What is the difference between a chemically reactive form release agent and a barrier-type release agent?

Barrier-type form release agents, also known as form oils, use heavy oils diluted in lighter oil such as kerosene oil or diesel fuel. When applied to a form surface, the lighter oil is designed to evaporate, or flash off, leaving a film of the heavier oil on the form surface. The heavy oil acts as a barrier, much the same as grease on a cookie sheet, to prevent concrete from sticking to form surfaces. Problems with barrier-type form release agents include the oil migrating vertically during the vibrating process, which results in form sticking, as well as unsightly oil stains on the cast concrete surface.

A chemically active form release agent contains an active ingredient that combines with the calcium hydroxide (lime) in fresh concrete to form a metallic, waterproof soap which prevents concrete adhesion to treated surfaces. Resulting concrete is uniformly bright white in color and free of surface discoloration. Chemically active form release agents are also typically reactive with wood and steel form surfaces, which prevents their migration before or during concrete placement.