Inside fire box, wood fired kiln, Pine Mills Pottery
Wood Firing: Gary's Opinion
An image gallery of our entire making process is located here, taking the pots from making, to glazing, to firing and into the studio gallery.
Aesthetic and philosophical aspects of wood firing run deep among potters. Some would say that wood firing is a cult or religion. I look at wood firing only as a means to an end, not an end in itself. I don't think it is a cult or a religion, only a compelling process that allows effects you can achieve by no other process of firing. Included below find more information about wood firing than most visitors to this site will be interested in reading. Nevertheless I have included it since potters (and maybe a few others)will be interested in the information that follows. In the first section, you can read a few of my ideas about wood firing. The second section contains text from an article published in and Australian ceramic publication in 2003 (Ceramics Technical, “The Bourry Box Kiln: Past and Present”, No. 17, 2003 Gary Hatcher on Wood Firing)
I feel it is my responsibility to help educate others about ceramic art and for that reason welcome inquiries about the process and do my best to explain the wood fire process to those interested. I am happy to go into detail about the wood firing process although, most people are primarily interested in the finished piece, not the making process. It is important that a ceramic artist not get caught in the trap of thinking that some intrinsic value is added to a piece just because of the way it was fired or because the process of making or firing was difficult. There is no magic in materials, making or firing processes. The magic is in the really exceptional finished piece. Only the results achieved are important.
I often remind students that it is immaterial how hard they worked on a piece. If it walks like a dog and barks like a dog, you need to call it a dog. Firing in a wood kiln for a week can not transform a dog into a diamond but wood firing may transform a good piece into a truly exceptional one. Wood firing connects one with the work through the entire process. Unlike gas or electric firing, if you are not fully engaged with the firing, a wood fired kiln will not fire. With other types of firing it is too easy to forget about the kiln, answer a phone call or check your e-mail since the kiln will continue to fire without your presence.
Firing with wood produces surfaces that are unmatched by any other form of firing. In our Bourry Box kiln we get light wood ashing that enhances the surfaces as well as the glaze. I have developed glazes that respond to the wood ash as well as the constant oscillation between reduction and oxidation. The oscillation from reduction to oxidation every 5 or 10 minutes is impossible to simulate with any other type of fuel. The fluctuation between reduction and oxidation is one of the main benefits wood provides other than the wood ash surfaces.
Wood burning 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 our personalities, 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 have harmony with tools and materials, and to have the knowledge that my pots are the result of decisions I have made. The pots are me. a projection of my spirit and energy. Wood firing is an added way in which I can influence the clay with my energy and unique characteristics. Making choices compatible with one’s way of working is important when building a wood firing kiln.
The Bourry firebox can make wood firing a feasible option for potters who may have ruled out wood firing 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. A sense of community develops around many wood firings. Larger wood kilns require teams to fire and the exchange that takes place between participants while firing is important and memorable. One of the unique aspects of all ceramics is the wonderful community of friends enjoyed. This is not so much the case with many other art mediums and the community of wood firers is more connected than most.
Daphne and I first became interested in wood firing while working as apprentices in North Devon, England in the 70’s. Our first two wood firing experiences were with Michael Cardew at Wenford Bridge Pottery. We had met Michael at the opening of Bernard Leach’s retrospective exhibition at the Victoria and Albert Museum in 1976. Being enthusiastic young students, Michael invited Daphne and I down for a firing of his round Bourry box kiln which was to be the first of many trips down to his pottery in Cornwall.
When we met Michael, Daphne and I were working with David Leach at Lowerdown Pottery where we spent the better part of two years as his assistants. The renaissance in wood firing that was beginning in England at that time was happening simultaneously in Australia, America and other countries as well. There were only a handful of potters in England at the time firing with wood and on the weekends when we had time off from work at David’s, we would head out to visit other studios especially those studios firing with wood.
The kiln Daphne and I fired twice with Michael was the kiln he built in 1949 which is still in use at Wenford Bridge today. Although the kiln worked well in some respects, it was also known as a kiln that was difficult to fire sometimes taking as much as 48 hours to fire, an unreasonable amount of time for a 150 cu.ft. Kiln with ware fired in sagers.
It was really not until we attended a firing with Ray Finch in Gloucester at his Winchcombe Pottery and saw the ease with which he fired his rectangular Bourry box kiln that we decided to build a Bourry box kiln when we returned to Texas. We arrived at Ray Finch’s studio after phoning up in advance and he was expecting us. The studio was clean and quite and it appeared we had come on the wrong day. We looked at the pots in the showroom; beautiful toasty functional pots with surfaces lightly kissed by a wood flame. Tenmoku spangled with gold flecks of fly ash and unglazed surfaces lightly ashed that felt good to the touch; surfaces smooth, not crusty. There was a smell of burning wood in the air and we wandered into the studio to find a lone man sitting in a chair reading a newspaper. Ray greeted us and then excused himself and from a stack of wood rolled next to the kiln by a pallet jack he took a bundle of wood and stoked the kiln. The stoking went on all day, increasing in intensity but never in a frenzy. This seemed to be the way we wanted to fire.
The kiln we built five years later in 1982 was very much like Ray Finch’s kiln. We follow the same routine for firing we encountered at Winchcombe 25 years ago. We pull up a couple of big chairs next to the kiln and enjoy the firing together. We take turns stoking, read and reflect on the previous making cycle, life and the future.
While we were in England in the late seventies, back in Texas, Karen Karnes led the construction of a Bourry box kiln in central Texas at the studio of Ishmael Soto. Karen and Ann Stannard had been over to England to visit Ray Finch and he had provided them a copy of plans used to build his kiln. Karen and Ann were later to build another kiln similar to Ray Finch’s at their studio in Vermont. Ann Stannard wrote an excellent article on her and Karen’s kiln for Studio Potter magazine in 1980 (vol. 8, no. 2) that is required reading for anyone considering building a Bourry box kiln.
Also in Devon, England in 1977, Clive Bowen built a two-chambered round Bourry box kiln similar to Cardew’s kiln at Wenford Bridge. Bowen’s kiln was built using plans drawn up by Michael O’Brien who had taken over the Pottery Training Center in Abuja, Nigeria after Cardew left for England in 1965. The Shebbear Pottery kiln was built along the lines of a high-fire kiln (as designed by O’Brien) but fired to earthenware temperatures. Daphne and I were working then for Michael Leach (Bowen’s father-in-law at the time) and had an opportunity to assist in the building of the large round kiln on weekends.
In France the Bourry box kiln was known to be in use by the late 1800’s in the factories of Sèvres and Limoges to fire porcelain to around 2500 F or cone 13. It is doubtful that Emile Bourry had a hand in the original idea for what has become known as the Bourry firebox but he was the first one to document the design in his book titled "A Treatise on Ceramic Industry” first published in 1890. (insert reference here) In 1942 Michael Cardew first incorporated the Bourry firebox in a downdraft stoneware kiln he built while pottery officer at the Achimota College in the Gold Coast Colony of Africa, now known as Ghana. In the hot tropical climate of Ghana, this type of fire box was favorable as it provided a means of firing that protected the kiln stoker from blasts of heat and eliminated the need for steel grates or fire bars. This kiln worked so well for Cardew in Africa that when he returned to England from West Africa in 1949 he built a round Bourry style kiln at his Wenford Bridge Pottery in Cornwall constructed with four fireboxes. (reference Pioneer Pottery).
Cardew never referred to the term “Bourry box” in his book Pioneer Pottery, but simply called it a “down draught firebox”. Ivan McMeekin traveled from Australia to England in 1949 to study pottery and found his way to Cardew’s studio just in time to assist with the first firing of his new kiln and subsequently stayed for three years. After returning to Australia in 1953 McMeekin designed and built several Bourry box kilns with multiple variations and improvements to the Wenford Bridge kiln, including several at Sturt Pottery in New South Wales. (McMeekin's first student-assistant was Gwyn John, later Hanssen Pigott, and when she left in 1957, her place was taken by Les Blakebrough.) More on McMeekin’s thoughts and experiences can be read in Ceramics Technical no. 15. Having several Bourry box kiln firing and building experiences under our belt, Daphne and I were confident about the building of a Bourry style kiln when we established our studio in east Texas in 1979.
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.
We usually make pots for 8-12 weeks accumulating pots for a wood firing. When the studio is full of pots and we have no more room for stacking work we begin the glazing and firing process.
We have a small 50 cu. ft. gas-fired car kiln that we bisque work in as well as fire occasional glazed ware. We usually do two bisque firings for one wood firing in addition to some raw glazing. When we first built our wood kiln, we fired bisque in it but stopped as ash had to be cleaned off the ware prior to glazing and this seemed to be an inefficient process that added no intrinsic value to the work.
I am usually the one to start the firing at about 4:00 am. Fires are started in the four lower fireboxes one at a time over a period of two hours. At about 8:00 am Daphne takes over the firing and continues to stoke until about 11:00 am or around 1000 degrees. At this point the ash pit doors are bricked up and we switch to the upper boxes.
As the kiln approaches 1000° F, these doorways are bricked up and stoking is transferred to the four 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 airtight. 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.
Much has been written about the use of primary air pre-heaters and we did incorporate them into our construction thinking that in theory, they were a good idea. We have stopped using them altogether as at about the time in the firing (over 1000 degrees F) that they begin to really work well we are beginning reduction and need to starve the kiln of oxygen retarding combustion. To inject super heated air into the kiln only seemed to increase efficiency and counteract attempts to reduce the wares.
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.
While preparing for a lecture on Bourry box kilns for NCECA in 2003 I conducted an informal survey of potters firing with Bourry box kilns. Along with other information provided by Bourry kiln firers, I discovered that many were dissatisfied with the quantity of ash effects provided by the Bourry box kiln. It is often thought that while the Bourry box kiln is an efficient kiln for firing it will not provide significant ashing to glazed and unglazed surfaces. This actually is the conclusion we came to at one point and had contemplated converting the kiln over to soda to achieve more surface effects from the firing. Through an extended firing cycle as well as some other adjustments we now consistently achieve firings in which the work has significant ashing of most surfaces. It is possible to achieve a moderately heavy ash deposit in some areas, but this is not something that I am looking for. Changes made to the firing in recent years are as follows.
The Bourry box design is not known for heavy ashing of pots but we have learned how to fire our kiln to provide just the right amount of ashing of unglazed surfaces as well as interaction with glazed surfaces on our pots. Our firings are longer. Ten years ago we fired the kiln in about 18 hours. Now we have extended the firing to about 24-30 hours.
Rather than extending the firing at the end, we stoke the lower fireboxes longer to deposit ash on the pots early while the velocity in the kiln is slower than at the end when the velocity is fast and the kiln hard to hold back with much of the ash being moved through the kiln and out the stack.
We have lowered the stack by about two feet. When we built the kiln we built the stack out of common house brick and over the years it had decomposed so much that it threatened to collapse at high temperatures. So when we rebuilt the stack with firebrick a few years ago we lowered it from 26 feet to 22 feet. This has slowed the velocity in the kiln down so that accumulation of ash is increased as well as making the kiln easier to control.
We have lowered the bag wall to only 12 inches from a height of 3 feet. We thought early on that a high bag wall would be conducive to even temperature distribution but this has not proven to be the case. Our kiln fires evenly within a half cone throughout to cone 12. We place tall pots on the bag wall that help direct and distribute the heat. These pots often are very heavily ashed and consequently get hotter than those in other areas of the kiln. We never fire the kiln in the heat of the Texas summer from July until September.
We have found that low humidity and dry wood results in a short firing that is difficult to reduce. We are, afterall, glazing the majority of the pots and have developed glazes that interact well with the wood ash. Some of our best firings have been during rainstorms where the air was thick and the humidity 100 percent.
We stir the ash with every stoke beginning early in the firing before we switch to the top fireboxes. This also seems to help with the accumulation of ash early before the velocity in the kiln increases.
We fire almost exclusively with hardwood, primarily oak. The hardwood ash is darker and tends to interact with clay and glazes in a way that is more appealing than pine, which is readily available.
The kiln is packed loose. Early on I was guilty of packing the wood kiln very tight in order get as many pots fired as possible. This focus on economy worked well using a gas kiln but much more consideration needs to be given to stacking a wood fired kiln. We try to leave at least one inch between the top of the pots and the bottom of the kiln shelf above.
We at one time cut all of our own wood for the kiln. We are no longer doing this as scrap wood is so readily available and since I began teaching in 1992, I just do not have the time to spend in the woods that I once did. At some time in the future, we may once again use wood from our forest, as light thinning is a good way to keep the trees healthy. Efficient energy use and conservation is another important facet of firing an efficient wood kiln such as ours. Rarely in wood firing circles is energy conservation discussed but it was one of the reasons we began firing with wood being what my son would call “old hippies”.
We began our pottery in 1979 still very cognizant of the energy crisis earlier in that decade. So firing with wood was not only an esthetic choice but also one motivated by practicality. Natural gas and propane increased in price right along with diesel oil, which made firing with wood, especially with scrap wood seem a wise decision. Projections by many at that time were for the world to grind to a halt due to a lack of oil by the year 2000. Of course this has not proven to be the case but that may be changing. A few years ago I noticed a headline that caught my eye in the Wall Street Journal, "Effects of Gas Shortage Rip Through Economy". I quote from the article, “Once thought plentiful, the U.S. is now facing a shortage of natural gas that could last for years, and the impact is just beginning.….” the article went on to reference the Wheeling-Pittsburgh Steel Corporation halting operations at three plants in Ohio because of spiking natural-gas prices. The company said that it usually pays between $4 and $7 per million cubic feet, but spot prices have climbed as high as $28 per million cubic feet, which equates to a 400% increase.
The Bourry box built correctly is a very energy efficient kiln for a functioning studio supported by sales of work. There are dozens of Bourry box kilns all over the world that are used reliably and consistently as an integral part of economically viable studio operations. I would speculate that most of those kilns are being fired with scrap wood from various sawmill or manufacturing operations. Free fuel not only makes firing with a bourry box practical, it utilizes scrap that in many cases would otherwise be burned or discarded.
Our forested land now comprises 130 acres and should supply all the fuel we could possibly use for life. In 1985 we planted 35 acres with about 20,000 pine tree seedlings thinking that we would use the wood for kiln fuel. In the end, we have been able to acquire kiln fuel from local pallet comapanies, using primarily culled hardwood 1x4's. There are many ways of looking at wood firing with no “right way” of firing that works for everyone. It is my feeling that the firing is only one step in the creative process and in and of itself adds no intrinsic value to the work. It is my opinion that the firing can make a good pot better but a bad pot cannot be made good simply by firing in a particular kiln. This is simply asking too much of any kiln no matter how well designed.
An image gallery of our timber management program is located here. We turned a derelict 20-years-neglected peach orchard into a pine forest starting in 1985, by having it cleared, prepared and planting 20,000 seedlings. We have had it selectively thinned twice. With the proceeds from the first thinning we built a 4-acre lake and stocked it with fish. The second time was very challenging for us emotionally, but the forest is so much healthier now, we know it was the right thing to do. We have left a portion of our woodland intact as a nature preserve (Daphne is a Texas Master Naturalist and we have several interesting features on our property that she is making an effort to preserve) and we have managed the acreage planted in 1985 as a crop, just as you would thin spring lettuce seedlings. It's just on a much longer timeline and larger scale. National Woodland Owners Association is a good source of information as is James Fazio's book The Woodland Steward.