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About (Pyrometric) Cones: Article or Project

Ceramic Kiln
Pyrometric cones

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(Pyrometric) cones have been used for around a hundred years as a way to control and monitor the firing of ceramic and pottery pieces. Pyrometric cones are used to measure heatwork during the firing process. They help determine if ware has reached maturity, if the kiln is operating properly, and if problems occur in a kiln during the firing process.

A cone is a slender pyramid or more recently bar of carefully controlled clay compositions that are developed to react repeatedly and consistently to varying degrees of heat. Their consistency has a range of about 50 degrees from one use to another. Cones are numbered so as to show how much heat they absorb before bending or ‘melting’. They bend when they begin to form glass from their composition melting. Cones can only be used once.

When originally developed by Orton, cones were numbered 1 through 20 with 20 being the hottest. When they developed cooler cones, they reversed the numbering by adding a zero in front of the number 01, 02, 03 etc. with cone 022 being the coolest. Cones currently range from coolest 022 to hottest 42. It is critical that when firing a kiln you be sure to check that instructions say cone 05 rather than 5, for example, in order to achieve success with the firing of your item. Firing an item at cone 5 that is supposed to be fired at cone 05 can only result in disaster.

The cone bending can be affected by temperature, time and sometimes even atmosphere. Temperature is the most common variant. Firing charts are available to aid in measuring final bending positions in relation to temperatures achieved. See cone temperature chart below.

There are two major types of cones. Those designed to sit on a shelf and those smaller ones designed to be used in a kiln sitter. A kiln sitter is a device developed to hold a cone and shut the kiln off when desired temperature is achieved. It operates by supporting the cone under a sensing rod. The weight of the sensing rod adds pressure to the cone and bends when it softens adequately from the heat. This in turn triggers a flange to turn off the kiln.

When using a kiln sitter to control the kiln, it is advisable to also incorporate routine use of witness cones and shelf cones to gauge accuracy of the kiln firing. Most kilns are subject to uneven firing. This can be caused by how the kiln is loaded or some fault in the elements or other factors. By placing witness cones on the shelves, the operator can se if all the ware has reached maturity and determine any problems that may be present.

Shelf cones can be used in stead of a kiln sitter. These cones are larger and designed to melt and bend by gravity. When using shelf cones, they are usually grouped in threes and placed near the peep hole so they can be monitored. They can be placed on all shelves as a means to monitor the temperatures throughout the kiln also. When using the three cones, you will pick one each of three sequential cones - example if you are wanting to fire your kiln to cone 05, you will use cones 04, 05, 06. The 04 cone is the guard cone, the 05 is the desired cone and the 06 cone is the guide cone. As you near your desired temperature the guide cone will begin to melt and bend. Shortly after, the desired cone will begin to bend. And finally, after achieving the desired temperature of 05, the 04 (guard) cone will begin to bend. As soon as the desired cone, 05 has bent to a 90 degree angle, you want to shut the kiln off - by then the 04 (guard) cone has begun to tip and the 06 (guide) cone has bent completely down.

Modern technology has created electronic controllers that operate by thermocouples placed throughout the kiln. A self-supporting witness cone should be used to make sure the controller is firing accurately. Place the cone on a shelf near the thermocouple. There should be no more than half a cone difference between the controller and the witness cone. It is most important to use witness cones when doing high fires repeatedly.

It is recommended to use self-supporting witness cones every firing and the three cone test periodically to check for changes in the kiln’s operation. Is it suggested to keep the cones from each firing to compare. When more than half a cone difference is observed, this may indicate a problem exists and the kiln should be checked out. Witness cones are less subject to bending variation and are easy to use. They are an inexpensive way to monitor your kiln.


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