Combine the juice of half a lemon with a teaspoon of baking soda and stir until it becomes a paste. Apply the paste with a soft cloth. If the tarnish is heavy, let the piece sit with the paste on it for 30 minutes. Rinse with warm water and dry. Repeat if necessary.
Slice a lemon in half and cover the cut section with a teaspoon of table salt. Rub the lemon on the tarnished piece, squeezing it as you go to release the lemon juice. Rinse with warm water and dry.
Combine equal parts of all three ingredients to create a paste. Apply a thin layer of that paste to the tarnished brass and leave it for an hour before rinsing with warm water and drying.
When you wondered how to clean brass, you probably didn’t think ketchup would be involved, but—surprise—it is! Tomatoes contain an acid that helps to remove tarnish on brass and other metals; that’s why applying a tomato-based product can work wonders on brass. Ketchup, tomato paste, and tomato sauce all work equally well. Apply a layer to your brass and leave it on for an hour. Then wash with warm water and dish soap. Let it dry.