Pyrometric Cones 101: How Cones Are Used in Clay Pottery to Fire Clay

By David M Jay | Submitted On August 23, 2011 Are Pottery Cones?

Pottery cones allow potters to know what's going on inside the kiln during the firing process. Pottery cones also allow potters to know if the glaze has reached maturity. In the world of pottery, cones are used to determine the amount of heat absorbed by the ceramic material inside the kiln.

What are cones?

Cones, also known as pyrometric cones or pyrometric rods, indicate the heat-range within the kiln during firing. As the name implies, cones are cone-shaped objects made of fluxing materials and melting agents that react to temperature changes.

Further, it acts as an indicator to the maturation point of the ceramic. Maturation or clay body maturation is the process wherein the clay reaches its optimal state. As the rod nears maturity, it will soften and the tip begins to bend.

Cones are designated by numbers. Those that start with zero are lower in temperature than those that don't e.g. 09 is lower in temperature than 9.

The Orton Pyrometric cone chart is a chart of temperatures that shows heat data with the corresponding cone number and color output. Potters use this as a guide in determining which cones to use in firing. Orton is the largest manufacturer of pyrometric cones and they have set the standard in the industry in respect of the heat temperature range in cones.

Potters use the symbol "^" to refer to cones in their notes.

Cones and Soaking

Rods are also used in the process of soaking. Soaking means keeping the temperature of the kiln constant for a specific period of time. This process allows the clay to absorb more heat and for the glaze to completely melt and smooth out on the surface.

When the cone starts to bend, soaking begins. At this point, the potter will have to adjust the kiln temperature and keep it at the same level of temperature. Once the cone is bent to a full ninety degrees, the soaking process is completed.

Pottery Cone Sizes

Rods come in different sizes but the basic ones come in large and small sizes. The type of rods depends on the kind of kiln to be used in firing.

For electric kilns, small rods are preferred. These are placed in the kiln sitter. When the desired temperature is reached, the rod bends then drops a lever which automatically shuts the kiln off.

On the other hand, fuel-burning kilns use large and self-supporting cones. The rods are placed where it'll be easy for the potter to check the distribution of heat inside the kiln through the peeping hole. It is important that the cones are placed correctly inside the kiln for the potter to ascertain the supply of heat during firing.

Unlike electric kilns where it automatically shuts off when the temperature reaches its desired level, the cones inside fuel-burning kilns cannot be read until it has cooled.

A word of caution when viewing cones through the peephole: Use welder goggles when viewing cones through the viewport. The above earthenware temperature emanating from the kiln can cause permanent damage to the naked eyes.

Reading Pyrometric Rods
Cones are numbered but it doesn't really indicate the actual heat range when it's inside the kiln. So how do potters interpret cones and temperature?

Pyrometric rods record temperature by bending. They are specially formulated to melt at a certain temperature. The higher the temperature inside the kiln, the more bend there is. When the cone starts to bend, that's the cue for the potter to check the kiln every fifteen minutes or so and until such time the firing process is completed.

Cones that bend at a ninety-degree angle indicate that the temperature has been reached. Less bend than that suggests that the ceramic material is under-fired. If the cone bends over the ninety-degree angle threshold, the clay is over-fired.

Generally, it takes about twenty minutes for the cone to bend to full ninety-degrees. The higher the cone number, the more heat is required to bend.

How do cones work?

When the lowest cone starts to tip over, you can start slowing down with the temperature. For electronic kilns, you have to get ready to turn off the kiln when the cone below is slightly bent, the middle cone is bent to ninety degrees and the cone above is bent over as though it has melted.

Choosing the right cone

Choosing the right type of cone depends on what type of kiln to be used in firing. If you have an electronic kiln, cones aren't necessary. But it is still recommended to use cones to make sure that the heat range inside the kiln is adequate. Even if the kiln automatically turns off, you can continue firing because the cones will determine which areas need additional firing.

If you have a manual kiln, on the other hand, using cones is the only way to find out when it's time to turn off the kiln.

The cone number you'll choose depends on the glaze's maturation point. It's best to use the cone number your glaze matures at, then two more cones - one cone above temperature and another one below temperature.

There are also different types of cones available in pottery supplies shops: standard cones, self-supporting cones and Jr cones.

Now, let's discuss these types in detail.

Standard cones are used in cone packs. It's composed of three cones embedded in a pack of clay or placed on cone plaques. The three cones include the firing cone, positioned in the middle; the guard cone which is placed above; and the guide cone located below.

Jr. cones are used in kiln sitters. Kiln sitters are two-stationary prongs sticking out from the wall of a kiln. It measures about half an inch in length and a quarter of an inch apart with a sensing rod in the center. The cone is placed at the center and when subjected to heat, it sags then lowers the center rod which, ultimately shuts off the kiln.

Self-supporting cones are used when you're not using a clay pack or clay plaque. All you need to do is place these cones inside the kiln and let it do its wonders until the end of the firing process.

Pyrometric cones are very important in the firing process. A knowledge of the basics will certainly make pottery an enjoyable hobby.

Now that you've gained insightful details on how cones work in pottery, it's time to create a beautiful work of art from clay.

Where to get clay [] is the one stop online pottery supply shop for all your clay needs. Find out more about pyrometric cones [] and how to pick the right clay for your clay pottery masterpiece.

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