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Glaze Fit Calculator (COE Mismatch)

Predict crazing or shivering risk by comparing the Coefficient of Thermal Expansion (COE) of your clay body and glaze.

Updated May 2026

Clay Body COE (× 10⁻⁶/°C)

Stoneware: 5.5–6.5 · Porcelain: 5.0–5.8 · Earthenware: 6.0–7.5

Glaze COE (× 10⁻⁶/°C)

Typical glaze range: 5.5–7.5. Check glaze data sheet or calculate from UMF.

Firing Range

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Results

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Enter your measurements above and click Calculate.

The Physics of Crazing and Shivering

All materials expand on heating and contract on cooling. The Coefficient of Thermal Expansion (COE) measures how much a material expands per degree. In ceramics, the critical question is whether the glaze and clay body contract at the same rate.

Crazing vs Shivering

Condition	Glaze State	Defect
Glaze COE > Body COE	Under tension	⚡ Crazing — network of fine cracks
Glaze COE slightly < Body COE	Slight compression	✓ Ideal — durable surface
Glaze COE much < Body COE	Severe compression	🪨 Shivering — glaze pops off

The Ideal State

A glaze in slight compression (glaze COE 0.1–0.3 lower than body COE) is the most stable. The compressed glaze cannot crack in tension. This is why glaze and body "fit" matters so much.

Typical COE Ranges

Material	COE (× 10⁻⁶/°C)
Vitrified porcelain body	5.0 – 5.8
Stoneware body (Cone 6–10)	5.5 – 6.5
Earthenware body	6.0 – 7.5
Typical glaze (mid-fire)	5.5 – 7.0

Fixing Crazing

Reduce alkali fluxes (Na₂O, K₂O, Li₂O) — they raise COE significantly
Increase silica (SiO₂) — lowers COE
Raise the firing temperature to increase body vitrification and its COE
Use a clay body with a higher COE

lightbulb Quick Tips

check_circle Crazing that appears only after dishwasher cycles or repeated hot/cold use is a sign of marginal COE mismatch — it's just barely passing fresh from the kiln, but failing under thermal stress.
check_circle A slight compression (glaze COE slightly below body COE) is the target state. Don't aim for exact matching — aim for your glaze to be 0.1–0.3 units lower.
check_circle Crazing at rims and foot rings first is classic glaze tension. These areas are thin and cool faster, amplifying the mismatch stress exactly where the clay body is under mechanical stress from use.

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Frequently Asked Questions

What causes crazing on pottery? expand_more

Crazing occurs when the glaze has a higher coefficient of thermal expansion (COE) than the clay body. On cooling, the glaze contracts more than the body, putting it under tension. When the tension exceeds the glaze's strength, it cracks in the characteristic crazed network pattern.

What is the difference between crazing and shivering? expand_more

Crazing (glaze cracks) occurs when glaze COE is higher than the body COE. Shivering (glaze pops off in flakes or chips) occurs when glaze COE is much lower — the glaze is under too much compression and physically spalls off the surface.

How do I find the COE of my clay body? expand_more

COE is typically listed on manufacturer data sheets for commercial clay bodies. For homemade bodies, calculate the COE from the oxide percentages using the Seger factor method, or test empirically using a dilatometer.

My pottery only crazes after washing — why? expand_more

Delayed crazing usually means the mismatch is marginal. Thermal cycling from hot water (dishwasher) stresses the glaze repeatedly until it finally cracks. A borderline mismatch that looks fine fresh from the kiln often reveals itself through repeated use.

Can I fix a crazing glaze without changing the recipe? expand_more

Sometimes. Increasing the firing temperature or extending the hold at peak temperature increases body vitrification, which slightly raises the body COE. This can correct a small mismatch without reformulating the glaze.