What Is a Kiln and How Does a Pottery Kiln Work?

What a kiln does to clay

Raw clay contains water, and even dry clay holds chemically bound water in its structure. When clay is heated, that water is driven out, and at higher temperatures the clay particles begin to fuse. This permanent change is what makes fired clay hard and waterproof in a way that dried clay is not.

Glaze is fired in much the same way. In the kiln it melts into a thin layer of glass on the pot, giving color, texture and a sealed surface.

How an electric kiln works

Most home and studio kilns are electric. Heating elements, coils of resistance wire set into the kiln walls, heat up when current passes through them. Soft insulating brick or ceramic fiber lines the chamber, keeping heat inside and protecting the outer shell.

A controller raises the temperature on a programmed schedule. Many controllers store ramp and hold steps, so the kiln heats slowly at first, then faster, then holds at peak. This gradual schedule helps prevent cracking and ensures even firing.

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How temperature is measured

Kilns use thermocouples, small sensors that measure temperature, and the controller reads them. Potters also use pyrometric cones, small pyramids of ceramic material that bend at a specific combination of heat and time. Looking at cones through a peephole shows how much heat work the pots received, which is more reliable than temperature alone.

Cone numbers identify the target. Low fire, mid range and high fire clays and glazes are matched to specific cones, so choose materials that match your kiln's capability.

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Other types of kilns

Gas kilns burn fuel and can create a reduction atmosphere, which changes some glaze and clay colors. Wood kilns fire with wood and produce ash effects. Raku kilns are small and fast, and are used for dramatic firings. Electric kilns are usually the easiest to use at home because they need no fuel burning and give repeatable results.

Each kind needs its own setup and ventilation. Gas and wood kilns especially require proper installation and safety measures.

Bisque and glaze firings

Most pottery is fired twice. The first firing, called bisque, hardens the clay while keeping it porous so glaze can be applied. The second, the glaze firing, melts the glaze. Some potters use a single firing for certain work, but two firings are the common method.

Shelves and stilts keep pieces off the kiln floor and prevent glazed pots from sticking. Kiln wash on shelves protects them from drips.

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Safety basics for kiln use

A kiln gets extremely hot on the outside and can ignite nearby materials. Place it on a non-combustible floor, keep it clear of walls and flammable items as the manufacturer states, and never leave it unattended without understanding the controller. Electric kilns may need a dedicated circuit, so have the wiring checked by a qualified electrician.

Kilns release fumes as materials burn off, so ventilation is essential. A kiln vent or a well ventilated room helps. Wear heat-resistant gloves and eye protection when handling hot items, and let the kiln cool fully before opening.

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Do you need your own kiln?

Not necessarily. Many studios fire work for a fee, and community classes often include firing. Starting without a kiln lets you learn forming and glazing before deciding whether the investment, space and electrical needs suit you.

If you do buy one, think about size, voltage and where it will live. Small kilns are suited to beginners and small spaces.

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What goes on inside the kiln during a firing

In the early stages of a firing, residual water leaves the clay as steam, which is why pieces must be completely dry and why firing often starts slowly. Rising temperatures then burn off organic matter and change the clay's structure. Around certain temperatures, quartz in the clay shifts in volume, so many firing schedules slow down at those points to protect the pots.

At peak temperature the clay vitrifies to some degree, depending on the body, and the glaze melts and flows into a smooth layer. During cooling, the kiln must come down gradually. Opening a kiln too soon can cause thermal shock, which may crack both glaze and clay.

Following the schedule suggested by the clay and glaze makers is the best approach for beginners, and keeping notes on each firing helps you improve results.

Loading a kiln sensibly

Leave space between pieces so heat can circulate, and keep glazed surfaces from touching each other or the shelves. In a bisque firing, pots can touch because there is no glaze to fuse them, but they should still be arranged so weight is supported evenly.

Place taller pieces where the kiln manual advises and keep all work away from the elements by the distance stated. A well loaded kiln fires more evenly and keeps your pots and the equipment safe.

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Frequently asked questions

How hot does a pottery kiln get? Depending on the type of clay, kilns commonly reach temperatures from about 1,800 to 2,400 degrees Fahrenheit, though some go beyond this.

Can I use a regular oven as a kiln? No. Household ovens do not reach firing temperatures and cannot fire ceramic clay. Special equipment is required for that.

How long does a kiln firing take? A firing can take many hours, and cooling takes additional time. Exact length depends on the kiln, the clay and the schedule used, so check your manual or ask your studio.