What's the difference between air-dry clay and BOHS foam clay?

Clay can be dried hard in two ways: air dry or bake dry.
All varieties of clay that may be dried hard by simply exposing it to air are referred to as "air dry clay," and BOHS foam clay is one such type. Another type of clay that requires an oven or kiln to bake to become hard is referred to as "baked clay" or "oven baked clay," and it is often polymer clay.

Air dry clay that has only partially dried can be softened by mixing it with BOHS air dry clay softener; however, polymer clay that has been baked cannot be recovered.