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Chariot of Fire: Principles of a Bourry Box Kiln by Gary C. Hatcher

Chariot of Fire: Principles of a Bourry Box Kiln

By Gary C. Hatcher

The Studio Potter, June 1991, Vplume 19, Number 2


Bring me my bow of burning gold!

Bring me my arrows of desire!
Bring me my spear! O clouds, unfold!
Bring me my chariot of fire!
            William Blake

A Kiln Named Wanda

Woodburning kilns have always fascinated me. Unlike other things I have built, they possess unusual personas and are inclined to have demands of their own. As a child I loved to take things apart and see how they worked. As I grew older I learned to put the parts together and make them do work for me.

Kilns are extensions of ourselves, an integral part of the creative process of pot making. Choices of fuel, temperature, design, and size are creative decisions that affect the outcome of the pots. If understanding and communication between potter and kiln is lacking, the quality of the pots is affected. If the kiln conjures feelings of frustration and negativity in the potter, attitudes towards making posts can be affected.

It is important to me to have harmony with my tools and materials, and to have the knowledge that my pots are the result of decisions I have made. The pots are me. Woodfiring is an added way in which I can influence the clay with my energies.

Making choices compatible with one’s way of working is important when building a woodfiring kiln. The Bourry type of firebox can make woodfiring a feasible option for potters who may have ruled out woodfiring as too unpredictable and labor intensive. The Bourry box kiln fires pots with a glazed surface only slightly affected by kiln ash, but not dominated by it. While the dramatic colorations produced by long firings in Eastern-style kilns are unrivaled, I personally prefer smoother surfaces on my pots.

Wanday Kay, our 128-cubic-foot kiln, fires to cone 12 in about eighteen hours on two cords of wood. It fires evenly throughout the setting when the chamber is stacked with proper consideration for the needs of the flame path. Though some unglazed surfaces are considerably ashed in parts of the kiln, the shelves are spared heavy ash deposits, that “Bizen” look is out of reach here.

Bourry Box Kilns

Beware of what you set your heart on for it surely will be yours.           

Ralph Waldo Emerson

The Bourry firebox utilizes wood in a unique way. It receives its name from Emile Bourry, a Frenchman who wrote in 1911 about this style of firebox in a book entitled A Treatise on Ceramic Industries. Bourry box kilns have been built recently by scores of potters since “rediscovery” of these construction principles by the late Michael Cardew.

Our kiln has nine doors. One large door gives access to the ware chamber and is shut tight during the firing. (The door is constructed of Z-Block ceramic fiber modules encased in a steel frame). Four small arched doorways lead to the ash pits, where stoking is done during the initial build-up of embers in the ash pits. As the kiln approaches 1000° F, these doorways are bricked up and stoking is transferred to the four Bourry box doors above. These hinged doors open and close for stoking until the end of the firing. (These doors are steel-framed on our kiln, and lined with one inch of ceramic fiber to make the kiln air-tight. Wood as long as 50 inches in length lies across the hobs at the top of the Bourry firebox above the ash pits. The heat from the embers and surrounding brickwork causes gases to be released from the wood as the primary air is drawn from the top of the firebox and down through the wood. This process works very much like a carburetor in an internal combustion engine, serving to blend the release of hydrocarbons from the wood with the air.

As the wood burns, ashes, embers and partially combusted wood fall into the ash pit below where they continue to burn and serve to ignite new wood added to the firebox. Elevating the wood above the embers allows the wood to combust better because the incoming primary air from above envelops the wood. This is a much more efficient firebox than one in which primary air travels over the top of the wood into the kiln. The hobs are made of brick and, unlike steel grates or bars, rarely need repair or replacement.

Fire and Water

The pot is the man, his virtues and vices are shown therein….no disguise is possible.
Bernard Leach

The design and operation of natural draft kilns are better understood by applying the analogy of water: physical principles governing the flow of water can be applied to those of fire when reversed. Gravity causes water to flow downward; heat causes flame to flow upward. Water can be siphoned downward; a chimney siphons flame upward, A pump moves water; a blower moves flame. However, although fire and water are similar in terms of flow, they differ in terms of behavior when compressed and heated. Water is reluctant to compress and does not expand dramatically when heated; flame (gas) can be compressed easily and expands dramatically as heat increases. Visualize water flowing through the kiln and allow for the fact that temperature has a significant effect on its volume.

The Bourry box works on the principle of a siphon. The tall chimney required on a Bourry box kiln (ours is 26 feet) siphons the flame through the kiln. Of course, there is also a significant pushing effect that takes place, but the action is essentially a pulling one. The flow of the flame through the kiln is controlled by restricting the air allowed to enter (primary air) or to exit (damper) the kiln. As with the siphon, the size of the hose (kiln dimensions) and the elevation of the water in relation to the siphon hose (chimney height) will determine whether or not the water (flame) will move and at what rate it will move.

I appears that an oriental-style climbing kiln may work like water draining from a higher level to a lower level. The firebox is placed low, like a water tenk being placed high, and the heat is pushed into the kiln just as water is pushed out of a low opening in a water tank by pressure and gravity. How do you stop water from draining out of an elevated tank? You must restrict the flow or prevent air from displacing the existing volume of water. How do you slow the flow on the upper end or prevent air from entering at the lover end. Using this model, if you have a climbing kiln that does not push at a high enough velocity, you could simply add a pulling chimney. If the ware is packed too tightly or the flame passageways are grossly undersized, little can be done to increase flame velocity.

These are general analogies, and there are exceptions and many variables. Remember: the climbing kiln works by pushing the flame and the Bourry box works by pulling the flame.

I might mention that a blower on a kiln is comparable to using a pump in water. But when pumping water, as opposed to siphoning it, the dimensions of the pipe through which the water flows are not critical. Conversely, the height of the chimney and the dimensions of the kiln are much less significant when using a blower on a kiln. This is why a poorly designed gas kiln can be made to work by using so-called power burners. If you are firing a natural draft gas kiln, you can apply the above analogy towards understanding how the kiln works. If you are using blowers, you are on your own-especially during power failures.

Some problems in gas kilns due to uneven temperatures are caused by too much velocity flowing through the kiln. Areas in the stacking space are either cooled off by excess air or the flame is forced out of the kiln before it has had a chance to totally discharge its calorific value. Kiln building and firing is an art, an expression of creativity, but underlying this subjective art there needs to be a framework of scientific theory from which those creative choices can spring.

Science vs. Empiricism

The ideograms indicate the sunny and shady sides of a hill….The art of life is not seen as holding to yang and banishing yin, but as keeping the two in balance because there cannot be one without the other.
            Alan Watts

Most potters are alchemists; lacking a desire to apply a definitive method to what we do, we are at once on the cutting edge of new experiences while being wasteful of time denying proven scientific facts. How boring it would be if we had a definitive textbook on kilns with no option of creatively choosing how to stack the bricks.

We begin, then, by balancing know scientific fact with what we have observed and experienced. There are kiln attributes that clearly defy scientific rules and axioms. Often something that applies to one kiln proves to be the inverse for another kiln with apparently the same design, construction, and fuel used. This is partly due to the fact that there are so many variables that it is diffucult to analyze what is going on.

Potters are amazingly resourceful people, however, and especially in America tend to follow the rule: “Whatever works.” Any box of bricks can be made to melt glaze, but when building a kiln that fires evenly with natural draft (wood or gas), the process is helped by the application of scientific principles that govern the properties of gases when heated and compressed.


Application of Gas Laws

The fish trap exists because of the fish.
Once you’ve gotten the fish you can forget the trap.
The rabbit snare exists because of the rabbit.
Once you’ve gotten the rabbit you can forget the snare.
Words exist because of meaning.
Once you’ve gotten the meaning, you can forget the words
            Chuang Tzu

When trying to convince gases to flow through the kiln and heat the pots evenly and efficiently, there are two gas laws that should be understood. Boyle’s Law say (among other things) that “pressure doubles when a gas is compressed to half its volume at a constant temperature.” This equation becomes more involved when we consider not only Boyle’s Law but also Charles’ Law, which states that “when gas is heated at a constant pressure, its volume increases in proportion to its absolute temperature.”

What does that mean? It means that the size of the passageways in the kiln must be proportional to the gas volume we expect to go through those passageways. As gases increase in temperature, they expand dramatically as temperature increases. Most kilns that do not fire well are the result of the builder’s not understanding those two rudimentary gas laws.

For every 366° C (approximately 690° F) that the temperature in the kiln rises the volume of the gas doubles (See Cardew, pp. 170-212). This means that as air outside the kiln enters the hot kiln, we must allow additional volume for the gases to expand. I use a method to calculate kiln areas based on surface areas of openings and not on actual volumes. Calculation of volumes would require the measurement of gas (air) velocity, which gets complicated and would vary throughout the firing. The surface are is easy to figure and is sufficient for our purposes of comparison and analyzing kiln passages. This works only on natural draft kilns, for when you add variables such as pressurized gas and venturi ports or venturi burners, these must be factored into the equations.

Example: At cone 11, or 1315° C (2399° F) when the gases in the kiln are near their most expansive volume, we have four primary air inlets open in our kiln measuring 2.5 in x 4.5 in each. The total surface area of the four inlets is 45 square inches, As air enters the kiln, its volume doubles at 366° C (690° F), so at that point we should allow 90 square inches in cross section inside the kiln for the expanding gases. Air volume doubles again at 732° C (1350° F), so we need to allow 180 square inches in cross section. Volume doubles again at 1098°C (2008° F), so we need to allow 360 square inches in cross section, and at cone 11, or 1315° C (2399°F), we need 576 square inches. We arrive at this last figure of 576 square inches because the temperature at con 11 rises only 60 percent of the 366° C doubling point: 360 square inches + 60 percent of 360 (216)=576 square inches. The surface area around the shelves at cone ll needs to be roughly 576 square inches, or approximately thirteen times our primary air surface area, Conversely, as the gases are exiting the kiln and cooling, the volume containing them (the exit flues and chimney) must also contract to maintain proper pressure.

The maximum temperature to which the kiln will be fired needs to be plugged into the equation also. If you fire to earthenware temperatures, the surface area around the shelves can be much less.

I have seen kilns that allowed only an inch between the shelf edges and the wall of the kiln, the builder expecting the flame somehow to snake its way through the pots in between shelves. This is one reason why kilns get “stuck” at higher temperatures. The kiln flame in an expanded volume is unable to flow through the areas it was able to flow through at lower temperatures. Visualize a river two-hundred feet wide being channeled into an area fifty feet across. One of the long-standing rules of kiln building applies here: You can always decrease flue areas if they prove to be too large, but it is difficult to enlarge them (Olsen, pp 27-29).

The idea that “bigger is better” applies to firebox size as well. Burning wood requires a large area for combustion, much larger than for gas, so it follows that fireboxes should be as large as possible. The total firebox volume for our 128-cubic-foot kiln is 22 cubic feet (above the hobs). This is proportionally more than most Bourry box kilns I have seen. Actually, a 150 cubic-foot kiln chamber could be fired with 22 cubic feet of firebox. Wanda Kay is slightly over powered. If you try to place fireboxes any larger than ours on a rectangular kiln, you can run into design problems due to lack of wall area. After 150 cubic feet, you need to go to a round kiln with a downdraft exit flue in the center of the chamber floor so as to provide more wall area upon which to put additional firebox volume. Then, of course, you have to figure out how to stack square shelves into a round chamber. Building a good firebox is only part of the job. If the kiln chamber, flue areas, and chimney are disproportional, the kiln will not fire well.

In a single-chamber Bourry box kiln, there are six basic relationships to consider:
1) primary air inlet, 2) firebox size, 3) passages from firebox to ware chamber (bagwall area), 4) ware chamber size, 5) exit flue from ware chamber, and
6) chimney stack height. Stack height and primary air inlet are not critical when building the kiln because they can be changed easily after the kiln is finished, but the other four relationships must be decided when building and cannot be easily changed.

Below is a comparison of three Bourry box kilns. The first and the smallest is our kiln, built essentially after Ray Finch’s rectangular kiln at Winchcombe Potter in England (Crafts, pp. 13-17) as was Karen Karnes’ and Ann Stannard’s Bourry box kiln in Vermont. The next is Michael Cardew’s round kiln built at Wenford Bridge Pottery in England (Cardew, pp. 170-212). The last is a round kiln built by Michael O’Brien in Abuja, Africa.

The ratio of firebox volume to chamber column is high on our kiln and low in the Cardew kiln, which I think accounts for the fact that our kiln never takes more than eighteen hours to fire and the Cardew sometimes requires twice that. I would say that the ideal range is between 1:6 and 1:9.

Bigger wood kilns are more efficient. Wood is wasted in a Bourry box kiln that is quite a bit under 100 cubic feet because, proportionally, much more heat goes toward heating the kiln structure itself in smaller kilns. It has been estimated that only about 2 percent of the heat when firing actually goes toward heating the pots, while 50 percent goes up the stack, 8 percent goes to heat the furniture, and the remaining 40 percent is used to heat the structure (O’Brien). The smaller the kiln, the more cubic feet of structure there is per cubic feet of packing space. In larger kilns, there is proportionally less structure to heat in relation to packing space.

Insulating brick or ceramic fiber can be used in selected places to improve this ratio, but the fluxing action of wood ash must be considered. Our arch is made of 2600°F insulating brick. The hard brick walls are backed up with insulating brick, and the door is made of Z-Block fiber modules.  After one hundred firings, deterioration of our arch and door has been insignificant, but the hard brick walls around the throat arches and bag wall have suffered ash attack. Areas low in the kiln where ash is likely to land should not be made of insulating brick or fiber because of their porosity and the acidic nature of ash.

There has been some discussion concerning preheating of primary air as it enters the kiln, thus causing a more efficient burn. In theory, it is a beautiful idea, but I am not sure it works on intermittent kilns. Preheated primary air has been used for a long time in continuous industrial car kilns, but there are problems associated with using it in an intermittent reduction kiln. We built air pre-heaters under our fireboxes but do not use them any more. By the time the fireboxes get hot enough to preheat the primary air, it is time to start reduction. The only way to reduce, is by adjusting for a less efficient burn, which obviates the need for more efficient combustion. In a continuous car kiln, the oxidation atmosphere is held sometimes for years, at a specific temperature.

There are two excellent sources for Bourry box kilns if you plan to build one. One is the article written by Ann Stannard in Studio Potter (Vol. 8, No. 2), entitled “Wood Firing with a Bourry Box Kiln.” The other is Michael Cardew’s book Pioneer Pottery. All you need to take is one known dimension for your new kiln and project it throughout your drawings, adjusting for shelf size (Wanda Kay uses about 110 20-in. x 14-in. shelves) and keeping in mind some basic principles. This trick also works when using a photograph as a base for drawings: seeing a 9-in. x 4.5 in. x 2.5 in. brick in the photograph of a kiln, one can project and entire working drawing from that by using an architect’s scale for any kiln built with exposed brick.

Then look at your kiln design and in cross section take each passageway (i.e., primary air inlet, firebox, kiln chamber, exit flues, chimney) and do some estimating. Using the model Cardew uses in Pioneer Pottery on pages 182-185, analyze every passageway the flame will travel through, comparing it to designs that you know work well.

With everything there is a balance. Although a bigger kiln can be more efficient, a realistic approach toward your desires and capabilities must be used. We decided several years ago not to have any employees and are comfortable firing our kiln every six or eight weeks. Daphne and I like making the quantity of pots we make. In a work cycle longer than eight weeks, I start to lose some sontinuity. In the future should we rethink having an assistant, we might want a bigger kiln. A big kiln can be a big problem if you ar constantly short of pots to fill it, and a real drag if you have bad firing.

Managing The Wood Lot:

Home-Grown Fuel

Years ago I remember reading that Hamada had his own forest to supply fuel for his kiln. The idea stuck in my mind as a good insurance against rising fuel prices or being subjected to the whims of the local sawmill manager. When our neighbor offered to sell us ten acres of woodland in 1981, we saw a vision of unlimited fuel for the kiln as a real possibility and bought into it. Three years later we acquired an additional larger tract of adjoining land.

Our forest is a gift from God. It is not for free, however, for everything is bought with a price, but it is a gift in the sense that we have the privilege of enjoying its life, beauty, and vitality. The land is our child, to be protected and nurtured. As with a child, there are expenses: monthly mortgage payments, taxes, upkeep. We are investing in an asset that appreciated, if slowly. Strictly as an investment, undeveloped land does not yield high returns, but it can add a dimension of joy and pleasure to living.

I would not encourage every woodfiring potter to buy timberland for the express purpose of obtaining wood for the kiln. Nor would I encourage building a woodfiring kiln unless she or he lived near a forested area. It is estimated that there are almost one-half billion acres of privately owned forest land in the United States, so it is not necessary to own your own supply of timber. Most of that one-half billion acres of timberland is not managed with good forestry practices, in fact most is not managed at all. The timber industry (as with most large industries) is permeated with a wastefulness that enables a potter to secure an abundant supply of wood. Check your local sawmill or tracts of forested land that have been thinned or need to be thinned; there are many ways to get wood for the kiln.

Now there are some who hold that cutting even a single tree is wrong. At one time I did too. But now I see timberland owners as stewards of the land, much as gardeners. Ecologically, the problem of depletion of woodland is not due exclusively to simply cutting trees. The problem lies in clear-cutting and with poor land management by property owners. Clear-cutting demonstrates gross stupidity and is usually the result of the nonresident landowner being seduced by a timber (timber pimp) who serves as an agent for the sawmill. Land that has been clear-cut is ugly and wasteful. Trees serve to clean the air we breathe; they inhibit erosion and provide animal sanctuary. We do not need more cow pastures or parking lots.

Pine Mills-where we live- was settled in the mid- to late- 1800s and was, up to that time, forested by virgin timber. I am told there used to be five sawmills in Pine Mills and that the surrounding land underwent a total removal of large trees. As the virgin forest was depleted, the sawmills disappeared, along with the brown bear and panther. Today it is rare to see a small sawmill anywhere in East Texas.

The timber industry in East Texas is now operated on a vast scale out of massive centralized sawmills. Unlike northern United States, timber in this area grows rapidly and is more suited to pulpwood, particleboard, and rough dimensional lumber, whereas in colder climates the trees grow more slowly and produce a dense wood more suitable for quality furniture and finished wood products.

Many aspects of the timber industry are alarming. Everyone has heard of the massive destruction done in Brazil to the forest, but smaller occurrences of wanton disregard for conservation are occurring worldwide. Although this cannot be blamed entirely on the industry, it is within each individual’s powere to help stop the effects of clear cutting, whether by refusing to buy exotic wood furniture or by planting trees. Everyone can do something. My way is to be responsible for what I can control.

The land we own has been logged many times over during the past one hundred years. Some areas, however are untouched by the logger’s saw because they are inaccessible. These are being preserved so that generations to come may enjoy the pristine beauty of virgin forest.

There are three primary areas on our land. One, a nature preserve, is a mix of hardwoods-oak, sweet gum, dogwood, hickory, to mention a few- and is graced with a stream and lake for the deer, bobcat, waterfowl, humans and other forms of life to enjoy. The second is an area whose primeval equilibrium was disrupted long before it became ours: it was clear-cut years ago and farmed until the 1970s.  It is an area with severe erosion problems. We planted 18,000 pine tree seedlings there two years ago to reestablish the forest. Trees combat erosion when their foliage absorbs the impact of raindrops, slowing water run-off and acting as wind-breakers.

The third area consists almost entirely of pine trees. Because much of this land was clear-cut within the past 100 years, the trees are all of the same age and compete intensely with one another for light, water, and soil. Only an optimum number of trees can thrive in a given area, much like a garden with too many seedlings stifling growth.

In a natural forest there are large trees, middle-sized trees, and small trees, with a continual cycle of rejuvenation. The large trees rise above the rest and receive tha most light, and their roots are deep and get the most nutrients. The leaves from the large trees fall to the forest floor to mulch and compost the earth. Their seeds fall into the natural seedbed, and small trees begin to grow. When the large tree dies, light comes through the over story and the young tree is allowed by nature to live and replace the large tree.

Many natural processes exist within this ecosystem that do not occur in a timber stand where all the trees are of the same age. I am sure you can go into any area of the United States and see this type of forest with its natural equilibrium disrupted. Most of the trees are of the same size, and no light comes through to the ground to encourage young trees. In later stages large ares occur where trees are dying for no apparent reason. This forest needs to be selectively thinned.

Ideally, we try to thin the trees sufficiently so that light can get to the forest floor and make it possible for another generation of trees to begin growing. The problem now is that there are few small pine trees on the ground. Moreover, layeres of pine needles represent a fire hazard. Here in the South, we have the Pine Bark Beetle that also destroys thousands of acres of woodland every year, as it attacks timber stands not in good health.

I thin trees with one employed helper, using a lightweight chain saw that does not wear me out while cutting trees under six or seven inches in diameter. It is the responsibility of my helper to lay a fifty-inch measuring stick on the log as I cut it. The wood is then stacked by my helper in a trailer pulled by a tractor, or left in piles near the road to be picked up later. The wood must dry for a year before use in firing the kiln.

If seriously considered timber management, I can heartily recommend a book entitled The Woodland Steward by James R. Fazio. Also consult your county agent about federal funding available for planting trees and developing timber management plans, And please, don’t cut that tree unless you’re sure it is necessary to improve the health of your particular wood stand.

Every event, every thought, every action has significance and meaning. Some events may appear to be without meaning, but only because of one’s limited awareness or inability to see connections. For me, pottery involves much more than just the final object; the process is just as important. Every job-whether mixing clay, cleaning the showroom windows, or preparing wood for the kiln-must be done in consciousness of how it fits into the whole.

Students achieving oneness will move ahead to twoness
            Woody Allen


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Studio Potter vol. 8, no. 2, 1980.

Daphne and Gary Hatcher apprenticed in England first with Michael Leach and later with David Leach. They operate the Pine Mills Pottery, and can be reached at 5155 FM 49, Mineola, TX., 75773

An American Woodfire Conference will be held October 23-26, 1991 at the University of Iowa, Iowa City, Iowa. Gary Hatcher will be among the panelists, which will include potters, writers, and critics discussing aesthetic and technical aspects of woodfire. For Further information, write: Center for Conferences & Institutes, 249 Iowa Memorial Union, The University of Iowa, Iowa City, IA 52242.