Burning coal can make you invent new words, it is a skill acquired along with the iron pounding. A small forge like my rivet forge, I can with some attention build a fire as large and nearly as deep as a margarine bowl. Easier to build a fire about the size of a softball. The bigger forges, them big square and rectangles with a recessed firepot/tuyere, one can get a fire going near as big as a small loaf of bread. The bigger the fire, the more coal consumed, but really only build a fire large enough to heat the work you are doing. Depth of the fire is as important as overall diameter, as iron laid on a fire two inches deep, and have two inches or more above the iron, heats things very well. A fire 3 inches or less in depth/height, the iron laid in there catches too much cold air from the blast, and takes longer to heat if it ever does. Iron does hot heat laying on top a fire, not into a red or even orange color that is. It almost always needs to be inside the (oven), the deep fire. Did I mention that coal, rather, coke can be cantankerous to even get lit at times? Other times I can have a smithing fire going ten minutes after the first match. In some ways, its like heating with wood, after awhile you learn what draft adjustment works best, how much wood, placed just so makes the best heat. Is all a learning experience.
