Firing ceramics: Basics, temperatures and technique explained

The two firing stages

In ceramic firing, there are usually two firings, which serve different purposes and therefore need different temperatures.

Bisque firing: the foundation for everything else

Bisque firing (also called biscuit firing) takes place at comparatively low temperatures, usually between 900 °C and 1,000 °C. First the remaining pore water escapes, then organic components burn off, and from about 450 °C the clay minerals release their chemically bound water. From this point on, the clay has permanently become ceramic that no longer dissolves in water. The bisque-fired clay body is stable enough for glazing and at the same time still porous, so it absorbs the glaze well and the glaze adheres.

Typical problems in this phase are pieces that burst due to residual moisture when heated too quickly, and cracks caused by uneven drying or very different wall thicknesses. So the rule is: let pieces dry completely before the bisque firing and bring the kiln up slowly in the lower range. We explain how long clay needs to dry in How many days should clay dry before firing?, and everything else about temperature, duration and cost in the article Bisque firing: what is it?

Glaze firing: when glaze and clay body become one

After bisque firing comes the glaze firing, also called the glaze firing. Depending on the glaze and clay body, the glazed pieces are brought to about 950 °C to 1,280 °C. The glaze melts, flows and bonds with the clay body at the interface. With stoneware and porcelain, the clay body also vitrifies in this firing, so it becomes much stronger and barely absorbent anymore.

There are countless glazes with different melting ranges. So always follow the glaze manufacturer's instructions and don't fire on a hunch. If the firing is too hot, the glaze can run and stick to the kiln shelf at the base. If it is too cool, it stays dry, rough or milky because it has not melted completely.

Understanding temperatures

Temperature is the most discussed topic in firing, and for good reason: not every clay body tolerates the same temperature, and not every display shows exactly what is happening in the kiln.

What temperatures does ceramic really need?

Ceramic clay bodies can be roughly divided into three firing ranges:

Clay body	Typical glaze firing	Properties
Earthenware (low fire)	approx. 950 to 1,100 °C	The clay body stays porous and absorbent. A vessel only becomes watertight through a closed glaze. Ideal for decorative pieces, tiles and flower pots.
Stoneware	approx. 1,180 to 1,280 °C	The body sinters almost completely dense, and water absorption is usually below 2 to 3 %, depending on the clay body. It is robust and well suited for functional tableware. Many hobby potters fire stoneware at around 1,220 to 1,250 °C.
Porcelain	approx. 1,230 to 1,300 °C (hobby), industrial up to approx. 1,400 °C	Not a pure clay, but a body made of kaolin, feldspar and quartz. Dense, hard and translucent when thin-walled. It demands careful temperature control and tends to warp.

The values are guidelines. The specifications from the manufacturers of the clay body and glaze always take precedence.

Besides the peak temperature, the hold matters, meaning how long the kiln stays at peak temperature. For many glazes, 10 to 30 minutes makes sense: during this time gas bubbles can rise out of the glaze, pinholes close up and the surface flows out smooth.

Cones: Why the display doesn't tell you everything

The controller measures the temperature with a thermocouple at one specific spot in the kiln. The actual temperature can differ from that, depending on position, load and the age of the thermocouple. On top of that, the result depends not only on the temperature reached but on the total heatwork, meaning temperature and time together. A slowly fired schedule with a hold melts a glaze out more than a fast firing to the same temperature.

That is exactly what pyrometric cones such as Seger or Orton cones show. They are made of precisely formulated ceramic mixtures and bend once a certain amount of heatwork has been reached. If you place them at different spots in the kiln, you can see right away whether your firing reached the target everywhere and where there are hotter or colder zones. You can find all cone values in our complete cone chart.

Firing Schedules & Time: The Underestimated Factor

Many beginners believe that the final temperature is what matters most. In practice, the pace of heating and cooling is at least as important, because clay is especially sensitive at certain temperatures. Modern controllers offer ramp programs for this, letting you set heating rates, holds and cooling in several segments.

Heating up: What happens in the clay

• Up to approx. 200 °C: The remaining pore water evaporates. If you heat too fast here, steam pressure builds up inside and pieces can burst. Heat slowly, and for thick pieces consider a hold around 100 °C.
• Approx. 200 to 900 °C: Organic components burn off, most strongly up to about 600 °C. This produces smoke and odors, so the kiln needs sufficient air supply.
• Approx. 450 to 700 °C: The clay minerals release their chemically bound water. Only after this has the clay permanently become ceramic.
• 573 °C: The so-called quartz inversion. Quartz in the clay body changes its volume abruptly here. Passing through this range evenly and not too fast prevents stress cracks.

Cooling down: Quartz inversion and cooling cracks

During cooling, too, the clay body passes through the quartz inversion at 573 °C, this time in the opposite direction, and with many stoneware bodies also the cristobalite inversion around 220 °C. If the kiln cools too fast in these ranges, for example because it is opened too early, cooling cracks form in the body. They are usually sharp-edged and often run straight across the whole piece.

These are different from fine crazing in the glaze, which sometimes only appears days or weeks after the firing. It is usually not caused by cooling too fast, but by glaze and body not fitting each other: if the glaze shrinks more than the body as it cools, it is under tension and cracks. The remedy is a glaze matched to the clay body, or a slightly higher firing that makes the body denser.

Kiln Technology & Kiln Types

Most hobby potters today work with electric kilns. Gas or wood kilns play a role mainly in specialized studios, especially when a reducing atmosphere is wanted, which an electric kiln is not designed for. To get started, you should know the most important differences:

• Top-loading kilns: Space-saving and often cheaper. Loading and unloading can be more strenuous with heavy pieces or a deep firing chamber.
• Front-loading kilns: Door at the front, more ergonomic to load, usually more expensive. Good for production runs and larger pieces.
• Energy source: Electric is simple, clean and highly repeatable. Gas allows reduction firing and special glaze effects, but needs installation, a flue and experience.
• Controller: Simple controllers or programmable controllers with several segments for firing schedules, holds and stored programs.

We carry kilns from Kittec, Nabertherm and Pyrotec along with accessories. Which models suit which use is shown in our comparison Kilns compared: The 5 best kilns for potters. If you are unsure which kiln fits your space, your power connection and your work, book our free kiln consultation: 20 minutes by phone or video, personal and with no sales pressure.

Choosing a Kiln: What Makes a Good Kiln?

When buying, price is not the only thing that counts. Look out for:

• Insulation: Insulating firebrick in the firing chamber and good backing with insulation boards keep the heat in the kiln, save energy and keep the casing cooler.
• Heating elements: High-quality heating elements and clean routing in the bricks extend their lifespan and reduce maintenance.
• Controller & repeatability: Stored programs with several segments make results reproducible.
• Service: Available spare parts, reachable customer service and repair options are worth their weight in gold.
• Size & power connection: Plan according to the size of your work and clarify the connection. Small kilns often run on a standard outlet, while larger ones need a three-phase connection (400 V). We calculated what a firing costs in electricity in How much electricity does a kiln need?

Common Mistakes and Practical Tips

Firing has a few pitfalls that come up again and again:

• Burst pieces: Almost always residual moisture. Let pieces dry completely, don't speed-dry them, and run the kiln slowly up to about 200 °C.
• Cracks or warping: Uneven wall thickness, uneven drying or heating up too fast.
• Cooling cracks: Cooling too fast or opening too early, especially around 573 °C and 220 °C.
• Running glazes: Applied too thick or fired too hot. Leave a glaze-free margin at the bottom of the piece and test new glazes on test tiles.
• Dry or rough glazes: Fired too low, hold too short or applied too thin. Some glazes are deliberately matte, though.
• Crazing in the glaze: Glaze and clay body don't match in their thermal expansion. Choose a different glaze or fire the clay body a little higher.
• Bubbles and pinholes: Often gases still escaping from the body during the glaze firing. A sufficiently high bisque firing and a hold at peak temperature help.

Many experienced potters keep a firing log with the program, cone result, clay body, glaze and special notes. That way you find causes faster and can reliably repeat successful firings.

Home vs. Studio: Environment, Safety & Floor Protection

At home there are special challenges: three-phase power is not always available, and the surface must be non-flammable and heat-resistant. Place the kiln on heat protection boards or tiles and keep the distances to walls and furniture specified by the manufacturer. Clay and glaze also permanently stain carpets and delicate floors, so plan durable floor protection for the entire work area.

Make sure there is good ventilation, because every firing produces fumes. Most smoke and odor is produced during the bisque firing, when organic components burn out. Depending on the clay body, compounds of sulfur and fluorine as well as carbon monoxide are added. During the glaze firing, components of the glaze can also outgas. For this reason, set up the kiln in a well-ventilated room, ideally with exhaust air vented outside, and don't stay next to it for long periods during the firing. A fire extinguisher, smoke detectors and a carbon monoxide detector should be a given.

In studios, the focus is more on efficiency: loading and unloading, capacity and a clean workflow. Which kiln fits depends on whether you produce series or mostly fire one-off pieces and classes.

Checklist Before the First Firing

• Are all pieces completely dry?
• Do the clay body and glaze suit the planned firing temperature?
• Is the program set: slow heating at the low end, peak temperature, hold, controlled cooling?
• Have you placed cones in several spots in the kiln?
• Are the kiln shelves clean and protected with kiln wash? Learn more in Kiln accessories explained.
• Are glazed pieces glaze-free on the bottom and not touching each other?
• Are the surface, clearances and ventilation sorted out?
• Do the power connection and fusing match the kiln?
• Are you firing together with others? Mark your pieces clearly.

FAQ

How long does a firing take?

That depends on kiln size, load, program and temperature. A bisque firing to 950 °C often takes 8 to 12 hours to heat up, and including cooling you are usually at 18 to 24 hours in total. A glaze firing to 1,230 °C often takes 20 to 30 hours including cooling. Only open the kiln once it has cooled below about 100 °C, otherwise you risk cooling cracks.

Can I run a kiln in my apartment?

In theory yes, in practice only to a limited extent. You need a suitable electrical installation, depending on the kiln a three-phase connection, a heat-resistant surface, sufficient clearance from flammable materials and good ventilation. Be sure to clarify this in advance with your landlord and insurance. For many people, a separate room, a ventilated basement or a garage is the more realistic solution. We explain what is allowed in a rental apartment in Kiln in a rental apartment: Is it allowed?

Do I need a bisque firing if I fire at a low temperature?

In most cases, yes. Bisqued pieces are safer to glaze, the glaze adheres better, and gases from the body have mostly already escaped by the glaze firing, which reduces bubbles and pinholes. There is also single firing, though, where leather-hard or dry pieces are glazed raw and fired to completion in a single firing. This saves time and energy, but requires suitable glazes and some experience.

Is stoneware automatically food-safe?

No. A densely fired body is a good prerequisite, but the glaze is what decides. For tableware, only use glazes that the manufacturer explicitly states as food-safe, and fire them exactly in the specified temperature range. A glaze that has not melted out completely can release components, even if it is listed as food-safe on the data sheet.

What are the most common mistakes in a glaze firing?

Running glaze (too thick or too hot), dry or rough surfaces (too cold or held too briefly), bubbles and pinholes (gases from the body, missing hold) and crazing (glaze doesn't fit the clay body). Always test new glazes first on test tiles made from your own clay body.

Further Reading

• Kilns compared: The 5 best kilns for potters: if you want to compare specific models.
• Can you fire ceramics twice?: when a second glaze firing makes sense and what to watch out for.
• How much electricity does a kiln need for ceramics?: consumption and cost per firing.
• The complete cone chart: all cone values at a glance.
• Kittec kiln lines explained: an overview of the Kittec model series.