Kiln cones

In a kiln, a set of three drooping cones can monitor the effects of temperature on the pottery items being fired.

Pylometric cones
Tinux, CC BY-SA 2.5, via Wikimedia Commons

When you’re firing ceramics – terracotta, porcelain, stoneware – you need to keep a very close eye on the temperature of the kiln because different materials and glazes require very specific heats. Porcelain, for example, needs to be fired at somewhere between 1,200 and 1,400°C, whereas some earthenware clays require a much lower temperature and won’t react properly at such a high heat.

However! It’s not enough to just monitor the temperature of the kiln. The important factor to measure here is not temperature but temperature over time. In pottery circles this is usually called heatwork. The thermometer measures the heat at a particular moment, but another tool measures the heat over the whole course of the firing. That tool is the pyrometric cone.

It is remarkably lo-fi. The pyrometric cone is a small pyramidal cone made of the same materials as other ceramics, plus a bonus material called flux. It is stood up in the kiln next to the items being fired. As the cone is heated, it droops over like it’s falling asleep. When a specific temperature equivalent heatwork is reached, that cone’s tip will touch the ground.

Usually such cones come in threes: one at the desired temperature equivalent, one a little bit below that temperature, and one a little bit above that temperature. If all three are drooped over, you know that the firing was way too hot. If even the lower one remains standing, the firing was way too cool.

Why doesn’t someone just design a thermometer that records temperature over time? Well, they have, but the cones are still useful for a few reasons. The success of the firing is dependent on other factors such as the air circulation within the kiln, the proximity of the items being fired, and the presence of hot spots and cool spots within the kiln. The cones, because they are made of the same materials and are in the same places as the items, react exactly the same as the items being fired. It’s just a really simple, elegant, and accurate way of measuring everything.

Some of the most famous potters in history used and developed pyrometric measures for their firings. One early proponent was Josiah Wedgwood (founder of Wedgwood). For more on the uses and benefits of pyrometric cones, this video is a very nice overview:

2 Replies to “Kiln cones”

  1. Thanks for sharing these!
    My beau is a potter, I’ve seen used cones in person! Have not had the right timing to watch a long enough part of a firing to see them work 🙁

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