Gary Wooding wrote:
.... The process of melting
the solder changes seems to change its characteristics and raise its
melting point. ...

Having just read Tubal Cain's "Soldering and Brazing", I can relate
something that I learned there. When brazing is done (soldering, too),
the filler metal mixes with the base metal to create an alloy. This
alloy can (generally does?) have a higher melting point than the filler.
This is commonly experienced when unsoldering copper pipe joints: it
takes more heat than soldering them.

BTW - I bought the Tubal Cain book based upon a recommendation posted
here. I'd like to second that - it is very good and I learned a lot.

Bob