You can easily create a homemade paste to clean tarnish from brass using non-toxic chemicals such as salt, vinegar, tomatoes and flour. To save time, use ketchup or tomato paste alone.
Before applying any cleaning paste to the item, scrub it thoroughly with soap and water, and then check to make sure the piece is solid brass by using a magnet. If the magnet sticks, then the item is made of brass-plated steel, and acidic pastes such as ketchup and tomato paste are not suitable for it.
You can use ketchup to remove tarnish by rubbing it lightly onto the item, allowing it to sit for about an hour, and then removing it with soap, water and a clean cloth. Another option is to mix equal parts of salt, vinegar and white flour into a paste, then apply it to the brass and allow it to sit on the item for about an hour before rinsing it clean with warm water and buffing dry using a soft cloth.
Another option is to combine one part lemon juice with two parts cream of tartar. Apply it to the item, and let it sit for 30 minutes before rinsing it clean with warm water and buffing with a clean cloth. After the item is free of tarnish, apply a light coat of mineral oil or clear furniture wax to delay future tarnish.