My old mentor swore by a coal forge and I finally tried a gas one last month
I ran 40 pounds of coal through my rivet forge for 3 months trying to get clean welds before switching to a borrowed gas forge and nailing my first solid billet in 20 minutes, so now I want to know if anyone else thinks coal is just nostalgia talking or if I'm missing something my mentor knew that I don't.
Read somewhere that coal gives you a hotter fire in a smaller spot, which helps with certain welds, so it might not just be nostalgia. Gas is way easier to control though, and I'd bet most guys making billets these days run gas. Your mentor probably learned on coal because that's all there was.