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Kiln Cooling Rate Calculator

Calculate safe kiln cooling rates for each temperature zone, with critical guidance for the quartz inversion at 573°C to prevent dunting and thermal shock.

Updated May 2026

Clay Body Type

Peak Temperature (°C)

Cone 06=999°C, Cone 6=1222°C, Cone 10=1305°C.

Piece Wall Thickness (mm)

Thicker pieces need slower cooling.

Crystalline glaze (very slow cooling needed)

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Results

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Enter your measurements above and click Calculate.

Kiln Cooling: Why It Matters

Cooling is as important as heating. Most kiln cracking ("dunting") happens during cooling, not heating. The primary danger point is the quartz inversion at 573°C.

The Quartz Inversion at 573°C

At 573°C, crystalline silica (α-quartz) undergoes a sudden reversible phase transformation. On cooling, beta-quartz converts back to alpha-quartz with a sudden 2% volume decrease. If the kiln cools too fast through this zone, the rapid volume change creates enough stress to crack — or shatter — the ware. This is called dunting.

Safe Cooling Rate Zones

Temperature Zone	Max Rate (°C/hr)	Reason
Peak → 1000°C	150–200	Glaze liquid, safe to cool fast
1000 → 700°C	100–150	Glaze stiffening / freezing
700 → 600°C	80–100	Approach quartz inversion
⚠ 600 → 530°C	50–100	CRITICAL — quartz inversion at 573°C
530 → 200°C	100–150	Safe zone again
Below 200°C	Free cool	Can open kiln carefully

Thickness Effect

Thicker pieces retain heat unevenly — the outside cools faster than the interior. This thermal gradient creates internal stress. For pieces over 20mm thick, reduce all rates by approximately 25–40%.

Crystalline Glazes

Zinc silicate crystals are extremely fragile. Crystalline glaze programs control the entire cooling schedule, often including holds at specific temperatures to grow crystals. Never free-cool a crystalline kiln.

lightbulb Quick Tips

check_circle The 573°C quartz inversion zone (600–530°C) is the single most common cause of kiln damage. Never fast-cool through it — even if you've been using the kiln for years without problems.
check_circle A thick kiln shelf is itself vulnerable to thermal shock. Cool your loaded shelves at the same rate you would for your thickest piece, not the smallest.
check_circle For electric kilns, program a slow segment from 600°C to 500°C into your controller. Even if the kiln is switching off elements, residual heat from the mass of shelves and bricks often keeps it in the danger zone longer than you think.

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Frequently Asked Questions

What is the quartz inversion and why is it so important? expand_more

At 573°C, crystalline silica (quartz) in the clay body undergoes a sudden phase change from alpha to beta quartz (on heating) or beta to alpha (on cooling). This transformation involves a 2% volume change and occurs suddenly, not gradually. Cooling too fast through this zone causes the thermal shock known as "dunting" — cracking or shattering of ware.

How fast can I cool my kiln safely? expand_more

The general rule: no faster than 100°C/hr through the 600–530°C zone (quartz inversion). Thick pieces (>20mm) need 50–60°C/hr maximum. Above 800°C and below 530°C, faster rates are safe for most bodies.

Do I need to slow-cool for crystalline glazes throughout? expand_more

Yes. Crystalline glazes require very controlled, extremely slow cooling from peak temperature all the way through. The crystals that form on cooling are fragile and will crack if cooled unevenly. Many crystalline programs hold at multiple temperatures on the way down.

Can I open my kiln at 200°C? expand_more

Most studio potters open the kiln bung or lid slightly at 100–200°C and fully open at room temperature. Glazed work can show thermal shock at temperatures above 150°C if exposed to a sudden cold draft.