Folks, I think I'm 98% sure of the answer here. My Osburn 1800i has pretty crappy firebrick (at least my opinion) in it. With the exception of two firebricks I replaced last year (bottom center two) The rest are the original sort of cinderblock type bricks. They're about half the weight of the bricks I used as replacements, and I'm wondering if it would be worth it to replace the whole lot. So, why would I do this? Well, the only reason I can think of is that the greater heat retention of a denser material may help the stove burn more efficiently. I think It's not worth it, but would like opinions.