Casting slip is liquid clay made with a deflocculant so it stays fluid with relatively little water. It is poured into plaster moulds for slip casting, where the plaster draws out water and a clay wall forms against the mould.
What Is Casting Slip?
Ordinary slip made by just adding water to clay needs so much water that it shrinks badly and cracks in a mould. Casting slip uses a deflocculant such as sodium silicate or Darvan to make the particles repel each other, so the slip pours freely while still being mostly clay.
Potters control casting slip by its specific gravity, the weight of a volume of slip compared with the same volume of water, and by its viscosity. A well-made slip casts an even wall in a predictable time, drains cleanly and releases from the mould without sticking or cracking.
You can buy casting slip ready-made in earthenware, stoneware, porcelain and bone china, or mix it from dry clay. It isn’t interchangeable with throwing clay recipes, because the balance of clay types and deflocculant is tuned for casting. Stir it well before every use and keep the lid on, since slip settles and thickens as it stands and loses water.
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.