My question is: in how many heatings will hard solder reach the
flow temperature of the silver itself? And wil
it won't, quite. The solder has more copper in it than does
sterling, so even if you've burned off all the zinc (which may not be
what's happening unless you're overheating the solder), the remaining
alloy won't be quite sterling.
But back to the "burning off the zinc" bit. Yes, some of the zinc
does vaporize, or oxidize and get removed by the flux. But the
increasing melting point with sequential solderings is also due to
the fact that in general, you've heated the solder to higher than
exactly it's melting point, and at this higher temperature, the
molten metal is capable of remaining liquid with a higher percentage
of silver and corrospondingly lower percentage of zinc and copper.
Given that this is stable, what happens is that the solder dissolves
some of the sterling silver in the metal you are soldering together.
In essence, you're not just loosing alloying metal, you're adding
silver. That's a bigger part of why the solder melts at a higher temp
each time you reflow it, than actual loss of the zinc, especially
when you start with higher melting (hard) grades of solder, which
don't contain much zinc to begin with. This characteristic of solders
dissolving some of the metal they are bonding is also part of why
solders flow towards the heat source.
The hotter the metal being soldered, the more easily the solder can
not just flow on it and bond it, but also penetrate it and dissolve
some of it, literally increasing the volume of molten metal. In gold
work, especially, one sometimes encounters soldering situations
where, despite the solder being molten and the metal appearing to be
hot enough, the solder doesn't much want to flow down a seam. In
these cases, sometimes what's to blame is that the metal alloys are
such that adding solder to the bonded metal's alloy would result in a
higher melting point than the bonded metal itself.
So then the above effect doesn't happen so much, making it harder to
get the solder to flow. There are also cases where solder, added to
the bonded metal, results in such a quick and drastic reduction in
melting point that the solder, instead of flowing much, just slumps
into the metal, often resulting in a messy puddle considerably larger
in mass than the volume of solder used. Not always just with solders,
a classic example of this happening is when one tries to solder brass
to sterling silver with too high a melting silver solder. If too high
a temp is reached, the simple contact of the brass with the silver
will result in the two melting together, essentially forming a puddle
of easy solder. It happens long before either the brass or silver
would have melted by themselves.