I think that as I was responsible for setting this particular hare running in the first place I should perhaps justify my intervention.
In the late 70s the importer of Columbus tubes in the UK invited his customers to a seminar presented by technical staff from the Italian factory to talk us, the frame-builders, through Columbus' preferred techniques for working their tubes.
None other than Felice Gimondi officiated, but was never seen with a torch in his hand..just there for the PR. The real lesson that we were to take away with us was that we should always use silver-solder/silver-brazing alloys on SL and SP tubes, the reason being that at the higher temperatures needed for brazing, important changes took place around the grain boundaries of certain alloying components of the steel, leading to a possibility of brass inclusion in the steel, which could give rise to cracking and, ultimately failure. Higher quality steels tend to have smaller grains in their structure but these expand as the heat input increases; large grains in steels tend on the whole to weaken the metal.
Apart from this factor and speaking generally about metals as they heat up, I understand that it is more likely that greater distortion will take place. I remember discussing the ways of working Reynolds 753 with Gerald O'Donovan, the top guru at Raleigh who was responsible in part for the development of 753. Not being a metallurgist he could not tell me whether the requirement to silver-solder with a 40% silver content was necessary to avoid changing the nature of the steel or whether it was to avoid building in, so to speak, distortion in the frame as 753, being a heat-treated steel, is notoriously difficult to set or track after being soldered together. It is imperative that the frame is built straight in the first place and not requiring any tracking.
In my experience firstly as a silversmith and then as a frame-builder, joints need to be thoroughly cleaned and prepared whether they are going to be joined with a silicon bronze alloy or a silver alloy. I can't subscribe to the view that its the builders who are too lax or idle to clean the joints who end up brazing their frames Again from experience, it seems to be the case that greater skill with a torch is called for when silver-soldering, if only to avoid destroying the flux which is less tolerant of heat than most brazing fluxes. It is for this reason that I prefer to use a compressed-air/natural gas torch for silver-soldered frames as one can obtain a larger "brush" flame that is not as hot as a similar sized oxy-acetylene one. I concur that skill with the torch and the choice of torch and gases used is very important, and I know of builders who would use a massive nozzle to braze up a bottom-bracket assembly, while another would use the smallest one that would do the job. One UK builder brags of his ability to braze up the whole main triangle of a frame in seven minutes ! I would need to wear an asbestos suit to get near enough his work-piece. He regularly built seven frames a day.
There is scientific proof that the heat tends to destroy or remove some of the "nature" of steel, particularly sophisticated alloyed-steels. Whether it is possible to detect this lack of "life", "response" or call it what you will in a bike frame I'm not certain..but tend to think like others that poor geometry and general bad design is likely to lead to a less responsive frame than a larger nozzle on a torch.
It is worth noting that some cycle tube steels such as Reynolds 501, Columbus Cromor, and Ishiwata Magny-V were structured to tolerate higher brazing temperatures, and I know that the first two of these series were extremely popular with certain frame-builders in France who used the even higher temperature nickel-bronze rods - these had superb flow capacities and higher tensile strengths than silicon bronze - to braze up their frames. And don't forget Reynolds 853..the hotter the brazing temperature the better thew frame.
Norris Lockley ..Settle, UK