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Message:: [QUOTE="physics-fan3.14, post: 2549888, member: 19165"]Tarnish is a thin layer of corrosion that forms on the surface of a metal. Only the top layers of metal interact, and the tarnish seals the underlying material. Numismatists don't usually use the term "tarnish", but that is exactly what is happening. "Toning" occurs when the tarnish is very thin, and an optical effect produces attractive colors. Toning is tarnish, which is corrosion. "Patina" is similar to toning/tarnish in that it forms a protective coating on the metal, but it is usually a thicker layer. You no longer get the optical effect from the thin film interference of "toning" - the color you see with a patina is the actual color of the corroded metal. Patina is still formed from the reaction of the surface metal with the environment, and is thus also corrosion. "Corrosion" is any process where a metal reacts with its environment to produce a more stable form, with coin metals these are usually oxides and sulfides. Toning, tarnish, and patina are all forms of corrosion. Numismatists use "toning" and "patina" to refer to attractive, usually stable, beneficial forms of corrosion. These are often desirable, but sometimes not - depending on its appearance. The way a coin was stored or handled can contribute to the type or appearance of the toning or patina - but they are still corrosive processes. Numismatists usually use the term "corrosion" to describe a reaction that is ongoing, or which has eaten into the surfaces beyond the top few molecular layers. Corrosion of this type needs to be removed through a conservation process to stabilize the coin and prevent further damage.[/QUOTE]

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