Preventing S-cracks: why bottoms split and how to stop it
S-cracks come from uneven compression and uneven drying. Here's where they start and the handful of habits that prevent them.
An S-crack is the S- or spiral-shaped split that appears in the bottom of a thrown form, usually after drying or after the bisque. By the time you see it, the cause is hours or days in the past — at the wheel.
The two root causes
- ·Insufficient compression of the bottom while throwing — the clay platelets aren't aligned and bonded, so they pull apart as they shrink.
- ·Uneven drying — a thick bottom and thin walls drying at different rates put the floor under tension.
Compress the bottom — twice
After opening and before pulling walls, compress the floor firmly with a rib or your fingers, working from center out. Do it again once the walls are up. Compression is the single biggest lever; most S-cracks trace back to skipping it.
Even the wall-to-floor thickness
A floor much thicker than the walls dries slower and shrinks later, fighting the rest of the pot. Aim for a floor close to wall thickness, and don't leave a heavy slug of clay in the base.
Slow, even drying
Dry upside down or on a wire rack so the foot and floor lose moisture at a similar rate to the rim. Cover loosely for the first day. Forced, one-sided drying — a heater on one face — is a reliable way to manufacture cracks.
Let Claybench do the math.
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