# Pinholes: Tiny Craters, Big Frustration

Unit: The Final Bake: Glaze Firing
Topic: Common Glaze Faults
URL: https://claybook.studio/learn/pinholes-tiny-craters-big-frustration/

# 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](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/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.

## Check your understanding

### Question 1: What is the most effective way to fix pinholes in your glaze?

- [ ] A. Apply a second coat of glaze before refiring
- [x] B. Add a hold time at peak temperature so the glaze can heal
- [ ] C. Fire at a higher cone than recommended
- [ ] D. Use a thicker glaze application

Tip: Adding a hold time at peak temperature (usually 10-20 minutes) gives the glaze time to stay molten and flow back over gas bubble craters before the kiln cools.

### Question 2: Why do pinholes form if the kiln cools too quickly after peak temperature?

- [ ] A. Rapid cooling causes the glaze to crack into a pin pattern
- [x] B. The glaze solidifies before gas bubble craters can heal over
- [ ] C. The clay body shrinks faster than the glaze during rapid cooling
- [ ] D. The kiln atmosphere becomes too oxidising during fast cooling

Tip: Gas bubbles from the clay travel through the glaze during firing. The glaze must stay molten long enough to flow back over the craters they leave. Rapid cooling freezes the glaze before this healing can happen.