Cooling
Part of Kiln Design
Managing kiln cool-down rates after firing to prevent cracking.
Why This Matters
The firing phase of a kiln cycle gets most of the attention — temperatures, atmospheres, hold times — but the cooling phase is where the majority of pottery losses occur. A pot that survived a flawless rise to 1,000°C can shatter into fragments during an uncontrolled cool-down. In a rebuilding scenario where clay, fuel, and labor are precious, losing half a kiln load to cooling cracks is a catastrophe you cannot afford.
Thermal shock is the enemy. Clay bodies and glazes expand when heated and contract when cooled. If different parts of a pot cool at different rates — the outside faster than the inside, the thin rim faster than the thick base — the resulting stress tears the piece apart. Crystalline changes in silica (quartz) at specific temperatures create sudden volume shifts that demand especially careful handling.
Understanding cooling science lets you design firing schedules that protect your work through the most dangerous phases. Whether you are cooling a pit kiln by simply leaving it buried overnight or managing a downdraft kiln with damper adjustments, knowing what happens inside the kiln during cool-down — and when the critical danger points occur — is the difference between a successful firing and a pile of shards.
The Physics of Cooling Stress
When a kiln begins to cool, heat leaves the structure from the outside in. The kiln walls radiate heat to the surrounding air, the outer surfaces of pots lose heat to the kiln atmosphere, and the interior of each pot retains heat longest. This creates a temperature gradient through the wall thickness of every piece.
As the outer surface contracts (cooling and shrinking), it pulls against the still-hot, still-expanded interior. This puts the surface in tension and the interior in compression. If the temperature difference exceeds the clay body’s ability to flex, a crack propagates from the surface inward.
Thermal conductivity of clay is low. Fired clay conducts heat roughly 100 times slower than metal. A pot with 8mm walls can easily have a 50°C difference between inner and outer surfaces during rapid cooling. That gradient is enough to crack most earthenware bodies.
Thicker pieces are more vulnerable. The temperature gradient is proportional to wall thickness. A thin-walled cup survives cooling rates that would destroy a thick storage jar. This is why uniform wall thickness is stressed in pottery instruction — it is not just aesthetics, it is survival during cooling.
| Factor | Effect on Cracking Risk |
|---|---|
| Wall thickness | Thicker = higher risk (greater gradient) |
| Wall uniformity | Uneven = higher risk (differential stress) |
| Cooling rate | Faster = higher risk |
| Piece size | Larger = higher risk |
| Clay body | Coarse grog = lower risk (absorbs stress) |
Critical Temperature Zones
Not all parts of the cooling curve are equally dangerous. Two specific temperature ranges demand slow, controlled cooling:
The Quartz Inversion (573°C / 1,063°F)
This is the single most dangerous moment in any firing cycle. At 573°C, the crystalline silica (quartz) present in virtually all clay bodies undergoes a phase change from beta-quartz to alpha-quartz. This transformation causes a sudden 2% volume contraction.
Two percent sounds small, but it happens instantaneously throughout the clay body. If the outer surface of a pot has already passed through 573°C while the interior has not, the outside has shrunk 2% while the inside has not — and the pot cracks. This is the origin of the characteristic “dunting” crack: a clean break, often through the thickest part of the piece, that appears during cooling even though the pot looked perfect at peak temperature.
How to manage it: Slow the cooling rate to no more than 50°C per hour when passing through the 550-600°C range. In a kiln with dampers, close them partially to retain heat. In a pit kiln, keep the covering intact and do not disturb it.
The Cristobalite Inversion (220°C / 428°F)
If your clay body or glaze contains cristobalite (common in stoneware and porcelain, or in bodies that have been fired to high temperatures multiple times), a second phase change occurs at approximately 220°C with a 3% volume shift. This is even more dangerous than the quartz inversion because it occurs at a lower temperature when potters often assume the danger has passed.
How to manage it: Maintain a cooling rate of 30-50°C per hour through the 200-250°C range. Do not open the kiln until well below 150°C.
The Glaze Fit Zone (600-400°C)
As glazed pots cool, the glaze and clay body contract at different rates. If the glaze contracts more than the body, it goes into tension and develops fine cracks called “crazing.” If the body contracts more than the glaze, the glaze goes into compression — which is actually desirable, as compression strengthens the glaze surface.
Cooling rate through this zone affects glaze fit. Slower cooling allows more complete stress relaxation and generally produces better glaze surfaces. Extremely fast cooling can cause immediate crazing even in well-formulated glazes.
Cooling Schedules by Kiln Type
Pit Kiln
The simplest cooling schedule: do nothing. A properly covered pit kiln cools slowly enough on its own because the earth insulation limits heat loss.
- After the final fuel addition and the burn-down period, ensure the pit is fully covered with earth or sand (minimum 10 cm / 4 inches).
- Do not disturb the covering for at least 12 hours for small loads, 24 hours for large ones.
- After the wait period, brush away covering material and test a pot at the edge by touching it briefly. If it is too hot to hold comfortably (above ~60°C), re-cover and wait another 6 hours.
- Remove pots one at a time from the edges inward. Set them on a dry surface — never on cold or wet ground, which can cause thermal shock even at relatively low temperatures.
The Temptation to Peek
The single most common cause of cooling losses in pit kilns is impatience. Opening the covering “just to check” introduces cold air that can drop surface temperatures 100°C in seconds. Every time you open and re-cover, you risk everything in the kiln.
Updraft Kiln
Updraft kilns cool faster than pit kilns because their vertical structure creates a natural chimney effect even after the fire is out. Active management is needed:
- At end of firing: Close the firebox opening completely with bricks and seal cracks with wet clay. Close or heavily damper the chimney opening.
- First 2 hours: Leave fully sealed. The kiln temperature will drop from peak to approximately 800°C.
- Hours 2-6 (quartz inversion zone): Keep sealed. Natural heat loss through the walls will cool the kiln at approximately 40-60°C per hour — an acceptable rate. If your kiln is poorly insulated and cooling faster (you can feel intense heat radiating from the walls), add external insulation by packing earth or sand against the exterior.
- Hours 6-12: The kiln passes below 500°C. You may slightly crack the damper (1-2 cm) to allow gentle air circulation, which helps equalize temperatures within the kiln.
- Hours 12-18: Open the damper halfway. Temperature should be below 250°C.
- Hours 18-24: Open the damper fully and crack the firebox door. Once you can reach inside and touch a pot without burning (below 60°C), unload.
Downdraft Kiln
Downdraft kilns offer the most control over cooling because their damper systems regulate airflow precisely:
- End of firing: Close primary air intake. Set damper to 25% open.
- First 4 hours (above 600°C): Reduce damper to 10% open. Target cooling rate: 60°C/hour.
- Hours 4-8 (600-400°C, quartz inversion zone): Close damper to 5% or fully closed. Target: 40°C/hour maximum.
- Hours 8-14 (400-200°C): Open damper to 15%. Target: 30°C/hour if cristobalite is present in your clay body.
- Hours 14-20 (200°C to room temperature): Open damper to 50%, then fully. Crack the door after hour 16.
- Unload when interior temperature is below 60°C.
Managing Cooling Without a Thermometer
In a rebuilding scenario, you likely do not have a pyrometer or thermocouple. Use these proxy methods to estimate kiln temperature during cooling:
Color observation. Peer through the spy hole (a small opening left in the kiln wall for observation). If the interior glows any shade of red or orange, the temperature is above 500°C and you are still in the danger zone. Once no glow is visible in a darkened space, you are below approximately 500°C.
| Interior Appearance | Approximate Temperature |
|---|---|
| Bright orange glow | 900-1,000°C |
| Cherry red glow | 700-800°C |
| Faint dark red (visible only in darkness) | 500-600°C |
| No visible glow | Below 500°C |
| Can hold hand at spy hole for 5 seconds | Below 250°C |
| Wall exterior warm but touchable | Below 100°C |
The paper test. Hold a piece of dry paper or thin wood near the spy hole. If it scorches or ignites, the temperature is above 250°C (the ignition point of paper is approximately 230°C). Once paper does not scorch, the cristobalite danger zone has passed.
The spit test. Spit on the exterior wall of the kiln. If it sizzles and evaporates instantly, the wall surface is above 100°C and the interior is much hotter. When spit merely dries slowly, the kiln is approaching safe unloading temperature.
Troubleshooting Cooling Defects
Dunting Cracks
Appearance: Clean, sharp cracks running through thick sections. Often a single crack splitting a pot in two. Cause: Too-fast cooling through 573°C quartz inversion. Fix: Slow the cooling rate between 600°C and 500°C. Add grog (crusite, ground fired pottery) to your clay body — 10-20% grog by volume reduces quartz inversion stress significantly.
Crazing
Appearance: Fine network of cracks in the glaze surface, sometimes visible only when the pot is wet or stained. Cause: Glaze thermal expansion coefficient higher than the clay body. Worsened by fast cooling through 600-400°C. Fix: Slow cooling through the glaze-set zone. Reformulate glaze to lower its expansion (reduce sodium/potassium, increase silica). Adding silica to the glaze lowers its expansion coefficient.
Shivering
Appearance: Glaze flakes lifting off the pot surface in curved chips. Cause: Opposite of crazing — glaze in excessive compression. Fix: This is a glaze formulation problem more than a cooling problem, but slower cooling can reduce severity. Increase the flux content of your glaze slightly.
Sound Test for Hidden Cracks
Tap each pot gently with a wooden stick or knuckle after unloading. A clear ring indicates an intact piece. A dull thud or buzz suggests an internal crack that is not yet visible — set that piece aside and do not use it for liquid storage, as it will eventually fail.
Record Keeping
Even without written records, develop a system for tracking cooling outcomes. Mark pots from each firing with a scratch mark indicating the batch. When cracks appear (sometimes days or weeks later), you can trace them back to the specific firing and identify what went wrong. Over time, this feedback loop is how you develop a reliable cooling schedule for your specific kiln, clay body, and conditions.