Tarnish is a thin layer ofcorrosion that forms overcopper,brass,aluminum,magnesium,neodymium and other similarmetals as their outermost layer undergoes a chemical reaction.[1] Tarnish does not always result from the sole effects of oxygen in the air. For example, silver needshydrogen sulfide to tarnish, although it may tarnish with oxygen over time. It often appears as a dull, gray or black film or coating over metal. Tarnish is a surface phenomenon that is self-limiting, unlikerust. Only the top few layers of the metal react. The layer of tarnish seals and protects the underlying layers from reacting.
Tarnish preserves the underlying metal in outdoor use, and in this form is called chemicalpatina,[2] an example of which is the green or blue-green form ofcopper(II) carbonate known asverdigris. Unlike patina advantageous in applications such as copper roofing and copper, bronze, and brass statues and fittings exposed to the elements, a chemical patina may be considered undesirable, as on silverware,[2] or a matter of taste or convention, as intoning on coins.
Tarnish is a product of achemical reaction between ametal and anonmetalcompound, especiallyoxygen andsulfur dioxide. It is usually a metaloxide, the product ofoxidation; sometimes it is a metal sulfide. The metal oxide sometimes reacts with water to make the hydroxide, or with carbon dioxide to make the carbonate. It is a chemical change. There are various methods to prevent metals from tarnishing.
Heavy tarnish can be mechanically removed by using tools such as afile or abrasive materials such assteel wool,sandpaper,emery paper, and heavypolishing compounds. Lighter tarnish may be abrasively removed with lighter polishing compounds or chemicals such asbaking soda. Gentler abrasives, such ascalcium carbonate, are often used by museums to clean tarnishedsilver, which will not scratch it or leave unwanted residues.[4] Objects such as silverware may have their tarnish non-destructively reversed electrochemically by resting them on a piece of aluminium foil in a pot of boiling water with a small amount of salt or baking soda.[5][6]