Cones: Explained

​When selecting a clay body or glaze, you will most likely see reference to its “cone” range. For example, Balcones White is a 'cone 10 clay'. But what does this even mean? In this lesson, we'll take a look at cones, the pyrometric chart, and help you get a basic understanding of what they are and how they fit into the ceramic process.

​What is a cone?

A cone (technically a pyrometric cone) is a physical object in the shape of a tall pyramid that serves as a visual tool for measuring heat work. When heated, the cone will begin to bend and eventually melt within a specific temperature range. Cones have a number assigned to them, typically 022-14 for ceramics, and thus a different chemistry for each number. Although there is a temperature related to each of these numbers, it is often presented as a range rather than one specific temperature. This is because heat work is dependent on time as well as temperature. A kiln fired quickly will require a higher peak temperature than a kiln fired slowly to bend the same cone. The chart to the left (click to enlarge) is split into three columns. You will notice that the temperature listed in the "Slow" column is lower than the ones listed for "Medium" or "Fast" firings. This is heat work in action.

​Modern kilns have come a long way. Today most electric kilns are controlled by a computer that interprets temperature via a thermocouple, a probe that translates heat into an electric current that is then translated into a specific temperature. Since the computer only reads the temperature and not heat work, an average temperature for each cone is used. Although digital kilns are very reliable it doesn’t hurt to double check with witness cones every now and then. You may find it necessary to put a hold on your kiln (maintain the peak temperature for a short time rather than shutting off immediately) in order to fully bend the cones.

Since their invention in 1892, pyrometric cones are still the most reliable indicator of the heat work achieved by your kiln.

4 Types of Cones

There are 4 different types of cones available.

Bars: These are used in a kiln-sitter type controller. When the kiln reaches the target temperature, the bar bends and triggers the kiln to turn off.

Small Cones: Also known as 'sitter cones'. They serve the same purpose as bars.

Large Cones: These are the traditional witness cones (for 'witnessing' the temperature of the kiln as it climbs). They are used to monitor heat inside the kiln.

Self-Supporting Cones: Witness cones with a broad base, eliminating the need for a cone holder. They serve the same purpose as large cones.

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Using Cones in Firings

Technology is great. Thermocouples, pyrometers (handheld temperature readers), and digital controllers have made monitoring your kiln's firing easier than it ever was in the past. There are plenty of scenarios, however, where you may want to use cones to verify that you hit the proper temp or just get more detailed information about how your kiln is performing. Potters do this by including cone packs in their firings.

A cone pack is a set of witness cones placed in a holder. This holder can be something as simple as a lump of clay (just make sure you let the clay dry completely before firing the kiln!). A cone pack typically contains 3 or more cones at and around the target firing temperature. These packs are placed directly in front of peepholes so they can be monitored throughout the firing.

Guide Cone: This cone traditionally acts as a guide to signal that the kiln is almost up to temp. It is usually one cone below (cooler) than the target temperature.

Firing Cone: This cone matches the target temperature for the firing.

Guard Cone: This cone serves as a warning that the kiln has gotten too hot. It is usually one cone above (hotter than) the target temperature.

For electric kilns, these packs can be placed anywhere throughout the kiln and checked after the firing is complete to glean useful information about hot and cold spots in the kiln.

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Do I really need to know all of this?

No. You don’t need comprehensive knowledge of pyrometric temperatures and the reactions that go on in clay and glaze, but you do need to know what you are working with. Clays and glazes fall into 3 main ranges:

• Low-Fire (cone 06-04)
• Mid-Fire (cone 5-6)
• High-Fire (cone 10+)

Notice there is a “0” in front of the numbers for low-fire clay/glaze. That is not to be ignored! A good rule to remember is “the “0” means low”. You must say “Oh” if it is present. Cone 6 is about 400 degrees hotter than cone 06! As you can see from the pyrometric chart (above), the “0” serves as a negative sign. Therefore cone 05 is cooler than cone 04 whereas cone 5 is hotter than cone 4.

For the most reliable results, it is best to match your clay with your glazes. If your clay’s recommended firing temperature is cone 06-04, then you should use low-fire glazes. These may say on the label something like “apply to shelf cone 04 bisque and fire to cone 06-05”. This means your clay has already been fired to cone 04 without glaze (bisque fired). Once glaze is applied, you will then fire the piece again to cone 06-05. If the label says fire to cone 5-6, do not use on low-fire clay! Remember the zero is important! At low-fire temperatures, a mid-fire glaze will not melt. At mid-fire temperatures, a low-fire clay could warp, slump, bloat or even melt. Of course there are exceptions to every rule but if you are a beginner, avoid confusing yourself and stick to one cone range. As you gain experience, you can experiment with different temperatures for a given clay or glaze, but remember it is exactly that: an experiment. Results will vary. Experimenting requires testing and a tolerance for failure. Usually this failure will only result in an unattractive pot, but it could ruin your piece and/or kiln shelf.

​If you are new to clay you will encounter a lot of terminology that is unfamiliar, even confusing. Just remember: 'cone' is a measurement of heat and whether or not there is an “0” in front is very important. Good luck and happy firing!

Download an Orton Cone Chart Poster

Download an Orton Cones-and-Firing Guidebook