Firing the Kiln

Firing turns raw clay and glaze into durable ceramic. Most studios use two firings:

1. Bisque Firing The initial firing that turns bone-dry (unfired) clay into “bisque” (a porous, sintered ceramic body) without fully maturing it.

2. Glaze Firing The second firing, where you apply a glaze to bisque-fired clay. Heat melts and fuses the glaze into a vitreous coating on the surface.

Firing Ranges and Cones

Ceramics are typically categorized into four firing ranges:

• Bisque Firing (Cone 08–04 / ~900–1060°C) Initial firing that converts raw clay into a porous ceramic state. Temperature is kept lower than the final glaze firing to maintain porosity for better glaze absorption.

• Low-Fire (Cone 06–04 / ~950–1060°C) Earthenware glazes mature at lower heat. Colors often stay vivid, but the clay remains somewhat porous. Great for decorative pieces or bright designs.

• Mid-Fire (Cone 5–6 / ~1180–1230°C) Very popular with electric kilns. Stoneware glazes get nicely vitrified here, producing durable, functional ware. There’s a broad selection of commercial cone 5–6 glazes.

• High-Fire (Cone 9–11 / ~1280–1310°C) Traditional realm of porcelain and classic reduction-fired stoneware. Glazes can become more subdued or earthy, with beautiful depth in color and surface (e.g., celadons, temmokus, copper reds).

Pyrometric Cones and Heatwork

Whether you choose low-, mid-, or high-fire glazes, you’ll likely see references to pyrometric cones (e.g., “Cone 6”) or Orton cones, a common brand name. These cones soften and bend at certain levels of heat plus time (called heatwork). Use witness cones in your kiln to confirm you’re hitting the correct maturity, especially if you suspect your kiln’s controller might be reading inaccurately or firing too fast. Note that different brands of cones may indicate different temperatures for the same cone number.

Bisque Firing

Why Bisque First?

Bisque firing before glazing gives you:

• Strength & Durability: Bisque is tougher than bone-dry clay, so it’s easier to handle and glaze without damage.
• Porosity for Glaze Application: Bisque stays porous, so it grabs glaze quickly and evenly.
• Burn-Off of Impurities: Moisture and organics burn out in bisque, reducing pinholing or black coring later.

Typical Bisque Firing Temperatures

Most potters bisque in the low-fire range, often cone 08 to 04 (~950–1060 °C or 1740–1940 °F). That’s hot enough to sinter the clay while keeping it absorbent. A “soft” bisque (cone 08–06) is more porous; a “hard” bisque (often cone 04) is sturdier but takes glaze a bit less readily.

Basic Bisque Schedule

A very common bisque schedule might look like this in an electric kiln:

1. Slow Ramp to 100°C (212°F) or so Allows any remaining physical water to evaporate without steam explosions.
2. Continue Slow/Moderate Ramps up to 600-800°C (~1110°F) Lets organics burn out gradually.
3. Regular Ramp to ~Cone 04 (~1060°C) Past red heat (600–800°C), you can increase speed slightly.
4. No Hold or Short Hold Many potters simply shut off at top temperature. Others hold briefly to ensure complete burn-out of carbon.
5. Cool Naturally Wait until the kiln is comfortably below 200°C before cracking it open to avoid thermal shock.

After bisque firing, your pieces will be firm and chalky in texture. They won’t dissolve in water anymore, but they’re still porous enough to accept glaze easily.

Glaze Firing

Once your ware is bisque fired, it’s time for the glaze firing, where the real magic happens. At higher temperatures, the glaze ingredients melt and fuse, bonding to the clay body to create a smooth, glassy surface.

What Happens During Glaze Firing?

1. Drying & Burn-Out (up to ~600°C) Any leftover moisture or added organic materials in the glaze may burn out. Heat slowly here to avoid bubbles or cracks.

2. Sintering (~800–900°C) The glaze particles begin to bond gently. By now, the underlying clay is already fully bisqued, so your pot remains stable.

3. Glaze Melting & Vitrification (from ~1000°C onward) Fluxes, silica, and other components in the glaze melt. The surface goes from powdery to glossy. The clay may also approach its own vitrification point, especially in mid- or high-fire.

4. Cooling A smooth, controlled cool (some potters also include a hold at peak temperature) helps even out the glaze surface, minimizing defects like pinholes or crazing, as well as promote crystallization.

Oxidation vs. Reduction

• Oxidation Atmosphere (typical in electric kilns) supplies ample oxygen. Glazes tend to be bright or consistent, perfect for most commercial glazes.
• Reduction Atmosphere (gas or wood kilns) deliberately restricts oxygen. Classic high-fire stoneware and porcelain glazes, like celadons or copper reds, rely on reduction to develop unique color and depth.

If you’re new to firing, oxidation in an electric kiln is simpler. Mastering reduction requires adjusting burners and damper, usually done in gas, wood, or specialized kilns. (Note that you can achieve reduction effects in oxidation using localized reduction techniques such as using silicon carbide.)

Cooling: Natural vs. Programmed

After the kiln reaches peak temperature and any desired hold time has elapsed, you have two main options for cooling:

1. Natural (Passive) Cooling Most potters simply turn off the kiln and allow the interior to cool at its own pace. Often the kiln is “closed up” by covering all holes (plugs/peeps), and for gas kilns the damper is pushed fully in. Natural cooling is typically sufficient for many glazes.

2. Programmed (Controlled) Cooling Some kilns let you program a ramp-down schedule (e.g., cooling at 80-150 °C/hour), while gas kilns allow control via the burners & air. This gives you more control during the critical stages where temperature changes can cause stress or produce glaze effects. Reasons to slow or shape the cooling curve include:

   • Reducing Crazing or Dunting Slowing the temperature drop through the quartz-inversion range (~573 °C) can reduce the risk of sudden dunting or glaze crazing.
   • Encouraging Special Glaze Effects Certain mattes, crystalline glazes, or other surface textures develop better if held at specific cooling steps.
   • Consistent Results If your kiln cools very quickly, or if you live in a colder climate, programming a controlled descent can even out results and minimize thermal shock.

Loading and Kiln Safety

• Bisque vs. Glaze Loading
  • In bisque firing, pots can be stacked or set close since nothing melts.
  • In glaze firing, leave space. Glazed surfaces shouldn’t touch each other (or the shelf!). Glazes can stick to anything they contact once molten.
• Kiln Furniture Use posts and shelves to create levels. For runny glazes, apply kiln wash or place catchers beneath pieces.
• Ventilation Even electric kilns produce fumes. A vent system or open studio windows can help. Always follow safety guidelines, and no flammable materials near a hot kiln.
• Cooling Let the kiln cool slowly and naturally. Opening it too soon risks cracking your pieces (thermal shock) and can damage the kiln’s lining or elements.

References & Further Reading

• Pyrometric Cones Resources (Orton) : A comprehensive list of documents and videos covering all aspects of pyrometric cones.
• Bisque Firing 101 by Dave Finkelnburg 
• A Bisque Firing Schedule to Help Prevent Glaze Faults by Steve Davis