Batt wash defect occurs when alumina batt wash residue is found on the surface of the glaze on a ceramic product. This is similar to a clay bits defect but the material causing the problem is alumina batt wash instead of clay.
Batt wash is a material made from alumina that is applied to the surface of kiln furniture such as shelves and bats. It is used to prevent the ceramic products from sticking to the kiln furniture during the high-temperature firing process. Without batt wash, the melted glaze can bond to the kiln shelf, causing damage to both the product and the kiln furniture.
Batt wash defect happens when loose alumina particles from the kiln furniture fall onto the glazed surface of the ceramic product during firing. This can happen because:
When these alumina particles land on the glaze surface during firing, they get embedded in the melted glaze. After cooling, they appear as white or rough spots on the surface of the ceramic product.
Batt wash particles on the glaze surface create rough spots and white patches that affect the appearance of the ceramic product. Products with visible batt wash defects are usually downgraded or rejected in quality inspection.

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Clay Bits Defect in Ceramics: Causes & Analysis
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