Make sure you have a very difficult, one-of-a-kind silver castingand all the extra bismuth will surely destroy your silver casting.
Personally, I'd just get another sleeve and use the one you have forstuff that doesn't matter.
One way would be to flux it well and heat until the flux is melted.Let it cool and leave it in a damp atmosphere for a while and thenremove all of the flux. The idea of leaving in a damp atmosphere isto devitrify the flux glass and make it easier to remove withoutdamaging the carbon sleeve. Another way would be to skim the sleevein a lathe.