Ketchup works to reduce tarnish because it has vinegar and a couple of other mild acids that give it a 'tang'. Using straight white vinegar is MUCH cheeper, easier, cleaner and faster than a ketchup soak!
Brass is not porous so the vinegar can't soak into the metal itself, all it can do is sit on the surface and combine with the copper oxide that makes the brass turn black. When the dark places have turned pinkish the vinegar has done all it's going to do, it's time to remove the cases, flush them off, dry and tumble them.
I once soaked a few scrap cases in straight vinegar for a week to see what would happen; so far as I could tell, nothing happened.
Last edited by wncchester; December 1, 2012 at 07:19 PM.