I'm no expert on clays and firing, but I do remember from my Soils class in college (a long, long, time ago) that Bentonite is one of the clays that has a very high shrink/swell ratio, so I wouldn't think it would be good for a forge lining unless you could get it dry without breaking up or distorting badly from excess shrinkage.
I could be wrong- it happened once before, I think about 1967...