Kiln Firing Guide: From Raw Clay to Finished Ceramic
Firing is the irreversible chemical transformation that turns fragile dried clay into permanent ceramic. It is the point of no return — and the point where all your work either comes together or falls apart. Understanding kiln behavior, temperature schedules, and the physics behind firing dramatically reduces the risk of losses.
Types of Kilns
Electric Kilns
The most common choice for studio potters and hobbyists. Electric kilns heat through resistance coils embedded in the walls. They fire in an oxidation atmosphere (plenty of oxygen), produce consistent results, and are controlled by digital programmers that automate the firing schedule.
Advantages: predictable, quiet, low maintenance, no combustion byproducts. Limitations: cannot achieve reduction atmosphere effects (certain glaze colors and surfaces require reduction), and element replacement is an ongoing cost.
Gas Kilns
Fueled by natural gas or propane, these kilns offer both oxidation and reduction firing. Reduction occurs when the fuel-to-air ratio is increased, starving the kiln of oxygen. In reduction, iron-bearing clays develop warm, toasted surfaces, and copper glazes shift from green to deep oxblood red.
Gas kilns require more skill to operate — monitoring pyrometric cones, adjusting dampers, and reading flame color are essential skills. They also require outdoor placement or dedicated ventilation.
Wood-Fired Kilns
The oldest kiln type, still used by potters seeking specific aesthetic effects. Wood firing produces natural ash glaze — wood ash settles on pieces during the multi-day firing and melts into a glassy coating. The prolonged firing (often 24-60 hours of continuous stoking) and atmospheric variation create surfaces that are impossible to replicate in electric or gas kilns.
Czech ceramicists at studios in Bechyne and the South Bohemian region maintain several wood-fired kilns for traditional stoneware production.
The Two-Stage Firing Process
Bisque Firing (First Firing)
Purpose: convert dried (bone-dry) clay into a porous but durable state that can absorb glaze. Typical temperature: cone 06 (998 C) for most stoneware and earthenware bodies.
Schedule for a standard electric kiln bisque:
- Phase 1 (Water Smoking, 20-200 C) — heat slowly at 55 C/hour. Residual moisture evaporates. If heated too fast, steam pressure can shatter pieces. Leave kiln lid cracked or peephole plugs out.
- Phase 2 (200-600 C) — increase to 80 C/hour. Chemically bound water releases from the clay crystal structure (around 573 C, quartz inversion occurs — a critical stress point).
- Phase 3 (600-998 C) — increase to 110 C/hour to reach target temperature. Organic material burns out, and the clay body begins sintering.
- Cooling — turn off the kiln and let it cool naturally. Do not open until below 100 C. Opening too early exposes hot ware to thermal shock.
Total bisque firing time: approximately 10-13 hours, plus 12-24 hours cooling.
Glaze Firing (Second Firing)
Purpose: melt the applied glaze into a glass layer fused to the clay body, and mature the clay to its final density. Temperature depends on the clay and glaze system:
- Low-fire (earthenware): cone 06 to cone 1 (998-1137 C)
- Mid-fire (stoneware): cone 4 to cone 6 (1162-1222 C)
- High-fire (stoneware/porcelain): cone 8 to cone 10 (1263-1305 C)
Glaze firings typically ramp faster than bisque (the ware is already ceramic and resistant to thermal shock), but require a controlled slow-down or hold near peak temperature to allow glazes to mature and smooth out.
Pyrometric Cones: Your Temperature Witnesses
Pyrometric cones are small pyramids of ceramic material calibrated to bend at specific heat-work levels (a combination of temperature and time). They sit inside the kiln on a shelf visible through the peephole. When a cone bends to touch the shelf, that zone has reached its rated heat-work.
Always use witness cones even with a digital controller. Controllers measure air temperature at the thermocouple location — the actual heat at shelf level may differ, especially in larger kilns.
Common Firing Defects
- Bloating — large bubbles trapped inside the clay body, usually from over-firing or impurities in the clay. Solution: verify firing temperature matches clay body specifications.
- Dunting — cracks that occur during cooling, especially at quartz inversion (573 C). Solution: slow the cooling rate through this temperature zone.
- Kiln shelf sticking — glaze runs onto the shelf. Solution: apply kiln wash (a refractory coating) to all shelves, and leave the bottom 5-8 mm of each piece unglazed.
- Uneven color — pieces in different kiln zones fire to different temperatures. Solution: fire test tiles at multiple shelf positions to map your kiln's hot and cold spots.
"The kiln is the final collaborator. You can control everything up to the moment you close the door — after that, you negotiate with physics." — Robin Hopper, Functional Pottery
Kiln Safety
All kilns produce significant heat and, in the case of gas and wood kilns, combustion gases. Basic safety measures:
- Install the kiln on a non-combustible surface with clearance from walls (minimum 30 cm for electric kilns)
- Ensure adequate ventilation — even electric kilns release fumes during bisque firing as organic materials burn out
- Wear heat-resistant gloves when opening peepholes or loading warm kilns
- Never leave a gas kiln unattended during firing
- Keep a fire extinguisher accessible
