NitroSteel, I have used all sorts of cast iron objects in wood fire conditions, but never had a pot like one of those. None of the pans or large pots ever cracked, but with something that thick and historically unique, I would error on the safe side and use some firebrick as others have suggested.
Richard the Instigator, as for your comment, 11" is our high limit in steel, but there is another way for someone to achieve construction of a 48 incher. I've always dreamed of doing a 36 incher in reinforced concrete with lots and lots of concentric rings of re-bar with walls about 3 feet thick and a concrete shot poured into a sand backed paper-mache and large beach-ball mold. The only trouble is the high cost of materials and the property upon which to build and shoot it. Too much for a one shot deal.
Pigs in space, indeed.
Tracy and Mike
P.S. We have been to Peekskill, NY where they made those rendering kettles. We have a customer in Croton-on-Hudson which is north of West Point about as much as Peekskill is south.