velocity thus convection currents are very high for ultra efficient complete and uniform heat transfer to the setting and less need for a high exit suction of the gases through the flue opening is required this eliminates the need for a flue taller than the kiln it should thus be evident that the present invention provides a new and improved down draft kiln that is completely controllable highly efficient and yet simple to construct referring now to fig6 there is shown a floor plan of a gas fired down draft kiln 73 which is similar to the kiln 11 except that a horizontal flue channel 75 is provided in the floor 23 39 as seen in fig6 and 7 the channel 75 communicates with the flue opening 51 39 and conventional refractory slabs 77 such as silicon carbide slabs are disposed in spaced relationship along the channel the slabs 77 cover desired segments of the channel so that circulating gases may be pulled more evenly throughout the depth of the kiln 73 of course the movable slabs may be replaced by a fixed or permanent horizontal flue opening floor pattern in longer tunnel type kilns it has been found desirable to provide a vertical flue at both the front and back ends of a kiln this is illustrated in the embodiment.