I've heard of freezing and/or heating up a brass instruments to extreme temperatures in order to better align the atoms for resonance and structural integrity. Is this a good idea? If so, is it still a good idea for a french horn with a detachable bell?

FREEZING
Detachable bell or not makes no difference. The concept behind the freezing is to even out the stress the instrument recieved during it's making. When you heat, and bend, and solder, and bang, and whatever else you do - you incur points of stress. The super duper freezing supposedly relieves this stress and when you bring the instrument back to room temperature the instrument is stress free allowing the instrument to vibrate more evenly and freely. Originally this was marketed as a way for professional players to get the new horns to sound and respond more like a worn in instrument. Does it work? It depends who you speak with, opinions vary.

My opinion (in two parts)- Part one: If you have a new instrument and want to try it, go for it. If you have an older instrument don't waste your money. As you play an instrument over time the vibrations will loosen up the stress that is naturally in the instrument. In fact it is important to play as much of the range of the instrumnet as you can to condition the instrument. Theoretically, the more frequencies you play the more well rounded the horn will be. I know people who have two of the same make and model horn who will use one for medium and low range playing and the other only for when they are playing lots of higher range music.

Part Two: If you are a new, begining, or intermediate player I seriously doubt you will notice the difference. Part of being a horn player is learning your instrument. And what better way to learn than to grow with it. If you want the Horn to be perfect without investing the time to grow with it and let it grow with you then you might as well go out and purchase a synthesizer.


HEATING
When you have work done on your instrument like a bunch of dents taken out of the bell you have tweaked the balanced of the molecules. You have bent them and shift their alignment, etc.There is uneven thicknesses and the metal is more brittle at these repair spots. In a case like this you will want to heat the entire bell, we call this process annealing. What annealing does is it gets the molecules exited and warm. Then they settle down and space themselves out more evenly as they cool down. This softens the hard spots making the bell one thickness and hardness throughout.