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These M390 mugs had progressed to stiff leather hard (after handles were applied, using just slip, and bases trimmed). A white engobe was then applied to the inside. It significantly softened the bodies of the mugs (to re-dry them to the point of being able to turn them over takes 4-6 more hours). This process sets up a big moisture differential between mug and handle. "Even drying" is the key to success, so slowing down the handles is obviously needed. How? I apply wax emulsion (actually I like Reed Wax), leaving just the inside handle-curves bare. That slows down drying enough to keep them even with the body of the mug. This method works so well that these do not even need covering during drying (even in our desert climate). Even drying is the goal - fast and even drying is much better than slow and uneven.
Handles expose all sides to the air and dry (and therefore shrink) much more quickly than the walls of the mugs (especially in our dry climate). To achieve more even drying we put the handles on as soon as possible and place them like this. We also cover them by wrapping a heavy cloth over and tucking it in all around.
Drying cracks are opportunistic, especially in highly plastic or fine-particled clays. They like to initiate inside sharp acute angles. The sharper the angle the greater the chance of crack. By doing this procedure before the clay gets too stiff (in the leather-hard stage) 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 too far 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.
Recipes |
L3954B - Cone 6 Engobe (for M340)
Dry and firing shrinkage fitted to Plainsman M390, M340 |
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Materials |
Wax Emulsion
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Glossary |
Drying Performance
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. |
Glossary |
Drying Crack
During drying clays and porcelains shrink as they become rigid. When this occurs unevenly, cracks are the result. |
Glossary |
Engobe
Engobes are high-clay slurries that are applied to leather hard or dry ceramics. They fire opaque and are used for functional or decorative purposes. They are formulated to match the firing shrinkage and thermal expansion of the body. |
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