Kilns and Firing

Many teaching studios focus solely on the making part of the ceramics process and never expose their students to the loading and firing of kilns. While the reasoning behind this is perfectly sound (one mistake can ruin an entire kiln load of work!), this exclusion leaves many fledgling ceramists with a massive hole in their clay education. In this lesson, we'll cover some basics about kilns and firing to fill in some of those gaps.

Please note: To best understand this lesson, it is important to have a decent grasp of the pyrometric chart. If cones are still a mystery to you, we recommend taking a look at the Cones: Explained lesson before reading further.

Types of Kilns

Electric Kilns

The most common type of kilns out there are electric kilns. These are just what they sound like: a kiln that uses electricity to produce heat via coils of special wire called elements that line the inside of the insulated chamber. Modern electric kilns are capable of reaching temperatures up to cone 10, though they are most often used for low- and mid-fire temperatures (cone 06-6). Even studios that run gas kilns (more on those next) will have electric kilns in-house for bisque-firing purposes.

Gas Kilns

Most universities and teaching studios use gas kilns for their glaze firings. These typically consist of a large square chamber and a tall chimney at the back. Natural gas or liquid propane is injected into the chamber through burners that mix the fuel with oxygen. The flames produced by this fuel are what heat the inside of the kiln.

​​Gas kilns can be purchased commercially through manufacturers like Bailey Ceramics, and feature digital controllers that make firing hassle-free. Many gas kilns, though, are homemade by experienced potters who prefer a bit more hands-on approach. These kilns are monitored using a pyrometer to keep tabs on the temperature and adjusting the gas and reduction level (more on that later) manually. ​Gas kilns are usually fired to cone 10, but can be used for low- or mid-range firing as well.

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Salt & Soda Kilns

Gas kilns can be modified for use as salt kilns or soda kilns, and sometimes as both in one. Portholes are created in various spots all around the kiln, allowing for a liquid mixture of sodium chloride (for salt) or sodium carbonate (for soda) to be sprayed into the firing chamber. These chemicals vaporize in the high heat and react with the clay, slips, and glazes to create unique aesthetic effects. You should always wear protective gear and a properly-rated respirator when injecting salt and soda.

Wood Kilns

One of the most beloved firing processes is wood firing, a time-honored traditional method that uses wood as its fuel source. A fire box - a combustion chamber at one end of the kiln - is constantly stoked for days (if not weeks) to gradually bring the kiln up to temperature. As the wood burns, ash is produced and travels through the kiln on its way to the chimney, sticking to the pots and creating beautiful, glassy surfaces, colorful flashing effects, crystal growth, and interacting with glazes in unique ways. The longer the firing, the more ash can build up. For this reason, some wood firings can last a week or more, though a day or two is much more common. Depending on the size, these kilns can take over a week to cool. They are typically fired to cone 10-12.

Given the immense amount of work that goes into this process, both in preparing copious amounts of wood and the long hours spent stoking and monitoring the kiln, wood firings are typically a community event. Participants work in shifts under the direction of one or two firing masters who are familiar with the kiln and experienced in the wood firing process. Every wood kiln fires slightly differently, and every firing produces different results.

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Alternative Kilns

While the above-mentioned kilns are the most common, they are not the only options. Kilns come in all shapes and sizes, with all manner of firing processes in mind. Experienced potters can get incredibly creative with their kiln-building. The "Glo Worm" pictured here is a temporary kiln (constructed for the firing and then broken down immediately after) that makes a periodic appearance down at the Eye of the Dog Art Center in San Marcos. It is built mainly of kaowool (ceramic fiber blanket), wire mesh framing, and an old barrel for the fire box, though salvaged electric kiln sections have sometimes been used in past years.

These alternative kilns can be a fun challenge, but always keep in mind that you are dealing with extreme temperatures that can cause 3rd degree burns in seconds. Appropriate safety measures should always be in place before attempting to build or fire any kiln, but these precautions are doubly important when dealing with alternative kilns and firing processes.

Oxidation vs. Reduction

You may have heard these terms in passing or noticed them on our cone 10 clay listings, but what exactly do they mean? Simply put, the when it comes to ceramics, 'oxidation' and 'reduction' refer to the oxygen content of the atmosphere inside the kiln during firing.

Oxidation

This term means that there is free oxygen inside the firing chamber throughout the firing process. Though it is possible to fire a gas kiln in oxidation, usually this term is used when discussing electric kilns. Since there is no fuel in an electric kiln to consume the oxygen inside the chamber, there is usually plenty of oxygen floating around in there.​ Therefor, you may hear the word 'oxidation' used interchangeably with 'electric' when discussing discussing kilns or firings.

Reduction

Reduction is the process of starving the firing chamber of oxygen during a firing. This requires the use of combustible fuels as a heat source, which just means 'flames'. Any kiln besides an electric kiln will meet this requirement. This oxygen-deprived atmosphere causes unique and sometimes dramatic reactions in clay bodies and glazes.

To achieve reduction, the burners and damper (a mechanism used to partially close off the chimney) are adjusted to maximize the efficiency of combustion. When a kiln is in reduction, you will often see flames shooting out from the top of the chimney. This is because the flames have consumed all available oxygen within the chamber and are forced to seek oxygen elsewhere. While it is possible to fire a gas or wood kiln in oxidation, it is far more common to aim for at least some level of reduction.

Other Types of Firings

Raku

This could easily fit under the "Kilns" portion of this lesson, but the design of raku kilns varies so widely that it is more helpful to talk about it as a firing process rather than a kiln type.

While firing relatively slowly is usually best for the work being fired, raku contradicts this entirely. The pots are heated quite quickly in a kiln, usually bringing them up to around 1800F to mature any low-fire glazes that may have been applied to the surface. Once at temp, the kiln is opened and the pots are removed while still glowing and deposited into a reduction bin. This is a fire-proof container (like a metal trash can) filled with combustible materials such as sticks, leaves, wool, paper, etc. When the red-hot ceramic piece touches these materials, they immediately catch fire. By closing the lid of the container, you starve the atmosphere inside of oxygen, resulting in heavy reduction.

The raku process has many variables and the results of these firings can range from brilliant metallics to elegant crackle effects depending on the glazes and combustibles used. Any pottery clay can be fired in raku, but the thermal shock can cause cracks in dense bodies like porcelains. More open bodies (ones that contain grog or sand) or clays that are formulated to resist thermal shock (like Armadillo Clay's Raku body) will have a better chance of surviving this process without cracking.

Protective equipment is imperative. Leather welding gear, kevlar gloves, raku tongs, and a heat-resistant face shield should all be employed to keep you safe from the extreme temperatures. We do not recommend this process to novice potters without direct, experienced supervision. Raku-fired work is typically not food-safe.

Pit, Smoke, & Barrel

Pit, smoke, and barrel firings are fun experimental processes that can be achieved without much upfront investment, however (like raku) the results are typically not food-safe. These rudimentary 'kilns' can be made from old BBQs, steel barrels, or just pits in the ground. They don't get hot enough to mature the clay or glazes, but you can achieve a number of interesting effects by experimenting with the types of combustibles you add or extending the length of the firing.

At its most basic, a kiln is just a box of fire, but the sizes, designs, and fuel sources are virtually endless. Electric is easy, safe, and reliable. Other fuels are a bit more involved, but can produce unique, truly one-of-a-kind results. Through classes, research, and experimentation, you can experience many of these methods for yourself and add to your ceramics knowledge over time.