Kilns of this class require permanent walls and a tight roof. The floor must be open and connected by flues with a chimney or stack. These kilns are more often made circular in plan and in the shape of a beehive, although they are also made of a rectangular shape. The heat is generated in ovens built outside of the main walls, and the flames and gases enter the kiln through vertical flues, carried to about half the height of the kiln. The heat, therefore, practically enters the kiln at the top and being drawn downward by the draft produced by the chimney, passes through the pile of bricks and the openings in the floor into the flues beneath, and hence to the chimney or shaft. It is claimed that all kinds of clay wares may be burnt more evenly in down-draft kilns, and terra cotta and pottery are almost always burnt in such kilns. For terra cotta and pottery the beehive shape is generally used, several kilns being connected with one stack.