Kiln Theory and Practice: Downdraft Kilns

by Frank Colson
A distinct advantage of the downdraft kiln is that it uses fuel more efficiently than the updraft. However, firing for reduction is a more complex operation. The downdraft kiln consists of three basic structural elements : The firebox, where fuel combustion takes place; the stack area, where pots are placed; and the kiln stack, which includes the flue damper system. Since the basic principle of a downdraft kiln entails directing flame downward inside the kiln, and not upward, the firebox is usually a separate section, located apart from the pottery stack area. Whereas in some kiln designs the firebox is on one side of the kiln, in other designs it may be split up into two or more areas on opposite sides of the kiln. Sometimes it may be tangent to the kiln in the rear or on the side, but in some cases it is several feet away. The function of this stack is to create a draft for the flame in the firebox so that the flame is drawn through the ware in the stack area. This draft commences either under the kiln floor or on the same level as the floor.

The height of the stack or chimney, plus its diameter, must be directly related to the size of the kiln if the proper draft is to be created. For the purpose of reduction, fuel back pressure within the kiln is controlled with the damper, which is located at the bottom of the kiln stack. The damper is handled in much the same way as for updraft kilns, but it is important to understand other characteristics of a downdraft kiln in order
to control reduction during the firing cycle.

Since the firebox in a downdraft kiln is normally separate from the pottery stack area, you must allow time in the early portion of the firing cycle for the firebox area to be heated. This must be done before the ware area will reach any substantial temperature. At the onset of a firing it may be difficult to acquire even a little draft within the kiln to draw the flame up from the burners because the kiln stack may be at some distance from the firebox. One actual advantage of such a firebox arrangement, however, is that the ware is protected from being licked by flame in the early part of the firing cycle. Once heat has begun to build up within the kiln, a substantial draft will usually occur if the damper is wide open. As heat continues to build up, with the damper set to maintain the greatest fuel efficiency as described earlier, you must watch for the signs of reduction conditions.

The first indication of reduction is in the kiln atmosphere itself. If when you look into the kiln, the inside is hazy or foggy, this means there is smoke inside the chamber and indicates reduction. It is difficult to observe obvious signs of reduction in a downdraft kiln, particularly in the early firing stages. In order to make these signs plainer, however, you can close down the damper control, thus creating a little more back pressure than necessary. When you do this, a forceful carbonization will take place around the peephole, and if you open a peephole (particularly a top peep), a forceful smoky flame will leap out. At this point, the back pressure should be let up a little by opening the damper slightly while you observe the nature of the flame from the peeps. The top peep should have a warm yellow-orange flame about six inches long with a very slight smoky tip. The middle peep should be the same color, but only about three inches long. The bottom peep will often be neutral - that is, there will neither be a draft (which can be checked by holding a lit match in front of the hole), nor a flame coming out of it to indicate any great back pressure. These conditions should be maintained by making damper adjustments as the heat buildup increases within the kiln and you reach the desired temperature. If at any point during the firing cycle you are uncertain about how reduction conditions are progressing, you can close the damper down a bit to overcompensate for reduction and then readjust by backing off the damper once again.

In making adjustments for reduction effects during a firing, keep in mind that the kiln must have at least a half hour to make adjustment. It is therefore easy to misjudge what is taking place during the firing if you make damper or fuel adjustments at intervals of less than thirty minutes. In fact, in many cases it is wiser to wait at least an hour between adjustments, but this depends on the size of the kiln, since large kilns take considerably more time to readjust to a setting than small kilns. Perhaps the greatest frustration you can experience in attempting to make a reduction firing comes when you anticipate what the kiln will do without giving it a chance to show its own traits. As already mentioned, it usually takes more than several firings to learn a fuel kiln’s traits and to be able to make predictions hat will contribute to the control needed for firing. After you have fired a kiln many times, your experience contributes to the predictability of a firing cycle, which, in turn, leads to your needing to give less attention to each firing. Each subsequence firing will then fall into a pattern, which may vary only slightly, according to the manner in which pots are stacked.
In reviewing the advantages of the downdraft kiln, note that although this type is a more complex structure to build and involves a great deal of labor and materials, the way it holds heat by passing the flame down through the ware and out of the floor is very efficient. It is more difficult to reduce with this type of kiln until you have learned and can predict its firing characteristics.

I have tried to make no specific recommendations as to what temperatures begin reduction or oxidation or at what point glazes or clay body are reduced, but have attempted to show instead how to set up the conditions for reduction firing with two basic types of kilns, the updraft and the downdraft.
Since the kiln itself is a retaining box for heat, a system that will generate heat must be provided. You must understand burners and the basic types of fuels that burners use in order to reach a given temperature within the kiln. As already noted, kilns constructed with ceramic fiber materials provide such excellent insulation that the heat input needed is considerably less than for conventional brick kilns and there is a resulting saving of fuel.