Time lapse photo after the kiln was stoked with wood
Pottery Making Illustrated Jan/Feb2003, Vol. 6 Issue 1, p45 by Eric Rempe
Wood-fired anagama kilns measuring hundreds of cubic feet chugging along at temperatures exceeding 2400 degrees can be an imposing sight. Fired for days at a time by teams of workers, these kilns possess an unquenchable appetite that consumes truckloads of wood in a single firing. In most cases building and firing this type of kiln simply is not an option. I set out to build a test wood kiln in a much more basic fashion from materials that were already in the kiln yard at San Diego State University. I wanted to build something that would deposit directional ash in a manner like an anagama, be as easy to stoke as bourry box kiln, and fire cleanly.
The kiln’s firebox incorporates three separate areas where the wood and coals break down. The air that enters the firebox only passes over the sections where you choose to allow draft. If coals begin to build up in one section, you simply remove the corresponding inlet bricks to introduce more oxygen. This improves the efficiency of the burn in that section and potentially increases the percentage of ash going into the chamber. The flexibility of this design can compensate for the varying factors of combustion throughout the duration of the firing.
Basic factors such as the quantity of bricks available or the size of a kiln shelf dictated some of the dimensions of the kiln; and the resulting simplicity of design easily allows for modifications based on your own needs or restrictions. Other factors like inlet flue areas, exit flue areas, chimney height and basic principles for kiln construction were taken from “The Kiln Book” by Fredrick Olsen.
The first part of the project was to excavate an area in the kiln yard that was slightly larger than the projected kiln would be. The ground was prepared at a slight slope to establish a natural draft towards the chimney. Using a level on a piece of lumber, this lengthwise slope was established taking care to maintain a level surface across the width. On top of the compacted soil a layer of broken scrap brick from old kilns was placed.
The first course of brick for the walls was put in place using a thick slurry of Lincoln Fireclay on the faces and ends of the brick. Once dry, the fireclay produced an impermanent bond between the courses and, with the help of fine grog, allowed for leveling at each course. This process was repeated until the approximate height of the entrance and exit flues for the chamber was reached. A layer of sand about an 1 1/2 '" was worked into the scrap brick and the final brick floor was put into place. Sand was repeatedly worked down into the cracks of the floor to create a solid base.
Final measurements were made and bricks were cut to create the spans across the chamber, the walls were notched using a grinder with a masonry blade, and everything was set into place using Tenax mortar. Courses were laid on the chimney up to the height of the damper and then continued on the walls of the chamber. Spy holes were created to monitor the temperature of the kiln in a number of spots, serving as a place to introduce soda if desired in future firings.
The construction of the firebox was the most time consuming part of the process. Tenax mortar was used on each course until the height of the first grate was reached. At that point the corresponding course of bricks was set back 1 3/8” to create a groove inside of the firebox where the brick grate could rest. Other kilns have used hollow triangular mullite posts for the grates. It should be noted at this point that the initial lower grate was made of 1” cold-rolled steel bars. These bars began to spall, and large deposits of dark metal created some rather crusty looking pots. The bars were subsequently replaced with a brick grate.
Kiln floor with a slight slope to establish a natural draft towards the chimney.
View into the firebox
For the upper grate, which consists of four widely spaced bars, the corresponding bricks were notched and the 1 1/4” stainless steel bars were set in place. When the final height of the firebox was reached skewback bricks were cut and ground in place to make the angled support for the arch. Before the arch was set the angle iron framework for the firebox was cut and bolted into place. It was tied together using all-thread running through pieces of hollow pipe welded to the angle irons. The firebox was now strong enough to support the arch.
The arch was constructed of 4” thick industrial chimney brick that had been donated to the university. Three arch bricks and the skewbacks made a 26” arch that went together with little difficulty. The faces of the arch brick were coated with the fire clay slurry and the ends coated with Tenax mortar. A door for the firebox was made from an old block of soft bricks cut to fit inside an angle iron frame. A steel rod was welded onto the door and a hollow pipe onto the frame of the firebox. The rod dropped into the hollow pipe and the soft brick door was ground to fit the face of the firebox.
This chamber door was constructed of soft firebrick with ½” holes drilled through the 2 ½” width. The holes on alternating courses lined up and the bricks, coated with Tenax mortar, were tied together with 3/8” all-thread bolted to two angle irons. The resulting door, though rather heavy, sealed the top of the 13 cubic foot chamber with few spaces. The door can be cumbersome, so a counterweighted door that would allow one person to load and fire this kiln would be ideal.
The same kind of support used on the firebox was also used to tie the chimney together. Extra bolts were used around the damper slot to prevent any movement that would jeopardize the integrity of the span at the damper slot. At this stage a metal chimney from an old kiln could be incorporated to save money, time, and effort.
This grainy video from 1999 is the only video footage that I could find. Richard Burkett asks the question and is filming.
Soft brick from an old kiln was piled around the walls of the chamber and firebox for extra insulation. Care was taken to keep the insulating brick off any of the metal framework so that the heat would not be able to break down the metal. This layer was subsequently coated with a combination of mortar and ground up soft brick to form a more permanent insulation for the walls of the chamber.
When stacking the pieces in the kiln careful attention must be paid to how the flames might travel through the work so that the entire chamber will receive ash deposits. The primary air sources that need to be open are different in each firing, and proved that the adjustable air sources were a definite asset. A single brick hole was cut out of the door, which allowed just enough air to enter the first grate where the wood begins its initial breakdown.
The most efficient method of stoking was to mix thin rippings with larger pieces of split 2x4. Each time the firebox was stoked an attempt was made to maintain that balance of small and large pieces. At the end of the firing we switched over to using only thin strips of wood to hold the kiln at cone 11. As each new handful of wood was placed in the firebox, it roared with renewed strength and in a few moments a three-foot flame was visible at the top of the chimney.
In both initial firings cone 11 was reached in every part of the chamber including the chimney, and the ability to stoke from a standing position made the entire process much easier. As expected, the ash deposit on the work was much heavier in the front of the kiln, and it became progressively lighter as you move towards the chimney.
From the time the pots are made, the fact that they will be wood fired needs to be kept in mind. They need to be well made to withstand the stress that they will undergo, and at the same time be designed in a way that they will be able to catch the flying ash. They must be glazed in a manner that will compliment the effects of the ash and not obscure them. The kiln needs to be loaded with careful attention to possible flame patterns and fired with constant attention. If all of this is kept in mind, opening the chamber of a wood kiln that you built can be a very rewarding experience.
I would like to thank Richard Burkett for his guidance, for answering all of my kiln building questions, and thanks to all of the students at San Diego State who were there to lend a hand.
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