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S-crack on the bottom of a pottery bowl. This pot was thrown off the mound.

S-crack on the bottom of a pottery bowl. This pot was thrown off the mound.

Photo © 2009 Beth E Peterson
Definition: S-cracks (noun) are S-shaped cracks that can occur in the middle of the floor of a pot as it dries. S-cracking is most common when a pot's floor was not compressed during throwing or when the pot is dried too quickly. They can show up during drying, during the bisque firing, or during glaze firing.

The best way to avoid S-cracks is to gently compress the floor of a pot several times during throwing and to keep excess water or slurry from sitting in the pot during and directly after throwing.

Why does the clay crack in this serpentine pattern? It is due to the way pressure is applied to the clay during the forming of the pot's floor. Uneven pressure, or not enough pressure evenly applied, allows the clay platelets to remain disorganized within the fabric of the clay. This, in turn, means that shrinkage will be uneven. The differences in shrinkage follow the differences of compression, or the patterns of the fingers as they formed the floor. Therefore, the crack changes direction as it goes through the center of the floor.

Pots thrown off the mound are especially susceptible to the formation of S-cracks. (noun)
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