Tarnish is a layer of corrosion that develops over copper, brass, silver, aluminum as well as a degree of semi-reactive metals as they undergo oxidation. It is analogous to rust, but with a slower rate of occurrence. It is mainly caused by chemicals in the air such as sulphur. It appears as a grey or black film over the metal.
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In objects which are primarily for display rather than use, the tarnishing process can be prevented in the long term by tinning, a process by which the reactive substance is coated in a non-reactive substance, such as tin or wax, and thus protected from oxygen.
"This kitchen version of electrostripping is safe and easy. It's especially useful for removing tarnish from flatware and holloware. In a pot lined with aluminum foil, mix a diluted solution of equal parts of baking soda salt, and liquid soap. A quarter cup of each to a gallon of water is a typical mixture. Set the sterling in the pot; bring the mix to a simmer and allow it to stand for 10-20 minutes as the oxides are transferred to the aluminum, which you'll see is darkened. Throw that away and wash the silver before using it".
Another way to treat tarnish is to put a drop of water and some fluoride toothpaste on a tissue and rub it on the silver (or metal, but silver is the most common to be targeted by tarnish).
- (McCreight, Tim. The Complete Metalsmith. Brynmorgen Press Inc., Portland, Maine. 2004.)