Does treating copper with vinegar and then using the resulting copper corrosion product as a pigment sound like a good idea? If you thought not, you'd be correct. The resulting pigment is verdigris (basic copper acetate), which was once popular for use in illuminated manuscripts and hand-colored maps. The pigment darkens, eventually turns brown, and begins to show through to the other side and off-set onto adjoining pages. Eventually, it can embrittle and weaken the support underneath it. No technique to reverse or halt this process has been developed.