And once the solder has hardened, it does seem (at least in my
scant experience) to have the same melting point as the silver. --
i.e. if the joint didn't solder, you can't just remelt the solder
that's already there.
Well, actually you CAN reflow the solder … Just clean (pickle),
wash, re-flux, and flow it again. You can also reflow a joint that
HAS properly soldered – that would be one way of disassembling a
piece or fixing a mistake. This is also the principle behind tack
soldering a piece (for example, in marriage of metals, you place your
solder snippets and only partially flow them – you can still see
lumps in the solder – until you’ve hammered the pieces to fill in
any gaps…then you go back and do a complete flow).
A good way to think about solder’s interaction with the metal being
joined is probably this…
As the 2 pieces of metal to be joined are heated, the heat “spreads
apart” their molecules, leaving gaps between them. The solder flows
at a lower temperature than the melting point of the pieces to be
joined, and “fills in” those gaps between the molecules in what’s
known as capillary action. Kind of the same way as your normal paper
towel absorbs water, but on a much smaller scale. The solder always
remains solder … it doesn’t melt away, alloy into the other metal,
or evaporate to any great extent. It has just filled in all those
tiny little molecular gaps.
Because it hasn’t alloyed or changed its composition, it retains its
own melting point and flow point. (although to be perfectly correct,
there is some extremely small component of the solder that does
evaporate or become oxidized and washed away, which contributes to
its flow point the second time around being about 10 degrees (F)
higher – a really negligible amount).
Does this help clarify things a bit?