Why Does Pottery Crack After Firing?

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Finding a crack in your pottery after firing can be frustrating, especially when the piece looked perfectly fine before it went into the kiln.

Unfortunately, pottery can crack during or after firing for several different reasons. The cause may be related to the clay, construction, drying, firing schedule, glaze, or the way the pottery responds to temperature changes.

The good news is that the location and appearance of the crack can often give you clues about what went wrong.

Why Does Pottery Crack in the Kiln?

Pottery goes through major physical and chemical changes during firing.

The clay loses remaining chemically bound water, burns out organic material, shrinks, and eventually becomes ceramic.

As the kiln heats and cools, the pottery also expands and contracts.

If stresses develop faster than the piece can tolerate, cracks can form.

That’s why a pot can look completely normal before firing and come out of the kiln with a crack.

1. The Pottery Was Not Completely Dry

One of the most important rules of firing is to make sure greenware is completely dry before it goes into the kiln.

Remaining moisture turns into water vapor as the kiln heats.

If moisture is trapped inside thick clay, the pressure can become a serious problem.

This is especially important with:

• Thick pottery
• Large sculptures
• Heavy bases
• Enclosed forms
• Pieces with uneven wall thickness

Allow thick or large pieces additional drying time rather than relying only on how dry the surface feels.

2. Uneven Clay Thickness

A pottery piece with extremely thick and thin sections can behave unevenly during firing.

Different areas may heat, cool, and shrink at different rates.

This can create internal stress and contribute to cracking.

Try to keep walls reasonably consistent and avoid unnecessary masses of clay.

For more on drying-related cracks, see Why Does My Pottery Clay Crack While Drying?.

3. Poorly Joined Clay Attachments

Handles, feet, spouts, decorative pieces, and other attachments can become weak points.

If an attachment isn’t properly joined or has a significant difference in thickness from the main body, stress can develop during drying and firing.

Common problem areas include:

• Mug handles
• Bowl feet
• Sculpture attachments
• Spouts
• Large decorative additions

Careful joining and even drying can reduce these problems.

4. S-Cracks in the Bottom of a Pot

An S-crack is a crack that develops in the bottom of a pottery vessel and often has a curved or S-like appearance.

They’re particularly associated with wheel-thrown pottery.

Several factors may contribute, including:

• Uneven compression of the clay
• Excessively thick bottoms
• Drying conditions
• Clay preparation
• Throwing technique

If you repeatedly get S-cracks, pay particular attention to how you prepare and compress the bottom of your pots.

5. Cracks Along Seams

Handbuilt pottery can develop cracks along seams where slabs, coils, or separate sections were joined.

If the sections aren’t properly integrated, the joint can become a weak point during drying and firing.

The problem can be made worse when the joined pieces have different thicknesses or moisture levels.

6. Thermal Shock

Pottery expands as it heats and contracts as it cools.

Rapid temperature changes can create thermal stress.

This is known as thermal shock.

Thermal shock can occur if pottery experiences an unusually rapid temperature change or if different parts of a piece experience temperature changes at different rates.

This is one reason kiln firing and cooling should be controlled rather than rushed.

7. The Clay Was Fired at the Wrong Temperature

Different clay bodies are designed for different firing ranges.

If a clay body is fired outside its intended range, its physical properties may not develop as expected.

An incorrect firing temperature can contribute to problems such as:

• Excessive shrinkage
• Warping
• Weakness
• Deformation
• Cracking

Always check the clay manufacturer’s recommended firing range.

8. The Clay Body May Not Suit the Project

Not every clay body is suitable for every type of pottery.

A clay designed for small wheel-thrown pieces may behave differently from one formulated for large sculptures or handbuilding.

If you’re making very large, thick, or sculptural work, choosing a clay body appropriate for that purpose can make a major difference.

9. Glaze and Clay May Not Be Compatible

Sometimes the clay itself isn’t the only problem.

The glaze and clay body need to work together.

If they expand and contract differently, stress can develop between the glaze and clay.

This can produce defects such as crazing, where fine cracks appear throughout the glaze surface.

Glaze cracking isn’t necessarily the same thing as the clay body itself cracking.

10. Rapid Cooling Can Cause Problems

The firing isn’t necessarily over the moment the kiln reaches its final temperature.

Cooling is also part of the process.

Rapid or uneven cooling can place thermal stress on pottery, particularly certain clay and glaze combinations.

Avoid opening a hot kiln simply because the firing has finished.

Allow the kiln and pottery to cool sufficiently before unloading.

Why Did My Pottery Crack After It Came Out of the Kiln?

Sometimes a piece may appear intact when removed from the kiln but develop a crack later.

This can happen because stresses within the ceramic continue to reveal themselves as the piece cools or is exposed to temperature changes.

Pay attention to when the crack appeared.

A crack discovered immediately after firing may have a different cause from one that develops days later during normal use.

Why Did My Pot Crack From the Bottom?

A bottom crack can have several possible causes.

Look at:

• The thickness of the base
• Whether the bottom was compressed properly
• How evenly the piece dried
• Whether the piece was completely dry
• The clay body being used
• The firing schedule

If the crack has a curved or S-like pattern, investigate possible S-crack causes.

Why Did My Mug Crack After Firing?

Mugs have several areas that can be vulnerable to cracking, particularly where the handle joins the body.

The handle and mug body need to dry and shrink together.

A large difference in thickness, an improperly joined handle, or uneven drying can create stress around the connection.

If the mug cracks during use rather than immediately after firing, thermal stress or glaze/clay compatibility may also need to be considered.

Can You Fix Pottery That Cracked in the Kiln?

Unfortunately, a pottery piece that has already been fired cannot simply be returned to its original clay state.

Some fired cracks can be repaired cosmetically or incorporated into an artistic repair, depending on the piece and its intended use.

However, repairs do not necessarily restore the original structural strength.

For functional pottery, especially pieces intended for food or drink, be cautious about treating a cracked piece as fully functional again.

How Can You Prevent Pottery From Cracking During Firing?

A good firing routine starts long before the pottery enters the kiln.

Make Sure Greenware Is Completely Dry

Don’t rush wet or thick pottery into the kiln.

Keep Thickness Reasonably Consistent

Avoid unnecessary masses of clay.

Join Attachments Properly

Handles, feet, and other additions should be securely integrated.

Dry Large Pieces Slowly

Large pottery generally needs more time and careful drying.

Use the Correct Firing Temperature

Match your clay body to the appropriate firing range.

Follow a Suitable Firing Schedule

Avoid treating every clay body and every piece exactly the same.

Allow the Kiln to Cool Properly

Don’t rush the cooling stage.

How to Troubleshoot a Cracked Pot

When a piece cracks, don’t immediately assume the kiln caused it.

Work backward through the process.

Ask:

Did it crack before firing?
The cause may be related to construction or drying.

Did it crack during firing?
Consider moisture, clay body, thickness, thermal stress, or firing conditions.

Did the glaze crack but the clay remain intact?
Investigate glaze fit and compatibility.

Did it crack during cooling?
Look at thermal stress and cooling conditions.

Did it crack later during use?
Consider thermal shock, mechanical stress, or an existing weakness.

This simple timeline can help narrow down the cause.

Frequently Asked Questions

Why does pottery crack after firing?

Possible causes include incomplete drying, uneven thickness, poorly joined sections, unsuitable clay, incorrect firing temperatures, thermal shock, and glaze-clay incompatibility.

Can pottery crack in the kiln even if it looked fine beforehand?

Yes. Firing causes major changes to clay, and weaknesses that weren’t visible before firing can become cracks during heating or cooling.

Why does pottery crack on the bottom?

Bottom cracks can be associated with thick bases, uneven drying, clay preparation, compression, and construction. Curved cracks may indicate an S-crack.

Why did my pottery crack during cooling?

Rapid or uneven temperature changes can create thermal stress. The cooling stage should be allowed to proceed appropriately for the kiln and clay body.

Can wet clay crack in the kiln?

Moisture trapped inside pottery can create serious problems as it turns into steam during heating. Greenware should be completely dry before firing.

Is a cracked pottery piece safe to use?

A crack can weaken pottery and allow liquids to enter the ceramic body. Whether a repaired piece is suitable for functional use depends on the type and location of the damage, so cracked functional ware should be treated cautiously.

Final Takeaway

Pottery can crack after firing for many different reasons, and the crack itself often provides clues about what happened.

Before blaming the kiln, look at the entire process: clay preparation, construction, drying, firing temperature, cooling, and glaze compatibility.

The most useful habit is to identify when and where the crack appeared. Once you know that, you can usually narrow down the possible causes and make a specific change to your next batch.

A cracked pot isn’t necessarily a wasted firing—sometimes it’s the best clue you have for improving the next one.