Most potters have experienced the effects of soluble salt migration in a clay body that can occur in the drying, bisque, or glaze firing stages. The industry term is ‘scumming’, and it presents itself as efflorescence, or white crystal powder, forming on the surface of the clay.
The industry term is ‘scumming’, and it presents itself as efflorescence, or white crystal powder, forming on the surface of the clay. It can be observed on random parts of the clay body surface but is most often seen on edges or high points which dry faster than other parts. Faster drying increases the ‘wicking’ action of the soluble salts that travel to the surface of the clay with the evaporating water. Textured surfaces or the ridges on cup handles can reveal a hard-discolored crust on the fired ware.
In the bone dry or bisque state, fingerprints
Why does my clay have a white irregular crystal surface when it is drying?
In the bone dry or bisque state, fingerprints can disturb the soluble salt deposit on the clay surface causing a noticeable flashing or discoloration on the fired clay. High concentrations of soluble salts on bisque clay surfaces can result in fused areas which retard absorption, resulting in uneven glaze deposits. Soluble salt migration can also disrupt the covering glaze surface. Essentially, it can create a mechanical disruption of the clay body/glaze interface which can result in the glaze crawling (the fired glaze rolls back revealing the clay body) or a series of small glaze blisters (sharp edged crater-like holes in the fired glaze) due to the flux. There are several options which can eliminate soluble salt migration.