Greg Hewgill (ghewgill) wrote,
Greg Hewgill

kitchen chemistry

Amy and I have a few pieces of silver passed down through the family - a tea service, a silver platter, and some silver spoons. We occasionally use them when we have guests over. Last time we used our silver, I spent a good amount of time with a silver polishing cloth, scrubbing and scrubbing to remove the tarnish. It's hard work, and not terribly effective because it's really difficult to get the polishing cloth into all the little cracks and crevices.

A bit of googling turned up this page which describes the chemical reaction that causes tarnish, and how to reverse it. It turns out that silver tarnish is just silver sulfide, and forms spontaneously in the presence of sulfur-containing compounds in the air. Furthermore, aluminium happens to have a greater affinity for sulfur than silver does. So, we apply a bit of chemistry and get:

3 Ag2S + 2 Al → 6 Ag + Al2S3

To make this reaction happen, boil a bunch of water in a large pot. Add some baking soda to the boiling water, about 30 g per litre of water (ie. about a cup per gallon). This will froth up a bit; stir until it's all dissolved. Then pour this solution into a pan containing your silver sitting on some aluminium foil. Make sure the silver is in contact with the foil, and the water covers the silver. You'll probably be able to detect the smell of sulfur almost immediately. After a few minutes, remove your silver, rinse it, and brush off any remaining gunk with an old toothbrush. Voilà, clean silver with no scrubbing!

This works because the hot baking soda solution helps liberate some of the sulfur atoms from the surface of the silver. The sulfur then binds more tightly to the aluminium, causing a tiny electric current to flow between the foil and the silver (this is why they have to touch). This electric current helps liberate more sulfur atoms, and the cycle continues. After a few minutes, virtually all the sulfur is no longer attached to the silver.

We did this today and not only is it very effective, but it's fun too. Armed with this technique, I don't think I'll ever pick up a silver polishing cloth again!

Interestingly, people have known about silver's antibacterial properties since the time of the ancient Greeks. For centuries since, people have used silver for food storage and tableware for that very reason. It's quite amazing stuff really.


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