I've been thinking about how I'm going to do the refractory lining for the new foundry. Many of the recipes I've found on line call for bentonite clay. The bentonite I've been able to locate is feed-grade stuff sold at agricultural supply places. It's small pellets which I assume have to be dried and ground to a fine powder so it can mix with the other ingredients and coat them. I searched around on the Google an got some ideas for a ball mill. I knocked this together with scrap I had lying around. I still haven't figured out how to drive it. Maybe an old hand drill.