Notice the water is relatively clear, which means there is a minimum of free bentonitic or colloidal material (that should minimize drying shrinkage). That being said, there could be colloids that are agglomerated. The absence of coloration in the water also means there is a minimum of soluble salts present (salts, like calcium sulfate, are normally iron-stained and reveal themselves by darkening the water). Shown again is the oversize material on the 30 mesh screen (after drying). These pebbles range in size from 7mm down to 30 mesh (remember there were also a few large ones removed at step 3). The existence of these pebbles means that this clay cannot be processed by grinding the dry powder, it likely has to be slurried and screened (that means only a potter would be able to utilize it).
Evaluating a clay's suitability for use in pottery
Would you like to be able to use your own found-clays in your production? Follow me as we evaluate a mystery clay sample provided by a potter who wants to do this.