> >>>Could anyone tell me why my students wheel thrown pots will get cracks
> >>> in the bottom of their ware?
There are several standard precautions to prevent the shrinkage cracks (also called S-cracks by potters) in the bottoms of thrown pots.
1. After opening and shaping the interior bottom, compress the clay by moving back and forth over the clay to lay the clay particles flat making a stronger bottom.
2. Be sure to remove all extra water or wetness from the inside of the pot. Water that remains will retard the drying of the bottom. Late drying shrinks the clay after the walls are too dry to shrink with the bottom.
3. Make the bottom thickness from 3/16 to 1/4 inch thick. If it is too thick it will dry too slowly. If it is too thin it will be too fragile. When opening, we use a pin tool to puncture the bottom to measure the clay thickness. For a cheap pin tool, you can poke a hat pin through a cork.
4. Take the wet pot from the wheel to a porous bat that will draw out some moisture. I use dry unglazed ceramic tile (commercial quarry tile) rather than plaster bats because plaster chips are a pain the rework clay--causing too many broken pieces from pop-outs when bisque fired.