Firing clay means heating it in a kiln to temperatures between roughly 1,800°F and 2,400°F, permanently transforming it from a soft, fragile material into hard ceramic. Before firing, clay can be dissolved back into mud with water. After firing, the heat triggers irreversible chemical changes that lock the clay into a durable, stone-like state. It’s the step that turns a shaped piece of clay into a functional pot, tile, or sculpture.
What Actually Happens Inside the Clay
Raw clay contains water in two forms: moisture trapped between particles and water locked into the mineral’s crystal structure. Firing drives out both. The first stage is straightforward evaporation. At 212°F, any remaining moisture turns to steam and escapes. This is why pieces must be completely dry before going into the kiln. If moisture is still trapped inside, especially in hollow pockets, the steam has nowhere to go and the piece can literally explode.
Above roughly 430°F, something more fundamental begins. The water that’s chemically bonded within the clay minerals starts breaking free, a process called dehydroxylation. This is the point of no return. Once this chemical water is gone, the clay can never be reconstituted into workable material again.
At even higher temperatures, around 1,250°F, quartz particles in the clay undergo a sudden expansion as their crystal structure shifts. Potters call this “quartz inversion,” and it’s one of the riskier moments in a firing because the rapid change can crack a piece if the temperature rises too fast. At the highest stoneware and porcelain temperatures, near 2,300°F, the clay’s components react with each other to form new minerals. Needle-like crystals grow throughout the clay body, bound together by a glassy phase. The result, especially in porcelain, is a dense, translucent material that’s genuinely closer to glass than to dirt.
Bisque Firing: The First Round
Most pottery goes through the kiln twice. The first firing, called a bisque fire, transforms raw dried clay (called greenware) into a porous but solid state known as bisque or biscuit ware. Bisque firing typically reaches between 1,828°F and 1,945°F. The goals are simple: drive out all remaining water, burn off organic impurities, and give the piece enough strength to handle without crumbling. Bisque ware is still porous, which is the whole point. That porosity lets it absorb liquid glaze evenly in preparation for the second firing.
Glaze Firing: The Finished Surface
The second firing is where glaze melts and fuses into a smooth, glassy coating on the clay’s surface. Glaze firing runs hotter than bisque, typically reaching 2,167°F to 2,345°F depending on the clay type. During this stage, the glaze ingredients melt and flow across the surface before cooling into a sealed finish. The clay body itself also becomes denser. At these higher temperatures, the clay particles fuse together in a process called vitrification, closing the pores and making the piece waterproof. Final color, texture, and surface effects all develop during this firing.
How Temperature Determines the Type of Pottery
Different clays mature at different temperatures, and the peak temperature defines the category of ceramic you end up with. Earthenware fires at the lowest range, roughly cone 06 to cone 02 (about 1,830°F to 2,050°F). It stays somewhat porous even after firing, which is why earthenware flower pots can absorb water. Stoneware fires in the middle range, around cone 1 through cone 6 (approximately 2,100°F to 2,230°F), producing a dense, durable body suited for dinnerware. Porcelain pushes to the highest temperatures, cone 8 to cone 12 (roughly 2,300°F to 2,470°F), where the clay vitrifies completely into a hard, often translucent material.
Shrinkage and Size Changes
One thing that surprises beginners is how much a piece shrinks during the entire process. From wet clay to finished fired object, total shrinkage typically falls between 8% and 12%, though some clay bodies can shrink as much as 15%. That shrinkage happens in stages. About half occurs during drying, before the piece ever enters the kiln. A small amount, around 0.5%, happens during bisque firing. The rest, often another 5% to 6%, occurs during the higher-temperature glaze firing as the clay particles pack tighter together. A mug you threw at six inches tall might come out of the final firing closer to five.
Oxidation vs. Reduction Firing
The atmosphere inside the kiln matters as much as the temperature. In an oxidation firing, plenty of oxygen circulates around the work. Metals in the clay and glaze stay in their fully oxidized state, producing bright, predictable colors. Copper glazes fire green, iron produces oranges and tans.
In a reduction firing, the oxygen supply is deliberately restricted. Metallic oxides in the clay and glaze give up some of their oxygen, shifting to a more metallic state. This changes everything about color. Copper turns from green to deep red. Iron becomes a powerful melting agent, producing intense earth-tone browns, celadon greens, and blues instead of its usual muted behavior. Natural iron particles in the clay body convert to their metallic form and melt, creating the characteristic iron speckling that many potters prize. Even materials like tin oxide behave differently, losing much of their opacifying ability in reduction.
The same glaze recipe fired in oxidation and reduction can look like two completely different materials. A glaze with about 12% iron oxide, for instance, will be stable and light-colored in oxidation. In reduction, the iron converts to a form that melts aggressively, creating the fluid, dark surface of a classic tenmoku glaze.
Electric Kilns vs. Gas Kilns
Electric kilns heat with coils and naturally produce an oxidation atmosphere. They’re popular for their consistency: if you develop a glaze you love, you can reproduce it reliably across dozens of firings. They also tend to heat and cool more slowly than gas kilns, which affects how some glazes develop. The tradeoff is that you’re limited to oxidation firing, which rules out copper reds and the unpredictable atmospheric effects many potters chase.
Gas kilns burn fuel, which consumes oxygen inside the chamber and makes reduction firing possible. Beyond reduction, the open flame itself creates effects that electric kilns can’t replicate: flashing, where the flame leaves red-orange marks directly on the clay surface; carbon trapping, which creates dark speckles and smoky textures; and subtle shifts in tone across a single piece where the atmosphere varied slightly. Each piece comes out distinct, even in the same kiln load.
Preparing Clay for the Kiln
The most common cause of kiln disasters is putting clay in before it’s fully dry. Two simple tests can save you from explosions. Hold the piece against your cheek: if it feels colder than the surrounding air, moisture is still evaporating from the surface, and it needs more drying time. Or set the piece on a sheet of paper for a few minutes: if the paper wrinkles, moisture is still migrating out of the clay.
Even with bone-dry work, the kiln temperature needs to climb slowly through the early stages. Many potters use a “candling” phase, holding the kiln at a low temperature (around 150°F to 200°F) for several hours to gently drive off any last traces of moisture. After that, a controlled ramp rate brings the temperature up steadily, often at 150°F to 300°F per hour, with holds at critical points to let the heat distribute evenly. A typical bisque firing takes around five to eight hours of heating, followed by a full day of cooling before the kiln can be opened safely. Rushing the cooldown risks cracking from thermal shock, especially during the quartz inversion zone on the way back down.