Active bentonite with high degree of swelling capacity; Together with water, which is added during conditioning, it induces the bonding capacity of bentonite-bonded molding materials.
The main constituent is the clay mineral montmorillonite. Due to thermal stresses during repeated recirculation of sand, part of the bentonite looses its swelling and bonding capacities; i.e. the bentonite is burnt and must be partly replaced by new material during conditioning. It must be ensured that the ratio between active bentonite and dead bentonite (oolitization, hard bentonite) is balanced.
Determination of the active clay contents is performed by means of methylene blue adsorption or through heavy-medium separation in an alcohol bromoform mixture.