Crawling is a common Defect encountered during the Glazing process, comparable in severity to issues like Cracking and Pinhole. It occurs when the Glaze layer shrinks or lacks sufficient Adhesion, causing the Glaze to pull back and expose the Green Body. The edges of the retracted Glaze may appear to have a higher Glaze Thickness.
This article aims to thoroughly analyze the root causes leading to Crawling and provide production-proven Remedies that can effectively resolve approximately 90% of these Defects.
Core Principle: Surface Tension Dynamics
The fundamental mechanism of Crawling lies in the action of Surface Tension within cracks in the Glaze. When the Glaze layer develops cracks during Drying or Firing, Surface Tension drives the Glaze on either side of the crack to contract , ultimately resulting in the clustering of the Glaze and exposure of the Body.
I. Process Control Risks in Body and Glazing
Factors affecting Adhesion are primarily focused on the state of the Green Body and the Glazing operation.
1. Green Body Moisture and Temperature Contol
Insufficient Drying: If the Sanitaryware is not Dried properly before Glazing , the high Moisture Content in the Body can cause Crawling during Firing due to Rapid Heating.
Remedy: Ensure the Body Moisture Content coming out of the Dryer is below 0.5%.
Overheated Body: When Glaze is Sprayed onto an Overheated Body (Ware Is Hot) , the Shrinkage Difference between the bottom and top layers of the Glaze can cause a Crack due to Surface Tension Difference.
Remedy: Glazing is performed only when the body temperature is appropriate. Additionally, Wetting the Body surface with a Wet Sponge helps ensure the Glaze shrinkage is not Rapid.
2. Surface Contaminants and Adhesion
Foreign Materials: The presence of Foreign Materials on the Body surface, such as dust, Talc used in Casting, oil, and grease, leads to Poor Adhesion between the Glaze and the Ware.
Remedy: Thoroughly Sponge the Body to Remove the foreign materials.
Sulphate Impact: Residual Sulphate content on the Body surface also causes Poor Adhesion between the Body and the Glaze.
Remedy: Use a Wet Sponge to Remove the Sulphates on the Body surface to achieve Proper Adhesion.
3. Body Geometry
Sharp Edges: When Glaze is Sprayed onto a Sharp Edge area, the Less Absorbing Capacity of the edges can result in Improper Adhesion , potentially causing Crawling during Firing.
II. Influence of Glaze Formulation and Preparation
Improper Glaze Formulation and Preparation are significant intrinsic factors causing Crawling.
1. Glaze Shrinkage and Thickness Control
Excessive Glaze Thickness: Excess Glaze Thickness can cause crawling, frequently encountered in the Sanitaryware Sump Area.
Remedy: Maintain the Same Thickness across all areas of the Ware.
High Shrinkage Materials:
High Clay Content: High clay Content (such as Kaolin) in the Glaze Formulation is a High Shrinkage Material, leading to High Shrinkage during Glaze Drying, which causes Crack and subsequently Crawling during Firing.
Remedy: Reduce the clay content in the Glaze, and consider Replace it with Calcined Clay.
High Zinc Oxide: Zinc Oxide has the property of High Hydration, resulting in greater Shrinkage during Drying, which can cause a Crack in the Glaze.
2. Glaze Fineness and Adhesiveness
Over Grinding: If the Glaze is Finer (containing More Fine Materials), the possibility of Excessive Shrinkage exists, which can lead to Crawling.
Remedy: Don’t Over Grind the Glaze; maintain the Required Particle Size Distribution.
Binder Issues: Insufficient Binder or Degraded Binder reduces the Glaze's Adhesive Property, failing to Improve the Adhesion between the Body and Glaze.
Remedy: Add the Required Amount of Binder and Anti-bacterial Additives to Avoid the Binder Degradation. Binder also helps to Reduce the Drying Time of Glaze.
III. Kiln and Pre-Firing Assurance Measures
Even with perfect Glazing, preparation before Firing and Kiln operation are critical.
Ensure Sufficient Pre-Drying: Before the Ware is sent to the Kiln, it must have Enough Time to Dry.
Correct Pre-Dryer Usage: The Correctly Using of the Pre-Dryer (often called the Glaze Dryer) inside the Kiln is very important. Its purpose is to Avoid the Rapid Heating of the Glaze, ensuring the Slow Drying of the Glaze.
By systematically managing the Body condition, optimizing Glaze Formulation, and strictly controlling the Glazing and Pre-Firing stages, Crawling Defect in Ceramic Wares can be effectively eliminated.