"I only cast sterling occasionally but every time it comes out black. Are there additives that can make it come out a nice frosty white or is there something I can clean it with easily to remove the oxidation? Thanks."

The black you're describing is perfectly normal with sterling. Accurately, the black is silver sulfide, not oxidation. Soak the castings in a hot "pickle" - dilute sulphuric acid or "Sparex #1" (which is a milder commercial chemical salt that, when mixed with water and warmed, forms acid that dissolves the silver sulfide and copper compounds from the surface of the silver, leaving it frosty white. Unfortunately you will encounter more copper in the form of "fire scale" as you polish the castings. If high shine is your aim, you'll have to cut through that fire scale layer. However, if you can live with a little less than a mirror polish - file, sand, or otherwise remove the flaws, bumps and sprue marks, and refine the surfaces lightly with extra fine sandpaper or with your buffer using spongy soft abrasive wheels (the extra fine version from 3M Corporation, St Paul Mn.) WASH OUT THE DIRT (use an ultrasonic cleaner if you have one) and then tumble the castings* in VERY clean and highly polished steel shot, What I think happens is the pure silver is smeared and burnished over the fire scale, leaving it unnoticable, (use absolutely no abrasive polish! - don't let anyone talk you into using lapidary polish. You will embed the abrasive into the silver's surface and then it will be much harder to buff clean.)

When you tumble with steel shot, use a rolling tumbler - its a smoother, sliding action than the vibrators, and THE FIRST THING TO DO is tumble just the shot over night to polish itself, in the anti- rust solution (available from "Rio Grande" and other suppliers) then wash the shot in a colander and IMMEDIATELY add new anti-rust solution. Keep the shot pristine by changing the solution often, otherwise you will be grinding and embedding a film of grit into the silver - if you tumble polish stones don't use that barrel for silver, or at least scrub the rubber liner twice, and rinse well.