The main additives for foundry sands are:

Bentonite: the main inorganic binder in green sand systems; it swells upon contact with water to bind the sand grains together.
Mineral black: an additive that releases volatiles to create a reducing atmosphere and forms a lustrous carbon layer that acts as a release agent and protective coating.
Starch and dextrin: organic auxiliaries that can be used in combination with other binders to enhance the properties of foundry sand. They serve as binding agents, helping to hold sand particles together and ensuring mold stability during the casting process.
Premixes: calibrated blends of the aforementioned additives that make it easier for foundry operators to incorporate these elements into the sand.