For me, I wouldn't fool with muriatic acid. The antique guys use ammonia for darkening brass. Here's how I did it on some furniture hardware.
1) Remove the clear lacquer with acetone.
2) Suspend the hardware with a coat hanger (or, what would have been even better would have been to use plastic tie wraps) in a coffee can, a couple of inches above the bottom.
3) Put a small amount (like 1/2") regular "household ammonia cleaner" into the coffee can, close the can up.
4) Check every few hours and when it reaches the darkness you want, take the hardware out.
5) Clean the hardware again with acetone
6) Spray with clear spray lacquer in a can (available at the paint store or hardware store). Because it's just a small piece of hardware, and there are lots of variable surfaces, the level of finish from a spray can will be fine.