The powders of HPM-20 bentonite (left) and National Standard 325 bentonite (right) fired to cone 6. Both have sintered into a solid mass. The HPM-20 is much more expensive because of the extra grinding done to make it micro-fine (for non-ceramic uses). However, its data sheet shows an Fe2O3 content double that of the National Standard material. That means the latter should be firing to alot lighter color. But they seem very similar.
National Standard 325 Bentonite
HPM-20 Volclay Bentonite