|Monthly Tech-Tip |
The foot ring on these leather-hard mugs has already been trimmed. At the stiff-leather-hard stage an engobe was applied to the inside. This rewets the bodies of the mugs, almost to the same point as freshly-thrown. But the handles did not get rewetted. To re-dry these mugs to the point of being able to turn them over will take 4-6 more hours. But by that time the handles will be too dry. To prevent that I waxed them after trimming (leaving a just the inside handle-curves bare). That slows their drying down enough to keep them even with the body of the mug. Keeping all parts of a piece at the same water content throughout the process, that is a key to successful drying.
Drying cracks are opportunistic. They will initiate inside sharp acute angles. The sharper the angle the greater the chance of crack. By doing this procedure after trimming you will deny a crack a place to start. Of course even drying is still important, the water content of a handle should now be allowed to get ahead of that of the main body of the mug at any time. In the pictures on the right, two tools are being used to compress and round the angle at which the handle meets the wall of the mug.
In ceramics, drying performance is very important to optimizing production. More plastic clays shrink more and crack more, but they are also better to work with.
Engobes are high-clay slurries that are applied to leather hard or dry ceramics and fire opaque. They are used for functional or decorative purposes.
During drying clays and porcelains shrink as they become rigid. When this occurs unevenly, cracks are the result.
L3954B - White Cone 6 Engobe Recipe
Dry and firing shrinkage fitted to Plainsman M390, M340