Slip is clay mixed with water to a creamy liquid. Potters use it as glue for joining clay, as a decorative coating in white or coloured layers, and, when made with a deflocculant, as casting slip for moulds.
What Is Slip?
The simplest slip is made by soaking scraps of the clay you are working with and mixing them to a smooth paste. Used with scoring, it bonds handles, coils and slabs. Using slip made from the same clay body matters, because it shrinks at the same rate as the pieces it joins.
Decorative slips are usually made from white or light clay, coloured with stains or oxides, and applied by brushing, dipping, pouring or trailing onto leather-hard work. They give a soft, matte layer of colour that can be carved through for sgraffito or covered with a clear glaze.
Slip is different from glaze, which is glass rather than clay. Slips that peel or flake usually shrink differently from the clay underneath; engobes are slips adjusted to fix that.
Casting slip is a special case. It uses a deflocculant so it can stay fluid with much less water, which lets it cast clean walls in plaster moulds without excessive shrinkage.
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.