An S-crack is a curved or S-shaped crack across the base of a thrown pot. Digitalfire calls it the classic sign of uneven drying: the rim dries and shrinks before the thicker base, which tears.
What Is an S-Crack?
S-cracks are one of the most common frustrations for new throwers, because they often only show up after the bisque or glaze firing, when it is too late to fix them. Ceramic Arts Network explains that the stress fracture is already there in the dry clay; the firing just opens it up.
Three things cause most of them. Uneven drying, where the rim firms up while the base is still soft. Loose, uncompressed clay in the base, especially when a lot of water was left sitting in the pot while throwing. And a base much thicker than the walls, which dries and shrinks at a different rate.
Prevention is mostly habit: compress the base with a rib after opening and again before pulling, sponge out standing water, trim the base to an even thickness, and dry pieces slowly, upside down once the rim is firm.
If S-cracks keep appearing in one clay despite good habits, try a different drying setup before changing the clay, as Digitalfire suggests.
This pottery glossary entry from Le Studio Art Space is general educational content. Always follow the clay, glaze and kiln makers’ data for your own materials and equipment.