Offers a classification of furnaces based on whether or not furnace fuel and contents (ore or object to be heated) are mixed e.g., bowl furnace vs. pottery kiln. Mathematical analysis of the respective operating characteristics of such furnaces is presented. When fuel and contents are kept separate almost any fuel can be used, and temperature is limited only by the quality of the refractory materials. When fuel and contents are mixed, natural draft furnaces need careful design, e.g., size of tuyere opening, size of charcoal, etc., to attain sufficient airflow. The addition of bellows gives a forced flow of air and allows larger output.