Pitting Defect in Ceramics: Causes & Analysis

Pitting Defect in Ceramics: Causes & Analysis

Pitting is a common surface defect that appears as small, crater-like holes on the glaze after firing, often referred to as the "orange peel" defect. Below are the key causes of glaze pitting and the best practices to prevent it.

Primary Causes of Glaze Pitting

Gas Entrapment During Firing

One of the primary causes of glaze pitting is the entrapment of gases during the firing process. Organic materials in the clay body or glaze release gases as they burn off. If these gases are unable to escape before the glaze hardens, they can form small pits on the surface. This is particularly problematic when the glaze matures too quickly, trapping the gases beneath it. Additionally, gases from the clay body, especially when the body contains organic impurities, may migrate through the glaze and cause pitting.

Raw Material Selection

The raw materials used in the ceramic body and glaze play a crucial role in minimizing glaze pitting. Ball clay/china clay containing lower levels of carbonaceous and sulfur compounds are preferred, as these materials reduce gas formation during firing. Selecting raw materials with minimal organic impurities will also help reduce the risk of gas entrapment and pitting, ensuring a smoother glaze finish.

Improper Firing Schedules and Kiln Temperatures

Improper firing schedules or uneven kiln temperatures are significant contributors to glaze pitting. Firing too quickly may prevent gases from escaping, resulting in pitting as the glaze solidifies. Conversely, firing at temperatures that are too low may prevent the glaze from fully maturing, while excessively hot temperatures can cause over-firing, leading to the volatilization of glaze components and the formation of pits. Ensuring proper firing conditions is key to avoiding these issues.

Glaze Binder Decomposition

The binder in the glaze helps to hold the glaze particles together. However, at higher firing temperatures, the decomposition of these binders can lead to pinholes or pitting on the surface. To mitigate this, it is advisable to either avoid using glaze binders or select those that decompose at lower temperatures—before the glaze begins to melt. This ensures a smooth and defect-free surface during the firing process.

Excessive Glaze Thickness

Applying thick glaze layers can trap gas bubbles beneath the surface, which rise during firing and form pits.

Best Practices to Prevent Glaze Pitting

• Ensure an appropriate and well-managed firing schedule with slow firing during initial stages, allowing sufficient time for gases to escape before the glaze begins to set

• Maintain gradual temperature increases throughout the firing process to ensure gases are eliminated

• Select raw materials with minimal organic impurities and lower levels of carbonaceous and sulfur compounds

• Choose or avoid glaze binders based on their decomposition temperatures—select those that decompose before the glaze melts

• Apply controlled glaze thickness to prevent gas bubble entrapment

• Maintain proper and consistent kiln temperatures to avoid firing defects

Ensuring a controlled firing process, using the right materials, and managing glaze thickness are all critical steps to avoid glaze pitting.