On Sun, 27 Feb 2011 10:59:16 -0500, mm
wrote:

One or two webpages suggested ketchup, which they said was made of
vinegar, salt, and tomatoes. But I figured I wouldn't be able to
see what was happening.

Soak a penny covered in ketchup for an hour and see if it cleans it.

I have a military 1970's era flashlight with copper inside and a brass
switch. After all these years, there is only one small corrosion on
the end cap spring I need to clean. I will either use ketchup, or a
paste of vinegar, salt and flour.

This may interest you:

http://www.howtocleanstuff.net/metals/