Welcome to the forum wrongplanet. I don't know about driller's mud, but sodium bentonite is a naturally occurring, mined clay, perhaps there are some chemicals added for specific use in drilling operations.
Rick, like Dave Davidson said, pure clay will shrink and crack when drying. As to "how much clay", that depends on the type available or used. Different types have different properties. Less bentonite clay is needed, because it swells so much when in water and sort of self-compacts to fill voids..I think 10% bentonite well tilled/mixed into marginal soils is standard, but I'm not positive there. In Ohio, if your water table is high and KEEPS your test holes filled, it would be cheaper to excavate deeper, and let the natural water table keep it at whatever it's water level is. Otherwise, a pond builder familiar with your soils will know your clay types and wherever clay is needed (outside the dam core) on sides and/or bottom, the clay blanket is spread in 6" lifts, then each lift is well compacted to "stitch" it all together.
