How to fix
Pottery Glaze Defects
If you’ve ever opened your kiln to find dull colors, pinholes, crawling, or glaze running onto the shelf, this guide is for you. In the following chapters, you’ll learn how to identify, prevent, and fix the most common pottery glaze problems. We’ll cover issues that occur before glazing, during glaze application, and during firing.
BEFORE YOU START
GLAZING
1. Clean Your Bisque Ware
Many glaze defects—especially glaze crawling and bald patches—are caused by contaminants on the bisque surface. Dust, oils, or residue prevent proper glaze adhesion and can cause the glaze to pull away during firing. Best practice:
- Remove dust with a soft brush or damp sponge
- Avoid touching bisque surfaces unnecessarily after cleaning.
2. Make Sure Your Bisque Is Completely Dry
If you clean your bisque ware before glazing, that’s excellent—but it must be fully dry before applying glaze. Damp bisque absorbs glaze unevenly, often leading to thin application and dull glaze colors after firing. Allow cleaned pieces to air-dry completely before glazing.
3. Check Glaze Consistency
Brush-on glazes vary in thickness. Proper consistency allows the glaze to glide smoothly across the surface without dragging. If a fully loaded brush does not apply smoothly, the glaze is likely too thick.
Important: Do not thin glaze with plain water. Over time, this weakens binders and can cause cracking, crawling, or uneven coverage. Instead, adjust consistency using binders such as:
DURING GLAZE
APPLICATION
4. Avoid Visible Brush Strokes and Uneven Coverage
If brush marks or uneven glaze coverage appear after firing, application technique is usually the cause. How to improve glaze application:
- Use brushes designed specifically for glazing—natural hair brushes hold more glaze and release it evenly.
- Alternate the direction of application between coats (vertical, then horizontal).
- Allow each coat to dry completely before applying the next. Applying glaze too soon will smear the underlying layer instead of building thickness.
5. Prevent Glaze from Cracking and Flaking off
Glaze cracking during application does not always result in a firing failure. If you are layering glazes and only the top layer cracks, this is usually harmless. However, if a single, stand-alone glaze cracks and exposes bare bisque, it should be corrected before firing. How to fix glaze cracking:
- Add a small amount of Gum Solution or another glaze binder to improve flexibility and adhesion
- Apply slightly thinner, more even coats
- Allow each layer to dry fully before applying the next
Cracking at this stage is an early warning sign—addressing it now helps prevent crawling and bare spots later.
ISSUES DURING
FIRING
6. ENSURE COLOR VIBRANCY
Muted or dull glaze colors are usually caused by one of two factors: glaze applied too thinly or excess heatwork during firing.
The first issue is easy to test—you can re-glaze the fired piece with an additional coat and refire it. If glaze thickness was the problem, color intensity should improve after the second firing. Controlling heatwork, however, is critical for consistent glazing results.
- Perform test firings to identify the glaze’s optimal firing range
- Use witness cones instead of relying solely on the kiln controller
- Keep firing records, including: clay body, glaze thickness, cone behavior and final fired results
Layered glazes are especially sensitive to heatwork—minor changes can significantly affect color.
7. Eliminate the risk of pinholes
Pinholes are tiny holes in the glaze surface caused by gases escaping from the clay body during firing. Layered glazes are particularly prone to pinholing because the glaze layer is thicker and gases must travel farther to escape. To reduce pinholes, it’s important to extend the time during which the glaze remains fluid.
We recommend slow cooling rather than soaking, as slow cooling allows gases to escape while minimizing additional heatwork. This reduces the risk of dull colors or excessive glaze movement. If pinholes appear primarily near the foot, taper glaze thickness toward the bottom—this area is especially prone to pooling.
8. Avoid Glaze Crawling
Glaze crawling occurs when glaze pulls back during firing, exposing bare clay underneath. This defect almost always originates earlier in the process, during bisque cleaning and glaze application. Even small surface contaminants can cause major defects after firing:
- Dust, oil, or fingerprints on bisque
- Excessive glaze thickness in certain areas
- Brushing over semi-wet glaze layers
How to prevent glaze crawling:
- Handle bisque ware with clean hands
- Wipe surfaces lightly with a damp sponge before glazing
- Let each glaze layer dry completely and avoid overworking the surface
9. CONTROL GLAZE RUNNING
Heavy glaze layering is often required for floating or reactive effects, but it increases the risk of glaze running onto kiln shelves. To protect your kiln:
- Use kiln cookies under glazed pieces
- Apply kiln wash to both shelves and cookies
- Reduce glaze thickness toward the bottom of the piece
You can read more about controlling glaze movement and layering glazes here.
Read more
Glaze layering
Layering glazes is one of the most rewarding ways to add depth, texture, and visual complexity to your ceramic works. Learn which glazes work well together, how to apply them, and how to control the results for consistent success.
How to use Orton cones?
Orton pyrometric cones measure heat work, firing time and temperature and will let you know that subtle changes are taking place in the kiln, allowing you to take corrective action before they affect your products.