![]() ![]() The metal oxide sometimes reacts with water to make the hydroxide and carbon dioxide to make the carbonate. It is usually a metal oxide, the product of oxidation. Tarnish is a product of a chemical reaction between a metal and a nonmetal compound, especially oxygen and sulfur dioxide. While tarnishing on other metal objects is generally cleaned off, some toning on coins may be considered aesthetically pleasing or beneficial to the coin's value, and is therefore kept in situ. Patina is the name given to tarnish on copper-based metals, while toning is a term for the type of tarnish which forms on coins.Ĭhemistry A Morgan dollar demonstrating a colourful form of toning on its reverse. The formation of patina is necessary in applications such as copper roofing, and outdoor copper, bronze, and brass statues and fittings. ![]() Tarnish actually preserves the underlying metal in outdoor use, and in this form is called patina. The layer of tarnish seals and protects the underlying layers from reacting. Only the top few layers of the metal react. Tarnish is a surface phenomenon that is self-limiting, unlike rust. It often appears as a dull, gray or black film or coating over metal. For example, silver needs hydrogen sulfide to tarnish, although it may tarnish with oxygen over time. Tarnish does not always result from the sole effects of oxygen in the air. Tarnish is a thin layer of corrosion that forms over copper, brass, aluminum, magnesium, neodymium and other similar metals as their outermost layer undergoes a chemical reaction. ![]() Tungsten rods with evaporated crystals, partially oxidized with colorful tarnish For 1924 silent film, see Tarnish (film). ![]()
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