Vegetable tanned calf, sheep and goat leather which has been speckled with iron black, dotted and marbled, as well as flat, light-colored and reddish brown calf bookbinding leather, can darken, shrivel and embrittle from contact with water, aqueous solutions, and alcohols. In order to prevent these reactions, only non-aqueous adhesives, consolidants, fixatives, cleaners or conditioners may be used.