Try drying the slabs on cloth which has been sprinkled with grog and then
placed on a large wood or plaster platform. The grog will allow the movement
of the clay as it shrinks and the cloth permits evaporation from the
underside. Dry very slowly by loosely draping a large piece of plastic over
the top. Depending on the thickness, a week to 10 days drying time is not
unusual and desired. Also adding extra grog to the clay while wedging will
help "open" the clay up and allows for more even drying and less warpage.