I suspect its copper oxide.
If you can beg borrow some stainless steel brazing flux from a
welding shop Johnson Matthey make, apply liberally to a small piece of
the pink silver, and heat till molten supported on some small piece
of cramic fire brick… Keep applying it in liberal amounts so the
flux runs away from the silver.
This flux will dissolve out copper oxide and it turns the flux red
leaving the silver white.
Its flouride based so ventilate your melting area well. As to keeping
sterling scrap free of oxides, you have to melt in an inert
to do that your melt has to be covered with flux, borax is ok but
againg a reactive flux will clean your melt. as you raise the temp
from cold to molten.
then you also need a larger torch, say 1in dia propane et to a
neutral flame to exclude oxygen. Then you need a big propane tank say
30lbs content to back up your torch.
I use a crucible in a proper melting furnace some 9in wide by 18in
high internal dia in which I have a 6in high by 3in dia graphite
crucible. Then when the silver is melted I pour through a reducing
propane flame, again to ensure oxygen is always excluded.
The cast iron mould is preheated then flooded with olive oil. This
also excludes air during pouring and results in clean shiny ingots
that roll well into sheet and wire.
In a bullion making plant after casting most ingots are milled both
sides to remove any surface contaminants and mould lines. then its
rolled to the required thickness, with appropriate annealing in
again inert atmosphere.
Keep oxygen out of your melts. its highly reactive at molten metal