Bentonite Clay (Calcium Bentonite) is used as a binder in water bonded foundry sands.
Add water (1% to 3% by weight of sand) to the sand and mix.
Foundry sands typically use between 2% to 10% bentonite by weight of sand as the binder. Add the bentonite to the wet sand and mix throughly.
Mixing in the water with the sand first promotes adhesion of the bentonite to the sand grains.
Let the sand sit overnight after mixing to allow the moisture to distribute evenly thru the mix. Store it in closed plastic buckets or covered with plastic to keep in moisture.