Why Is My Glaze Bubbling After Firing?

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You open the kiln expecting a smooth, beautiful glaze, only to find tiny bubbles, blisters, or raised spots across the surface.

So, why is your glaze bubbling after firing?

Glaze bubbling is a common pottery problem and can have several causes. Trapped gases, glaze thickness, firing temperature, clay body, and firing schedules can all play a role.

The good news is that once you understand what’s happening inside the kiln, you can usually make changes to reduce the problem.

What Does Bubbling Glaze Look Like?

Bubbling can appear as:

• Small raised bubbles
• Blisters
• Pinholes
• Craters
• Rough patches
• Small holes in the glaze surface

Some bubbles may remain intact after cooling, while others can burst and leave small holes or craters.

The appearance can give you clues about what happened during firing.

Why Does Glaze Bubble During Firing?

One common explanation is gases being released from the clay or glaze while the glaze surface has already started melting.

Clay and glaze materials can release gases as they heat.

Ideally, those gases escape before the glaze becomes fully molten.

If the glaze has already formed a relatively sealed surface, gases may become trapped underneath it.

As the gas tries to escape, it can create bubbles or blisters.

Can Trapped Gases Cause Glaze Bubbling?

Yes.

Organic materials, carbonates, sulfates, and other components within ceramic materials can release gases during firing.

If those gases don’t escape before the glaze seals over, they can become trapped.

This is one reason the firing schedule can be important when troubleshooting bubbling.

Can Applying Too Much Glaze Cause Bubbling?

Yes.

A glaze that’s applied too thickly can be more likely to develop surface defects.

A thick glaze layer may take longer to melt and can make it more difficult for gases to escape before the glaze becomes fluid.

Excessive thickness can also contribute to other problems such as running, crawling, and uneven surfaces.

Follow the glaze manufacturer’s recommended application thickness rather than assuming more glaze will produce a better finish.

Can the Clay Body Cause Glaze Bubbles?

Yes.

The clay body itself can contribute to glaze bubbling because materials within the clay can release gases during firing.

Different clay bodies behave differently.

If the same glaze bubbles repeatedly on one clay body but performs well on another, the clay body may be part of the problem.

This is why testing a glaze on the specific clay you use is so important.

Can Firing Too Quickly Cause Bubbling?

It can contribute to the problem.

If the kiln moves through certain temperature ranges too quickly, gases may not have enough time to escape before the glaze begins sealing the surface.

A firing schedule that’s appropriate for one clay and glaze combination may not be ideal for another.

If bubbling is persistent, review the manufacturer’s recommended firing schedule and consider whether your heating rate is appropriate.

Can a Slow Firing Fix Glaze Bubbles?

Sometimes adjusting the firing schedule can help.

A carefully controlled firing can give gases more opportunity to escape before the glaze fully melts.

However, simply making the entire firing slower isn’t necessarily the solution.

The appropriate schedule depends on the clay, glaze, kiln, and firing temperature.

Avoid changing several firing variables at once because it makes troubleshooting much harder.

Can a Soak Help With Bubbling?

In some situations, a controlled hold or soak near the end of a firing can help certain glaze surfaces heal after bubbles have formed.

However, a soak isn’t a universal solution.

If the underlying cause is excessive glaze thickness, contamination, incompatible materials, or excessive gas release, holding the kiln at temperature may not solve the problem.

Follow the recommendations for your particular glaze and firing process.

Can Overfiring Cause Glaze Bubbling?

It can contribute to glaze surface problems.

If a glaze is fired beyond its intended range, it may become excessively fluid or behave differently from the expected result.

Some glazes can develop blistering or other defects when overfired.

Always use the firing range recommended by the glaze manufacturer.

Could the Bisque Firing Be the Problem?

Yes.

The condition of the bisque ware can influence how the glaze firing behaves.

If organic material or other substances haven’t been adequately burned out before the glaze melts, gases may be released during the glaze firing.

This is one reason proper bisque firing is important.

Can Dust Cause Glaze Bubbling?

Dust and contamination can contribute to glaze defects, although they aren’t the only possible cause.

Before glazing, make sure the bisque surface is clean and free from loose debris.

Keep your workspace clean and avoid contaminating the glaze or pottery during application.

How Can You Prevent Glaze Bubbling?

Start with the basics.

Apply the Correct Glaze Thickness

Follow the manufacturer’s instructions and avoid excessive glaze buildup.

Use a Suitable Firing Schedule

Make sure your firing schedule is appropriate for the clay and glaze.

Fire the Bisque Properly

A properly prepared bisque surface can help reduce problems during the glaze firing.

Keep Pottery Clean

Remove loose dust and avoid unnecessary contamination before glazing.

Test Your Glaze

Fire a test tile before glazing a large batch of pottery.

Keep Firing Records

Record your firing temperature, schedule, glaze, clay body, and any unusual results.

This makes troubleshooting much easier.

Can You Fix Bubbling After Firing?

Sometimes.

If the bubbling is minor, a second firing may occasionally allow some surface defects to heal.

However, refiring isn’t guaranteed to work.

If the glaze is already too thick or the firing temperature is already too high, another firing could make the problem worse.

Always identify the likely cause before deciding to refire.

Can Sanding Fix Glaze Bubbles?

For some minor surface imperfections, potters may carefully smooth rough areas after firing.

However, sanding glazed pottery creates dust, and the surface should be handled appropriately.

If the pottery is intended for food use, don’t assume that simply sanding a defect makes the piece suitable for functional use.

For serious glaze defects, it may be better to treat the piece as a learning experience and correct the process on future work.

Why Does My Glaze Bubble in Only Certain Areas?

If bubbles appear only in certain parts of a piece, look for differences in:

• Glaze thickness
• Clay thickness
• Surface contamination
• Shape of the pottery
• Application technique
• Kiln position
• Local firing conditions

For example, glaze that pools heavily in one area may behave differently from a thinly coated area.

Localized defects can provide useful clues about what went wrong.

Is Bubbling the Same as Pinholing?

Not exactly.

Bubbling refers to raised or blistered areas in the glaze.

Pinholing refers to small holes or openings that remain in the glaze surface.

The two defects can be related because both can involve gases escaping through the glaze, but they aren’t identical.

Understanding the difference can help you troubleshoot more accurately.

Final Thoughts: Why Is My Glaze Bubbling After Firing?

If your glaze is bubbling after firing, don’t immediately assume that your kiln is defective.

The problem may be related to trapped gases, glaze thickness, clay body, bisque firing, contamination, firing temperature, or the firing schedule.

Start by checking how thickly the glaze was applied and whether the clay and glaze were fired according to their recommended specifications.

If the problem continues, make small test tiles and change one variable at a time. This will help you determine whether the issue comes from the glaze, clay, application, or firing process.

With careful testing and consistent firing practices, you can usually get much closer to the smooth glaze surface you’re aiming for.