)
Cone 012:   (Centigrade - 884, Farenheit -1623)
Cone 011:   (Centigrade - 894, Farenheit -1641)
Cone 010:   (Centigrade - 900, Farenheit -1652)
Cone 09:     (Centigrade - 923, Farenheit -1693)
Cone 08:     (Centigrade - 955, Farenheit -1751)
Cone 07:     (Centigrade - 984, Farenheit -1803)
Cone 06:     (Centigrade - 999, Farenheit -1830)
Cone 05:     (Centigrade - 1046, Farenheit -1914)
Cone 04:     (Centigrade - 1060, Farenheit -1940)
Cone 031/2: (Centigrade - 1080, Farenheit -1976)
Cone 03:     (Centigrade - 1101, Farenheit -2014)
Cone 02:     (Centigrade - 1120, Farenheit -2048)
Cone 01:     (Centigrade - 1137, Farenheit -2079)
Cone 1:       (Centigrade - 1154, Farenheit -2109)
Cone 2:       (Centigrade - 1162, Farenheit -2124)
Cone 3:       (Centigrade - 1168, Farenheit -2134)
Cone 4:       (Centigrade - 1186, Farenheit -2167)
Cone 5:       (Centigrade - 1196, Farenheit -2185)
Cone 6:       (Centigrade - 1222, Farenheit -2232)
Cone 7:       (Centigrade - 1240, Farenheit -2264)
Cone 8:       (Centigrade - 1263, Farenheit -2305)
Cone 9:       (Centigrade - 1280, Farenheit -2336)
Cone 10:     (Centigrade - 1305, Farenheit -2381)
Cone 11:     (Centigrade - 1315, Farenheit -2399)
Cone 12:     (Centigrade - 1326, Farenheit -2419)
Cone 13:     (Centigrade - 1346, Farenheit -2455)

B) How is a cone used?

Cones are used in kilns to monitor temperature and maturity of the ware. Electric kilns often have a "kiln sitter" that is connected to the kiln's power. It uses a small bar-shaped cone that the operator places under a metal rod. When the cone bends, the rod drops and the kiln shuts off.

In kilns without a sitter, larger cones called "witness" cones are used. These may be freestanding, or set at a slight angle in a piece of damp clay. See the packaging for detailed instructions, as they may vary from one manufacturer or configuration to another.

Instructions for using witness cones:

A "cone pack" is a series of cones set in a piece of clay or a commercial holder. It consists of at least 3 cones: the cone below your target temperature (placed on the left), the target-temperature cone, and then the next higher cone on the right. For example, in a cone 10 firing, you would use cone 9, cone 10 and cone 11 in your pack. The lowest temperature cone may melt a bit, so it's a good idea to leave some room around the pack.

A cone pack is placed in front of each kiln spyhole. The pack must be far enough into the kiln that the cones are unaffected by cooler air near the spyhole. Some people like to put cone packs in various other parts of the kiln to more closely monitor how evenly the load fires. This is always a good idea with a new (or new-to-you) kiln.

When you can see that the cone on the left of your pack is bending, your load is reaching target temperature; at this point you should monitor the kiln more closely. When the cone in the middle is completely bent over and the tip of the cone on the right is just beginning to bend, it's time to turn off the kiln.

(The attached photo shows a pack from a cone 10 firing in my gas kiln. I used a 4-cone pack for this load.)

Be sure to wear protective gear when looking into the spyholes of a kiln! A heat-resistant face shield and welder's dark goggles are an essential part of any kiln-owners equipment, along with a good pair of heat-resistant gloves long enough to protect your forearms.

Thumbnail by imapigeon

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