Because all that heat will cause the joints to un-solder.
Do I listen?
I think NOT
I was making a tool canister this weekend and I wanted to have the red oxide patina on it. Again, I blame Jan. I really only had the solder joints for the top and bottom and the slip sleeve on the top. I figured that IF I used hard solder and was really really careful, I could work on getting said patina. Remember, I ONLY had 1 or 2 solder joints on each piece. I would gather that if you had LOTS of tiny little solder joints that could wiggle and move, this won't work - your milage may vary.
I made the canisters and after soldering on all three pieces, I trimmed off the excess brass, for the top and the bottom and put a steel brush texture on the outside of the canister. Today during lunch, I setup the fire bricks and a pot of HOT borax water. I fluxed the outside really well and turned on the torch. I had to position the torch such that it did not flame out and kept my eyes on the solder. When I could see it flow, I backed the heat off a bit but I was able to get it really RED hot by keeping the heat on for a long time, using bricks to help hold in the heat and occasionally turning the piece.
Here are the results. I am rather quite piece and what I was surprised to see what that the oxide started to form on the brass end pieces. Brass has some copper in it but I was not expecting the red oxide to appear with any great color.
The first two pictures are before I waxed it.