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Posted by Admin on July, 12, 2026
What Causes Cracks in Ceramics — And How Feldspar Cures It?
Cracks in ceramics are caused mainly by thermal shock during firing/cooling, uneven mesh grading of raw materials, excess free silica triggering quartz inversion stress at 573°C, insufficient flux content (K₂O + Na₂O) leading to incomplete vitrification, and iron or organic impurities that cause uneven shrinkage across the body. The cure involves correcting the raw material and firing profile:
- 1. Use graded 200/325 mesh feldspar — consistent particle size from Aalok Overseas ensures even melting across the body.
- 2. Balance the flux ratio — correct K₂O+Na₂O content lowers vitrification temperature and reduces internal thermal stress.
- 3. Slow the cooling schedule — especially through the 573°C quartz inversion point where silica expands/contracts.
- 4. Fix the feldspar:quartz:clay ratio — an imbalanced body composition is a leading cause of lamination and dunting cracks.
- 5. Switch to low-iron, high-purity feldspar — impurities cause black coring and differential shrinkage that crack the body.
The Complete Diagnostic Guide to Ceramic Cracking
Since 1992, Aalok Overseas has supplied mesh-graded, low-iron potash and soda feldspar to tile, sanitaryware, and tableware manufacturers across the world — engineered specifically to reduce firing defects like cracking, warpage, and black coring.
Understanding Why Ceramic Bodies Crack
Ceramic cracking is rarely caused by one single factor — it is usually the compounding of raw material inconsistency and firing schedule mismatch. Below we break down each root cause in detail, followed by the diagnostic table manufacturers use on the shop floor.
1. Thermal Shock During Firing and Cooling
Rapid temperature changes create stress gradients between the surface and core of a ceramic piece. If the outer layer contracts or expands faster than the interior, the resulting tension exceeds the material's tensile strength and a crack forms — often at the weakest point in the body, such as a poorly compacted edge or an air pocket.
2. Quartz Inversion at 573°C
Free silica in the clay body undergoes a crystalline phase change at 573°C, expanding roughly 1% in volume almost instantly. If cooling passes through this point too quickly, the sudden volume change causes a distinctive crescent-shaped "dunting" crack. Bodies with excess uncombined quartz are especially vulnerable.
3. Uneven Mesh Grading of Feldspar and Other Fluxes
When feldspar particle size is inconsistent, some grains melt earlier than others during firing, creating localized zones of glassy flux next to unmelted, porous zones. This non-uniform vitrification builds internal stress as the body cools at different rates in different areas.
4. Insufficient Flux Content
Feldspar acts as the primary flux in most ceramic bodies, lowering the temperature at which the clay and quartz begin to vitrify and bond into a dense, glassy matrix. A body with too little feldspar remains porous and mechanically weak, making it far more prone to cracking under thermal or mechanical stress.
5. Iron Oxide and Organic Impurities
Low-purity feldspar carrying iron oxide or residual organics can cause localized gas release during firing and uneven coloring known as black coring. These pockets shrink differently than the surrounding matrix, generating internal stress lines that propagate into visible cracks.
6. Incorrect Feldspar:Quartz:Clay Ratio
Every ceramic body — wall tile, floor tile, vitrified tile, sanitaryware, tableware — has an optimal ratio of flux to filler to plastic clay. Deviating from this ratio, even by a few percentage points, changes the shrinkage behavior of the body and is one of the most common root causes of lamination and edge cracking reported by tile manufacturers.
Symptom → Root Cause → Fix: Diagnostic Table
| Symptom Observed | Likely Root Cause | Fix |
|---|---|---|
| Crescent-shaped crack after cooling | Quartz inversion / dunting | Slow cooling rate through 573°C; reduce free silica |
| Cracks near edges/corners | Uneven drying or thermal gradient | Control kiln zoning; pre-heat uniformly |
| Random hairline cracks across body | Uneven feldspar mesh grading | Switch to 200/325 mesh graded feldspar (Aalok Overseas) |
| Black coring with crack lines | Iron/organic impurity in flux | Use low-iron potash/soda feldspar |
| Porous, weak, crack-prone body | Insufficient flux (feldspar) content | Increase feldspar ratio to 25-35% of body |
| Lamination cracks / delamination | Incorrect feldspar:quartz:clay ratio | Reformulate body composition with ceramic engineer |
Aalok Overseas Feldspar Specification for Crack Prevention
| Parameter | Potash Feldspar | Soda Feldspar |
|---|---|---|
| SiO₂ | 66-70% | 68-71% |
| Al₂O₃ | 17-19% | 18-20% |
| K₂O | 9-11% | 1-2% |
| Na₂O | 2-3% | 8-10% |
| Fe₂O₃ (max) | 0.08-0.12% | 0.10-0.15% |
| LOI (Loss on Ignition) | 0.3-0.6% | 0.3-0.5% |
| SO₃ | <0.05% | <0.05% |
| Mesh Size | 200 / 325 mesh | 200 / 325 mesh |
| Whiteness | High | Very High |
Low LOI and low SO₃ content directly correlate with reduced gas evolution during firing — a key contributor to internal stress cracking. Both grades are available ex-Beawar/Indore, Rajasthan for global export.
Frequently Asked Questions
Why do ceramic tiles crack after firing?
Post-firing cracks usually occur from rapid cooling through the 573°C quartz inversion point, where silica changes crystal structure and expands. A slower cooling schedule and a feldspar-rich body with lower free silica content reduces this risk.
Does feldspar quality affect crack resistance in ceramics?
Yes. Low-iron, consistently graded feldspar vitrifies more evenly across the body, reducing internal stress points that lead to cracking. Impure feldspar with variable particle size causes uneven shrinkage.
What mesh size feldspar is best for reducing cracks?
200 mesh is common for wall tiles and general bodies, while 325 mesh finer grades are preferred for vitrified tiles and sanitaryware where uniform melting and minimal cracking is critical.
What is dunting crack in ceramics?
Dunting is a crack, often crescent-shaped, that forms during cooling due to thermal shock or quartz inversion stress. It is prevented by controlling cooling rate and using a flux-balanced body with quality feldspar.
How much feldspar should be in a ceramic body to prevent cracking?
Typical vitrified tile bodies use 25-35% feldspar as flux. The exact ratio depends on the clay and quartz content and the desired firing temperature, and should be adjusted with your ceramic engineer.
Can impure feldspar cause black coring or cracks?
Yes. Iron and organic impurities in low-grade feldspar can cause black coring, uneven gas release, and localized stress that leads to cracking during firing. Aalok Overseas supplies low-iron feldspar to minimize this defect.
Related Ceramic Defect & Feldspar Guides
- Potash Feldspar Product Page
- Soda Feldspar Product Page
- Quartz & Silica Powder
- Kaolin Clay AQLK-94
- 200 vs 325 Mesh Size Guide
- Warpage in Ceramics: Causes & Fix
- Shrinkage in Ceramics: Causes & Fix
- Glaze Crawling Defect Guide
- CIE L*a*b* Colour Values Guide
- Whiteness: Potash vs Soda Feldspar
- HS / HSN / CAS / EC Code Reference
- Engineered Stone Market & Mesh Selection
- AIFTA Minerals Trade: Vietnam
- SAFTA Minerals Trade Guide
- Quartz Powder for Vietnam Buyers
- Potash Feldspar for Vietnam Buyers
- Export Markets Overview
- Certifications & Quality Control
- Loading Ports & Logistics
- Packaging Specifications
- About Aalok Overseas
- Mining & Processing Facilities
- Lab Testing & QC Process
- Bulk Shipping Options
- Container Loading Guide
- Request a Free Sample
- Current Price List
- Feldspar for Glass Industry
- Feldspar for Paint & Coatings
- Feldspar vs Nepheline Syenite
- Tile Body Formulation Guide
- Sanitaryware Body Composition
- General FAQ
- Contact Export Team
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Source Low-Iron, Mesh-Graded Feldspar for Crack-Free Ceramic Bodies
Aalok Overseas has exported potash and soda feldspar, quartz/silica powder, and kaolin from Rajasthan, India since 1992. Request a lab-tested sample with full LOI, SO₃, and mesh-size certification for your next production run.
Request Sample & Spec SheetAalok Overseas · Beawar / Indore, Rajasthan, India
Ms. Ankita Agrawal, Director – Exports & International Relations · WhatsApp: +91-9004229525 · exports@aalokoverseas.com
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