SODSIL
A solution of silica and soda in water, not a dry powder · Thins a casting slip by making clay particles repel each other · Past the right amount it thickens the slip again and slows the cast · Freezing damages it
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Sodium silicate is an alkaline solution of silica and soda in water, and it is what a casting slip is deflocculated with. A very little of it makes a slip pour freely with much less water than the same clay would need without it.
Sodium silicate works on the charge a clay particle carries. The flat faces of a platelet are negative; the edges are not always, and where they run positive the platelets link edge to face into an open structure that traps water and thickens the slip. Sodium silicate raises the pH and turns the edges negative too; the structure collapses and the particles push apart rather than linking, so the same solids pour freely with much less water. Past that point more of it thickens the slip again, the casting slows rather than quickens, and the cast comes out hard and brittle. Brushed onto a thrown wall and dried, it hardens the surface enough to crack when the wall is stretched.
