Tarnish occurs when surface molecules of the silver react with sulphur from the air or food to form a compound called silver sulphide. With repeated polishing over many years there is a danger of wearing completely through the plating and exposing the base metal. The only solution then is to have the item re-plated.
A better idea is to remove the tarnish by converting the silver sulphide back into silver. This is quite easy to do and doesn't require any sophisticated equipment. You will need a bowl large enough for the silver item to be completely submerged, some hot water, some aluminum foil and some baking soda.
- Line the bottom of the bowl with aluminum foil.
- Place the silver item into the bowl, making sure it is in contact with the foil.
- Boil enough water to immerse the item and pour it into the bowl.
- Sprinkle the baking soda into the water, using about 1 tablespoon for each pint of water.
It will foam up and could spill over the top of the bowl, so best do this in the sink.
You should see the tarnish begin to disappear. For lightly tarnished items, it should all be gone in a few minutes. For heavily tarnished items, you may need to re-heat the water and repeat the treatment.
So how does it work?
What you are seeing is an electrochemical reaction. In the hot water and baking soda solution, a small electric current is generated between the aluminum and silver. The electric current causes a chemical reaction between the aluminum and the sulphur. Aluminum has a greater affinity with sulphur than silver. The sulphur in the tarnish is attracted into the solution and towards the aluminum, leaving the silver behind, where it belongs. The reaction happens faster when the solution is hot. The compound formed when aluminum and sulphur react is called aluminum sulphide and that's what you'll find floating in the bottom of the bowl or stuck to the foil when you are finished.
Now you don't have to see your silver to polish it back to its original shine! Isn't science great?