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Whilst the aluminium foil/sodium chloride/ sodium bicarbonate method is very effective at removing tarnish it will leave deposits behind in the small cracks and interstices of older metal items which will lead to further corrosion in future. Please don’t use it on anything which is not a modern item of your own which you don’t intend to pass on to others.
Archaeological silver, such as a coin, is seldom safe to treat by immersion in a liquid, as it will have holes and weakened crystal boundaries and some areas may be more severely affected than others. Liquid treatments are not easy to control (or undo). This includes cleaning agents such as dips or foams which are applied to the surface, but which will penetrate into weakened areas which may be invisible to the naked eye.
The safest way to clean it is to take it to an archaeological conservator who will examine it under a microscope and clean it using controlled, localised methods, often by physical, not chemical cleaning.
Only when you have asked an archaeological conservator and found out whether the cost of treatment is beyond your budget (and that of any potential donors, fundraising activity etc) and been assured that the item is of relatively little historical importance should you consider cleaning it yourself.
First you must make sure you understand what it is you are removing and why. Then having satisfied yourself that you are not removing evidence of the object’s past, an intentional treatment or a modern disguise (like the dark brown and green paint or wax sometimes applied to bronzes to make them look antique), document the item thoroughly with photographs and description. Cleaning is irreversible.
Then you could try a Goddard’s Silver Cloth http://www.goddards.com/html/products/productDetail.php?id=8 or similar product, taking great care of the surface. You can use small pieces of the cloth wrapped around a toothpick to gently clean details in the decoration. This will remove much of the corrosion which is tarnishing the surface. Keep watching the surface in case you uncover some unexpected change.
When you have finished, document what was done to the object and preserve it as part of the object’s history. Keep the item in clean, dry conditions, away from sources of sulphur or acidic materials which could cause further rapid retarnishing.
Hope this helps