The extent of the devastation described makes is sound like moisture in the
clay. How did you determine that the clay was bone dry? Place the bottom of
the greenware or thickest part to your cheek and if feels cool or cold it is
not dry, even though it may look dry. If it has been raining or there is
high humidity, it can take a long time to dry clay. If you are in a humid or
rainy area, leave the kiln on low for several hours with the lid open before
raising the temp. for the normal firing cycle.