1. . 
A concentrated slurry of bentonite clay mixed in fresh water. The maximum practical concentration of bentonite is about 30 to 40 lbm/bbl because greater concentrations of bentonite are difficult to mix and pump. Water is put into the rig's prehydration tank and the pH raised to 10 or 11 with caustic soda. Soda ash is added as required to remove hardness. Bentonite is slowly added through the mud hopper. Continual energetic mixing and stirring helps the clay particles fully disperse. In some muds, lignosulfonate should be added shortly before mixing the slurry into the active system to protect the colloidal clay particles from flocculation.