The best bonding clays for metal casting are sodium bentonite and calcium bentonite. Sodium bentonite occurs naturally in the Western United States and thus is referred to as Western bentonite. Green sand produced with Western bentonite will have medium green strengths and high dry strength, which increases resistance to erosion of metal but can cause issues at shakeout. Western bentonites are commonly used for steel casting.
Calcium bentonite is mined in Alabama and Mississippi and is thus referred to as Southern bentonite. Calcium bentonite is more widely distributed. Green sand produced with Southern bentonite will have high green strengths but low dry strength, causing low erosion resistance and prone to scabbing and other expansion defects.
Most of the available bentonites used for green sand molding are blends of Western and Southern and include specific additives to achieve the desired mold characteristics. In addition, clays absorb water from the atmosphere, so they should be stored in dry conditions.