How do I do antique metal finishes?

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I'm all set to build some bookshelves that will look like the attached photo. I don't know if you can tell from the picture, but the metal frame has a brownish antique sort of finish that does not look painted. I'd like to know how to do these sort of finishes. I've seen it on various metal furniture pieces quite a bit lately, but can't find any place that says how to do it. Does anyone here have any ideas where I might find out? Thanks.