Why Sterling Silver Tarnishes? 

Sterling silver is often called pure silver, but in fact it's a silver alloy. Pure silver, also called “fine Silver”, is defined as 99.9% pure silver, but is too soft for most uses. Sterling silver contains 92.5% silver, with the remaining 7.5% being another metal, most often copper. The addition of the cheaper metals, often called a base metal serves to harden the resulting alloy so that the metal can be cast into shapes that it will retain with use. Sterling silver made in Ireland is marked with a stamp on the inside of the ring, stamped 925 (fineness mark), the Dublin assay office mark , a letter defining the year of manufacture and the makers mark (initials).

Sterling silver can simply tarnish by being in contact with the air or other chemicals (such as sulphur in human perspiration). Pure silver is, like gold, impervious to tarnish the surface. It is the alloy metal (copper) with attracts the tarnish. Rub your thumb over an apparently shiny piece of sterling silver. You may find a black or dark green smudge on your skin that indicates that the sterling silver is beginning to tarnish.Your sterling silver ring can be kept polished simply by buffing regularly with a cotton cloth.

You could try this old trick to remove silver tarnish. Line the bottom of an old pot with aluminium wrap (tin foil), and fill with hot water. Add some salt and baking soda, proportions aren't important here, just throw in a couple teaspoons of each. Next, put the sterling silver piece into the water, with it touching the aluminium wrap. The tarnish will be pulled off the silver to the aluminium wrap ,in cases of heavy tarnish, you will be able to see it flaking off. Leave in for no more than five minutes, rinse and dry.

For more information on silver tarnish we recommed you search wikipedia.com using the search term "sterling silver".