What is saponification?
Saponification is the reaction in which an alkali converts a fat or fatty oil into a water-soluble soap. It is one of the two main ways an alkaline degreaser removes oil: animal and vegetable-derived oils are chemically converted rather than merely suspended, so they rinse away cleanly.
Alkaline builders in a degreaser — silicates, carbonates, hydroxides — react with the fatty ester component of a soil to produce soap and glycerol. Both are water-soluble, so the oil is genuinely gone from the surface rather than sitting in the bath waiting to redeposit.
This only works on saponifiable soils. Fatty oils, animal fats and many soluble cutting fluids saponify readily. Straight mineral oils, paraffins and greases do not — they have no ester group to react. Those have to be removed by emulsification instead.
In practice a real machining soil is a mixture, so a good degreaser does both jobs: alkalinity to saponify the fatty fraction, and surfactants to emulsify the mineral fraction. That is why a cleaner's builder package and its surfactant package both matter.
