Rik Midgley - Ceramics - Cob Ovens - NVC - Qigong
The clay is usually dug from the ground nearby, and then soaked over night. If this clay is not smooth enough some terra cotta clay may be mixed in, or if it is too smooth then some sand is added. To make the kiln, after removing the turf, place a few newspapers on the ground as insulation. On top of this, carefully place the pots. It doesn't matter if they are still a bit wet, but if any are both heavy and wet they are best placed at the down wind end. To add a bit of colour to the pots, mix in a bit of salt, or perhaps some banana skins, seaweed or bits of copper. Carefully place a couple of layers of logs of varied sizes over the pots - the smaller bits help the fire to spread through the kiln. On the downwind side place a barrier of large logs so that the fire is slow to catch into the main body of the kiln and giving it a chance to warm up first.
To make the kiln walls, slip must first be made by soaking some clay overnight in ample water and then mixing it well into a slurry. Dip sheets of paper into the slip and place over the wood. Using newspaper, about 20 layers are necessary. Magazines require slightly fewer layers, since they contain china clay. This wall enables the fire to be controlled as well as being insulating - the paper will burn out leaving layers of clay and air. Leave a flue hole about 5cm in diameter in the middle on the downwind end. A small fire is lit at ground level on the upwind side and allowed to spread slowly. The idea is that insufficient air is allowed into the kiln to burn the wood fully, turning into charcoal. During this process a thick white smoke rises from the flue. Much of this is steam as within the kiln the water vapor pressure is 100%, thus protecting the wet pots from exploding. After some hours this smoke will go clear. At this point all the wood has been converted to charcoal. In a charcoal kiln you would now block off all the airways and come back in the morning to collect your charcoal. Here however, the aim is to get to the highest temperature possible. Increase the size of the flue hole, or even add a metal tube for a chimney. Open holes at the base of the kiln, particularly on the downwind side. Thus air will be forced over the hot coals as with a forge. I have managed temperatures approaching 1000C in large kilns.
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