Firing Process
Part of Kiln Design
Step-by-step kiln firing from cold start to peak temperature, including fuel management and atmosphere control.
Why This Matters
Building a kiln is only half the challenge. Firing it correctly — managing the temperature ramp, controlling atmosphere, timing fuel additions, and reading the kiln’s behavior — is the skill that separates successful production from wasted fuel and shattered ware. A single mismanaged firing can destroy days of work: pottery cracks from thermal shock, bricks bloat from overfiring, or an entire load comes out underfired and crumbles in use.
In a rebuilding scenario, every firing represents a significant investment. A typical wood-fired kiln consumes 200-500 kg of dry wood for a full stoneware firing. That is multiple days of cutting, splitting, and drying. The ware inside — whether bricks for construction, pots for food storage, or tiles for roofing — represents additional days of forming and drying. A failed firing wastes all of it. Mastering the firing process means your community gets reliable output from every firing, builds confidence in ceramic production, and develops the skills to train others.
The firing process also teaches thermodynamics in the most practical way possible. Understanding how fuel, air, and heat interact in a kiln gives you transferable knowledge for operating forges, smelters, lime kilns, glass furnaces, and any other high-temperature process your community needs.
Preparation
Fuel Preparation
Fuel quality determines firing success more than almost any other variable.
Wood requirements:
- Moisture content below 15%. Freshly cut “green” wood can be 40-60% water. This water must be boiled off before the wood can burn, wasting enormous amounts of energy. Split wood and stack it off the ground under cover for a minimum of 3-6 months.
- Size grading. Prepare three size categories:
- Kindling (2-3 cm diameter): For starting, warm-up, and pushing through temperature plateaus
- Medium splits (5-8 cm diameter): The workhorse fuel for the main climb
- Chunks (10-15 cm diameter): For sustained heat during soaking phases
- Species. Hardwoods (oak, maple, ash) produce more heat per kilogram and longer-lasting coals. Softwoods (pine, spruce) burn fast and hot — useful for rapid temperature climbs but consumed quickly. A mix is ideal.
- Quantity. Plan for 150-250 kg per cubic meter of kiln chamber for earthenware (1000°C), 250-400 kg for stoneware (1250°C). Always prepare 20% extra.
| Fuel Type | Heat Value (MJ/kg dry) | Burn Rate | Best Use |
|---|---|---|---|
| Pine / Spruce | 18-20 | Fast | Warm-up, quick climbs |
| Oak / Ash | 20-22 | Slow, steady | Main firing, soaking |
| Bamboo | 16-18 | Very fast | Small kilns, quick bursts |
| Charcoal | 28-32 | Moderate | Highest temperature work |
| Dried dung | 14-16 | Slow, smoky | Emergency fuel, low-temp only |
Loading the Kiln
Loading patterns directly affect firing results:
- Bone-dry ware only. Test by holding a piece against your cheek — it should feel warm (room temperature), not cool. Cool feeling means residual moisture.
- Leave circulation gaps. Every piece needs 1-2 cm clearance on all sides. Hot gases must reach every surface.
- Bottom-heavy loading. Place the densest, thickest pieces at the bottom. Thin, delicate pieces go higher where temperature is slightly lower and heat-up is gentler.
- Stagger rows. Never stack pieces directly behind each other in the airflow direction. Offset them so gases weave through the load.
- Protect glazed surfaces. Glazed pieces must not touch each other or shelves — they will fuse permanently. Use kiln stilts (small ceramic supports) or wads of fire clay.
- Place witness cones. Set pyrometric cones at front, middle, and rear of the kiln at eye-level spy holes.
The Seal
Brick up the loading door with dry-stacked bricks and seal joints with fire clay. Leave one or two small spy holes (2-3 cm diameter) at different heights for visual temperature monitoring. Plug these with small clay balls when not in use.
The Firing: Phase by Phase
Phase 1: Candling (Ambient to 120°C)
Duration: 2-4 hours Damper: Fully open Fire size: Small — a few sticks of kindling
Purpose: Drive the last traces of moisture from the ware and kiln structure. Water boils at 100°C, and steam expanding inside a clay wall generates enough pressure to shatter it. This phase must be gentle and thorough.
Technique:
- Light a small fire in the firebox using kindling
- Maintain a visible but small flame — you should be able to hold your hand at the stoke hole for a moment
- If you see steam rising from the chimney or condensation on the spy hole plugs, the kiln is still wet — continue candling
- A properly candled kiln shows dry, warm air at the chimney with no visible steam
The Newspaper Test
Hold a sheet of paper near the chimney top. If it dampens at all, there is still moisture in the system. Continue candling until the paper stays completely dry.
Phase 2: Smoking / Burnout (120-600°C)
Duration: 3-4 hours Damper: 3/4 open Fire size: Growing — medium splits, regular stoking
Purpose: Organic matter in the clay (roots, carbon, organic binders) burns out. This is visible as smoke — initially heavy and dark, gradually lightening as organics are consumed. By 500-600°C, the clay has passed through its critical chemical transformation (dehydroxylation) and is permanently changed from clay to ceramic.
Technique:
- Increase fire size gradually — add 3-4 medium splits every 15-20 minutes
- Target a rise rate of about 80-100°C per hour
- Do not rush this phase. Rising too fast causes carbon coring — a black core inside the clay body where organics were trapped before they could burn out. This weakens the finished piece permanently.
- At around 200-300°C, steam from chemically-bound water exits. You may hear faint hissing or popping — this is normal as long as you are rising slowly.
- By 573°C, quartz in the clay undergoes alpha-to-beta inversion, expanding by about 2%. Rise at no more than 100°C/hour through this zone.
Phase 3: Ramping (600-900°C)
Duration: 2-4 hours Damper: 1/2 to 3/4 open Fire size: Heavy — frequent stoking with medium splits
Purpose: Now that the critical low-temperature hazards are past, push the temperature upward steadily. The clay body is sintering — particles are fusing together, and the piece is becoming harder and more durable with each degree.
Technique:
- Stoke every 10-15 minutes with 4-6 medium splits
- Keep the firebox fuel bed 15-20 cm deep
- The kiln interior should glow dark red by the end of this phase (visible through the spy hole in a shaded environment)
- If temperature rise stalls, switch to smaller wood pieces and increase stoking frequency
- Monitor chimney exhaust — it should be clear or lightly hazy. Black smoke means incomplete combustion; open the damper slightly and ensure fuel pieces are not too large
Phase 4: High Fire (900°C to Target)
Duration: 3-6 hours (varies greatly by target temperature and kiln efficiency) Damper: 1/3 to 1/2 open for oxidation, 1/4 to 1/3 for reduction Fire size: Steady and consistent
Purpose: Reach and maintain the target temperature for your specific clay body and glaze.
Key temperatures:
| Temperature | Significance |
|---|---|
| 900°C | Minimum for functional earthenware |
| 1000°C | Standard earthenware, common brick |
| 1100°C | Hard earthenware, some stoneware |
| 1200°C | Stoneware, firebrick |
| 1250-1280°C | High-fire stoneware, fully vitrified |
| 1300°C+ | Porcelain range (requires exceptional fuel and kiln) |
Technique:
- This phase demands the most attention and the most fuel
- Stoke with uniform-sized splits every 8-12 minutes
- Each stoke should maintain or slightly increase temperature — never let it drop between stokes
- Close the stoke door between additions. Open door = heat loss
- Read the kiln color through spy holes (see color chart below)
- If pursuing reduction, restrict the damper and stoke with smaller, more frequent charges. Look for wisps of flame at the chimney top as confirmation.
The 900°C wall: Most kilns hit a plateau around 900°C. The quartz inversion is complete, and the kiln seems to resist further climbing. This is normal. Counter it by:
- Switching to thin, dry kindling (3-4 cm diameter)
- Stoking every 5-7 minutes
- Ensuring maximum draft (damper 3/4 open briefly)
- Maintaining this aggressive approach for 20-30 minutes until the kiln breaks through to 950°C+
Phase 5: Soaking (At Target Temperature)
Duration: 30-90 minutes Damper: 1/4 to 1/3 open Fire size: Maintenance — just enough to hold temperature
Purpose: Allow heat to equalize throughout the kiln. Even in well-designed kilns, temperature varies by 30-50°C between the hottest and coolest spots. Soaking gives the cooler areas time to catch up.
Technique:
- Reduce stoking frequency but maintain temperature — add 2-3 splits every 15-20 minutes
- Watch your cones. The target cone should be bending but not flat. If it goes flat quickly, you may be overshooting.
- Check spy holes at different levels — top, middle, bottom should all show similar color
Phase 6: Cooling
Duration: 12-48 hours depending on kiln size Damper: Closed Fire size: None
Purpose: Controlled cooling prevents thermal shock damage to the ware. Cooling too fast causes dunting — cracks that form as the ceramic contracts.
Critical cooling points:
| Temperature | Risk | Rate |
|---|---|---|
| Peak to 1000°C | Glaze solidification, can tolerate moderate rate | Natural cooling, damper closed |
| 1000-700°C | Cristobalite inversion (if present) | Slow — do not open kiln |
| 573°C | Quartz inversion (beta to alpha, contracts 2%) | Very slow — critical, do not disturb |
| 573-200°C | Gradual contraction | Natural cooling |
| Below 200°C | Safe to open | Crack door, wait 1 hour before unloading |
Patience Saves Ware
The single most common cause of cracked pottery is opening the kiln too early. A large kiln can take 36-48 hours to cool safely. Do not open the door until you can hold your hand inside without pain (below 60°C at the opening). Cracking the door at 300°C to “speed things up” will crack your best pieces.
Reading the Kiln Without Instruments
Color Temperature Guide
| Color Observed (through spy hole) | Temperature |
|---|---|
| Black (no visible glow) | Below 500°C |
| Faint red (only in total darkness) | 500-550°C |
| Dark red | 650-750°C |
| Cherry red | 750-850°C |
| Bright cherry | 850-950°C |
| Dark orange | 950-1050°C |
| Orange | 1050-1150°C |
| Light orange | 1150-1200°C |
| Yellow-orange | 1200-1280°C |
| Yellow | 1280-1350°C |
| Light yellow / white | Above 1350°C |
Other Indicators
- Chimney exhaust: Clear = good oxidation. Light haze = neutral. Visible smoke/flame = reduction or incomplete combustion.
- Sound: Steady low roar = good draft and combustion. Puffing = unstable draft. Sharp pings or cracks = ware failing (reduce ramp rate immediately).
- Fuel bed behavior: White ash = complete combustion. Black chunks = fuel too large or air too restricted.
Fuel Consumption Log Template
Track every firing. This data improves your efficiency over time.
| Data Point | Record |
|---|---|
| Date | |
| Kiln type and volume | |
| Ware loaded (type, quantity, total weight) | |
| Fuel type and estimated dry weight | |
| Firing start time | |
| Time to 600°C | |
| Time to 900°C | |
| Time to peak temperature | |
| Peak temperature (cone or color) | |
| Soak duration | |
| Total firing time | |
| Cooling time to unload | |
| Results (% good, % seconds, % loss) | |
| Notes and observations |
The Learning Curve
Expect your first 3-5 firings to be learning experiences with imperfect results. By firing 10, you should be consistently hitting your target temperature with predictable fuel consumption. By firing 20, you will be teaching others. Document everything — your kiln log is more valuable than any textbook.