Ceramic glaze defects can affect both the appearance of a ceramic piece and its technical properties. The presence of pinholes, cracks, craters or surface irregularities can compromise the uniformity of the finish, the continuity of the glaze layer and its performance during use.
Although these defects become visible on the surface, their origin does not always lie in the glaze itself. Raw material composition, application conditions, the characteristics of the ceramic body and the firing cycle all influence the final result. Understanding how these variables interact is therefore essential to identifying the causes of each defect and implementing solutions that help maintain process stability.
Why do ceramic glaze defects occur?
Ceramic glazing defects occur when material properties or manufacturing conditions prevent the intended finish from being achieved.
The influence of composition and raw materials
The chemical composition of a glaze determines properties such as its maturation temperature, melt viscosity, thermal expansion and resistance. The proportion of fluxes, glass formers and stabilisers influences its behaviour during firing and its compatibility with the ceramic body.
The characteristics of the raw materials also affect the final result. Variations in particle size distribution, the presence of impurities or changes in the composition of certain components can alter glaze behaviour, even when the same formulation is used. Controlling these variables helps achieve greater consistency between batches.
Glaze preparation and application
Glaze preparation determines how the suspension behaves during application. Incorrect density can change the amount of material deposited, while variations in viscosity can affect its distribution and levelling on the ceramic body.
Suspension stability, the presence of agglomerates and the condition of glazing equipment also play a role. Uneven thickness or non-uniform application can lead to visible differences after firing, even when the glaze composition is suitable.
The role of the ceramic body and firing cycle
The ceramic body undergoes various transformations during heating, some of which release gases that must pass through the upper layers. If the glaze begins to melt and seals its surface before these gases can escape, the risk of pinholes, bubbles or craters increases.
The heating rate, peak temperature and soaking time influence these transformations. During cooling, differences in contraction between the ceramic body and the glaze can also generate stresses that affect the integrity of the surface.
Common ceramic glaze defects and their causes
Recognising the appearance of each defect helps narrow down potential causes and identify the process parameters that need to be analysed.
Glaze crazing
Crazing appears as a network of fine cracks on the glaze surface. Its most common cause is a mismatch in thermal expansion between the glaze and the ceramic body, which generates tensile stresses during cooling.
To correct it, we need to check the thermal compatibility of both materials and, if necessary, adjust the glaze composition to reduce the stresses responsible for the cracks.
Pinholes and surface porosity in ceramics
Pinholes are small openings that appear when gases pass through the molten glaze and the resulting holes do not close before solidification. These gases may originate from carbonate decomposition, the removal of organic matter or air incorporated during application.
We should review the degassing behaviour of the ceramic body, the applied glaze thickness and the firing cycle. Adjustments to the formulation may also be necessary to promote pore closure during melting.
Glaze craters
Craters are surface cavities, generally circular in shape, that can form when bubbles burst or when contaminating particles locally disrupt the melting process.
If they appear in specific areas, we should check for possible contamination or application irregularities. When they are distributed across the entire piece, it is advisable to review suspension homogeneity, sieving and glaze behaviour during firing.
Bubbles and blistering during firing
Bubbles form when gases become trapped within the molten glaze. If they increase in size and deform the surface, they can produce visible blisters.
High melt viscosity or insufficient time for gas release can encourage their formation. To reduce these defects, we need to examine the maturation temperature, soaking time and the glaze's ability to release gases without becoming excessively fluid.
Glaze crawling and shrinkage
Crawling leaves areas of the engobe or ceramic body exposed. It may result from excessive shrinkage during drying, insufficient adhesion or the presence of contaminants that interfere with wetting.
The solution involves checking the condition of the ceramic body, controlling glaze thickness and adjusting the rheological properties of the glaze. In certain cases, wetting agents can help improve wetting of the ceramic body. If the unfired glaze layer already shows cracks, these must be corrected before firing.
Adhesion problems between glaze and ceramic body
Poor adhesion causes separation between ceramic layers. If this occurs before firing, it may be related to insufficient cohesion of the dried glaze, the condition of the ceramic body or incorrect binder dosage.
When detachment occurs after firing, particularly along edges and corners, it may be caused by excessive compressive stresses. In this case, we need to review the thermal compatibility between the glaze and the ceramic body.
Glaze running and overfiring
Glaze running occurs when the glaze becomes excessively fluid during firing and flows across the ceramic piece, causing material build-up or loss of definition in the finish.
It may be caused by a formulation containing excessive fluxes, an excessively high firing temperature or excessive glaze thickness. To prevent it, we should adjust the glaze's melt viscosity and ensure that the firing cycle falls within its maturation range.
Orange peel and other surface irregularities
Orange peel is characterised by small surface undulations that disrupt surface uniformity, particularly noticeable on glossy finishes.
It may result from insufficient levelling during application, unsuitable particle size distribution or incomplete melting. If the texture is already present before firing, we need to review the rheology and application conditions. If it develops after firing, we should examine the maturation temperature and the glaze's levelling behaviour.
How to correctly diagnose ceramic glaze defects
To identify the origin of a defect, we need to determine when it appears, which variables have changed and whether it affects the entire production run or only certain batches. Comparing defective pieces with reference samples helps narrow down potential causes before modifying the formulation.
Identify the stage of the process where the defect appears
We should inspect the pieces after application, following drying and after they leave the kiln. This comparison helps distinguish defects that originate before firing from those that develop during the firing process.
We should also check whether the defect coincides with a batch change, an equipment adjustment or a specific position on the production line.
Check density, viscosity, thickness and application conditions
We need to ensure that density, viscosity and applied glaze thickness remain within the established parameters. Deviations can affect glaze distribution and cause surface irregularities.
If differences are detected, we should also review suspension stability and the operation of the application equipment.
Analyse compatibility between the ceramic body, engobe and glaze
We can carry out comparative tests using different combinations of ceramic body, engobe and glaze to identify which layer contributes to the defect. When cracks or detachment occur after firing, dilatometry tests help assess the thermal compatibility of the materials.
Check firing temperatures, curves and conditions
We need to compare the kiln's actual temperature with the established firing cycle, paying particular attention to heating rates, soaking times and cooling conditions.
If the defect appears only in certain areas of the kiln, we should investigate possible temperature differences. When its origin is not immediately apparent, we can use surface analysis techniques, such as digital microscopy for ceramic defect analysis, to examine irregularities in greater detail.
How to prevent ceramic glaze defects during production
Preventing ceramic glaze defects requires maintaining stable manufacturing conditions and identifying potential deviations before they affect the final finish. Four key areas deserve particular attention:
Controlling glaze properties before application: check the density, viscosity, particle size distribution and suspension stability. We should also monitor possible changes during storage, such as sedimentation or foam formation. Depending on the requirements of each formulation, dispersants, defoamers and rheology modifiers can help maintain suitable application properties.
Stability and repeatability on the glazing line: maintain consistent flow rates, line speeds and quantities of deposited material. Regular maintenance of nozzles, bell applicators and pumping systems also helps achieve uniform application and reduce differences between pieces.
Controlling the drying and firing process: adapt drying temperature, ventilation and drying time to the characteristics of the ceramic body and glaze. During firing, we need to monitor the kiln's temperature distribution and maintain an appropriate firing cycle to ensure proper glaze maturation.
Testing and adjustments before introducing production changes: carry out controlled tests before modifying a formulation or process parameter. Comparing results with a reference sample helps verify whether the adjustment corrects the defect without affecting other properties. Once validated in the laboratory, its performance should be verified under actual manufacturing conditions.
At Kerafrit, we work with you to understand the specific needs of your process and develop solutions adapted to your real production conditions. Our expertise in frit, glaze and additive formulation allows us to evaluate different alternatives and test their performance before introducing changes into production.
Ongoing monitoring is also an essential part of this approach. Verifying that results remain consistent between batches and during normal production line operation helps consolidate improvements and reduce the occurrence of new defects.
Frequently asked questions about ceramic glaze defects
What are the most common ceramic glaze defects?
The most common defects include crazing, pinholes, craters, bubbles, glaze crawling and surface irregularities. Their frequency depends on the type of product, glaze formulation and manufacturing conditions.
Why do pinholes appear in ceramic glazes?
Pinholes appear when small openings formed during firing remain on the surface after the glaze solidifies. Although they are often associated with gas release, air incorporated during application and melting conditions can also contribute to their formation.
What causes ceramic glaze crazing?
Crazing is generally associated with excessive tensile stresses in the glaze, often caused by a mismatch in thermal expansion between the glaze and the ceramic body. When cracks appear after a period of storage or use, it is also advisable to assess how the ceramic piece responds to temperature changes and other factors that may contribute to stress development.
How can you tell whether a defect is caused by the glaze or the firing process?
One way to guide the diagnosis is to compare samples of the same glaze fired under different thermal conditions. If the defect disappears after modifying the firing cycle, this suggests a relationship that should be investigated through further testing.
Tests can also be performed using a reference formulation to determine whether the defect occurs under the same conditions. These comparisons help distinguish material-related problems from those associated with kiln operation.
How can ceramic glaze defects be prevented in industrial production?
Prevention relies on raw material control, suspension stability, equipment maintenance and continuous monitoring of manufacturing conditions. Maintaining historical production records makes it possible to identify deviations and link defects to changes in batches, formulations or production parameters. This information supports a faster response when problems arise and helps reduce their recurrence.
