More recently, we've been made aware of the danger of cosmetics containing aluminium. And any combination containing this substance will definitely ring an alarming bell.
However, the two compounds could not be more different in terms of origin, chemical stability, particle size and effects.
The potassium alum molecules have negatively charged ions, and cannot be absorbed by the skin. Potassium alum is 100% inoffensive, the oxides and hydroxides released at skin contact are chemically stable. They don't react with the skin, are not able to release the aluminium, are not soluble and molecule size is big enough so not to penetrate the skin.
Not to be mistaken with ammonium alum, synthetically manufactured from aluminium salt.