I spent the weekend at the home of jeweller and ceramic artist Linda Unsworth (Pampeliska) in the Preselli hills where a group of us had got together under the leadership of potter and expert kiln builder Joe Finch to build Linda a small wood kiln in the garden.
We began with a flat concrete base into which holes had been drilled to let the moisture escape. It’s also possible to build onto a base of hollow concrete blocks with insluating blanket over the top. Firebricks had been laid out on top of the concrete in the shape of the kiln’s base and we built up onto this one layer at a time, photographing each layer. On the outer brick layer were light insulating bricks and heavier firebricks on the inside wall against where the flames will be. The chimney was built in a more aesthetic red reclaimed firebrick.
Joe’s kiln design is five bricks across and four wide and he’d built a model from Lego to guide us. Joe advises working out the size of the kiln you want after deciding how many kiln shelves you want to fit inside. The two chambers either side are fireboxes into which the wood will be fed through two openings at the front creating a ‘fast fire’ down draft style of kiln – the chamber in the middle will pull the flames back down and out the chimney. You can see in the photo that the second layer of bricks is pushed out slightly in the firebox to create a ledge. This is for the perforated brick layer to sit on. The idea is that the embers from the logs burning on the firebars above will drop down onto these and the oxygen through the holes will help combust them, meaning you don’t have to rake out the embers so often like in other kilns. In the chimney you can see a space where the bricks are missing – this is where the damper will be placed and bricks can be pulled out here to create reduction.
As the kiln got taller we added the firebars – hollow tubes of refractory fireclay onto which the logs will be placed. These need to be loose enough so they can be pulled out and replaced if needed. We continued to build up the bricks layer by layer, insulating on the outside and firebricks on the inside, sometimes having to saw bricks in half to fill in cracks and filling smaller gaps with gaskets of insulating ceramic fibre. Things became more complicated when we began the kiln chamber floor. We placed the flattest kiln shelves we could find on top of the fireboxes, leaving two gaps for the back for the flames to flow through and one at the front in the middle. We covered the flue to the chimney too.
Things picked up after this stage when the job got easier – we simply built the insulating bricks up in layers around the perimeter of the kiln chamber. Once the desired height was reached (about 2m high for the entire kiln) Linda painted numbers on the bricks that will make up the door using watered down red iron oxide. Joe then sawed out the door.
We began Sunday morning by mixing up a mortar for the reclaimed molar bricks for the outside kiln cladding. This second layer of insulating bricks isn’t necessary but makes the kiln look more aesthetic. As a cheaper alternative it’s possible to cover the inner layer of bricks in insulating fibre then add a corrugated iron shell.
The mortar was a mixture of ball clay, sand, water and red iron oxide (the iron colours the mixture pink so it blends in better with the red bricks). The bricks were each soaked in water for a few seconds before building with so they absorbed the mortar better. Since the bricks were reclaimed we spent some time scraping the layer of old mortar off the surfaces before we could begin the next stage.
The cone spyholes in the back had to be adjusted for this second layer of bricks so we made new longer ones which can be pushed in at an angle. After completing one wall of outer layer we added the supporting angle irons on the outer corners and secured these together with a 12mm threaded rod (could use 5mm), then slid in horizontal ones between them.
The roof was constructed with three layers of insulating bricks cut at angles to make an arch. A D shaped wooden arc support frame was held up by planks underneath and we built the arches over the top, supported either side by bricks cut in half length-ways. Once the other outer walls have been completed the frame should be able to drop and slide out through the kiln entrance leaving a freestanding roof. Unfortunately by the end of the weekend we’d run out of the reclaimed bricks so we couldn’t complete the outer layer fully but Linda has promised to send photos of the finished kiln – I’m looking forward to see the results! More information about kiln building can be found in Joe’s book ‘Kiln Construction: A Brick by Brick Approach’.