In looking back, I had tried to shine up the steel shot with coke.
The black appeared after that.
In my book, the best thing to use to clean grungy shot or tumblers
is Draino (the dry granular type). I've been using it for years 1st
with carbon steel & then stainless steel shot in a vibratory
tumbler. I use about 2 tablespoons of Draino & 2 oz of water for 5#
of shot. Let the tumbler run for 1/2 - 1 hr, dump the liquid down
the drain, rinse in running water, dry the tumbler out, return the
shot to the tumbler & I'm ready for the next load to be polished. If
you want you can use lye (sodium hydroxide) in place of the Draino.
Just be careful with the Draino or lye, if you get it on you, wash
it off with plenty of water, it can cause burns.
Just yesterday I helped a friend recover from a situation of a black
covered sterling chain she'd made. It had been left in the tumbler
too long & came out black & stiff. She had put it through the 'Coke'
treatment a couple of times, but it was still grungy. We put it in
the pickle pot (PH Minus) for about 20 minutes. It came out lots
cleaner, but still wasn't really clean.
Next I put gave it a boric acid/alcohol dip for firescale prevention
& heated the entire piece to a black heat with a torch. While it was
still warm it went into the pickle. When it came out in about 5
minutes, it had the nice white look of well pickled sterling.
The amount of heating has some part to play in the whole operation.
A small section of the chain near one end didn't come out as white
as the rest the 1st time. I gave it the boric acid treatment &
heated that section a second time. After it came out of the pickle
the entire chain was the same whiteness.