Most commercially built fuel kilns on the market today are updraft kilns, primarily because of the simplicity of building such a kiln as well as packing and shipping it. An updraft kiln consists of tow basic elements which contribute to its simplicity of design : the first is a firebox, where most of the flame is concentrated, and which is always located directly under the bottom shelf of the stack area. The second is the flue damper system where the flame exhaust is controlled; this is actually a part of the top of the kiln and consists of little more than a hole in the roof.

In some cases, the burner system for an updraft kiln is located under the structure, pointing upward. But sometimes the system is on its side or even on the back in a horizontal position. Such a kiln may also have burners within it at floor level. Regardless of the burners' placement, the direction of the flame in an updraft kiln always commences at floor level and goes up through the kiln and out the flue opening at the top. This being so, the problem is to acquire the best efficiency of flame while the flame is in the kiln and to control it for reduction. When the flame enters the kiln at the firebox, it makes its primary circulation. The objective, then, is to hold as much as possible of this flame within the kiln before it is drawn up and out of the flue. This can be done by placing a small wedge shaped brick in front of the burners inside the kiln.

Depending upon where and how such a brick is placed, flame can be directed inward towards the center of the firebox or in another direction of more efficient use. The flame is split up into segments and directed into areas that might not otherwise be covered in the firebox. Burners that enter through sides of the kiln may have such circulation devices, while kilns with burners directed vertically from underneath are often baffled with a refractory plate located a few inches above the burner openings within. In the latter case, the flame, which is dispersed out over the firebox bottom, spreads the heat as evenly as possible. These devices, as well as the density with which the ware is stacked, affect the length of time that the heat of the flame is held within the kiln. With experience in firing as well as stacking a kiln, you will be able to determine the flame's best level of efficiency.

Initial reduction during the firing cycle will depend upon the amount and type of flame that goes into the firebox. If an overabundance of fuel is pushed into the updraft kiln, the draft system will simply accelerate the passage of the flame through the kiln and out the flue, with the result that proper fuel combustion (the right amount of air and raw flame) will take place outside the kiln at the point where the flame leaves the flue. One of several signs indicating combustion going on outside rather than inside the kiln is difficulty in gaining temperature. You can usually notice this by the excess time and fuel consumption required for firing. Another indication will be the extremely active flame coming out of the flue, often in the form of fiercely waving tails along with a low muffled sound of roaring fire. What is deceiving is that this type of flame will often present a great deal of smoke, leading the potter to believe that a good reduction is taking place. It is - but outside the kiln! If these effects are encountered early in the firing cycle, before cone 6, for instance, there often is a distinct odor of carbon dioxide which tends to smart the nasal passages and indicates incorrect fuel combustion. The object is then to set up the flame input and the damper control so as to make combustion occur within the kiln. This will bring about a heat rise, thus making possible the conditions needed for a reducing atmosphere.

Unlike a downdraft kiln, an updraft has no chimney stack; its one great advantage in reduction firing, therefore, is that there is a means of seeing and controlling the amount of reduction. Excessive reduction, which also produces excessive back pressure within the kin, will result in an extensive flame (more than 12 inches) coming out of the top of the flue. Thus the flame you see coming out of the flue and the way you adjust your kiln settings for that flame are the most important factors in a successful firing.

In the early cycle of firing an updraft kiln, (before heat color occurs) the burners are kept soft and low so as to confine the flame within the boundaries of the firebox. Flame from the burners should not be allowed to lick against any pots until after all the chemical water has been removed. ( 500°F to 900°F ) The damper is wide open at this point. Once the temperature has passed beyond 900°F, you may close the damper nearly three quarters while bringing the flame up more and more during the early stages of the firing cycle. The function of this damper position is to hold as much heat in the kiln as possible during the early oxidizing stages of firing. Once color in the kiln has begun, you can pay close attention to the flue.

It is helpful to plan the timing of a firing so that the color within the kiln will coincide with dusk or early evening; darkness will facilitate seeing flame emerge from the flue. If the first flame out of the flue appears blue, this will indicate that the temperature within the kiln is not yet high enough to prompt proper reduction conditions. At this pint, the damper should be opened substantially, which will result in either a disappearance of any visible flame or a change in color from blue to orange-yellow. As heat continues to build up within the kiln and fuel input continues (up to the point of greatest efficiency, so as not to push fuel through the kiln), the flame coming out of the flue will increase. Once you can see a strong orange color within the kiln - anywhere from cone 8 to cone 1 - reduction should begin. A visible flame should be coming from the flue now : orange, about 12 inches long, and very active. If viewing conditions are good, you may observe a small amount of smoke coming off the top of the flame, and this should indicate that the type of flame going through the kiln is naturally smoky from the burners right through to the flue opening. When you reach the high temperature range of the firing cycle - cone 5 and above - check the kind of flame coming out of the flue often.

Make damper adjustments as needed. The greater the fuel input into the kiln, the more important it is to look at the color and kind of flame coming out of the flue. The back pressure of fuel in the kiln is kept in check by adjusting the flue damper opening. This damper is the primary device for controlling proper combustion, sufficient reduction, and the increase of heat, once the fuel input is at a constant setting.

During the peak of the firing, a secondary characteristic of proper reduction may be observed : the accumulation of slight carbon deposits around the peepholes in the door and around the damper openings.

Summarizing the advantages of an updraft kiln : First and foremost, it is simple to construct, being nothing more than a refractory box in which there is a fuel input in one end and a fuel outlet on the other. Also, with such a kiln, the characteristics of reduction firing can be observed through the entire firing cycle, making it relatively easy to learn reduction firing techniques that can be controlled with considerable consistency.

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