Slip

Slip is simply clay and water mixed to a liquid or semi liquid state, and the exact consistency changes with the job. Thin, cream like slip works for joining two pieces of leather hard clay, such as attaching a handle to a mug, where it acts as a kind of clay glue once the joined surfaces are scored and slipped together. Thicker slip is used for casting into plaster molds, where the plaster's absorbency pulls water out and builds a wall of clay against the mold surface over time.

Colored slip is also one of the oldest pottery decorating techniques, brushed, dipped, poured, or trailed onto greenware to add pattern and color that becomes a permanent part of the clay body itself rather than sitting on top like a glaze. Slip trailing, squeezing colored slip through a fine tipped bottle to draw raised lines and dots, is a distinct technique built entirely around this material, closer to cake decorating than painting.

For joining pieces, the classic mistake is relying on slip alone without scoring, lightly roughening both surfaces with a needle tool or fork first, since a smooth to smooth glue joint with no mechanical grip is far more likely to pop apart during drying or firing as the two pieces shrink. Slip made from the exact same clay body being used, sometimes called the magic water or deflocculated version for extra smoothness, generally bonds and shrinks more predictably than a mismatched slip mixed from a different clay.