What Are Common Glaze Problems and How to Fix Them?

Opening a kiln after a glaze firing is one of the most exciting moments for any ceramic artist. It feels like unwrapping presents—until you pull out a beautifully formed mug only to find it covered in blisters, cracks, or bare patches. If you have ever felt this heartbreak, you are not alone.

Understanding common glaze problems and how to fix them (ceramic glazing) is a fundamental part of mastering pottery. The transformation from raw clay to a vitrified, glass-coated piece is a complex chemical process. When variables like temperature, application, or material compatibility fall out of balance, flaws occur.

In this guide, we will break down the most frequent issues potters face, why they happen, and exactly how you can correct them.

Understanding the Root of Glaze Defects

Before diving into specific troubleshooting, it is essential to look at the relationship between your clay and your glaze. Most glaze defects happen because these two materials do not play well together.

A vital step in any ceramic practice is checking glaze to clay body fit. This refers to how securely the fired glaze attaches to the fired clay. Achieving a perfect fit requires understanding coefficient of thermal expansion (CTE). Simply put, CTE measures how much a material expands when heated in the kiln and shrinks as it cools. If the glaze and the clay shrink at significantly different rates during the cooling phase, structural tension builds up, resulting in surface failures.

Crazing and Shivering: A Battle of Tension

When the thermal expansion of your clay and glaze are mismatched, you will usually encounter one of two primary issues: crazing or shivering.

Fixing Glaze Crazing in Pottery

Crazing looks like a fine network of spiderweb cracks covering the surface of your fired piece. It happens when the glaze shrinks more than the clay body during cooling, causing the glaze to stretch and crack under tension.

Fixing glaze crazing in pottery requires altering the recipe or your firing routine. One of the most effective methods is increasing the role of silica in glaze stability. Because silica has a low thermal expansion, adding more of it to your glaze recipe (or to your clay body) can reduce the shrinkage gap. Alternatively, firing your pieces half a cone hotter can sometimes fuse the materials more thoroughly, minimizing the risk of cracks.

Shivering vs Crazing in Ceramics

While crazing is the glaze shrinking too much, shivering is the exact opposite. When comparing shivering vs crazing in ceramics, shivering occurs when the clay body shrinks more than the glaze. The glaze is compressed so tightly that it buckles and flakes off the pot, typically along sharp rims, handles, or edges.

How to fix shivering:

• Decrease the silica content in the clay body.
• Increase high-expansion materials (like sodium or potassium) in the glaze recipe.
• Round off sharp edges on your pottery during the trimming phase to give the glaze a safer surface to grip.

Surface Imperfections: Crawling, Pinholes, and Blisters

Sometimes the chemistry is right, but the application or the firing environment is slightly off, leading to frustrating textural flaws.

How to Prevent Glaze Crawling

Crawling occurs when the glaze pulls away from the clay during firing, leaving patchy, bare spots of exposed bisque.

Learning how to prevent glaze crawling comes down to cleanliness and proper glaze application. If your bisque ware is covered in studio dust or oils from your hands, the glaze cannot adhere.

• Wipe your bisque: Use a damp sponge to remove dust before glazing.
• Watch your application: If you apply a second coat of glaze over a completely bone-dry first coat, the moisture can cause the bottom layer to lift and crack.
• Avoid overly thick glaze: Thick layers are prone to cracking as they dry, which almost always leads to crawling in the kiln.

Causes of Pinholes in Ceramic Glazes

Pinholing leaves the surface of your pottery looking like it was pricked with a tiny needle. The main causes of pinholes in ceramic glazes involve trapped gases. As organic materials in the clay burn away, they release gases that bubble up through the melting glaze. If the glaze freezes before those tiny bubbles can smooth over, a pinhole remains.

To fix this, try bisque firing slower to ensure all organic matter burns out completely before the glaze firing.

Remedies for Glaze Blistering and Cratering

Similar to pinholes but much larger, blisters and craters look like sharp, frozen bubbles. These are also caused by violently escaping gases.

The best remedies for glaze blistering and cratering involve adjusting kiln firing schedules. Implementing a “drop-and-hold” technique—where you allow the kiln to reach its peak temperature, drop down by about 100°F (38°C), and hold for 15 to 30 minutes—gives the melted glaze time to heal and smooth over the popped bubbles before it hardens.

Application and Firing Flaws

Your technique in the glazing room and your control over the kiln are the final hurdles to a perfect finish.

Runny Glazes and Thickness

The impact of glaze thickness on finish is monumental. A glaze that looks stunning at two coats might turn into a messy, running puddle at three. Preventing runny glazes in the kiln is crucial to protect your kiln shelves.

• Always test the specific gravity (water-to-powder ratio) of your glaze.
• Leave the bottom quarter-inch of your pots unglazed.
• Apply generous wax resist to the foot of your pots.
• Use a “catch plate” (a disposable bisque disc coated in kiln wash) under experimental or notoriously runny glazes.

Temperature Troubles: Over-firing and Under-firing

Hitting the correct temperature is non-negotiable. If you notice dull, rough, or milky surfaces, you are likely looking at the signs of under-fired ceramic glazes. The glaze simply didn’t get hot enough to melt into glass.

Conversely, over-firing causes its own distinct set of issues. Have you ever wondered what causes clay body bloating? When a clay body is fired past its maturation point, trapped gases expand, causing the clay itself to bubble, warp, and form large, ugly lumps under the glaze.

Finally, opening the kiln too early when it is still hot can result in thermal shock. Identifying cooling cracks in pottery (often called dunting) is easy: these cracks are usually sharp, go completely through the clay body, and make a dull thud when tapped, rather than a high-pitched ping. Always let your kiln cool to below 150°F (65°C) before cracking the lid.

The Functional Impact of Glaze Flaws

As ceramic artists, we must care about more than just aesthetics. We must strongly consider the food safety of unstable ceramic glazes.

When a piece of functional ware has crazing, those microscopic cracks can harbor bacteria from food particles and moisture. Shivering is even more dangerous, as tiny, sharp shards of glass can flake off into a user’s food or drink. Furthermore, under-fired glazes may not be chemically durable, meaning acidic foods (like coffee or citrus) can leach heavy metals from the glaze into the food. Ensuring your glazes are perfectly fitted, properly applied, and fully fired is the only way to guarantee your dinnerware is safe for daily use.

Final Thoughts

Encountering common glaze problems is a rite of passage for every potter. Rather than viewing a flawed piece as a failure, view it as a diagnostic tool. By meticulously tracking your recipes, monitoring your application techniques, and tweaking your kiln schedules, you will soon learn to speak the language of ceramics. With patience and practice, you can transform unpredictable kiln openings into consistent, beautiful results.