This occurs when expansion or contraction of the glaze does not fit the
clay body. The problem that you are describing with the glaze is called,
"shivering". If the low fire clay that you are using has sand in the clay
this may change the rate of shrinkage in the kiln. Bisque firing at a
higher temperature may increase the shrinkage of the body and this may
eliminate the problem. \
-Numo