What is the equivalent to niello for brass? I have some brassware with nice engraved designs that I'd like to bring out. If it were silver or gold, I'd use niello, but that apparently doesn't work on brass. I could try and do something to oxidise the whole surface and polish the oxidation off of the high points of the engraving, but ideally I'd like something darker.

I can see from pieces like this:


Engraved lamp stand from the Met Museum, public domain

that such a finish is achievable, but the entry just lists it as 


  Brass; cast, engraved, and inlaid with black compound


So, what is the equivalent to niello which can be used to selectively darken recessed areas of a brass object?

I'd prefer more traditional methods, as opposed to simply spray painting the whole thing black and rubbing it off the high points, if possible.
I think you will have to age the alloy first, and clean the higher areas afterwards.

For tarnishing, you can use Palmolive, or any kind of washing liquid that has sulphates in it.
Another, more aggressive option, but only viable for solid, thick brass objects, is to use ammonia.
The most traditional choice, however, would be to use a mixture of salt and vinegar.
In all cases, you can leave the brass object in the liquid for an extended period of time, or rub it in with a piece of cloth, which will give you more control.
After the tarnishing, you can use either ketchup, a mixture of vinegar, salt and flour, lemon juice, or soap, to brighten the brass again.