A eutectic is an alloy or solution, such as solder, with its component
ratios optimized for the lowest melting point.  The eutectic mixture
of 63% tin (Sn) and 37% lead (Pb) melts abruptly at 183 deg. C (361 F).
The popular solder for electronics of 60% tin & 40% lead has a less abrupt
phase change temperature, occuring between 183-191 C (361-376 F).

Since higher melting temperature is often desired -- and also because 
tin is expensive -- 50/50 solder (50% tin & 50% lead) is commonly used 
in industry; it melts at about 225 deg. C (437 F).

Brother John Rhodes wrote to me that 50/50 solder may be thought of
as a eutectic plus an excess of one component.  One result of this
imbalance is that the phase change, as the temperature falls and the
metal goes from liquid to solid, isn't as abrupt.  He wrote:

"The old auto body shops took advantage of this when they leaded-in
dented panels; they used a non-eutectic Sn/Pb alloy (perhaps it was
50-50) which exhibited a definite "mushy" state where it could be
paddled around and shaped.  A eutectic mixture would turn from liquid
into solid within a few degrees, and was much more difficult to work."

Beware of soldered components made during WW1.  An old tuba I owned was
precisely dated by the repairman, who told me, "When I dropped the horn
on the workbench every joint failed!  All brass instruments made during
WW1 are like that, because there was a tin shortage, and the solder with
less tin work-hardens quickly from the vibration as the instrument is
played."

John also notes that many heat guns will deliver air temperatures of
370-540 C (700-1000 F).  Makes popcorn real quick !

Robbie Rhodes