G’day; I’ll begin by saying that I know nothing about casting.
I get a black color, sometimes in metal, instead of a
clean sterling cast........ Four came out a nice color of silver cast
and four came out black. The black can be removed by soaking in
pickle and a short dip in silver tarnish remover.
I can only think of one form of blackening that would produce the
effect you describe, which is that of silver sulphide. Are you sure
that you didn’t have any sulphur compounds in the investment or
somewhere close? Place in some mix a bit of freshly sanded (not
polished silver; it will bear grease from the polish), preferably
sterling, leave it for a while, remove it and if there is the
slightest trace of darkening on it, then your problem has to do with
sulphur. Otherwise someone else will doubtless come up with a better
answer. Cheers, – John Burgess