A coal forge is a hearth that uses coal as fuel for a fire that will allow a blacksmith or other worker to heat metal to high enough temperatures that the metal becomes malleable. The system includes several components that help stoke the fire, including a moving air fan or bellows, a fire pot in which the coals will burn, and a hearth where fuel is stored prior to burning. The primary source of fuel for a coal forge is, of course, coal.
In order to keep the coal forge hot, fuel must be delivered constantly to the fire pot, and air must stoke the flames consistently. To accomplish this stoking, a fan is mounted near the system, and it blows air through a pipe known as a tuyere. The air delivered through this system will cause immense heat at the center of the fire pot, thereby burning the coal into coke. Coke will surround the hot core of the fire, and this is where the metal worker is likely to heat the metal, as the temperature of the coke can be regulated more consistently than the fire itself.
Blacksmiths throughout history have used the coal forge to create tools, horseshoes, and other metal goods used for the home or business. Various metals can be worked in a coal forge, and they are heated until they are malleable. The blacksmith can then shape the metal into the required form; as the metal cools, it sets into a hard shape. In some cases, the blacksmith might cool the piece quickly by dousing it in water, which can increase the strength of the piece. This process can also cause cracks in the metal, however, so it is not always the best practice.
The overall function of a coal forge has changed very little over the course of centuries, though the specific designs have varied. In the past, the best method for delivering air to the fire was the use of bellows, which are essentially hand-operated blowers. More modern variations of the coal forge design use electric fans to deliver a constant and easily regulated stream of air to the fire. The configuration of the fire pot and the hearth itself can vary according to the blacksmith's needs, though many modern designs are still based off traditional models.