All Glossary

Kiln Firing

All types of ceramic are fired in a kiln to cement particles together to produce a hard and water and temperature resistant product.

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At it most basic level, firing is a process of heating a clay (or recipe of clays and minerals) to a temperature sufficient to fuse the particles together. However today, each type of ceramic has not only its own firing temperature but also schedule (control of the rate of rise and fall of the kiln). In addition, the atmospheric pressure and atmosphere itself within the kiln are controlled for many types of firing, either by restricting the amount of oxygen in the chamber or replacing it entirely by another gas (like nitrogen). In addition, kilns subject the load to drafts to help even out temperature and atmosphere and carry away water vapor and products of combustion and decomposition (of bodies and glazes). Firing also varies in the types of fuel that are used (e.g. coal, gas, wood, sawdust, oil, electric) and the type of kiln (kilns vary widely in the way they deliver heat to the ware and channel it out).

Related Information

A small test kiln: More control, faster learning

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A question about a glaze or body should be a reason to run a test. Having your own small programmable kiln means you can do that without waiting for a production load or tolerating someone else’s firing schedule.

This is a Cone Art 119D: an 11-inch-diameter, 9-inch-deep, 0.57-cubic-foot cone 10 test kiln. It runs on 208 V and has a Bartlett Genesis touchscreen controller. Being able to create, edit and repeat firing schedules easily is central to how it's used and what can be tested. It holds four or five mugs, or a dozen clay and glaze test specimens. Small loads make frequent testing practical: compare a recipe adjustment, investigate a defect or check a new material shipment. Controlling the cooling schedule is among the most fascinating of tests (and a way to emulate a slow-cooling production kiln). With most firings below cone 7, a set of elements can last a thousand firings. Start with a kiln like this and you will discover how much you can accomplish before needing a larger one. Even then, the little kiln keeps its job.

Two standalone electric wall-mount kiln controllers. But they are very different.

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The red controller on the right is a Skutt Kilnmaster, the blue controller to the left is an Orton Autofire. As of 2022 both of these are now ancient devices, having been replaced with newer touch panel units. The principle of operation for both is the same: They turn the power on and off in a duty cycle to control temperature rise. A 50% duty cycle, for example, sees the power on for 50% of the time. The length of individual bursts increases with kiln temperature. The controllers monitor a thermocouple in the kiln to determine the length and frequency of power bursts needed. Both of these devices are external to the kilns (but there is a big difference). The KilnMaster controller is attached to the 220V power line and the kiln power line attaches to it (there are heavy-duty electrical relays inside). The blue Autofire controller does not have internal heavy-duty relays or switches, they are in the kiln. The KilnMaster is thus more flexible since it can connect to any kiln, but it is also triple the price. In 2022 the AutoFire now has its own relays like the KilnMaster.

A gas kiln built by Luke Lindoe in the 1960s is still used at Plainsman Clays today

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It fires very evenly from top to bottom and front to back. We have used it for quality control to fire thousands of porosity and shrinkage test bars to monitor the maturity of the clay bodies. Oh, we also fire pottery in this!

Front of soda and salt kilns

At the Medalta International Artists in Residence

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In Medicine Hat, Alberta, Canada. Designed by Aaron Nelson.

Firing an impossible-to-fire piece was possible!

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These porcelain pieces were cracking, even during very slow firings at cone 03. The radiant heat from the elements heated the outside edge first, creating a temperature gradient. That produced a firing shrinkage gradient that the piece could not absorb. Even with very slow firings and various other strategies, the potter could not solve the problem. So is it really true that anything can be fired if you can do it slow and even enough? Yes, even this. Three factors made it possible: 1) The low firing temperature. 2) Even slower firing speed (24 hours instead of 12 hours to ramp). 3) Elimination of the direct radiant heat. This clever potter fires using only the elements in the bottom one/third of the kiln (the top 2/3 elements never go on) and puts the rings only in the top 2/3 of the kiln. Again, that was possible because it is only a cone 03 firing.

Temporary Raku Kiln

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Used for firing raku test specimens at Plainsman Clays.

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