A furnace includes: a furnace housing; a perforated tray mounted in the furnace housing; and a stirring rod disposed rotatably on the perforated tray. The stirring rod has top and bottom ends, two opposite lateral ends, a middle portion, and opposite first and second portions, each of which extends from the middle portion to a respective one of the lateral ends. Each of the first and second portions has an inclined face that extends between the top and bottom ends of the stirring rod from the respective one of the lateral ends toward the middle portion of the stirring rod, that is inclined relative to the perforated tray, and that faces toward the perforated tray.