There are ways that the silver content could be reduced, but not
through any normal casting operation. What’s more likely is that
there could be slightly less copper than there was before, but this
would be a really, really small difference, and the metal wouldn’t
stop being fine enough to be sterling (925 parts per thousand, or
higher). It sounds like a case of someone who has been told that
alloy contents can be changed by reprocessing the metal, and treating
that as an absolute truth that must be applied to all situations. It
is perfectly reasonable, however, if she doesn’t want you to mix your
metal in with her supplies, as she can’t verify the contents of your
metal.
Jamie Hall
http://primitive.ganoksin.com