It's fire hardening. We used to make giant shillelagh cudgels in college to compete in St. Patrick's day every year out of entire trees. We just used a makeshift propane flamethrower up the middle of the tree, which was cored out with an auger. It took an hour or so but that thing was like a granite fossil after finished. Temperatures inside were around 1100 deg F, you could throw an empty beer can in it and it would disintegrate before it could blow out the other end.