Just been reading this, and as a former chemist/corrosion scientist I feel compelled to add a couple of other points, which may or may not be useful.
Firstly, caustic soda is VERY nasty stuff. Most people kind of ignore that because it's not an acid, but in lots of ways it's worse. So - if you tip it town the sink when you're done, make sure you flush it away with loads (and loads) of water. Don't want it sitting in the pipes, especially if they're copper...
Secondly, and somebody touched on this - the coefficients of thermal expansion for steel and aluminium are very different. For a nice steel frame with a decent paintjob, getting a blowtorch onto the outside isn't too useful since it will make the alloy post expand even more than the steel. And burn off the paint
But you can still use that property - if you live anywhere near an industrial gases plant, you'll probably be able to get hold of some liquid nitrogen. A couple of thermos's full should be enough to shrink the seatpost out, if you organize a plug and put it in via the BB etc etc with the frame upside down. I did it once years ago at university (where LN2 is freely available
) and worked a treat.
This might be my second or third post in the four or so years I've been a member - it's the first time I've felt like I knew something useful...there are a lot of smart people on this forum!!
Just Enough Overkill
1993 Mountain Cycle San Andreas - up and running, 1993 Carrera Team EL-OS - awaiting rebuild, 1994 Manitou FS - being rebuilt, 1995 Manitou HT - wife's daily, 2001 Santa Cruz Bullit - earthquake bike, 2001 Foes DHS Mono - for going big, 2010 Santa Cruz Nomad - up and running, 2011 Cannondale Super Six - commuter