What Are Pinholes?
Pinholes are tiny craters or holes in the glaze surface after firing. They look like someone poked the glaze with a pin while it was still soft. They are one of the most common defects and have a specific, fixable cause.
Why Pinholes Form
During firing, gases are released from the clay body and from organic materials in the clay. These gases travel through the clay and glaze, forming bubbles. Ideally, the glaze stays molten long enough for these bubbles to pop and the glaze to flow back over the craters and heal.
If the kiln cools too quickly after peak temperature, the glaze solidifies before those craters can heal. The result is pinholes.
Common causes:
- Kiln cooled too fast after reaching peak temperature.
- Glaze applied too thickly (the bubbles have further to travel).
- Clay body with high organic content or moisture.
- Firing ramp too fast (gases released too suddenly).
Fixes
- Add a hold time at peak temperature (typically 10-20 minutes) to give the glaze time to heal.
- Slow the cooling rate for the first 100°C below peak.
- Apply glaze in thinner layers.
- Ensure bisque pieces are fully cooled and clean before glazing.
Firing Adjustment Checklist
If pinholes or other defects persist:
- Add or extend peak hold
- Slow the first 100–150°C of cooling
- Recheck glaze thickness and application method
Change only one variable per firing so you know what actually helped.
When Defects Are Features
Intentional pinholes and crawling are exploited in some historic glaze styles. The famous oil-spot and temmoku glazes from Chinese ceramics rely on controlled pinhole-like effects. As you advance, you will learn to tell the difference between a defect and a texture.
The Bigger Picture
The famous oil-spot and tenmoku glazes from Chinese Song dynasty kilns are a striking example of controlled defects becoming prized aesthetic features. Understanding the full range of glaze defects and their causes is one of the most practical skills in studio ceramics.
Common Mistakes and Quick Fixes
- Firing too fast through burnout stages: Slow early ramps if gas release is high.
- No hold at peak with pinhole-prone glazes: Add a measured hold and retest.
- Changing glaze and schedule at once: Keep one variable fixed per trial.
- Ignoring bisque maturity: Under-bisqued ware can increase outgassing defects.
Practice Exercise
Run two otherwise identical glaze firings on test tiles: one with no hold and one with a 15-minute peak hold. Compare pinhole healing. This creates a direct, actionable schedule decision for your studio.
