I wonder if you get good results with mill scale because it's iron oxide?
I believe you are correct as rusty steel works just as well.
Does that white oak lighten a bit when you put the ammonia on?
No, it gets "browner" (I put it in a plastic bag with a can of household ammonia). Nothing like the FA change but pronounced (and sapwood is quite different from heart). The sapwood change is underwhelming at first but with a finish and sun/time, is quite pleasant.
Here are the same pieces with finish:
Seven months later, the ladle is almost the color of lightish heartwood (finish was BLO + lacquer) and the black spoon hasn't changed.