chemistry question #334

Iain Hunter, a 56 year old male from the Internet asks on February 10, 1998,

Q:

What is the chemistry behind the method of cleaning silver where you place it on some aluminum foil in water to which washing soda has been added? I was told this would eventually damage the silver and that I should buy a special metal plate to use in place of the foil. I bought the plate as I have no wish to risk doing irreversible damage to the silver, but I think aluminum is just a catalyst in which case the foil should suffice. Can you please help me?
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the answer

Reg Mitchell answered on February 10, 1998, A:

Aluminum dissolves in the soda-water solution, and silver reprecipitates on the articles. In other words, the silver sulphide (black) gets reduced to the metal by the aluminum, which is more electropositive. In order to conduct, the solution has to be a bit alkaline (sodium bicarbonate or carbonate). Your metal plate is possibly made of zinc. Aluminum foil works just as well and is cheap, but in either case some silver will be lost, so don't overdo it.

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