Why Did My Pottery Crack in the Kiln?

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Few things are more frustrating than opening a kiln and discovering that a piece you spent hours making has cracked.

You may immediately wonder:

Was it the clay? The firing? The drying? The kiln?

The truth is that pottery can crack for several different reasons, and sometimes the cause isn’t obvious.

The good news is that understanding the most common causes can help you prevent many cracks in future firings.

1. The Pottery Wasn’t Completely Dry

One of the most common causes of problems during the first part of firing is moisture remaining inside the clay.

Even when a piece feels dry on the outside, thicker areas can still contain moisture.

As the kiln heats up, that moisture turns into steam. If it cannot escape gradually, the pressure can damage the piece.

Solution: Let pottery dry thoroughly before firing. Pay particular attention to thick sections, enclosed areas, handles, and joined pieces.

2. The Piece Was Too Thick

Very thick pottery can be more difficult to dry and fire evenly.

Large differences in thickness can also create uneven shrinkage and stress within the piece.

This doesn’t mean every piece needs to be extremely thin. Instead, aim for reasonably consistent wall thickness and avoid unnecessary masses of clay.

3. Uneven Drying Caused Stress

Clay shrinks as it dries.

If one part of a piece dries faster than another, the different areas can shrink at different rates. That can create stress and eventually lead to cracks.

This is particularly common around:

• Handles
• Attachments
• Thick bases
• Rims
• Large flat surfaces
• Areas where two pieces of clay were joined

Solution: Dry pottery slowly and evenly, especially when pieces are large or have different thicknesses.

4. Handles and Attachments Are Common Trouble Spots

A mug can look perfect before firing and still develop a crack where the handle meets the body.

Why?

The handle and mug body may dry and shrink at different rates.

The same principle applies to decorative additions, feet, spouts, and other attached parts.

Good joining techniques and careful drying can significantly reduce this type of cracking.

5. The Clay Wasn’t Properly Joined

When adding one piece of clay to another, the connection needs to be strong enough to survive drying and firing.

Simply pressing two pieces together may not create a reliable bond.

For attachments, properly preparing the surfaces and using an appropriate joining method can help prevent separation and cracking.

6. The Clay Body May Not Be Suitable

Different clay bodies behave differently during drying and firing.

Some are more prone to certain types of cracking or warping than others.

If you’re experiencing repeated cracks even though your construction and drying seem consistent, check the clay manufacturer’s information.

Make sure you’re firing the clay within its recommended range.

7. The Firing Schedule May Be a Factor

Clay undergoes significant changes during firing.

The kiln heats the pottery gradually, and different stages of firing involve different physical and chemical changes.

If pottery is heated too aggressively at a sensitive stage, existing weaknesses can become problems.

This is one reason kiln firing should follow an appropriate firing schedule for the clay being used.

If you’re using a studio kiln, ask the studio about the firing schedule and whether it is appropriate for your clay.

8. Thermal Shock Can Cause Cracking

Pottery expands when heated and contracts as it cools.

Rapid temperature changes can create stress in ceramic pieces.

That’s why the cooling stage matters just as much as the heating stage.

Don’t rush to open a hot kiln.

Allow the kiln and pottery to cool according to the manufacturer’s and firing program’s recommendations.

9. The Piece May Have Been Under Stress Before Firing

Sometimes the crack you see after firing actually started before the kiln was turned on.

A tiny drying crack, weak attachment, or structural stress may not be obvious when the piece is greenware.

Firing can make that weakness much more visible.

If the same type of crack keeps appearing in the same location, inspect that area before the next firing.

How to Reduce Cracking in Your Pottery

You can’t eliminate every possible firing problem, but you can reduce the risk.

Dry Slowly and Evenly

Give your pottery enough time to dry completely.

Keep Thickness Reasonably Consistent

Avoid large differences in wall thickness whenever possible.

Strengthen Attachments

Pay extra attention to handles, feet, spouts, and decorative additions.

Use the Correct Clay

Make sure your clay is suitable for the firing temperature you’re using.

Follow an Appropriate Firing Schedule

Don’t assume that every clay body should be fired using the same schedule.

Don’t Rush Cooling

Allow the kiln to cool properly before unloading.

What If the Crack Appears in a Specific Place?

The location of the crack can provide a useful clue.

Crack around a handle:
Look at how the handle was attached and whether it dried at the same rate as the mug.

Crack across the bottom:
Consider uneven drying, differences in thickness, construction stress, or the way the base was formed.

Crack along a joined section:
Inspect the quality of the connection between the pieces.

Random cracks in several pieces:
Look at your clay, drying conditions, firing schedule, and kiln conditions rather than focusing on one individual piece.

Cracking is often a process problem rather than simply a “bad kiln” problem.

What About Cracks After Glazing?

Not every crack happens during the same stage.

A piece can survive bisque firing and develop a problem during glaze firing.

If your pottery is fine after bisque but cracks during glaze firing, consider the clay body, glaze compatibility, firing conditions, and any stresses already present in the piece.

If you’re seeing repeated problems, keep notes about your clay, glaze, firing temperature, and firing schedule. Patterns become much easier to identify when you have something to compare.

Final Thoughts

When pottery cracks in the kiln, it can feel like the firing ruined your work.

But a crack is also information.

Look at where the crack appeared, when you noticed it, how thick the piece was, how it dried, how it was constructed, and what firing it went through.

Once you start identifying those patterns, you’ll often be able to make small changes that dramatically improve your results.

And remember: even experienced potters lose pieces in the kiln.

The goal isn’t to never have a pottery crack.

The goal is to understand why it happened so you can make the next piece stronger.