Plaster definitely is NOT a long term foundry material, that much is obvious now. It's more of a one-time use thing before it starts crumbling.
Anyway, stuffed with charcoal it looks like this:
Worlds biggest penny:
got a couple of them
Speaking of the next furnace, I see there is a whole section on this forum about building them. I think i know what i'm going to read next. The couple of books i've read talk a lot about different designs, but it seems focused on commercial sized operations or at least not small hobbyist ones.