Diagnosing a Crack: Why Did That Happen?
Okay, so this is the lesson where we stop building for a minute and do something that feels backwards: we look at broken stuff on purpose.
Every handbuilder ends up with cracks. Doesn't matter how careful you are, doesn't matter how many lessons you've sat through. The difference between someone who gets better and someone who stays frustrated is whether they can look at a crack and actually explain it. So that's the whole point today. Not preventing cracks. Reading them.
Start with where it cracked
Cracks tell you where they came from if you look at the location honestly.
Cracks along a seam or join almost always mean the join failed. Either it wasn't scored and slipped properly, or the two sides were at different moisture levels when you put them together, or both. We covered both of those already, and I'm not going to let this lesson turn into a re-lecture, but this is the payoff of those lessons. This is what it looks like when you skip them.
Cracks in the middle of a flat wall, nowhere near a seam, are usually a drying problem. The piece dried unevenly, one section shrinking faster than the section next to it, and something had to give.
Cracks at the base where a wall meets the floor of the piece usually mean the walls were too thick relative to the base, or the piece dried from the outside in without enough attention to the thicker parts underneath.
Different locations, different causes, different fixes. This is why I keep telling you guys to slow down before you patch something. A quick fix on the wrong diagnosis just means you get to have the same argument with the same pot again in a week.
The story I keep on my windowsill
My very first slab box cracked clean down one corner in the kiln. I still have the pieces. They've been sitting on my windowsill for about five years now, not because I'm sentimental about a broken box, but because I look at it sometimes and it keeps me honest.
Here's what happened. I built the walls at two different levels of dryness. One panel had been sitting out longer than the other, and I didn't check, I just joined them because they looked close enough. They were not close enough. The drier piece pulled against the wetter piece as everything dried and fired, and the seam gave out exactly where the stress was worst, which was the corner.
I was confident. That's the part that still gets me. I'd done a few pieces by then and felt like I knew what I was doing, so I stopped checking the thing that actually mattered. The box didn't fail because I was bad at this. It failed because I got a little casual about moisture, which is the one thing in this whole process that doesn't forgive casual.
How to actually diagnose your own crack
Next time you've got a crack, walk through this instead of just sighing and starting over.
- Find the exact location. Seam, open wall, or base junction.
- Ask when it happened. Did it crack while still wet and being handled, at leather-hard, bone-dry before firing, or in the kiln? Wet cracks are usually handling damage. Bone-dry cracks are usually drying stress. Kiln cracks are almost always something that was already there, invisible, that the heat just finished off.
- Check for the moisture mismatch. If it's a seam crack, ask yourself honestly whether both sides were the same stiffness when you joined them. Be honest even if the answer's embarrassing. I have been there.
- Check wall thickness with your fingers, not your eyes. Eyes lie. A wall that looks even can have a thin spot that dried faster than everything around it.
This is a good place to say the thing I say a lot in this class: score and slip every join, no exceptions, no matter what anyone tells you. I'll be honest with you, most of the crack diagnoses in a beginner class come back to a join that got skipped or rushed. It's not a glamorous rule. It's just the one that's true.
What you can and can't fix
If you catch a crack while the piece is still leather-hard, you can often re-score, re-slip, and press it back together, then blend it with a coil like we practiced. Once something's bone-dry, your options shrink fast, and once it's fired, that crack is permanent, full stop, no amount of hoping fixes a fired crack.
This is also why I don't sweat symmetry the way some of you want to. A slightly uneven coil pot with good joins will outlive a perfectly even one with a bad seam. Fix the crack. Leave the personality alone.
A caution worth repeating
If you're cutting into a piece to inspect or repair a crack, use your tools the way we've talked about, blade or wire moving away from your hand, not toward it. Diagnosing is slow, careful work, not a moment to rush because you're annoyed at the pot.
Before next time: find one piece you've got lying around with a crack in it, even an old one, and try to diagnose it using the steps above before you show up. We'll compare notes.
~erin