Those coins are no longer with me, Sgard, but I'll keep that in mind for any future iron disasters.
Meanwhile, here's my current complication. Treating red rot on brass with my wife's brass pots 'n pans "shampoo" worked wonderfully. The first treated corroded coins are already starting to regain their natural beautiful brass shine. Then, I came across this awesome foursome, all pretty Peruvians. The very same treatment did not work well on the largest. Some muck came off, but probably as a consequence of restless rubbing only. The sediment is orange, rather than red, with some baby blue mixed in, so, for lack of a nice alliterating name, I have settled for "Trump treacle". They will be Trump toast if I can't find a smart solution for their sorry state.