actually it's the chemical reaction of the oxalic acid in those polishes (and Mothers is just one of many brands that contain OA) that produces the black residue on the rag. If there are abrasives in the polish, too, it's those that "remove aluminum"...most of these polishes (Super7, Happich Simichrome, MAAS, Autosol, etc., etc.) make a big effort to point out that they are NON-abrasive.