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, along with two less obvious causes that show up later in the process.

At a Glance

• Trapped air pockets expand rapidly as the kiln heats and are the most common cause of a dramatic kiln explosion.
• Water turns to steam around 212°F (100°C) and expands to roughly 1,700 times its liquid volume, so any residual moisture in the clay needs a slow, even ramp through that zone.
• "Bone dry" means room temperature to the touch, not just dry-looking; a pot that still feels cool still holds moisture.
• Quartz inversion, a sudden volume change in the clay's silica content around 1063°F (573°C), can crack ware if the kiln heats or cools through that window too fast.
• Cracks that appear hours or days after a firing, in the glaze layer only, point to a glaze-fit problem (crazing or shivering), not a firing mistake.
• Uneven wall thickness is a quiet cause behind cracks that survive careful wedging and drying; the base-to-wall junction is the usual weak point.

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 works out pockets of air and inconsistent moisture; potters treat it as close to non-negotiable, in the same way bakers treat kneading bread dough as an essential prep step. If you rush it or skip it, bubbles remain, and wedging also removes the layered stresses left by machine-pugged clay, which can cause their own drying and firing cracks even without a visible air pocket. The second way air gets trapped 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: 20 to 30 passes of the standard ram's-head or spiral technique is a reasonable target for a beginner working with a couple of pounds of clay, though dense, stiff clay can take longer to fully homogenize. Do this check on every fresh ball before you build with it, not just the first one from a new bag.

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 (evaporation pulls heat from the surface, which is why damp clay always feels cooler than dry clay, the same reason a wet towel feels cold). 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; a base twice as thick as the walls can easily need two to three times longer to reach the same dryness.

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. Candling matters most for thick or complex forms, large platters, closed shapes, anything with a lid, and it costs almost nothing in kiln time compared to the risk of skipping it.

Thermal Shock and Quartz Inversion

Past the steam zone, another hazard appears: quartz inversion. The silica (quartz) content in most clay bodies undergoes an abrupt structural change right at 573°C (1063°F), where the crystal shifts from its alpha to its beta form. According to the ceramics reference site Digitalfire, this is "the sudden volume change in crystalline quartz particles" as they pass through that temperature window, and it becomes dangerous specifically "if there are sufficient large particles of free quartz available and heat is not evenly distributed through a piece of ware in the kiln." In other words, thick or unevenly built pieces are the ones most at risk, because uneven heat means one part of the pot is inverting while another isn't, and that mismatch is what cracks the wall.

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. Industrial sanitaryware manufacturers manage this same risk at scale by slowing the cooling cycle and using finer-particle quartz in their clay bodies; a hobby potter's version of that fix is simply a slower kiln schedule and, where possible, a clay body with less coarse silica.

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, a mismatch ceramicists call glaze fit. As Digitalfire's glaze-fit reference puts it, "a glaze should have an expansion that is slightly lower than the body so that contraction during cooling puts it under compression." Glaze that shrinks more than the clay goes under tension instead and cracks into a crazing pattern. Glaze that shrinks less than the clay goes under compression and pops off in shards, the shivering failure. Notably, Digitalfire also points out that thermal expansion mismatches this small happen "in thousandths of an inch," which is a useful reminder that a brittle, fired ceramic surface has almost no tolerance for movement that a flexible material would shrug off.

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. Tapping the pot gently near the crack and listening for a dull versus ringing tone can help you judge whether the body itself is compromised; a crazed piece usually still rings close to normal.

Quick Reference: Causes and Fixes

Problem	Likely Cause	Fix
Explosion in the kiln	Air bubbles in the clay	Wedge more thoroughly (20-30 passes); check with wire cut
Crack through the wall	Moisture not fully gone	Dry to bone dry (5-7+ days); 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 (days later)	Glaze-fit mismatch (crazing)	Adjust glaze recipe or switch to a compatible commercial glaze
Glaze flaking off at edges	Glaze-fit mismatch (shivering)	Raise glaze expansion or switch glaze
Crack only at the base	Base restrained during drying	Dry on newspaper/fabric so the base can slide as it shrinks

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, which is one reason many shared studios are strict about it before anyone loads a communal firing.

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, sometimes two weeks or more for a large platter or a lidded jar. 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, and it's also more likely to still contain enough free quartz to trigger a quartz-inversion crack while the thinner walls have already passed through that zone safely. 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, especially once it goes through repeated freeze-thaw or dishwasher cycles.

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, since less surface area is in direct contact with the shelf.

Does the type of clay body change how much risk I'm carrying?

Yes. Clays with more free silica and less grog tend to be more prone to both quartz-inversion cracking and drying cracks, while a grogged body is more forgiving on both counts because the grog particles interrupt shrinkage stress. If you keep losing pieces to cracking despite careful technique, switching to a body with a bit more grog is often a faster fix than changing your firing schedule.