Hi Net, Yes the surface black is removed with pickle however,
there is a reaction with the sulfur in the investment that goes
deep into the surface molecules especially if your flask and
metal are too hot. The worst fire scale is the red kind. It is
much deeper and can be caused by “tired”, dirty, or over heated
sterling. Depending whether you are torch melting or induction
melting in an inert atmosphere, ie: Argon, Nitrogen or Forming
gas( a mixture of Hydrogen and Nitrogen) you can still get
varying degrees of the “grey ghost”. You may be aware that
sometimes the grey fire scale is easier to polish than other
times. With a torch melt it IS possible to keep the firescale to
a managable minimum. Whereas even with a Neutec, Galoni, Memco or
other induction melt machine w/Oxygen free atmosphere, you can
still get some of the “grey ghost” from the sulfur in the
investment. When torch melting use a large envelope of reduced
flame kept constantly on the crucble and sterling and use only
fresh grain from a reliable source, and will you get the best
results possible. Will you still have a chance of a little “grey
ghost”? Yes, but you will find it is the easily removeable kind.
One of the problems can be; having done the best possible you can
have some of this slight grey stuff deep witin a textured piece.
At that point if you are searching for cast-sterling-nirvana, you
add the final step of “Bombing”. John, J.A.Henkel Co.,Inc.
Moldmaking Casting & Finishing