Why Does My Pottery Crack?

Pottery cracks for one underlying reason: stress in the clay. The most common source is uneven drying. Clay shrinks as it dries, and if a rim dries faster than a base, the tension pulls the piece apart. The other big culprit is thermal shock: rapid temperature change in the kiln, the oven, or even a hot dishwasher cycle on a cold mug.

The good news is that almost every crack is preventable once you know which stage it happened at. Below I’ll walk through cracks at the drying stage, cracks in the kiln, and the cold-to-hot cracking that ruins finished pieces, plus how to fix what’s already cracked.

The main reasons pottery cracks

Every crack traces back to one of these causes. Figuring out when the crack appeared tells you why it appeared.

When it cracks	Most likely cause	Typical crack
While drying	Uneven drying, attachments drying faster than the body	Cracks at handles, rims, joins
During bisque firing	Moisture left in the clay, heating too fast	S-cracks in the base, blowouts
During glaze firing	Glaze/clay fit mismatch, cooling too fast	Crazing, shivering, dunting
Weeks or years later	Thermal shock in daily use, moisture expansion	Hairline cracks, clean breaks

Uneven wall thickness makes all of these worse. A mug with a 1/4-inch wall and a 3/4-inch base dries and heats at two different rates, and the boundary between thick and thin is exactly where the stress concentrates.

Cracks during drying

Drying cracks are the ones I see most in beginner work. Clay can shrink 5 to 8% just going from wet to bone dry, and any part of the piece that shrinks ahead of the rest puts the slower section under tension.

The usual suspects:

Handles and attachments. A thin handle dries hours ahead of the mug body. Wrap handles, spouts, and rims in plastic so they dry at the body’s pace.
Rims drying before bases. Air hits the rim from all sides while the base sits on a board holding moisture. Flip pieces over once they’re stiff enough, or dry them on a wire rack.
Drafts and direct sun. A piece near a heater vent or sunny window dries lopsided. Slow, boring, even drying is what you want.
Poor joins. Attachments that weren’t scored and slipped properly pull away as both parts shrink.

Drying a piece evenly usually takes about a week, sometimes longer for thick work. When in doubt, loosely tent the whole piece in plastic and add a day or two.

S-cracks

An S-crack is the curved crack that appears in the bottom of wheel-thrown pots, usually showing up after bisque firing even though the stress was created on the wheel. It comes from poorly compressed clay in the base and from the base staying wetter than the walls. Compress the bottom firmly with a rib or your fingers after opening, and don’t leave a puddle of water sitting inside the pot.

Cracks during firing

If a piece survives drying but cracks in the kiln, the cause is usually one of three things:

Residual moisture. Clay that feels dry can still hold water in its core. Steam expanding inside the walls will crack a piece, or blow it apart entirely, which I cover in why pottery explodes in the kiln. Candling the kiln (holding it around 180 to 200°F / 82 to 93°C for several hours) drives off that last moisture safely.
Heating too fast through quartz inversion. At about 1063°F (573°C), the quartz crystals in clay suddenly change size. Pass through that zone too quickly, heating or cooling, and the piece can crack. Slow your ramp to around 150 to 250°F (65 to 120°C) per hour through that range, especially with thick work.
Cooling too fast. Cracking caused by rushed cooling is called dunting, and it often produces a clean crack with sharp glazed edges right through the pot. Resist the urge to crack the kiln lid early. Let the kiln cool to at least 250°F (120°C) before opening, and wait longer for large or thick pieces.

Will ceramic crack going from cold to hot?

Yes. This is thermal shock, and it’s the number one killer of finished pottery. Ceramic is strong under compression but weak under tension. When you pour boiling water into a cold mug, the inside surface heats and expands instantly while the outside is still cold. That mismatch creates tension, and if it exceeds what the piece can absorb, it cracks — sometimes with an audible ping.

The risk depends less on the absolute temperatures than on the speed and size of the swing:

A mug going from room temperature to hot coffee (a swing of about 130°F / 72°C) is fine for most stoneware.
A dish going from the freezer (0°F / -18°C) straight into a 400°F (200°C) oven is asking for a crack, even in good stoneware. That’s a swing of around 400°F (220°C).
Earthenware and pieces with existing crazing are far more vulnerable, because porous clay and micro-cracks give a fracture somewhere to start.

To protect a piece: warm it gradually. Put oven dishes in a cold oven and let them heat with it, run hot tap water into a mug before adding boiling water, and never set hot ceramic on a cold stone countertop or wet surface. The same logic applies to microwaving pottery, where uneven internal heating is just thermal shock from the inside out.

At what cold temperature does ceramic crack?

There’s no single temperature at which ceramic cracks. Cold alone doesn’t break fired pottery, and a well-vitrified stoneware or porcelain piece can sit in a freezer at 0°F (-18°C) indefinitely without harm. What cracks ceramic in the cold is one of two things:

Frozen absorbed water. Porous, low-fired ceramic (earthenware, flowerpots, anything that wasn’t fired to full vitrification) soaks up water. When that water freezes it expands about 9%, and repeated freeze-thaw cycles flake and crack the piece. This is why terracotta pots left outside over winter spall apart, often once temperatures cycle around the freezing point of 32°F (0°C).
Rapid swings from cold. Taking a dish from the freezer to a hot oven, or pouring boiling water into a glass or mug straight from a cold car, creates the same tension described above. The cold just makes the swing bigger.

Practical rules: vitrified stoneware and porcelain are freezer-safe; unglazed earthenware outdoors should be emptied and brought in (or at least lifted off wet ground) before freezing weather; and anything coming out of the fridge or freezer should rest toward room temperature before meeting high heat.

How to prevent cracks: my checklist

Wedge clay thoroughly to remove air pockets and even out moisture.
Keep walls and bases a consistent thickness. Under 1 inch everywhere, and ideally matched within the piece.
Compress bases on thrown work to head off S-cracks.
Score and slip every join, and consider magic water or slip made from the same clay body.
Dry slowly under loose plastic, wrapping rims and handles, away from drafts and sun.
Make sure work is bone dry before firing. Hold it against your cheek; cool means damp.
Candle the kiln, ramp slowly through 1063°F (573°C), and let it cool fully before opening.
Match your glaze to your clay body’s expansion. Persistent crazing means the fit is wrong, and I cover that in how to fix crazing in pottery.

Can cracked pottery be repaired?

It depends on the stage. Greenware cracks can sometimes be healed with magic water and paper clay made from the same clay body, though deep cracks tend to reopen in the firing. Bisque cracks can be filled with a paste of paper clay, finely ground bisque, and a few drops of glaze before the glaze firing. Results vary, and structural cracks usually come back.

Once a piece is glaze-fired, the kiln can’t fix it; you’re into cold repair with epoxy or, for decorative work, kintsugi-style fills. I’ve written full guides on fixing cracks in fired pottery and how to fix broken pottery. One honest note: a repaired crack is never as strong as the original, and repaired pieces shouldn’t go back into the oven or microwave. If the piece is past saving, there are still plenty of good uses for broken pottery.

FAQ

Will ceramic crack from cold to hot?

Yes. Moving ceramic quickly from cold to hot (freezer to oven, cold mug to boiling water) causes thermal shock, where the heated surface expands faster than the rest of the piece. Stoneware tolerates moderate swings; earthenware and crazed pieces crack easily. Warm pieces gradually and start oven dishes in a cold oven.

At what cold temperature does ceramic crack?

Cold by itself doesn’t crack fired ceramic. Vitrified stoneware survives 0°F (-18°C) and below. Cracking happens when absorbed water freezes and expands in porous ceramic (a problem starting at 32°F / 0°C), or when a cold piece is heated too fast. Keep porous earthenware dry and indoors over winter.

Why did my pottery crack while drying?

Part of the piece dried and shrank faster than the rest, usually a handle, rim, or thin wall outrunning a thick base. Dry work slowly under loose plastic, wrap attachments, and keep pieces away from drafts, heaters, and sunlight.

What is an S-crack and how do I stop it?

An S-crack is a curved crack in the base of a wheel-thrown pot caused by uncompressed clay and a base that stays wetter than the walls. Compress the bottom firmly with a rib after opening, remove standing water, and dry pots upside down once they’re leather hard.

Can I put cold pottery in a hot oven?

No. Going straight from fridge or freezer into a preheated oven is the fastest way to crack a dish. Let the piece approach room temperature, then place it in a cold oven and let it heat up with the oven.

Why did my pottery crack in the kiln?

The usual causes are leftover moisture turning to steam, ramping too fast through quartz inversion at 1063°F (573°C), or opening the kiln before it cooled. Make sure work is bone dry, slow your firing schedule, and keep the lid shut until the kiln is below about 250°F (120°C).