If you've been working with clay for more than five minutes, you've met cracks. They show up uninvited, ruin your best pieces, and make you question everything. But here's the good news: most cracks are preventable once you understand why they happen.
The Big Two Causes
Almost every crack comes down to one of two things: uneven drying or uneven thickness. That's it. Once you wrap your head around these two, you'll save a lot of heartbreak.
You've probably also been told that air bubbles cause cracks and explosions. Let's talk about that.
The Air Bubble Myth
This is one of the most widespread misconceptions in pottery. We've all heard it: "Wedge out every air bubble or your piece will explode in the kiln!" It's taught in almost every beginner ceramics class — but it's not quite right.
Air itself doesn't cause explosions. When air heats up in the kiln, it expands gradually and escapes right through the porous clay walls. Clay is not airtight — especially at bisque temperatures. The air simply works its way out.
What actually causes kiln explosions is moisture (water) trapped inside the clay. When water hits 212°F (100°C), it converts to steam and expands to roughly 1,600 times its original volume. If there's still moisture trapped deep inside the walls of your piece, that steam has nowhere to go — and the pressure builds until something gives. That's your explosion.
So why does the air bubble myth persist? Because bubbles can create pockets where moisture gets trapped and has a harder time escaping during drying. And they can create inconsistencies in wall thickness. But the bubble itself — the air inside it — is harmless. It's always the water.
Every potter wedges — it's part of the process. But the reason it matters isn't because of the air. It's because thorough wedging creates a uniform clay body, which dries more evenly, which means less trapped moisture. The real secret to avoiding cracks and blowouts? Give your pieces the time they need to dry completely — all the way through.
1. Uneven Drying — The Silent Killer
Clay shrinks as it dries. That's normal. But when one part of your piece dries faster than another, the shrinking happens at different rates — and that tension creates cracks.
Common culprits:
- Thin rims drying before thick bottoms
- One side facing a fan or heater
- Handles drying faster than the body
- Leaving pieces uncovered on a dry day
The fix: Slow and even is the mantra. Cover your pieces loosely with plastic. Dry them on wire racks so air circulates underneath too. Flip plates and bowls upside down halfway through drying. And for the love of clay — keep pieces away from direct heat sources.
2. Uneven Thickness — The Hidden Problem
If your walls are 1/4 inch thick but your bottom is 3/4 inch, you're setting yourself up for trouble. The thin parts shrink and stiffen while the thick parts are still wet inside — holding moisture that can turn to steam in the kiln. Something's gotta give, and it's usually a crack right through the base.
The fix:
- Aim for consistent wall thickness throughout your piece
- Compress the bottom of thrown pieces with a rib
- For hand-built work, roll your slabs to an even thickness (use guide sticks!)
- Check thickness by gently pressing — if one area feels much softer, it's thicker
S-Cracks on the Bottom
If you throw on the wheel, you've probably seen the dreaded S-crack — that squiggly line running across the bottom of your pot after bisque firing. This usually happens because:
- The bottom wasn't compressed enough during throwing
- The piece was left on the bat too long (the bat holds moisture while the walls dry)
- You pulled the piece off-center during cutting
The fix: Compress, compress, compress. Use a rib on the inside bottom at the end of every throw. Wire-cut your pieces off the bat and let them stiffen on a wire rack or board so the bottom can dry evenly with the rest.
Cracks After Bisque — Glaze Won't Fix Them
Real talk: if your piece has a crack after bisque, glaze will not magically seal it. The crack will still be there, just shinier. If it's a hairline crack and you love the piece, you can try filling it with a bisque fix — but honestly, most potters just accept the loss and learn from it.
Your Anti-Crack Checklist
- ✅ Wedge your clay for an even, consistent body
- ✅ Keep wall thickness consistent
- ✅ Compress bottoms with a rib
- ✅ Score and slip all joints
- ✅ Dry slowly and evenly (cover loosely with plastic)
- ✅ Dry on wire racks, flip halfway
- ✅ Keep away from direct heat and drafts
- ✅ Make sure pieces are bone dry before firing — not just surface dry
- ✅ Don't rush — clay knows when you're in a hurry
Cracking is frustrating, but it's also one of the best teachers in pottery. Every crack is telling you something about your process. Listen to it, adjust, and your success rate will climb. You've got this. 🏺
Tracking which pieces crack (and which don't) is one of the best ways to spot patterns. The Potter's Mud Room makes it easy to log every piece, every clay body, and every firing — so you can finally figure out what's working and what needs to change.