"jim rozen" wrote in message
...
In article , Ed
Huntress
says...

Be aware that if you place the solder on copper, or on any other metal
with
which it will amalgamate, and if you heat it slowly, you will wind up
with
intermetallic compounds in the solder that will keep raising its melting
temperature as your experiment proceeds.

Err, not *always*. Try to solder some gold wire
with lead/tin solder, Ed! You might be suprised
at the result.

I'm not likely ever to encounter gold wire in my soldering experiences. g
Try it with some copper water tubing that was a bit overheated when the
original joint was made, and almost everyone is surprised. It can be a b*tch
to pull apart; it squeeks from the friction of the hard intermetallics; and
it just may freeze up on you and refuse to come apart until you nearly melt
the parent metal.

Most common metals that solder can wet will alloy to some degree with the
solder. The result is a tutti-fruiti mess of intermetallic compounds. They
embrittle the joint in many cases; they lead to unpredictable freezing and
crystalization; and they can either strengthen or weaken the joint depending
on what's mixed and at what temperatures. Usually, the result is bad. But
the phenomenon can be carefully engineered to produce a stronger,
higher-temperature joint, when it's done intentionally.

The extreme example of this actually is a brazing technique, in which a
small percentage of bismuth is added to what actually is a parent-metal
alloy, and the joint is soaked at a slowly increasing temperature. The end
result is a joint that melts at the same temperature as the parent metal.

Anyway, the most common way most of us run into this is when we try to
disassemble old soldered copper or brass plumbing. It can be very surprising
if the original joint was overheated.

Ed Huntress