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This bisque mug has been glazed on the inside. But the bisque has absorbed water from that glaze, and this thin-walled mug is now waterlogged as a result (except at the thicker base). It does not have the absorbency needed to build up a thick enough layer of glaze on the outside. Even if it did, the water from the two glazes would wet the bisque so much that its drying time would be greatly extended. This is a problem because the mechanism of attachment of glaze to body is fragile and works best when the glaze dries quickly. When drying is too very slow, bubbling and cracking often occur (leading to crawling in the firing).

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A light coating of ordinary ceramic dust is made of the same minerals as the ware and glaze itself. During dipping, the incoming water instantly penetrates and incorporates surface dust. Glaze are dust mixed with water! However, washing trades a tiny and often harmless surface contaminant for a more serious problem: partially saturated pores. Bisqueware is a porous ceramic sponge. Its job during dip glazing is to suck water out of the slurry quickly and evenly, leaving behind a stable layer of particles on the surface. If the bisque is damp from washing, the suction is weakened. Of course, there are common-sense exceptions, but compressed air, a soft brush, or a barely damp sponge used sparingly are better cleaning solutions to preserve the essential dryness.
These thin-walled mugs demonstrate: Just pour-glazing the inside has waterlogged them, they must be dried before applying the outside glaze. Imagine if they had been washed also!
| Glossary |
Glaze thickness
Many ceramic glaze benefits and issues are closely related to the thickness with which the glaze is applied. Many glazes are very sensitive to thickness, so control is needed. |
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