Firescale on sterling silver will produce the results your friend
Firescale developed from casting is much more sever than that
produced when soldering.
To prevent firescale you must prevent oxygen from getting in
contact with sterling.
There are commercial anti-firescale fluxes that will prevent
firescale when soldering by shielding the silver from oxygen. Paste
fluxes will not prevent firescale.
If you friend is vacuum casting her work have her look at the paper
I wrote on a method of preventing firescale when vacuum casting. The
process described in the paper will not prevent firescale on silver
that is cast by centrifugal means. It takes too long to get the hot
flask with the silver from the centrifugal arm into the reducing
atmosphere produced by my process. I have an illustrated copy of
this article which I can e-mail to you.
This will lead you to an article entitled "Firescale Prevention
During Vacuum Casting - An Inexpensive Method"
If you do a search at Orchid's "Jewelry Making Articles" then "fire
scale" in The tip's from the Jeweler's bench article archive" you
will find an illustrated, improved, anti fire scale article entitled
Improved Anti-fire Scale Casting Procedure
The photo of the bear at the beginning of the article shows a
casting that was brushed with a brass brush to remove investment
after quenching. There are two photos at the end of the article. One
photo shows the comparison of a casting cooled normally and one
cooled with my process. No fire scale on the castings cooled by this
The process is very simple. It produces a reducing atmosphere around
the silver as it cools after pouring the metal. Oxygen is combined
with the burning wax that is placed at the base of the flask and on
the sprue and cannot combine with copper in the sterling silver.
The Archives have a large body of on how to rid the work
of firescale. Eliminating firescale is not an easy task so it is
better not to get it in the first place.
If I can be of any more help contact me.