NNP

White Rust.  Zinc corrosion, a powdery white surface contaminate, actually zinc oxide, ZnO. The zinc tarnishes in moist air or water, particularly when heated (at 205° zinc rapidly turns to powder). On pure zinc coins and medals white rust should be removed immediately and transferred to a cool, dry atmosphere. Zinc oxide is soluble in

acids.

            Removing white rust. Once a zinc coin or medal develops its white powdery surface contaminate this should be removed at once, as soon as it is first observed. Otherwise the corrosion will continue, worsen, and ultimately destroy the surface of the item. First, use a Cratex abrasive. This is a rubbery substance with light particle abrasive imbedded, much like a pencil eraser (which can be substituted, use a soft new eraser, not one that has hardened or worn down where the metal holder would scratch the item). Rub back and forth like erasing a pencil line. In the event even Cratex does not remove all the white rust, the item will then have to be  chased, preferably be a professional.

excerpted with permission from

An Encyclopedia of Coin and Medal Technology

For Artists, Makers, Collectors and Curators

COMPILED AND WRITTEN BY D. WAYNE JOHNSON

Roger W. Burdette, Editor