The flux is soaked off after the frame is brazed, so hopefully there's no flux remaining on a finished frame.
You can reheat a brazed joint to separate it, but for a brazed joint using brass filler, the reheat temperature required will be greater than the heat required to do the initial brazing. Depending on whether brass filler or silver filler was initially used (brass filler melts at a higher temperature) the heat required to melt the filler so the joint can be separated may overheat the tubing, weakening or distorting it.
Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Raleigh Pro, Trek Cycle Cross, Mongoose tomac ATB, GT Bravado ATB, JCP Folder
Additionally some mass production frames use either pins or weld tacks to hold the tubes in place while razing is done. So any disassembly might have to take these into account. It's best to fully remove all paint (a messy job in it's self) before going onto the heating to ascertain the assembly methods. because the heat levels needed to melt brass/bronze is pretty high you might find the tube or lugs will tear or break apart in a manor that you don't expect. This is why some will cut away the bulk of a tube's length and then grind out the portion left in the lug or shell's socket, if they're replacing a tube. Expect a lot of bad fumes from the paint of grunge inside the frame to vent off. This is best done outside and with some breathing protection. Andy.