Sodium Silicate

Sodium silicate changes the electrical charge on clay particles. The flat faces of a clay platelet are negatively charged, but the edges can be positive, and when they are, the platelets link edge to face into an open structure that traps water and thickens the slip. Sodium silicate raises the pH and makes the edges negative too, so the structure collapses and the particles push apart instead of linking. The same solids then pour freely with much less water. A slip commonly needs a fraction of one percent of the dry clay weight, often paired with soda ash, and past that amount the slip thickens again. The cast forms more slowly, and the cast comes out hard and brittle. Sodium silicate is also brushed onto a thrown wall and dried to harden the surface, so that it cracks in a controlled way when the wall is stretched.