An insoluble or poorly soluble residue formed when soap reacts with hardness minerals and mixes with oils, soil, or other material.
GlassRenu · Educational illustration
Meaning and boundaries
Soap scum is not simply dried soap, and it should not be assumed to be the only cause of haze in wet-area glass.
Why it matters in glass restoration
Correctly identifying a soap-and-mineral film can prevent premature classification as etched glass while still recognizing that long exposure may coexist with surface alteration.