I've done some blacksmithing as a hobby. The two most common ways of heating the metal are a gas or a coal forge. The coal forge normally has some sort of forced air coming from the bottom to feed the fire. The coal starts burning real smoky like, but then turns to coke and burns hotter the more air you force through it. Typically you pile some coal around the sides of the fire so it converts to coke then you scoop it into the fire as needed. Also it produces a waste product called clinker that builds up at the bottom of the fire at the tuyere (the nozzle or grate the air is forced through). It's kind of like stone or metal and it needs to be cleaned out to keep the fire going.