Western sodium bentonite. Montmorillonite. An ultra fine, clay-like mineral of volcanic origin added sparingly to dramatically improve clay body plasticity and to augment the suspension properties of clay in glazes while improving strength. Additions of up to 2% in either clay or glaze are typical; more raises the risk of cracking due to attendant high shrinkage.
For more information on bentonite, see the Digitalfire.com Reference Library of ceramic materials.