Watch this science video tutorial from Nurd Rage on how to restore silver with electrochemistry. You can restore old silver with aluminum foil or a battery by simple electrochemistry.
Old silver is often tarnished by a thin layer of silver sulfide that appears as a brown color. The silver sulfide can be electrochemically converted back into pure silver if a negative charge is applied.
To do this, first make a solution of equal parts baking soda and table salt (NaCl) and adding enough hot water until they dissolve. Then simply place a silver object in the solution and contact it with a large piece of aluminum foil. In a short time the silver will be restored.
A very cool way to do the same thing is to use a battery. Connect the negative end of the battery to the silver and the positive end to a large electrode, like a copper plate, and dip them both in the solution.
This process occurs due to electrochemical reduction of the silver sulfide to silver.
THIS ONLY WORKS WITH ELEMENTAL SILVER! This restoration method does not work with silvery things like stainless steel or chrome. So make sure the item you want to experiment on really is silver before trying this.
A low voltage battery is best for this experiment, around 3 volts. Higher voltages do not improve the process and may actually damage the item you're trying to restore. If you don't want to mess up, then just use aluminum foil and skip the battery entirely.
Brought to you by one of WonderHowTo's favorite scientists NurdRage.