|Monthly Tech-Tip |
Partially dried clayware. Leather hard pieces can still be trimmed, fettled and are still flexible enough that pieces which have gone slightly out-of-round, for example, can be squeezed carefully back into shape.
These mugs are in the leather-hard 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.