Why Pottery Cracks or Explodes in the Kiln

Finding a pile of shards at the bottom of the kiln is demoralizing, but the cause is almost always traceable. Pottery explodes or cracks in the kiln for a short list of reasons, and once you understand them, they are largely preventable. The two main culprits are trapped air bubbles and moisture that has not fully left the clay before the temperature climbs. Let's look at each one in detail.

Air Bubbles: The Most Common Cause of Kiln Explosions

A pocket of air sealed inside the clay wall has nowhere to go when the kiln heats up. As the temperature rises, the air expands rapidly. If the pocket is big enough and the surrounding clay is not yet porous enough to vent it, the pressure builds until the wall gives way. The result is a dramatic crack or an outright explosion that can damage neighboring pots too.

How Air Gets Trapped

Air enters clay in two main ways. The first is incomplete wedging before you build or throw. Wedging is the kneading process that homogenizes the clay and drives out pockets. If you rush it or skip it, bubbles remain. The second way is from the throwing or hand-building process itself: folding clay back on itself, trapping a fold, or sealing a coil joint without pressing it firmly can all leave voids.

How to Check for Air Bubbles

Slice a test ball of wedged clay with a wire tool and look at the cut face. A well-wedged body shows a smooth, even surface without visible holes. If you see gaps, keep wedging.

Moisture Left in the Clay

Steam is the other major reason pottery cracks or explodes in the kiln. When water in clay turns to steam around 212°F (100°C), it expands to roughly 1,700 times its liquid volume. Clay that is only surface-dry but still damp inside cannot vent that steam fast enough. The result is a crack, a blow-out, or both.

What "Bone Dry" Actually Means

Bone dry clay feels room temperature against your cheek or the back of your wrist. It no longer feels cool, because cool clay still holds moisture that is evaporating. If your pot feels even slightly cold, it needs more drying time. Thick walls and heavy bases dry slower than thin ones, so give them extra time.

Rushed drying is one of the most common beginner mistakes. Placing a pot that dried for only a day or two straight into a firing is risky. A good target for most beginner work is at least a week of open-air drying, longer for anything thick or large.

For a detailed breakdown of safe drying technique, see how to dry pottery before firing so it doesn't crack.

Kiln Loading and the Candling Period

Even a properly dried pot can pick up ambient moisture if the weather is humid or the pot sits in a damp studio. Many potters use a low-temperature hold, called candling, at the start of a firing. The kiln sits at around 200°F (93°C) for an hour or two before the normal ramp begins. This evaporates any residual surface moisture before the temperature crosses the steam-expansion zone.

Thermal Shock and Quartz Inversion

Past the steam zone, another hazard appears: quartz inversion. Silica in clay undergoes a structural change at around 1063°F (573°C), expanding suddenly on the way up and contracting on the way down. If the kiln climbs or cools too quickly through this window, the clay may crack.

This is why firing schedules include slow ramps through key temperature ranges, and why you should never open the kiln before it cools below 200°F (93°C). Thermal shock on the way down causes dunting cracks, which run through the wall in smooth lines and typically happen near the foot of the pot.

For context on why bisque and glaze firings use different temperature curves, see bisque firing vs glaze firing: what each one does.

Glaze-Fit Failures That Cause Cracks After Firing

Sometimes the kiln itself is not the direct cause. A finished, apparently whole pot can develop hairline cracks hours or even days after the firing. This is called crazing (a network of fine cracks in the glaze) or, in more severe cases, shivering (glaze flakes off and the body can split).

Both crazing and shivering happen when the glaze and clay body expand and contract at different rates during cooling. Glaze that shrinks more than the clay goes under tension and cracks into a crazing pattern. Glaze that shrinks less than the clay goes under compression and pops off in shards.

This is a glaze-chemistry problem, not a wedging or drying problem, but beginners sometimes mistake crazing cracks for kiln damage. If the crack is in the glaze surface only and the clay body beneath is intact, glaze-fit is the issue.

Quick Reference: Causes and Fixes

Problem	Likely Cause	Fix
Explosion in the kiln	Air bubbles in the clay	Wedge more thoroughly; check with wire cut
Crack through the wall	Moisture not fully gone	Dry to bone dry; use a candling hold
Crack through the wall	Too-fast temperature ramp	Slow the firing schedule through key ranges
Crack after the kiln cools	Quartz inversion / too-fast cooling	Keep lid closed until under 200°F
Fine network of cracks in glaze	Glaze-fit mismatch (crazing)	Adjust glaze recipe or switch to a compatible commercial glaze

To understand what your kiln is actually doing at each stage, see how a pottery kiln works: a beginner's guide.

Frequently Asked Questions

Can a small air bubble actually destroy a whole kiln load?

A large explosion can send shards into neighboring pots and crack them. It rarely destroys the kiln itself, but it can chip shelves and ruin surrounding work. Thorough wedging protects your neighbors as much as your own pot.

How long should I dry my pottery before firing?

Most hand-built and thrown pieces need at least five to seven days of open-air drying at room temperature. Thick work, closed forms, and anything with an attached base or handles needs longer. When in doubt, add another day. The cost of waiting is nothing compared to the cost of a kiln explosion.

My pot cracked in the kiln but I wedged carefully. What else could have caused it?

Uneven wall thickness is a common hidden cause. A thick base attached to thin walls dries and heats unevenly. The thick section holds moisture longer and expands at a different rate. Keep walls and base as even as possible, and pay attention to the base-to-wall junction where thickness often spikes.

Is a crack on the outside of a bone-dry pot a problem before firing?

A hairline crack that appeared during drying will not heal in the kiln. You can fill it with a slip made from the same clay and let it dry again, but it remains a weak point. Decide whether the piece is worth firing; functional ware with a crack in the base is at risk of failing later.

Why does my pottery crack only on the bottom?

The base often cracks because it cannot shrink freely during drying. If a flat pot is sitting on a solid shelf or bat, friction holds the base in place while the rim pulls inward. Use a piece of newspaper or thin fabric between the pot and the bat during drying, so the base can slide slightly as it contracts. Foot rings and trimmed bases also tend to behave better than flat, untrimmed bottoms.