· Cracks form in the cooling zone when different parts of a ceramic product contract at different rates.
· The surface may cool and contract while the interior remains hotter, generating temperature gradients and tensile stresses.
· Existing microcracks can propagate when these stresses exceed the local strength of the fired body.
1. Excessive cooling rate
Ceramics have limited thermal conductivity. Large products, thick sections and enclosed cavities therefore develop greater temperature differences during rapid cooling.
Research on thermal-shock cracking in ceramics shows that thermal stresses generated by rapid temperature changes can initiate and propagate microcracks.
2. Quartz transformation near 573°C
Quartz undergoes the α–β inversion at approximately 573°C, accompanied by a dimensional change.
Research published by the American Ceramic Society reports an inversion temperature of approximately 573°C for pure quartz. If different areas of a sanitary ceramic pass through this critical range at different rates, localized stress can increase significantly.
3. Thermal-expansion mismatch
After firing, a porcelain ceramic body contains quartz, mullite and a glassy matrix. These phases do not contract identically during cooling.
A study of clay-based ceramic microstructures observed cracks associated with thermal-expansion mismatch between quartz particles and the surrounding glass phase during cooling.
Avoid excessive cooling around the quartz-inversion range;
Use gentler cooling for large and thick-wall products;
Maintain uniform airflow across the kiln car;
Prevent concentrated cold air from striking one area;
Correlate crack position with kiln position and wall thickness;
Revalidate the cooling curve after major product-design changes.
Cooling must match the ware and process conditions rather than pursue a shorter cycle indiscriminately.
Cooling cracks are fundamentally caused by temperature gradients, phase transformations and non-uniform contraction. The solution is not to make every cooling stage as slow as possible. The critical task is to control cooling through sensitive transformation ranges and whenever large internal temperature differences are likely to develop.
Q1: Is slower cooling always better?
A: No. Cooling should be slowed mainly through critical temperature ranges; excessive slowing elsewhere reduces productivity without necessarily improving quality.
Q2: Why are thick products more likely to crack?
A: Their interiors cool more slowly, producing larger temperature gradients.
Q3: Are cooling cracks caused only by the kiln?
A: No. Wall thickness, quartz particle size, pre-existing microcracks and kiln-loading conditions can also influence cracking.