Is there any way to harden brass other than work hardening? I'm working with a small piece of brass that has to be soft enough to form into a barrel shape, but then has to be hardened enough to prevent this piece from opening. I can't just start whacking the finished shape, of course. I tried reheating and allowing a slow cooldown, but that didn't seem to help. Any tips?

If you work the surface with a small light hammer you can harden the surface wihout distrorting the whole shape.
You might have to do some light filing afterwards (only removing very little, or you are back to square one!)
Work the surface with many light blows so it becomes smooth and compressed.
It will induce some hardnes to the piece.
It helps having a support that is close to the inside shape while you work or you could distort the shape by accident.

Not difficult. Just go slowly and carefully.
Do not heat up again afterwards. That will make the bronze/brass soft and your work is undone.

Thanks! It sounds like what I need is a small mandrel and custom-made hammer.

Toxic!? I had no idea. Is getting the brass to cherry-red for annealing releasing toxic gas as well, or does it have to be actually flowing to be dangerous?

Brass toxicity is dependent on the alloy, Sean. It will release gases at cherry red, but not in the quantities released during, say, full melts for casting. The immediate danger is from the zinc, which melts at considerably lower temperatures than copper (419C degrees for zinc versus 1083C for copper). Thus, when brass (alloy of copper and zinc) is heated past 907C degrees (boiling point of zinc), metal vapor is released. Obviously, zinc vapor is released, but trace elements in the alloy may also be highly toxic. SEE LINKS BELOW

When heating metals, especially non-ferrous alloys, I make it a rule of thumb to have adequate ventilation. Keep ventilation in mind for high-temp soldering and brazing as well. Some soldering media for precious metals, and cupric alloys themselves, may contain CADMIUM. http://risk.lsd.ornl.gov/tox/profiles/cadmium.shtml and http://www.cadmium.org/app_allo.html

ALWAYS use caution when working with "found" (ie: scrap) metals. They are a bargain, but there are so many different alloys that we generically call 'copper' or 'brass' or 'bronze' that safety should be on anyone's mind who works with them.

Cheers!

Great info, Carl! Thanks to you and everyone else for saving my brain.