Pinholes, crazing, crawling and unexpected finishes can result from more than the glaze alone. Your clay body, application technique, firing schedule and kiln performance all influence the finished surface.
This guide explains how to recognise common glaze faults, investigate their causes and test practical changes before your next production firing.
1. Pinholing: Tiny holes in the glaze
Symptoms
Small holes or tiny craters appear in the fired glaze, either individually or across larger areas.
Possible causes
Pinholing often occurs when escaping gases form bubbles that do not heal before the glaze solidifies. Contributing factors include:
- Gases released by the clay body or glaze during firing.
- Incomplete burnout during bisque firing.
- Excessively thick glaze application.
- A firing schedule that gives insufficient time for gases to escape and the surface to heal.
- A clay and glaze combination that produces more gas during firing.
Checks and prevention
Review your bisque firing and glaze thickness first. A controlled bisque firing with sufficient time for burnout can help reduce gas-related faults.
An appropriate soak or controlled cooling stage may help the glaze surface heal, but test any schedule change carefully. Additional heatwork can also cause overfiring or running.
If pinholing repeatedly affects one combination, test the same glaze on another clay body to help identify whether the issue relates to the clay, glaze or their interaction.
2. Crazing: Fine cracks across the glaze
Symptoms
A network of hairline cracks develops across the glaze. It may appear immediately or days, weeks or months after firing.
Possible causes
Crazing is generally a glaze-fit problem. If the glaze contracts more than the clay body during cooling, it can be placed under tension and crack.
- A mismatch between the thermal expansion of the clay and glaze.
- A glaze formulation incompatible with the chosen clay.
- An insufficiently matured clay body, which may contribute to moisture-related expansion and delayed crazing.
- Changes in glaze thickness or firing conditions.
Checks and prevention
Establish a compatible clay and glaze combination. Check that both materials reach their recommended firing maturity and test alternative bodies where necessary.
Changing the firing temperature alone may not resolve a glaze-fit problem. Allow time to assess delayed crazing rather than judging a test only when it leaves the kiln.
For functional ware, crazing can make cleaning more difficult and may expose porous clay beneath the glaze. Do not assume a crazed surface is suitable for food contact without appropriate assessment.
3. Crawling: Glaze pulling away from the surface
Symptoms
Bare patches of clay are surrounded by raised, rounded areas of glaze rather than a continuous coating.
Possible causes
- Dust, grease or other contamination on the bisque.
- Excessively thick application.
- Poor adhesion between the glaze and clay.
- Applying further coats before earlier coats have dried sufficiently.
- Excessive shrinkage or cracking of the unfired glaze layer.
Checks and prevention
Ensure bisque ware is clean, dry and free from dust. Follow the manufacturer's application instructions and allow coats to dry appropriately before adding further layers.
Check areas where glaze collects, including internal corners and around handles. Localised build-up can increase the risk of crawling.
4. Shivering: Glaze flaking or chipping away
Symptoms
Small flakes or chips of glaze detach, particularly around rims and edges. The glaze may appear to lift away from the body.
Possible causes
Shivering may occur when a thermal expansion mismatch places the glaze under excessive compression. It is effectively the opposite of crazing.
- An incompatible clay and glaze combination.
- Excessive compression in the fired glaze.
- Changes in clay or glaze formulation that affect fit.
Soluble salts may be present on clay ware and these salts migrate through moisture evaporation to edges of ware such as rims. During the bisque firing the salts cause localised composition changes which disrupt the glaze fit on subsequent glaze firings.
Checks and prevention
Review clay and glaze compatibility and the firing conditions. Changing thickness or temperature may not resolve the underlying problem; a different formulation or clay body may be needed.
In the workshop, ensure water is regularly replaced to prevent soluble salts becoming concentrated in water used for making and sponging clay ware. Consider adding barium carbonate to clay reclaim (1 gram per 1000 grams of clay), especially where reclaim is dried on plaster of paris batts (calcium sulphate).
Pieces showing shivering should not be used for food or drink because glaze fragments can detach during use.
5. Blistering: Raised bubbles and larger craters
Symptoms
Raised bubbles or blisters form in the glaze. Some burst, leaving larger craters or rough areas.
Possible causes
Blistering can develop when gases become trapped beneath molten glaze or when the glaze receives excessive heatwork.
- Excessive firing temperature or heatwork.
- Gases released by the clay body.
- Excessively thick glaze application.
- An incompatible clay and glaze combination.
- A schedule that does not allow bubbles to clear before cooling.
Checks and prevention
Check the recommended firing ranges for the glaze and clay. Review application thickness and whether the bisque firing allows sufficient burnout.
Test a revised firing schedule before making larger material changes. Distinguish small pinholes from raised blisters and larger craters to help narrow down the fault.
6. Running: Excessive glaze movement
Symptoms
Glaze flows further down the piece than intended, collects at the base or fuses the ware to the kiln shelf.
Possible causes
Some glazes are designed to move. Excessive movement can also result from:
- Too much glaze.
- Firing above the recommended range.
- Additional heatwork from a prolonged soak.
- Layered glazes becoming more fluid when combined.
- Reactive glazes applied too close to the base.
Checks and prevention
Follow the recommended thickness and firing range. Leave sufficient clearance above the base when testing new glazes or combinations, and use suitable test tiles or kiln protection.
Two glazes that share a firing range do not necessarily remain stable when layered. Test the combination before applying it to a production batch.
7. Unexpected colour or finish
Symptoms
The glaze fires lighter, darker, duller, glossier or a different colour from the expected result.
Possible causes
- Firing temperature and total heatwork.
- Oxidation or reduction atmosphere.
- Application thickness and technique.
- The colour and composition of the clay body.
- Cooling rate.
- Interactions between layered glazes.
Checks and prevention
Start with the manufacturer's recommendations and test on the clay body you intend to use. Compare application, firing schedule and kiln conditions with earlier successful firings.
The underlying clay can influence the appearance, particularly with translucent glazes. A glaze shown on white porcelain may look different on darker or iron-rich stoneware.
8. Underfired glaze: An undeveloped surface
Symptoms
The surface appears dry, rough, grainy or unexpectedly matt, or lacks its intended colour and finish.
Possible causes
An underfired glaze has not received sufficient heat work to develop its intended characteristics. Possible reasons include:
- Firing below the recommended range.
- A schedule that is too short.
- Uneven temperature distribution within the kiln.
- A controller reading that does not represent the heatwork received by the ware.
Checks and prevention
Check the firing range and use witness cones to assess the heatwork achieved. Kiln loading, element condition, thermocouple accuracy and firing duration can affect the result.
If results differ consistently between the top and bottom of the kiln, investigate temperature distribution before changing the glaze.
An intentionally matt glaze is not automatically underfired. Compare the result with the manufacturer's intended finish.
Quick reference: where to start
- Pin holing: check bisque firing and glaze thickness.
- Crazing: check clay and glaze compatibility.
- Crawling: check bisque cleanliness, adhesion and application.
- Shivering: check clay and glaze compatibility.
- Blistering: check firing schedule and application thickness.
- Running: check thickness, layering and firing heatwork.
- Unexpected colour: compare test tiles and firing records.
- Underfired finish: check witness cones and the firing schedule.
Check the clay body, not just the glaze
Clay composition influences firing behaviour and glaze interaction. Vitrification is particularly important for functional ceramics: as a body matures, it becomes denser and generally less porous.
A glaze can appear fully developed while the clay remains insufficiently matured and retains higher water absorption. Conversely, firing a body beyond its recommended range to mature a glaze can cause warping, bloating or other faults.
Choose compatible firing ranges and aim for the correct maturity of both materials, not the highest possible temperature.
Are glaze faults only cosmetic?
Some faults are mainly visual; others affect durability, cleanability or suitability for the intended use. A surface acceptable on decorative ware may be unsuitable for functional ceramics.
Pinholes, crazing, shivering, exposed porous clay and unexpectedly rough surfaces require particular attention on ware intended for food and drink.
A smooth, glossy finish is not proof of food-contact suitability. Assessment concerns the finished ceramic, including the clay, glaze, application, firing and relevant contamination risks. Where suitability needs to be established, independent testing of representative finished ware is the appropriate approach.
Five checks before your next glaze firing
- Clay and glaze compatibility: check that the recommended firing ranges overlap and the combination has been tested.
- Application: follow the manufacturer's guidance for method, coat count and thickness.
- Bisque condition: check cleanliness and sufficient firing burnout.
- Heatwork: use witness cones rather than relying solely on the controller reading.
- Testing: treat a new glaze, clay, schedule or layering method as a new variable.
Test, record and repeat
Record the clay body, glaze, application method, firing schedule and finished result. When investigating a fault, change one variable at a time wherever possible.
Test tiles and clear firing records provide a practical basis for resolving problems before committing to a larger batch. Use witness cones to confirm the kiln programme is delivering an appropriate level of heat-work. Consistent results depend on the clay, glaze and firing working together.