NNP

Incrusted Patina.  A thick, hard, natural coating on buried or sunken objects, formed by a chemical reaction on the surface of the object and its atmosphere of soil or salt water, usually over a long period of time. The copper in the bronze slowly reacts with sulfur in its environment to form copper sulfate. Incrusted patina is somewhat self-sealing, prohibiting corrosion or further chemical attack.  The Latin term is aerugo nobilis (literally, noble rust).  See patina.

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