I use the same clay as in the clay body. As bentonite has approximately six times the shrinkage of clay. Bentonites are volcanic clays which have the ability to absorb huge quantities of water into their structure and hence shrink considerably on drying. Not all bentonites shrink six times as much as clay but most come somewhere near it. So far the Hunter Valley ones seem fine to me.
Bentonites are difficult to add to water.
There are two approaches. Magnesium Matt 3. 33 Bentonite 3 Ball Clay 35 Red Iron oxide 10 4. Pseudo Shino 5. Opaque White 6.