For an S-shaped crack, compare the drying of the base with the walls and rim. A thick center, exposed edge or restrained base can create uneven shrinkage. Compression is worth reviewing, but the crack shape alone cannot identify one missed throwing step.
When did the crack become visible?
Inspect as the pot firms, before firing and after firing. A crack first noticed in bisque may already have existed in the dry clay. Photograph it before any attempted repair so that its extent is recorded.
Note whether the crack lies only underneath, passes through the floor or connects with another defect. Keep separate records for cracks at handles or rims; those joints and shapes may involve different stresses. Grouping every defect as an S-crack makes troubleshooting less precise.
Compare thickness and moisture
A broad floor may dry differently from an exposed rim. A heavy base may remain wetter than a thin wall. Direct drafts, sun and the supporting surface can increase those differences. Examine the piece itself rather than assuming that a certain number of days produces even drying.
Inspect the thickness of comparable practice work. A substantial block beneath a delicate wall calls for reviewing the opening depth and trimming allowance. The form need not have identical thickness everywhere, but abrupt changes can make drying harder to manage.
Check the supporting surface
Clay shrinks while it dries. A base that is restrained on a supporting surface may experience a different condition from the exposed wall. Review whether the piece was released appropriately for its bat and whether it could be moved without distortion.
Keep a plate or wide bowl supported until it can be handled without bending, then consider how its underside will receive air. Protect areas that are drying faster. Check inside the covering as well: the aim is more even moisture loss, not a rim that stiffens while the base remains wet.
Compression and the next batch
Smoothing and compressing the floor are useful forming steps. Ceramic Arts Network’s S-crack discussion emphasizes unequal shrinkage, however, so repeated cracking deserves a drying investigation even when you are satisfied with the compressed surface.
This is a useful problem for a labeled comparison batch. Keep the clay body and form similar, then change a specific condition such as base allowance or protection of a fast-drying rim. Record the first visible crack and the moisture condition of the center and edge. “Rim firm, center still soft” identifies a difference that “dry slower” does not.
A cracked base before firing
Glaze does not make a cracked base structurally sound. For routine beginner work, reclaim a failed unfired piece through an appropriate process. Ask the firing studio before submitting a cracked piece or attempting a repair.
Questions about this topic
Is every S-crack caused by insufficient compression?
No. Unequal drying and shrinkage are important causes. Compression alone does not establish what happened in a particular pot.
Should I dry every pot for a fixed number of days?
No. Clay body, thickness, humidity and support affect drying. Judge the actual condition of the whole piece and follow the firing studio’s requirements.