I melt with an electric melt furnace. I do not get any yellow
coating. Some times the surface is a little dull.
Fire scale is forming as soon as your pour the metal. It is
imperative you place the casting flask in the reducing atmosphere as
rapidly as possible.
I use wax filings for the reducing atmosphere. I find wax filings
produce a better reducing atmosphere. The smaller the particles of
wax the better.
Do you get the yellow layer all the time?
Normally you do not need to pickle your casting after quenching if
you use the anti fire scale vacuum casting process. However have you
tried to remove the layer by pickling? A brief time in a pickle pot
might remove the yellow. You might try coating the piece with anti
fire scale flux heat it up to the solder temp and then quench.
I don't believe the wax is causing the yellowish layer on your