NNP

Corrosion.  In numismatics the advanced state of metal deterioration beyond tarnish. Iron coins rust, lead medals form a white disease, tin and zinc objects erode, all under different chemical or environmental conditions. The best prevention for corrosion is to lacquer the piece. While this is done for most medals, it is impractical for coins or blanks. The best protection for coins is to choose metal alloys least liable to tarnish or corrode in most environments; these are, obviously, gold, silver and somewhat bronze.

Corrosion is the extensive tarnish or chemical reaction to a metal surface in a detrimental environment. Most coinage alloys tone or darken in time, as copper or bronze ages; should coins and medals be exposed to harsher environment they may form tarnish.  Should this detrimental condition persist, it may become corrosion. Should a numismatic or medallic specimen reach this stage it is not worthy, perhaps, to be collectable. It is assumed the detrimental condition is treated long before a specimen can be called corroded.

See tone, toning; tarnish.

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