If you have gummed up iron or steel, you can buy liquid paint stripper and let it soak in that for a couple of weeks in a glass or metal container (but it must have no plastic or rubber lid seal; card stock only). That should rid you of any trace of trace of remaining hydrocarbons and grease. If you burned the stuff during casting attempts with it, actual hardened carbon will be present. Probably the fastest way to get rid of your problem is Slip 2000 Carbon Killer. I expect it will remove hydrocarbons and grease, too. It smells a bit like Berryman's carburetor cleaning solution.
If you are more patient, just get a small spray bottle of Gunzilla BC-10 CLP and wet it with that and let it sit for a month under cover. That stuff actually breaks the bonds in the carbon, gradually turning it into a kind of soft sludge. Much of the carbon then falls off or may be wiped off.
Boretech also has a good carbon remover called C4. I've not tried it on baked-on carbon, but it may do the job. A fellow who shoots black powder told me that he cleaned his bronze cap nipples with it and it made them bronze-colored again for the first time since he'd bought them. So it takes off some kinds of old carbon well.
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