Ed. Kruspe, Erfurt baritone. looking for info.

[Dan Schultz]

...... is it citric acid in ketchup or vinegar? I also read somewhere to use one part vinegar and 3 parts Dawn dish washing liquid.

Dunno whether it's the citric acid in tomatoes of the acetic acid in vinegar that does the work.... maybe both. Most acids leave behind traces of red on brass. The redness is actually a very thin film of copper where a tiny bit of zinc in yellow brass has been leached out on the surface. For some reason, the ketchup trick leaves behind a very bright brass finish. BTW.... I don't recommend this treatment. It's just an example of what some of the 'old timers' may have done. This old guy played a tiny Couesnon Eb upright tuba that always reminded me of cheeseburgers for some reason!