This is actually two questions:

1. I have a nickel crucible in which some bismuth was melted, plus other contaminants. (Specifically, as part of a traditional Marshmallow Peep torture party, a peep was eviscerated, filled with molten bismuth, and the remains put back in the crucible. Fun.) The question is, what's the best way to clean out the detritus (bismuth oxide, random schmutz) and restore the crucible to near-pristine state? To what reagents is it particularly resistant but that might tackle the dregs?
2. On the other end of the spectrum, I'm trying to remove the metal jacket from some large dry cells. (I'm trying to reclaim the encircling carbon relative intact.) They're magnetic, but I don't think they're steel -- FeCl₃ seems to scarcely touch it -- so I'm suspecting they might be nickel. To what reagents is nickel particularly susceptible?

Thanks!

From prior experience with nickel-plated magnets, nickel has excellent resistance to hydrochloric acid, barely dissolving at all.

As for what it's weak against, I'd try nitric acid or even aqua regia - those will dissolve just about anything.

I suspect that one of the things which will dissolve or attack nickel quite well is molten bismuth.

I suspect that one of the things which will dissolve or attack nickel quite well is molten bismuth.

I'd be quite surprised, given the temperatures involved. I'd guess the crucible never exceeded 800⁰C. I've melted bismuth in it before with no problems.