PITFIRE

The Ceramics Guild and the Raku class sponsor two Pitfires each year. Dates and locations vary. More information of these events will be offered though your instructor.

Oxidation and Reduction

In fuel burning kilns the atmosphere can be easily controlled. Various atmospheres have an important effect on glaze and body colors and textures. (For information on reduction effects, see Clay and Glazes for the Potter by Daniel Hobes)

In oxidizing fire, plenty of air is let into the burners to oxidize or burn the fuel thoroughly. This air, which enters the burners and mixes with the gas before combustion is called primary air. Air that enters through the burner itself, and adds oxygen to the flame is called secondary air. It is pulled or sucked into the kiln by the pull of the draft. The sign of a clear or oxidizing fire is a clear atmosphere in the kiln, everything being sharply visible. There will be a total lack of visible flame at the damper or coming from the spy holes. the flame at the burners should be burning with a predominantly blue color, with little yellow flame appearing.

If too much air enters the kiln from the secondary air accesses, there may be a cooling effect that prevents the kiln from gaining temperature. Only enough air for proper combustion should be allowed to enter. Even in the case of air which supplies oxygen for combustion, the fraction of nitrogen must be warm and passed through the kiln.

If the kiln appears to be oxidizing, yet no temperature gain is noted, it is probable that too much air is being admitted.

Perfect oxidation is hardly attainable in ceramic kilns. An analysis of the flue gases will always reveal the presences of some carbon dioxide. But, for all practical purposes, if the kiln is burning clear without flame or smoke, an oxidizing effect will be achieved.

If the kiln is oxidizing, a satisfactory rate of climb usually results. To advance the temperature, either the valves are turned up from time to time, or additional burners started. The damper is ordinarily left open, but if the kiln is pulling in too much air, the damper may be partially closed to diminish the draft. To advance the heat, it is good to follow some sort of schedule for a regular pattern of temperature climb. The operation simply involves feeding sufficient fuel in through the burners to maintain the desired rate of climb. Beginning at cone t , carbon in the wares (and there is always some) bums, and there is an endothermic reaction that may cause a rise in temperature not attributable to the burner settings. Also, at about this heat, radiation from one surface to another seems to make for a more rapid climb.

To reduce, the air supply is cut back. Either the primary air or secondary air supply may be diminished, or both may be art back until the flames begin to bum with a yellow color. The damper should be closed somewhat until a back pressure develops in the kiln. This will be evidenced by some flame at the spy hole. Flame will be observed at the damper.

Extremely heavy reduction does no good. It is quite unnecessary to have great belching clouds of black smoke coming from the spy holes and chinks in the kiln. The eternal questions are when to reduce and how much to reduce. As a general rule, a neutral to light reduction gives a good color and texture. It will be necessary, however, to experiment with a new kiln to determine just what symptoms of sufficient reduction are. Once a satisfactory firing pattern is arrived at, it can be repeated successfully.

Since reduction involves an excess of unburned carbon in the firing chamber, too much reduction is a waste of fuel. Heavy reduction will usually haft the advance of temperature , or even cause a loss of temperature. If the temperature in the kiln is not advancing, admitting more air at the primary or secondary sources of air will sometimes bring about satisfactory rate of climb.

Reduction in the earlier stages of firing, from 750‑900 C.(1382‑1652 F.), will cause a deposit of some carbon in the clay being fired, the so‑called "body reduction." This may produce warm browns or orange color in stoneware clays. Too much reduction at this stage, however, may cause bloating or cracking, especially if the later of the firing is rapid.

Heavier reduction toward the end of the firing tends to slow the kiln down and give some "soaking," usually beneficial. At this stage, reduction may also favor the development of celedon and copper red glaze colors.

Kilns always tend to re‑oxidize on cooling, because they are not really airtight enough to keep the oxygen out. Clay colors are developed toward warm brown by the effect of this re‑oxidation on the iron of the body. Some potters give the kiln a brief period of sharply oxidizing fire at the end to "clean it up," but is rather doubtful that this has any real effect.

For reduction firing, best results are obtained in a downdraft kiln that circulates gases to every part of the setting. Kilns burning wood can be reduced by adding an excess of fuel. To maintain temperature advance, and to bring about a pattern of intermittent reduction, this overstoking must be afemated with lighter stoking. Reduction firing may be damaging to the firebrick of the kiln, especially if the brick contains considerable iron. The iron may catalyze the deposition of carbon within the pores of the brick, causing a tendency to disintegrate. In practice, this is nothing to worry about because in all ordinary reduction firings there is ample opportunity for the carbon to burn during the long cooling period under oxidizing conditions.