I've lumped these together as they all mainly contain copper and have the same care recommendation. I love using these metals as they help add beautiful contrasts and variety to my work.
Brass: 87% copper, 13% zinc
Bronze: 92% or more copper, ~ 5% tin
Copper: 99.99% copper
Allergy: Something I want to clarify here is allergies to copper. If you break out into a rash or itchy skin or worse then you have an allergy to copper (most likely culprit) and should stay away from these and sterling silver. If you're one of the unlucky people whose skin turns green, while annoying, it is completely harmless. Limiting how much you wear your piece - when going out or once in a while - should help with this.
Care: These metals do patina fairly quickly. Storing your piece by itself in a plastic baggie will help slow this process. Storing with an anti-tarnish tab will help even more.
To Clean: The below is a bit of a process but incredibly effective and will leave your jewelry looking brand new. I also did a "stress test" on colored metal rings, leaving them in the paste for 24 hours straight and saw no changes to the colored rings. So, for those of you who have mixed metal pieces, this is an excellent method to freshen up your jewelry. Also, reducing this recipe by half should give you plenty of paste to work with if you're only cleaning a single item.
What you'll need:
2/3 cup white vinegar
2/3 cup flour
1/2 cup salt
hair dryer (recommended)
Mix flour and vinegar together until well blended. Add salt and blend well again.
You can either wipe the paste onto your jewelry or set it in the paste. Leave for 10 minutes (maybe 20 minutes if it well past due for this freshening up).
Remove your jewelry and rinse under warm water; this is where the toothbrush might come in handy as the paste might try to hide in some of the thicker or more complicated pieces.
Dry with a soft cloth. As an extra precaution, using a hair dryer on the lowest setting might be a good idea to make sure it completely dries. (And do remember metal can get hot. Be careful.)