Dave and the others,
I guess I'm somewhat guilty also upon occassion giving the "impression" that silver brazed frames are better. I'd like to add my .02 on this subject before we let it drop.
Wheather a frame "should" or "should not" be brass brazed; or vice versa, isn't really the question in my opinion. Obviously both methods have their time and place. The framebuilder must choose which one to use on any given frame. Sometimes the frame material is the deciding factor. A framebuilder who ordinarily uses brass is forced to silver braze if they take up using Reynolds 753. I also think it best to braze thinwall OS tubes on lugged frames with silver; and yet thousands of frames like this were brass brazed by factories using carosel brazing stations without distatorous effects. Silver brazing is better for this application but not practical for the factory atmosphere which is why Reynolds very wisely formulated brass temp. air hardening alloy (853). Tig compatable tubing has been developed because it is in greatest demand.
There are two basic schools of brass brazing bike frames. The "old school" which is Mario and friends where the torch is oxy/propane emitting a large but diffuse heat flame. The frame is coated with chipped boric acid after it comes out of the tacking fixture and is aligned before brazing. The tubes and lugs were fluxed with the paste form of brass brazing flux when it was assembled in the tacking fixture.
When ready to braze the frame is suspended from a rack over a brazing table of fire bricks with a back wall of bricks also. The joint is preheated with the large flame (which is what the preheaters do in a carocel system) until it is ready to introduce brass to the joint. With a hand brazer the preheating is skillfully done and only as necessary until ready to braze. The brazer is actually brazing through a layer of moulton boric acid (which is the primary component of brass flux anyway) which prevents the heated tubes from oxidizing during brazing. The flux is also a heat sink which keeps the outer areas of the tubes cooler while brazing. Care must be taken not to braze in a draft, nor to allow frames to cool too quickly (the frames worst enemy in my opinion) by setting them on a cold shop floor afterwards, etc. My guess is frames probably break most often from cooling too quickly, not overheating.
The second brass brazing method is using oxy/acy to braze with. Again a relatively large orifice is used but as you know, the flame will be a more concentrated one and somewhat "hotter". The technique is basically the same, it's just done with a different type of torch. My feeling is that mastering brass brazing is more difficult and more "precise" control of the heat is required to be great at it. Getting perfect penetration, avoiding bulges and chokeing of the tubes, getting super clean lug edges, are all more difficult with brass. If you really know your clearances, prep the stuff well (clean stuff brazes at a lower temp.), have good fixturing that allows you to not have to align stuff too much, and you're a good (or excellent) brazer then brass brazing is fine for tubing that can tolerate the temps. Mario was pretty good at brazing; he's also the only old school brazer I ever saw braze. As a technician, the best brass brazer (cleanest edges and solid penetration) I've seen in action is Tesch. I've seen masters in both schools. Ironically, I've never brazed a brass lugged frame in my life. I've done and will do again brass fillett brazing but not lugged frames.
Silver brazing has its place amongst the small timer who maybe is working with light tubes or delicate designs (although Weigle and Erickson use brass) or for some other reason find it practical to use. For me it makes it easy to modify lugs using brass joints which allow me to silver braze the end results to good effect. Each person will use what works best for them. If the builder focuses on doing a quality job and has a good level of skill then it shouldn't matter which material he uses.
Brian Baylis
La Mesa, CA
Good brazing is good brazing.