A deflocculant is a chemical added in tiny amounts to clay slip or glaze to make the particles repel each other, so the mixture becomes fluid with much less water. Sodium silicate and Darvan are the most common.
What Is Deflocculant?
Clay particles naturally clump together, or flocculate, in water. Deflocculants give the particles a like electrical charge, so they push apart and slide past each other. The result is a slip that pours freely at a high clay content, which is the basis of every good casting slip.
Less water means less shrinkage, faster casting and fewer cracks, because a mould only has to draw out a small amount of water to form a wall. Typical additions are a fraction of a percent of the dry clay weight, and adding a few drops at a time while checking how the slip flows is the safest approach.
Too much deflocculant has the opposite effect, making slip thick, gelled or slow to drain, and it can make casts stick to the mould or crack. Sodium silicate also slowly wears down the surface of plaster moulds. Most potters who mix their own casting slip measure both its specific gravity and its flow before every session, so batches behave the same way.
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.