Common Glaze Defects and How to Fix Them

Glaze20 Apr 2026

Every potter has opened a kiln and found something unexpected. A glaze that crawled away from the surface. A network of fine cracks across an otherwise beautiful bowl. A pinhole right in the middle of a plate you were proud of. These things happen, and they're frustrating — but they're also informative. Glaze defects are diagnostic. Each one has a specific cause, and once you understand the mechanism, you can fix it.

This guide covers seven of the most common glaze defects. For each one, we'll explain what you're actually seeing, what caused it, and what to change next time.

A Quick Reference

Before diving in, here's a map of when each defect occurs in the process. This matters because the stage tells you where to look for the cause.

Defect	Stage	Primary cause
Crawling	Drying (glaze micro-cracks) or bisque (poor adhesion); retraction during early firing	High clay content in recipe; dirty bisque
Crazing	During cooling	Glaze CTE too high for the clay body
Shivering	During cooling	Glaze CTE too low for the clay body
Pinholing / Pitting	During firing	Gas escaping through an almost-sealed glaze
Blistering	During firing	Larger gas pockets sealed under the glaze
Underfiring	During firing	Kiln didn't reach maturity temperature
Colour running	During firing	Over-fluxed glaze, overfiring, or thick application

Crawling

What you see: The glaze has pulled back from areas of the clay surface, leaving bare patches. The glaze edges are sharp and the retracted areas may be beaded or ridged.

What actually happened: Crawling happens in two stages. First, during drying, the raw glaze layer develops micro-cracks — the glaze shrinks as it dries, and if there's enough shrinkage stress, the dry coat fractures before it ever reaches the kiln. Second, during early firing, these cracks open further. As the glaze begins to melt, its surface tension pulls the softening material away from the cracked edges rather than spreading it back flat. The result is retraction.

Root causes:

High raw clay content in the recipe — kaolin, ball clay, and bentonite all shrink significantly during drying. A recipe heavy in these materials will crack the dry coat reliably, regardless of water content. This is a formulation issue.
Contaminated bisqueware — oils from handling (fingerprints), kiln dust, or wax residue prevent the glaze from bonding to the surface. This is probably the most common cause potters encounter in practice.
Excessively thick application — a thick dry coat has more shrinkage stress. Over-dipping, slow dipping, or double-dipping are all contributors.

Prevention:

Calcine a portion of the kaolin or ball clay in the recipe before batching — this pre-shrinks the clay and dramatically reduces raw-glaze shrinkage.
Handle bisqueware with clean hands or gloves; wipe with a damp sponge just before glazing to remove dust.
Control application thickness — check your specific gravity before each session and aim for an even single coat.
A small addition of CMC gum strengthens the dry glaze layer, reducing crack formation during drying.

A note on water: Crawling is not caused by too little water in the glaze. Thick application (which too little water can contribute to) is a risk factor, but if your glaze crawls consistently at correct specific gravity, look at the clay content of the recipe or the cleanliness of your bisque first. Those are the real levers.

Crazing

What you see: A network of fine cracks in the fired glaze — like a shattered windscreen or a map. The cracks may appear immediately as the kiln cools, or they may develop days, weeks, or even months later after the piece is put into use. You can often hear a faint tinkling sound as a piece crazes.

What actually happened: Crazing is a thermal expansion mismatch between the glaze and the clay body. As the kiln cools below roughly 500–600°C, the glaze is a rigid glass and can no longer flow to relieve stress. If the glaze contracts more than the clay body during cooling — because its thermal expansion coefficient (CTE) is higher — it is placed under tension. When that tension exceeds the glaze's tensile strength, it fractures in a network of cracks. Delayed crazing happens the same way, but triggered by thermal cycling in everyday use (dishwashers, temperature changes between hot liquid and room temperature).

Root causes:

Glaze CTE too high relative to the clay body — typically caused by high alkali content in the glaze (sodium, potassium, and lithium all raise CTE significantly).
Insufficient silica or alumina — both lower CTE and increase durability.
Using a glaze formulated for a different clay body — a glaze that works perfectly on one clay may craze on another.
Underfired clay body — a body that hasn't reached maturity has different dimensional behaviour than a properly fired one.

Prevention:

Reformulate to lower the glaze CTE: reduce high-alkali fluxes (Na₂O, K₂O); substitute with lower-CTE fluxes like calcium (CaO) or magnesium (MgO); increase silica and alumina.
Always test a glaze on the specific clay body you intend to use it with — a glaze isn't glaze-body compatible in the abstract.
Fire to the correct maturity temperature for your clay body.
Slow cooling through 600–500°C helps borderline cases, but does not fix a significant CTE mismatch — that requires reformulation.

For functional ware: Crazed glazes on food surfaces are a genuine food safety concern. The cracks harbour bacteria that cannot be effectively cleaned. Decorative crazing on vases or sculptural pieces is a different matter — it can be beautiful and intentional — but craze-fix glazes should not be used on food surfaces.

Shivering

What you see: Chips or slivers of glaze flake off the clay body, sometimes spontaneously, sometimes after the piece has been in use. The fragments are sharp. On functional ware, this is a safety hazard.

What actually happened: Shivering is the compressive counterpart to crazing. If the glaze's CTE is lower than the clay body, the body contracts more than the glaze during cooling. The glaze ends up in compression. A glaze under slight compression is actually desirable — it's more durable than one under tension. But if the compressive stress is excessive, the glaze fails by spalling: fragments physically shear off the surface.

Root causes:

Glaze CTE significantly too low — excessive silica or alumina, or materials that strongly lower CTE (lithium compounds in particular: spodumene, petalite, lepidolite).
Overfired clay body — a very vitrified body may behave differently from the same body at its rated temperature.
Glaze applied too thick — more stored compressive energy means more risk of catastrophic shivering.

Prevention:

Reformulate to raise glaze CTE: add or increase alkali fluxes; reduce silica or high-silica materials. This is the opposite correction to crazing.
Fire to correct temperature.
Apply glaze at appropriate thickness.

Worth knowing: Shivering is less well-known than crazing and is sometimes misdiagnosed as crawling or poor adhesion. The distinction: crawling happens during firing and leaves the bare patch while the piece is still in the kiln; shivering happens during or after cooling and the flakes detach after the piece is out.

Pinholing and Pitting

What you see: Pinholing — small, sharp-edged holes penetrating through the glaze to the clay body, like pin pricks. Pitting — wider, shallower craters with a rounder edge. Both are variations of the same underlying mechanism.

What actually happened: During firing, both the clay body and glaze ingredients release gases — water vapour, carbon dioxide from carbonate decomposition (whiting, dolomite, magnesium carbonate), sulphur compounds, and gases from any organic material that wasn't burned out during bisque. This outgassing is normal. The problem occurs when gas is still escaping at the point where the glaze has become viscous enough to seal the surface but hasn't yet become fluid enough to heal itself. A gas bubble reaches the surface, bursts, and leaves a crater. If the kiln cools before the glaze flows back into the crater, it freezes in place.

Root causes:

Insufficient soak at peak temperature — without adequate hold time at peak, the glaze doesn't have time to self-heal. This is the single most controllable factor.
Bisque temperature too low — unburned organic material or carbonates remain in the body and outgas during the glaze firing at the wrong time.
Firing too fast through the carbonate decomposition range — allowing adequate time for gas to escape before the glaze seals is critical.
High-carbonate glaze materials — whiting, dolomite, and other carbonates release CO₂ during firing. A glaze heavy in these materials generates more gas and needs more time.
Organic contamination of bisqueware — fingerprints, dust, or paper marks produce gas when the kiln heats.

Prevention:

Hold (soak) at peak temperature for 15–30 minutes to allow the glaze to self-heal.
Some potters use a "drop and soak" — cooling slightly below peak temperature and holding there — to maximise healing time without overfiring the body.
Bisque to a temperature high enough to complete organic burnout (at least cone 06 / approximately 999°C is a commonly recommended starting point).
Fire slowly through the temperature range where carbonates decompose — this spreads the gas release over a longer period and reduces peak gas pressure.
Keep bisqueware clean.

Blistering and Bloating

These are related but distinct defects. It's worth knowing the difference.

Blistering

What you see: Bubbles or domes on the glaze surface — like pinholing but larger, sometimes with an intact bubble rather than a burst crater. The surface looks raised, and broken blisters leave rough, crater-like openings.

What actually happened: Similar mechanism to pinholing, but involving larger volumes of gas or a more viscous glaze that traps the gas rather than allowing it to escape. The glaze seals before all gas has escaped; the trapped gas inflates a dome. If the glaze is stiff enough, the dome may remain intact or rupture to leave a rough opening.

Sulphate-bearing materials in the glaze recipe — barium carbonate, strontium carbonate — can be sources of blistering gas at high temperatures. Very thick glaze application also concentrates the problem.

Prevention: Fire slowly through the temperature range where volatiles release; ensure adequate soak time; apply glaze at appropriate thickness; be cautious with sulphate-bearing materials.

Bloating

What you see: The clay body itself has internal voids, swelling, or distortion. Walls may look puffy or cracked from within. This is a body defect, not a glaze defect.

What actually happened: The clay body has been overfired to the point where glass phases form rapidly, trapping residual gases inside a semi-molten matrix. The body effectively becomes a foam. This is particularly associated with high-iron bodies in reduction firing (iron reduction significantly lowers the effective flux temperature) and with inadequate bisque leaving organics that outgas at the wrong stage.

Prevention: Do not exceed the rated temperature range of your clay body. Bisque adequately. In reduction firing, monitor iron-rich bodies carefully and be conservative about final temperature.

Underfiring

What you see: The glaze looks dry, rough, powdery, or undermelted — more like a sintered powder coat than a glass. Colours may be off, and the surface is often not smooth to the touch. The clay body may be pale, chalky, and weak.

What actually happened: The glaze (and clay body) did not receive sufficient thermal energy to complete the melt. At insufficient temperatures, the glaze batch sinters — particles bond at contact points — but the full eutectic fusion that creates a homogenous, waterproof glass layer is never achieved. The result is porous, structurally weak, and not suitable for functional ware.

Root causes:

The kiln did not actually reach the target temperature — thermocouple drift, element failure in electric kilns, or damper error in fuel kilns.
The ware was in a cold zone; the thermocouple location may read correctly while the actual shelf temperature is lower.
Dense kiln load without adequate soak time to reach equilibrium.
Using the wrong cone target for the glaze and body combination.

An important distinction: Underfiring is sometimes confused with a matte surface. A well-formulated matte glaze at correct temperature is smooth, consistent in colour, and has a particular tactile quality — velvety or stone-like. An underfired glaze is rough, powdery, and inconsistent. The diagnostic test: run a finger across the surface. Underfired glazes feel gritty and may even feel fragile.

Prevention:

Always use witness cone packs placed in the actual firing zone, not just at the thermocouple. A cone pack with one cone below target, the target cone, and one above is the standard approach.
Check thermocouple calibration regularly — thermocouples drift with age and use in electric kilns.
Extend the soak at peak temperature for dense kiln loads.
Pieces that are underfired can often be successfully refired.

Colour Running and Excessive Flow

What you see: The glaze has moved significantly during firing — drips, streaks, and runs down the walls, possibly pooling at the foot or adhering to kiln furniture. Adjacent glaze colours may have bled into each other.

What actually happened: All glazes flow to some degree at maturity. Running occurs when the glaze's viscosity at peak temperature is too low to stay in place on a vertical surface. The melt becomes thin enough that gravity overcomes surface tension and the glaze slides. Colour bleeding between adjacent areas happens when both glazes are fluid enough for their boundary to become mobile and the two melts intermix.

Root causes:

Over-fluxed formulation — too much alkali, alkaline earth, or other fluxes relative to alumina and silica.
Overfiring — even a well-balanced glaze becomes excessively fluid if fired significantly beyond its intended cone.
Glaze applied too thick — more mass means more flow.
High colourant additions — some metal oxides act as supplementary fluxes. Iron oxide in particular at high concentrations, especially in reduction, can dramatically increase fluidity.

Prevention:

Reformulate to increase alumina — this raises melt viscosity without dramatically raising CTE.
Apply at correct thickness; consistent specific gravity control matters here.
Fire to the correct cone for the specific glaze.
When testing an unfamiliar glaze, use a catch tray or sacrificial tile underneath the piece — kiln shelf damage from running glaze is expensive.
For intentionally fluid or flowing glazes, apply only to the upper two-thirds of the piece, leaving the lower section unglazed or in a stable base glaze.

Diagnosing Your Own Kiln

When something goes wrong, the sequence above is your first diagnostic tool. Ask:

When did it happen? Crawling starts in the drying stage. Crazing and shivering happen during cooling. Running and pinholing happen during firing.
Is it the glaze recipe or the process? Crazing and crawling from clay content are recipe issues. Pinholing from insufficient soak is a process issue. Many defects can be either.
Is it consistent or random? A defect that happens on every pot points to a recipe or systematic process problem. A defect that appears on one pot in a full kiln suggests localised issues — position in the kiln, contamination, or thickness variation.

Keeping records is the most underrated habit in studio pottery. Note your specific gravity, application method, kiln position, firing schedule, and results every time. Patterns that are invisible in your memory become obvious in a notebook.

For more on glaze recipes and how to read them, see Reading a Glaze Recipe and Glazes 101.