Steve Harrison describes taking home a ‘perfect system’ Bourry box kiln
Firing a Bourry box down-draught kiln can be quiet, calm and civilised. Most of the time is spent sitting and watching the firebox work. When larger pieces of hardwood are used, a single stoke of wood can last up to one hour. I like this kind of firing; it makes for a very laid-back firing experience, but of course it isn’t for everyone.
There are those who feel that they must do something, in order to make their mark on a firing. They interfere with a perfect system, bring it undone, and then work hard to rectify what they’ve ruined. They spend a lot of time and energy fixing what wasn’t broken and claim they are marvelous for reaching temperature, when it would have happened anyway, quietly and calmly without them.
In 2009, Janine King and I spent the northern summer building and firing our wood-fired kilns in Europe. We were employed to design and construct kilns using our particular design modifications that utilise the Bourry down-draught firebox. We borrowed this innovative design concept from the French, via Cardew, developed it in our own way and are now handing it back. The great beauty of this design is that it is very efficient, clean, and easy to fire with minimal physical effort, while delivering excellent ash deposits.
I have adapted this firebox concept to fire all sorts of kilns. Col Levy has two of these wonderful inventions adapted to his Bizen style kiln1. Since the 1970s I have spent a lot of time developing the potential of this firebox concept in my own idiosyncratic way. Because the burning wood is suspended in the air, the ash from combustion falls downwards with the flames. This allows the ash to be carried well into the kiln without any extra agitation being required to lift it up off the floor or fire bars.
I have developed a firebox and chamber combination that encourages a lot of ash onto the work without the necessity to rake and flick the ember pile to get the ash onto the pots, as is necessary with an anagama style kiln. In my kilns I get heavily ashed, carbon-included and ember-enhanced pots, as well as more delicate flashed and lightly ashed pieces, without raking the embers. Another great advantage this firebox offers is that it doesn’t need to make much smoke, especially during reduction. This will become a very important issue in the future. Another of my designs has a down-draught firebox incorporated into an anagama chamber. I call it the ‘enhanced anagama’. An anagama without the smoke!
One of the kilns we built in Europe was a salt kiln for the German potter Markus Bohm. Markus’s brief was that the kiln should be approximately 1 cubic metre in total capacity, easily fired in one day, capable of delivering good ash deposit onto the pots, constructed of materials that resist salt degradation, and as fuel efficient as possible, given the other requirements.
Salt kilns are usually built of quite heavy and dense high-alumina refractories so as to resist the corrosive effects of the salt vapour at high temperatures. Markus favoured the use of castable refractory as a hot face lining for the chamber. Fortunately, I pioneered the use of castable refractory-lined salt kilns here in Australia back in the seventies and had built a considerable number of them here. (See Pottery in Australia, Vol 14, No 1, pages 26-30, 1975; “A calcium-aluminate cast kiln for salting”.)
In the seventies, salt glazing was quite popular and I realised that castable refractory had all the requirements for building very long-lasting salt kilns. The basic chemistry of calcium and alumina does not have any noticeable eutectic with sodium. I was able to travel widely building my wood-fired kilns, meeting all sorts of potters in a variety of places all over the eastern states.
Castable refractories have improved over the years. There is now a lightweight high alumina insulating refractory castable, which is quite fuel efficient while being just as salt resistant as the older dense mixtures. This improvement incorporates small, hollow alumina spheres into the mix in place of dense high-alumina grog as an aggregate. The kiln chamber is entirely cast with only the door constructed from very refractory, RI28 insulating bricks. The result is a very light and fuel-efficient yet very refractory kiln that will resist salt attack for many firings.
The kiln building/firing workshop was scheduled for one week. Timing was critical, with Markus and I casting the concrete slab the day before the workshop was to begin. We had three days to construct the kiln, one for packing, one for firing, one for cooling and then on the final day, unpacking. The whole exercise went along with Germanic precision, with the kiln being packed as the brickwork was being finished.
We started the firing early and finished around midnight, with the whole thing still steaming on the outside. Seventeen hours to 1360ºC! (Markus fires his salt firings to 1360ºC.) We fired the raw, wet kiln with raw pots achieving temperature in the evening, leaving a few hours to do the salting and then to reoxidise well.
The results were very good. I was quite pleased with the amount of ash on the pots and the surface qualities. Overall, the event proved to be a great learning experience for everyone, including me. We had made a nice little kiln with very low neighbor-impact that will make lovely work in a clean, efficient way for years to come.
1 Pottery in Australia, Vol 16 No 2, page 38