Compression is pressing clay firmly with a rib, sponge or fingers to pack the particles together, especially across the base of a thrown pot and along rims and slab edges. It is the main defence against S-cracks and edge cracks.
What Is Compression?
When clay is opened and pulled on the wheel, the particles in the base are left loosely arranged and full of water. Compressing the base with a rib or the side of your fingers, often in a spiral from the centre out, aligns the particles and squeezes out excess water, so the base shrinks evenly as it dries.
Ceramic Arts Network lists compression alongside particle alignment and even wall thickness as the three main ways to prevent S-cracks. Many teachers ask students to compress the base twice: once after opening and again before lifting the walls.
Compression helps elsewhere too. Running a chamois or sponge over a thrown rim firms it up and helps stop tiny rim cracks, and compressing the edges of slabs with a rib makes them less likely to crack as they dry. It also leaves a smoother surface for glazing. Clay that was wedged well and thrown without too much water needs less compression afterwards, which is another reason to keep the wheel tidy.
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.