Troubleshooting

Every potter encounters glaze defects. The key to solving them is understanding whether the root cause is application, chemistry, or firing — because the fix for each is completely different. Below, each defect is listed with causes ranked by likelihood and specific fixes.

What it looks like: Molten glaze pulls away from areas of the pot during firing, forming beads or islands of glaze with bare clay patches between them.

Causes (ranked by likelihood)

1. Dusty, greasy, or contaminated bisqueware — oils from handling or dust create barriers that prevent adhesion. Hamer notes that crawling often begins with poor dry-glaze adhesion before the kiln gets hot.^[1][2]
2. Excessive drying shrinkage — glazes with high raw clay content (especially uncalcined kaolin or ball clay) shrink excessively as they dry, cracking the raw glaze layer.^[3]
3. Excessively thick application — thick layers crack more during drying and take longer to melt through.^[1]
4. High surface tension in the melt — zirconium, tin oxide, and high zinc increase surface tension, causing the melt to bead up rather than wet the body.^[3]
5. Thickened/evaporated slurry — a slurry that has thickened by evaporation is more likely to crawl because the melted glaze prefers to pull together.^[1]
6. Highly alkaline glazes — Rhodes notes these can have high surface tension when melted, making them vulnerable to crawling even while being fluid enough to run.^[4]

Fixes

• Clean bisqueware thoroughly before glazing (damp sponge wipe)
• Apply glaze at appropriate thickness (thinner coats)
• Calcine high-shrinkage materials — replace a portion of raw kaolin with calcined kaolin
• Add a small amount of gum (CMC) to improve raw-layer adhesion
• Reduce zirconium and tin oxide if possible
• Recondition thickened slurry — don't just add water; re-sieve and check SG

Cone/kiln notes: More common in matte glazes (higher alumina = higher viscosity/surface tension) and high-clay low-fire glazes. Some potters intentionally exploit crawling as a decorative effect with purpose-formulated crawl glazes.

What it looks like: Pinholes are tiny holes (about pinhead size) in the fired glaze surface, sometimes penetrating to the clay body. Bloating is a more severe form where trapped gas causes the clay body itself to swell.

Causes (ranked by likelihood)

1. Incomplete burnout of organic matter or carbonates — gases escape from the clay body during glaze firing, after the glaze surface has sealed. Steve Davis (Aardvark Clay) identifies incomplete carbon burnout as a shared root cause of bloating, black coring, pinholing, and blistering.^[3][2][5]
2. Insufficient soak time at peak temperature— the glaze doesn't have enough time in its fluid state to heal over burst bubbles.^[3]
3. Excessively fast ramp through 600–900°C— volatiles don't escape before the surface seals.^[6]
4. Thick glaze application — more material to outgas through.
5. Rough, grogged, or trimmed body surfaces — pinholes often form when glaze dries over surface voids from trimming or coarse grog.^[3]

Fixes

• Slow bisque firing, especially through 600–900°C burnout zone
• Add a soak at peak glaze temperature— 10–30 minutes allows the surface to heal. The “drop-and-hold” technique (fire slightly past peak, then drop temperature and soak) is effective for some glazes.
• Apply glaze at appropriate thickness
• Smooth rough trimmed areas before glazing
• Ensure proper kiln ventilation during early firing

Cone/kiln notes: Low-fire (cone 06) is more prone because bisque and glaze temps are close together. Gas kilns in reduction need extra attention to early ventilation. High-fire stoneware is less prone if properly bisqued.

What it looks like: A network of fine cracks in the glaze surface, similar to crackle in old china.

Primary cause: thermal expansion mismatch.The glaze's coefficient of thermal expansion (CTE) is higher than the clay body's. As the kiln cools, the glaze contracts more than the clay, putting the glaze under tension until it cracks.^[3][1][7]

Process tweaks (slow cooling, thinner application) are secondary mitigations only. The primary cure is changing the glaze chemistry to fix the expansion mismatch.

Contributing factors

1. Too much sodium, potassium, or lithium in the glaze formula (high-expansion fluxes).^[7]
2. Thick application — thicker glaze has less body compression acting on it.^[8]
3. Underfired glaze— silica hasn't fully dissolved into the glass matrix.
4. Delayed crazing — appears weeks or months after firing. Hamer explains that porous earthenware absorbs atmospheric moisture and expands permanently, while the glaze does not.^[1]
5. Thermal shock — rapid cooling through quartz inversion (~573°C).

Primary fixes (chemistry)

• Increase silica (flint/quartz) in the glaze — lowers thermal expansion
• Increase alumina (add more clay/kaolin to the recipe)
• Decrease high-expansion fluxes: reduce sodium, potassium, lithium
• Substitute low-expansion fluxes: increase calcium (whiting), magnesium (talc, dolomite), zinc, or barium

Secondary mitigations (process)

• Apply glaze thinner
• Fire to full maturity
• Slow the cooling rate through quartz inversion (~573°C)

Why it matters for functional ware: Digitalfire reports measured strength losses of roughly 300–400% in freshly fired ware with crazed glazes — cracks act as failure initiation sites. Crazed surfaces also harbor bacteria and can allow leaching of colorants.^[3][9]

Intentional crazing: Some traditions (raku, Chinese Guan/Ge ware) use crazing decoratively, rubbing ink or tea into the cracks. This is an aesthetic choice, not a defect, in non-functional ware.

What it looks like: The opposite of crazing — the glaze lifts off the body in sharp, razor-edged flakes or slivers. This is a safety hazard for functional ware because glass slivers can end up in food or drink.

Hamer states plainly: shivering is the worse defect because sharp flakes of glaze detach from the pot, and the result is dangerous on table and kitchen ware.^[1]

Cause:Glaze CTE is much lower than the body's, putting the glaze under excessive compression. The glaze can't stretch enough and pops off. Compression concentrates at sharp edges, lips, and rims.^[3][6]

Fixes

• Decrease silica in the glaze (raises expansion)
• Increase high-expansion fluxes — more soda feldspar or nepheline syenite.^[10]
• Apply glaze thinner, especially on edges
• Round off sharp edges on forms (compression concentrates at sharp angles)

Note: A small amount of compression is actually desirable— it makes the glaze stronger and more durable (like tempered glass). The problem is when compression exceeds the glaze's strength.^[8]

What it looks like: Glaze becomes too fluid during firing and flows down vertical surfaces, pooling at the base and potentially fusing the piece to the kiln shelf.

Causes (ranked by likelihood)

1. Overfiring— firing beyond the glaze's intended cone makes it too fluid.^[3][2]
2. Excessively thick application — more material + gravity = more flow.
3. High flux / low alumina recipe — too much flux creates an overly fluid melt. Dramatic cone-6 running often traces to high sodium and lithium coupled with low silica.^[11]
4. Layered glazes creating eutectics — two glazes can create a lower melting point at their interface, causing excessive flow even at the correct temperature.
5. Highly alkaline glazes — can be both runny and prone to crawling simultaneously.^[4]

Fixes

• Fire to the correct cone (don't overfire)
• Apply glaze thinner, especially on the lower half of vertical pieces
• Reduce flux / increase alumina or silica in the recipe
• Test layered combinations on vertical test tiles before committing
• Keep the bottom ¼ inch of the pot glaze-free as a safety margin
• Use a catch tray (bisque dish or kiln shelf cookie) under pieces with known runny glazes

Cone/kiln notes: Especially common with combination/layered glazes at cone 6 — the eutectic effect makes individually-stable glazes very runny when layered. Many cone 6 potters routinely use kiln cookies as insurance.

What it looks like: Uneven color across a piece, or inconsistent results between firings.

Causes (ranked by likelihood)

1. Uneven glaze thickness — thicker areas appear darker/more saturated; thinner areas lighter.^[3]
2. Poorly mixed glaze— Rhodes warns that heavier materials settle, and if the bucket isn't stirred frequently, portions of glaze used have the wrong composition.^[12]
3. Cross-contamination— copper is notorious for “traveling” in the kiln. Even tiny amounts create green spots on other glazes. Manganese and chrome are also contaminators.^[3]
4. Kiln atmosphere/placement variation — different locations experience different heat, airflow, or reduction.
5. Batch variation — different batches of raw materials can have slightly different chemistry.^[2]

Fixes

• Apply at consistent thickness (use SG measurement)
• Stir/remix glaze thoroughly before each use
• Sieve regularly to ensure even colorant distribution
• Clean tools between colors
• Document kiln placement and correlate with results
• In shared studios, fire copper-glazed pieces separately or where drips won't reach other work

What it looks like:Large bubbles (larger than pinholes) form in the glaze surface, often leaving a “moonscape” of burst craters.

Causes (ranked by likelihood)

1. Excessively thick application — more material = more trapped gas. Rhodes specifically notes blisters form where glaze pools on the inside bottom of bowls.^[13][14]
2. Underfired bisque / body outgassing — incomplete organic burnout means gases escape through sealed glaze at peak temperature.^[2][5]
3. Volatile flux components — sodium oxide and borate frits become volatile above ~1200°C.^[14]
4. Residual moisture — pieces not fully dry before glaze firing.
5. Excessively fast firing — gases released too quickly.

Fixes

• Apply glaze thinner (especially watch bowl interiors)
• Ensure pieces are completely dry before loading
• Slow the firing, especially in the upper temperature range
• Add a soak at peak temperature to allow bubbles to heal
• Some potters find slower cooling helps more than a peak soak — gives the glaze more time in its fluid state to heal
• Ensure thorough bisque firing

Cone/kiln notes: Low-fire more susceptible. Rhodes notes that reduction makes lead-glaze blistering more likely — fritted lead glazes are less prone than raw lead.^[13]

What it looks like: A rough, matte-looking glaze that should be glossy. The glaze particles have sintered but not fully fused into smooth glass.

Causes (ranked by likelihood)

1. Kiln didn't reach target temperature — element degradation in electric kilns, insufficient gas pressure in gas kilns.^[2]
2. Wrong glaze for the cone range — e.g., a cone 10 glaze fired to cone 6.
3. Cold spots in the kiln — uneven heat distribution (bottom, near doors).
4. Insufficient flux in the formula — Pitelka notes that if the kiln reached temperature but the glaze still looks dry, the formula needs more effective fluxing.^[15]
5. Too-thin application — not enough material to form a continuous glass layer.

Fixes

• Verify kiln temperature with witness cones (not just the controller)
• Check kiln elements (electric) regularly — they degrade over hundreds of firings
• Ensure glaze matches your firing temperature
• Refire — often the simplest fix. Most underfired glazes fire perfectly on a second pass at the correct temperature
• If the formula is the problem, add flux or reformulate for your cone

Carbon Coring / Black Coring: Dark discoloration in the center of the clay body cross-section, caused by incomplete carbon burnout during bisque. Common with dense bodies, fast bisques, and kilns with poor early oxidation. Fix: slower bisque with good ventilation through the 600–900°C range.^[5]

Dunting: Cracking of the fired piece due to thermal shock during cooling, particularly through quartz inversion (~573°C) and cristobalite inversion (~226°C). Fix: slow cooling rate, especially through these critical temperature points.

Leaching (Food Safety):Unstable glaze chemistries can release colorant metals (copper, lead, barium, manganese) into food and drink, especially in acidic conditions. Digitalfire distinguishes watertightness from chemical durability — “doesn't leak water” is not the same as “food-safe glaze chemistry.” Crazing makes leaching worse because cracks expose more surface area and porous clay can harbor contamination. Functional ware should have a dense body (typically under ~1% porosity) and a stable, well-fitted glaze.^[16][9]