powdered Ammonium Alum. Most drug stores (sorry, couldn't spell farmacy) carry it near the witch hazel, glycerin, and astringent stuff. Kroger here in ft worth has it in 2.75 oz. bottles on the spice rack aisle, for whatever reason.
Anyway, mixing the alum is supposed to draw up the pores of a skin during tanning so the hair doesn't slip out.