|Monthly Tech-Tip |
Partially dried clayware. Leather hard pieces can still be trimmed, fettled and are still flexible enough that pieces that have gone slightly out-of-round, for example, can be squeezed carefully back into shape. Engobe and slip decoration are best done at the leather-hard stage (for optimal adherence and so they dry and shink together). Incising of designs is also done at this stage.
They have just been trimmed. The clay has dried firm enough to handle them without pushing them out of shape too much, but not so much that it is difficult to tool and flute the foot ring.
The body needs to shrink as it dries. Typical glazes have low clay content and shrink very little. So as the body shrinks underneath the glaze just flakes off. Brushing glazes contain significant amounts of gum, that gum bonds them securely to bisque ware, but not to unfired ware. As you can see here. the glaze bond with the body could not withstand the differential during drying.
This porcelain becomes quite brittle as it gets stiffer making it difficult to make these cuts in the foot ring. This creates extra sponging work when it is dry. It also means that dry strength will be low. Porcelains do not need to be this way, plenty of white burning bentonites are available (although they increase cost).
This is done at the leather hard stage. The lines are in a contrasting color because a black-burning engobe was applied, by dipping, earlier. The clay is Plainsman M340. The tool being used is a Kemper R4 ribbon tool.
|By Tony Hansen|
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