Surface problems with Argentium casting

When Casting Argentium with a Kerr (hand) Electro Melt, I am using
the suggested temperature of 1780 F ( 971 C) I am getting a scale
that does not come off. After pickle, tumble and polish. It appears
as a inconsistent cloud on the surface of the finished piece. At the
same time I am getting cold fill at the bottom of the tree. I have
raised the temp. 10-20 degrees F and get the same results. The burn
out is clean, and free of any sulfur smell / darkness that goes with
it. Could it be a reaction to the graphite crucible? Or the lack of
gas cover during the melt and pour?

I am from the new school of casting (closed system bottom pour)
using the old hand held process is new to me.

Any help would be appreciated.
Thank you