A forge is a large furnace made for the purposes of softening or melting metals. Such technology is extremely old. Many real-world pantheons contain a god or goddess who has power over the forge, such as Hephaestus from Greek mythology. As shown in the above image, leaves would serve as useful fuel in a world without widespread use of coal. In ancient times, a forge would be made of earth and stone.
The above image was taken from the book De re metallica libri XII… by Georgii Agricolae.