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Posted by Admin on July, 20, 2026
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What Causes Crawling in Ceramic Glaze — and How Is It Cured Through Kaolin and Feldspar?
The 5 fixes for crawling through kaolin and feldspar:
- Reduce excess plastic clay in the glaze — high-plasticity clay shrinks more during drying, increasing the chance the glaze layer cracks and pulls away before firing.
- Use consistently graded kaolin — uniform particle size gives more even drying shrinkage across the glaze layer, reducing pre-fire cracking.
- Select feldspar with reliable, low-viscosity melt — a feldspar that flows evenly helps a glaze re-heal minor pre-fire cracks instead of pulling further apart.
- Ensure clean, dust-free bisque surface — this is a process step, but glaze formulation (via kaolin content) affects how forgiving the recipe is of minor surface contamination.
- Verify consistent LOI and mesh grading — inconsistent raw material behaviour compounds marginal application technique into visible crawling.
Source: Aalok Overseas Technical Laboratory, Rajasthan, India · Manufacturer & Exporter of Kaolin and Feldspar to 50+ Countries Since 1992 · www.feldsparindia.com
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AALOK OVERSEAS INDIA · CERAMIC DEFECT GUIDE #02
Glaze Adhesion · Kaolin Plasticity Control · Feldspar Melt Consistency · NABL COA · Exporter Since 1992
REQUEST KAOLIN & FELDSPAR SAMPLE → VIEW KAOLIN SPECIFICATIONSWhat Is Glaze Crawling — and Why Does It Happen?
Glaze crawling is a defect where the glaze layer pulls back from parts of the fired surface during melting, leaving bare, unglazed patches surrounded by beaded-up glaze — like water beading on a waxed surface. It happens because the glaze failed to bond evenly to the body before or during early firing, usually from micro-cracks that formed in the dried glaze layer (from excess clay shrinkage), from contamination on the bisque surface preventing adhesion, or from a glaze melt that doesn't flow evenly enough to re-heal those small gaps.
Crawling Risk Factors
| RISK FACTOR | EFFECT | MITIGATION |
|---|---|---|
| Excess ball clay/kaolin in glaze | Higher drying shrinkage, more pre-fire cracking | Balance clay content; use graded kaolin |
| Dust or oil on bisque surface | Poor initial glaze adhesion | Clean, dust-free bisque before glazing |
| High-viscosity feldspar melt | Glaze can't re-heal small pre-fire cracks | Select feldspar/frit combination with adequate flow |
| Thick glaze application | Greater shrinkage stress during drying | Control glaze application thickness |
6 Causes of Crawling Traceable to Kaolin and Feldspar
CAUSE 01
Excess or Inconsistent Kaolin/Clay Content in Glaze
Kaolin and ball clay give a glaze slip its suspension and application properties, but too much clay increases drying shrinkage, which can crack the dried glaze layer into "islands" before firing even begins — the direct precursor to crawling.
CAUSE 02
Coarse or Uneven Kaolin Particle Size
Inconsistent kaolin particle size produces uneven drying shrinkage across the glaze layer, increasing the likelihood of localized pre-fire cracking in some areas but not others.
CAUSE 03
High-Viscosity or Inconsistent Feldspar Melt
A feldspar melt that doesn't flow evenly and quickly enough during early firing can't re-heal the fine pre-fire cracks in the dried glaze, letting them widen into visible crawled patches instead of closing up.
CAUSE 04
Mismatched Feldspar Type for Glaze Recipe
Soda feldspar's lower viscosity generally gives better crack-healing flow than potash feldspar at equivalent firing temperatures — a recipe originally designed around one type can crawl more if switched to the other without adjustment.
CAUSE 05
High LOI Causing Additional Shrinkage Stress
Elevated LOI in the kaolin or feldspar component adds an extra shrinkage component during firing on top of the glaze's normal drying shrinkage, compounding the stress that leads to crawling.
CAUSE 06
Batch-to-Batch Raw Material Inconsistency
A kaolin or feldspar lot with different plasticity, particle size, or melt behaviour than the batch a recipe was tuned around can turn a previously stable glaze into a crawling problem — without any visible change to the raw material itself.
Crawling Diagnostic — Symptom → Cause → Fix
| SYMPTOM | LIKELY CAUSE | FIX |
|---|---|---|
| Bare patches surrounded by beaded glaze | Pre-fire drying cracks in glaze layer | Reduce clay content; use consistently graded kaolin |
| Crawling worse on thickly glazed areas | Greater drying shrinkage stress with thicker application | Control and standardize glaze application thickness |
| Crawling appears after switching feldspar type | Melt viscosity mismatch with existing recipe | Reassess feldspar type/ratio for adequate crack-healing flow |
| Intermittent crawling with no formula change | Batch-to-batch kaolin or feldspar variation | Demand NABL COA (chemistry, LOI, mesh) on every lot |
The Aalok Overseas Solution — Consistent Kaolin & Feldspar for Crawl-Free Glazes
Consistently Graded Kaolin
Ceramic and glaze-grade kaolin with controlled particle size, giving predictable drying shrinkage and adhesion behaviour batch after batch.
Reliable-Flow Potash & Soda Feldspar
Mine-direct, mesh-graded feldspar with consistent K₂O/Na₂O content, giving the predictable melt flow needed to heal minor pre-fire glaze cracks.
NABL COA on Every Shipment
Chemistry, LOI, and mesh data verified on every lot — so a glaze recipe that works today keeps working with your next shipment.
Technical Recipe Support
Aalok Overseas' technical team can advise on kaolin/feldspar ratio adjustments to reduce crawling risk in your specific glaze formulation.
Why Manufacturers Choose Aalok Overseas
Since 1992, Aalok Overseas has supplied mine-direct, NABL-certified kaolin and feldspar to ceramic manufacturers across 50+ countries. Consistent particle size and melt behaviour — not just correct chemistry on paper — is what keeps a glaze recipe crawl-free shipment after shipment.
FAQ — Crawling in Ceramic Glaze
Q
What causes glaze crawling in ceramics?
Crawling is caused by poor glaze-to-body adhesion before firing, usually from micro-cracks in the dried glaze layer due to excess clay content, surface contamination on the bisque, or a feldspar melt that flows too unevenly to heal small pre-fire cracks.
Q
How does kaolin content affect glaze crawling?
Kaolin and ball clay increase a glaze's drying shrinkage. Excess clay content raises the risk that the dried glaze layer cracks into islands before firing, which is the direct precursor to visible crawling.
Q
Can the right feldspar prevent crawling?
A feldspar with reliable, adequate melt flow can help a glaze re-heal minor pre-fire cracks during early firing, reducing the chance they widen into crawled patches, though surface preparation and clay content also need to be addressed.
Q
Is crawling the same as crazing?
No. Crawling is the glaze pulling back to expose bare body, caused by poor adhesion before or during firing. Crazing is a fine crack network in an otherwise intact glaze layer, caused by a thermal expansion mismatch after firing.
Q
Does surface cleanliness matter more than raw materials for crawling?
Both matter. A clean, dust-free bisque surface is essential, but raw material consistency (kaolin particle size, feldspar melt behaviour) determines how forgiving the recipe is of minor surface or application imperfections.
Related Ceramic Defect & Raw Material Guides
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