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Your actual question was what is going on. My understanding of what
happens is, with a little too much heat, the metals form their
eutectic alloy-- the combination of the metals that has the lowest
possible melting point. Since the new, local alloy has a lower
melting point than the surrounding metal, it tends to create a pit
where the metal has alloyed.
I am very familiar with this problem from my own attempts to fuse
gold and silver. With practice and extreme focus, this can be done
very successfully, but it is not easy.
Soldering silver and brass together is particularly likely to
produce collapse, since together they tend to turn into a puddle of
This is not nearly as heartbreaking as when it happens with gold, of
Overheating gold-filled even a little causes the gold to alloy into
the brass-- and you are left with very expensive brass.
If I were going to try to solder granules of gold onto silver, I
think I would do it with plumb gold solder, and preferably by heating
from underneath the silver. Possibly the safest (though not quickest)
way to do it would be to bur or dap a little divot where each granule
was to go, then melt a speck of solder (gold or silver) into the
divot. Add the granule(s) and re-heat, being very careful to stop the
moment the solder flowed.