Text and photos: Markus Böhm
My wood-fired kiln is now 17 years old and will, so to speak, come of age next year. The last firing in October, in which I tested my wood-fired porcelain on a larger scale, was an occasion for me to look back at its construction. I have made plenty of mistakes in my life — but this kiln is not one of them. It was built as part of a kiln-building seminar organized by kalkspatz e.V., led by Australian kiln builder Steve Harrison. Since then, more kilns with Bourry-Box fireboxes have been built in Germany and Switzerland, some with Steve’s help or based on his kiln-building book. In 2009 I wrote an article about it, which was published in the Töpferblatt of kalkspatz e.V.. The cameraman (and by now also potter) Andreas Münz made a short film about it, which is also available as a DVD.
An Ambitious Schedule and a Perfect Result
No, I’ll never do something like this again. I promise. Start on Monday, cast the arch on Tuesday, insulate and weld the frame on Wednesday, set the kiln on Thursday, fire on Friday, cool on Saturday and unpack on Sunday. “Today I’ll bake, tomorrow I’ll brew, and the day after…”
What went wrong at the last moment in Rumpelstiltskin worked here — thanks to the extraordinary commitment of all the kiln builders. But it was so tight that Steve and I constantly feared something might suddenly stop running as perfectly as it had so far. He therefore gave the kiln the name “Dave,” after the astronaut in Space Odyssey, to whom the onboard computer HAL keeps telling in his sonorous voice: “Everything is going perfectly well, Dave,” while at the same time calculating how best to get rid of Dave into the depths of space.
Half a year earlier, Steve and I had talked on the phone about the time frame for the seminar — seven days seemed like a long period. After all, potential participants have to free up their schedules, and having to cancel a seminar due to too few registrations is always a bit embarrassing. But things turned out quite differently: even while I was translating one of Steve’s articles for Neue Keramik in January — partly to promote this seminar (unfortunately the article only appeared half a year later, right during the seminar) — all the places were already taken.
Perhaps the idea that wood-firing can be efficient, simple, fast and uncomplicated is particularly interesting here in Germany. However, I had to put up with Stefan Lehner’s question why the title “Laid Back Woodfiring” was chosen, since the kiln building itself had been anything but relaxed. Well, I could still defend myself by saying that the title, after all, was not “Laid Back Kiln Building.” And indeed, the firing itself, in contrast to the construction, turned out to be very relaxed.
When the 1380°C Pyrometric Cone Fell During Dinner
Only after dinner did things get hectic. Wood had only occasionally been added; everyone was more focused on eating than on firing and was enjoying the evening beside the kiln. Suddenly Katrin Otolski shouted:
“The 1380 cone is down — I can’t believe it — is it really down? No, that can’t be true …”
In fact, the 1380 °C pyrometric cones were no longer visible in almost all spy holes — and since those were actually the cones that were supposed to remain standing, almost nowhere were any cones to be seen at all. We then decided to start salting, and the temperature dropped just as desired.
I must admit, something like this has never happened to me in all my years as a wood-firer: in a high firing, having all the cones accidentally bent over during dinner. Normally, once you reach 1320 °C, increasing the temperature further becomes real work — one wheelbarrow of wood after another disappearing into the now very hungry kiln. But evidently it can be done differently. Already that afternoon we had been trying not to let the temperature rise too quickly — after all, the arch was not even three days old at that point.
The Bourry-Box: Efficiency through Clever Firing
In my experience, this has far less to do with whether the kiln is built from heavy firebricks or insulating firebricks — the much more important factor is the efficiency of the firebox. The Bourry-Box was used mainly in the French porcelain industry and is ideal for achieving high temperatures using hardwood. (Well into the last century, hardly anyone in Europe would have thought of using softwood for stoneware temperatures.)
This efficiency is visible — or rather, not visible — at the chimney: hardly any flames emerge. Jens-Peter Planke likes to quote Wilhelm Pukall in this context:
“The emergence of flame from the flue always indicates a faulty firing system.”
You probably have to experience it yourself to understand how the box, toward the end of a firing, seems to make the kiln “fire by itself.” No more wood is added, the primary air supply is gradually reduced, and yet the temperature on the pyrometer keeps climbing.
Misinterpretations and Misunderstandings in Germany
Why hardly anyone in Germany ever thought of using such a firebox will probably remain a mystery to me. Perhaps it has something to do with the wood-fired kiln known as the K 18 at the University of Kassel. During the planning phase of my own kiln, Jens-Peter asked me in astonishment:
“What? You want to build a Bourry-Box? That didn’t even work in Kassel!”
I called Michael Üffing who was pretty much involved in that kiln building at this university.. According to him, nobody there had really been interested in firing from the top. Looking at the drawings, the spacing between the wood support (hob) and the kiln floor was far too large — they even built a firebrick grate where the floor should have been, effectively sabotaging the firebox’s intended function right from the start.
To their credit, firing doors had been integrated in such a way that one could also stoke from the front — which was then done and worked very well. But imagine how that kiln would have fired with both fireboxes operating as designed! This approach was probably also due to the fact that they were using offcuts from a sawmill as fuel, to save the work of splitting logs. In any case, I can easily imagine that this is how the Bourry-Box in Germany earned the label “doesn’t work anyway,” and that afterwards nobody bothered to try again.
Nevertheless, the importance of the K 18 — as one of the first larger wood-fired kilns at a university in Germany — should not be underestimated. As Ralf Busz (ceramics professor at this time) noted, the kiln was built “despite the obstruction of the Department of Fine Arts Council.”
Two Variants, One Principle: Open or Closed
Some colleagues considered it a problem that the firewood has to be cut to a uniform length. However, in Germany firewood is usually sold in lengths that are an exact multiple of one meter. Cutting off the meter ends is really no big deal. And if shorter pieces turn up — for example, from self-harvested firewood — they can easily be used at the beginning of the firing, when the fire still burns on the floor of the firebox and the ember bed is being built up.
By now, two main versions of the Bourry-Box are in use: one with a closed top (as, for example, in the kiln at Guldagergaard, built by Robert Sanderson) and one with an open top, as in Steve’s usual kilns. At first, I wanted to build the closed variant. But then, the ash is not so easy to remove, the wood cannot be placed as easily on the ledges, and Ute Dreist had told me that with this version, the upper area of the firebox becomes quite hot, and the radiation when stoking is unpleasant, especially around the head and neck area. In addition, heavier pieces of wood are then thrown in with force, which usually affects the durability of the opposite wall. On the other hand, I was a bit afraid that a completely open firebox might be difficult to handle when stoking. But I thought: closing the box later would be no problem.
However, the box works so well that I will continue to use it exactly as Steve built it. And you can immediately tell that he has built many of them: the air holes for checking the ember bed are exactly the right size and in exactly the right place, and the height of the wood supports is perfect. If these are too low, the ember bed clogs the passage for the combustion gases, and if they are too high, the heat from the ember bed cannot reach the wood above properly during the early phase of the firing.
Construction and Flame Path
When it came to designing the firing chamber itself, I had no direct models. Normally, the floor of the Bourry-Box lies lower than that of the firing chamber — except when building what is called a Train Kiln. In chamber kilns, this difference in height redirects the flame and forces it to pass through the ware instead of simply sweeping along the kiln floor and leaving the arch cool. My goal, however, was to get pots with different sides and thus to let the flame’s path draw itself across the vessels — which requires a horizontal flame. Therefore, in my kiln, the floor runs continuously at the same level. To still achieve a good temperature distribution, I wanted to incorporate a perforated fire wall and a perforated back wall (the Japanese call this kind of design Sutema).
Steve said that this was a new construction method, but I can hardly imagine that no one else on this planet had thought of it before. In any case, it was precisely this novelty that had tempted him to run the kiln-building seminar with me. When building the fire wall, I relied completely on Steve’s experience. I would have left far more gaps — but the temperature distribution and the pattern of ash deposits turned out to be optimal.
New Ideas Cast in Concrete
Another innovation was the lid for covering the Bourry-Box. I had emailed Steve a sketch showing a lid sliding upward on rails, with counterweights and a lever. The idea was that it would lift so that the hot lower side would face the kiln rather than the stoker. Steve reacted politely but was skeptical about the complexity — as were most of the seminar participants.However, thanks to Leo’s and Ulrich’s help, the system worked — just in time, as the Bourry-Box was put into “real” operation during the firing. Of course, the success of the seminar as a whole was due to the wonderful mix of very different people, each contributing specific skills — whether in bricklaying, cooking, welding or wood splitting.
For Steve and me, it was a joy to experience how, despite the long workdays, there was such a good atmosphere. A spirit that eventually captivated us, too. When the kiln was finally loaded, more or less sealed, and the gas burner ignited, Steve said he wouldn’t be at the kiln before eight the next morning — unusually late for him. But when I arrived shortly after seven, I found that Steve had beaten me to it: the wooden form for the firing-door casting, poured the evening before, had already been removed.
It should also be mentioned that this was in fact a seminar with two kiln builders: Andres Allik from Estonia found the project so interesting that he came especially for it. Normally, he builds wood-fired kilns in the Baltics, Finland and the USA, where he is currently working again. With his calm, competent and hands-on manner, he quickly won the sympathy of all participants. I’m not sure we would have met the schedule without him.
Refractory Concrete and Firing Results
Another experiment was to cast the side wall from refractory concrete. This insulates far better than heavy firebricks and, because of its high alumina content, hardly takes on any ash deposits — which suggests a long lifespan in salt firing. Jens-Peter was very skeptical about the firing results: such a kiln wall surface, he said, would produce “dry” surfaces on the pots. But this turned out to be wrong in the very first firing. Overall, this refractory concrete is my material of choice when complex arches and short construction times are required. It contains hollow-sphere corundum and lightweight chamotte, which of course insulate better than dense firebrick.
Nevertheless, the kiln has impressively disproved Bernd Pfannkuche’s thesis about the inefficiency of heavy linings: if I relate firing time and wood consumption to the amount of ware fired, my large, old, heavy kiln is more efficient — and clearly so! Wood-firing is a highly complex matter where simple rules and black-and-white thinking are not very useful. Only when I factor in my own energy and the steeper learning curve from more frequent firings into the cost-benefit analysis does the kiln justify its existence.
At the second firing, not even fourteen days later, the final temperature was reached at 2:30 p.m.; we had started early at 7 a.m. Seven and a half hours therefore represent a “normal” firing cycle compared with electric or gas kilns.
The First Firing and the Result
Before opening the kiln, I was a little nervous. Not only had this very first firing reached previously unachieved temperatures, but “Project Dave” had gone so “perfectly well” that it almost seemed impossible the firing itself could also turn out well. But that fear proved unfounded. There were many “Oohs” and “Aahs” as the pieces came into the daylight. For me, it was especially gratifying to see how the kiln’s construction had actually produced “light and shadow sides” on the pieces.
Epilogue
At the time I wrote:
“I hope that the Alt Gaarz kiln builders didn’t just take home some nice pieces from this seminar, but also the courage and the ideas to build their own kilns.”
And that is exactly what happened. To my knowledge, a total of four kilns with Bourry-Box fireboxes have since been built by participants of the seminar!
Further information and additional drawings can be found in the book Laid Back Woodfiring, available directly from Steve Harrison at www.hotnsticky.com.au.
Participants were: Stefan Lehner, Katrin Otolski, Ulrich Elberding, Ragnvald Leonhardt, Petra Müller, Ilja Strittmatter, Dagmar Wichmann, Elena Krafft, Lennart Eisenhart, Annika Schüler, Karin Schweikhard, Michael Limbeck, Frank Richter.
Materials used included stock from older supplies (Kaolin lightweight bricks KL 12 and heavy firebricks III p), as well as lightweight firebricks (L6 s and LAL 8) from Feuerfest-Vertriebsgesellschaft mbH, Weg an der Bahn 9, 16259 Bad Freienwalde. The refractory concrete GLS 60 used also came from there.
