Another Brick in the Wall

Pottery is pottery by virtue of fire; without fire it is mere clay. A potter is thus a potter by virtue of fire, and a potter without a kiln is no potter at all. Last year I watched as the seven tonnes of my kiln danced like bamboo in the breeze. It has taken fourteen months, but I am a potter once more!




Since the kiln shed roof was completed we have been building the new kiln. With the help of friends we brought the 2000 odd fire bricks from the old kiln in Ichikai here to Minakami last summer. There they waited, through Autumn and the long Winter, but with the coming of Spring they have found a new home. The children all helped as we carried load after load of bricks in the little truck from the green house, where we had originally planned to build, to our new kiln shed. We sorted them by size, type and relative crustiness, and stacked them neatly, with calls of "I can build a quick trick brick stack!" punctuating the process. The practical Dr Seuss!





I published the plans of the original kiln along with an essay on responsible wood firing in the Ceramics Technical magazine some years ago, and since then there have been at least a dozen of them built in Japan and many others across rest of the world. But for all its efficiency, practicality and environmental benefits, it was not prepared for the worst earthquake in Japanese history. Back to the drawing board!





Essentially there were four weaknesses in the original kiln; the foundations were too shallow, the kiln was not anchored to the foundations, the chimney was independent from the kiln for its whole four metre height, and the sprung arch mingled straight bricks and arch bricks, making it susceptible to sliding under vibration. This time we have built the foundations specifically to carry the seven tonnes of kiln, with concrete thirty centimetres deep around the edges and fifteen centimetres through the centre, with extra steel reinforcing bar.



I have redesigned the kiln with the chimney interlocked into the back wall of the kiln. That means that the chimney is now part of the body of the kiln up to 2.2 metres, leaving only 1.8 metres independent above the kiln. This should reduce the possibility of sway in the event of another earthquake. It should also increase the efficiency of the kiln by shortening the horizontal distance that the chimney needs to pull the flame through the kiln...we will see in the first firing!







The entire kiln has been braced with an angle iron frame which has been anchored to the reinforced concrete with a bakers dozen of bolts. The flame may dance within the kiln, but the kiln will dance no more!



Most of the bricks have been recycled from the old kiln. I have always used fire clay to build the kilns, as I was taught at university in Bendigo, not mortar. Fire clay makes repairs to the kiln much easier, and cleaning the bricks was less of a challenge than I have had with mortared bricks before! Fire clay is also more forgiving in the laying of the bricks, as it stays plastic longer.



I made a slurry of fire clay, 25 kilograms of dry clay to 21 litres of water, and dipped the bricks in the slurry to give a one or two millimetre thickness of fire clay between bricks. The less fire clay or mortar the better, as it is the brick, not the mortar, that insulates the kiln.



It is imperative that the kiln be square and level through out the construction, as the sprung arch must have a level, square and parallel surface from which to spring. A spirit level and a straight edge to span the kiln is essential. A set square will square the corners of the kiln, but over the length and breadth of a kiln it is hard to keep the kiln completely square. We gratefully accept the help of our old friend Pythagoras. The depth of the kiln was an untidy 1387mm, and the breadth was 1545mm. For a right angle triangle, "The square on the hypotenuse equals the sum of the squares on the other two sides", says Pythagoras. Thus; if you square the depth of the kiln, and square the breadth of the kiln, add them together and take the square root, the diagonal of the kiln from corner to corner should be 2076mm in both directions ! Viola, a square kiln!



Many of the bricks were seriously degraded after over a hundred wood/soda firings. The insulating fire bricks which lined the inside of the old kiln were somewhat..thinner?..than a fire brick should be, particularly those at the fire face. I reversed the bricks, so that the cold face was now the fire face, and back filled the gap with a mixture of 25kg fire clay: 36litres vermiculite: 21litres water. This is effectively a castable refractory filler which can be made cheaply with materials from the garden centre. No good at the fire face, but perfect between the two layers of the wall.





A soldier row of hard bricks links the two layers of the wall together, then the wedge shaped spring bricks are set, the frame is attached and the arch begins. The arch is only four brick lengths deep in the new kiln, rather than five as it was. It now butts up against the back wall rather than overlapping it, and allows the door bricks the butt up against the front of the arch. Which means 22 less arch bricks than the original kiln. Each row of arch bricks was cut level (once again I was reusing degraded bricks from the old kiln) and a mixture of Y1 and Y2 angle bricks makes up the whole arch. The danger of slippage under the vibration from an earthquake is now considerably reduced! Unfortunately the arch is also thinner, because I could not afford to buy new bricks. Fear not! We still have fire clay and vermiculite! Two batches of castable on the arch, with insulating firebricks to house the front edge, and the arch is complete.





The last 1.8 metres of chimney now pierces the roof, and angle iron braces it to the top. A kiln shelf to stop the reduction flame shooting to far out the top of the chimney, and we have a new kiln! I am a potter once more!





The cherry blossom has blown, and the trees are green with new leaves. The studio is waiting for me, Hamada's wheel is waiting for me, the kiln is now waiting for me. With the help of god, of friends, of family and, yes, of virtual strangers, I am a potter once more.







Thank you.