Is it the silver that has a high affinity for oxygen or the
copper part of a sterling alloy?
Silver doesn't oxidize, but oxygen will dissolve into it as CO2 will
dissolve into water. You may note sometime in the future find that
your fine silver is bubbling when it is molten. My understanding is
that this is dissolved gasses escaping, particularly oxygen.
Many thanks to Tino, as I got a whole new perspective from your
explanation in regard to casting.