The wood kiln is a medium large kiln with an internal volume of 3.36 cubic metres. (Inside dimensions of the glaze chamber – height to bottom of the arch 110cm, depth 170 cm, width including the bag wall 180 cm).
It is effectively a one-chamber kiln as the second chamber merely allows the flame to be collected evenly across the back to be conveyed to the chimney. The kiln has a shelf capacity of 63 silicon carbide shelves, ranging in size, generally 45 x 30 cm.
The kiln is a trolley kiln where the part of the wall that is removed to give access to the inside (this is called the wicket) is attached to a trolley. The trolley with its shelves is pulled out in order to load the kiln and, after firing, to remove the pots. The kiln is fired to stoneware temperatures – 1320 degrees centigrade closest to the firebox to 1280 degrees close to the exit flues. This range of temperature allows a range of effects and glazes to be produced by a single firing.
The kiln takes between 15 and 18 hours to fire depending on climatic conditions and the glaze effects desired. It uses 12 – 15 bundles of wood (about 3 tonnes). These bundles are tied with wire and placed near the firebox with a tractor. These bundles can be seen in the ‘wood storage’ section. Generally stoking uses larger pieces of wood early in the firing and smaller pieces towards the end.
As the kiln approaches full temperature it needs more constant feeding to produce enough fuel to raise the temperature. During the later parts of the firing the combustion becomes more efficient as the Dutch Oven gets hotter and vaporises the wood quickly into ‘producer gas’ which is burnt in the main chamber.
As can be seen the chamber of the kiln is raised off the ground about 45 cm to accommodate the trolley. This height also allows the firebars in the firebox to be at floor level with an ash pit below floor level.
The kiln is powered by a large firebox (called a Dutch Oven) with a volume of 1.35 cubic metres (inside dimensions – 100cm wide, 150 cm deep and 90 cm high from the fire bars).
The fire bars are inverted railway irons placed about 12 millimetres apart. The Dutch Oven only partially combusts the wood but converts it into gas which burns as it mixes with air in the main chamber. The Dutch oven is the engine house for the kiln. This one is large enough to burn logs up to 25 cm in diameter and 120 cm long even towards the end of the firing when .
Showing the inside of the firebox, upside down railway irons used as firebars and the flue into the main chamber. This opening is 60cm x 60 cm. The perforated wall in the middle of the photo is the ‘bag wall’ which deflects the flame upwards so that the flames slowly move downwards through the kiln and are vented out at the bottom of the far side of the kiln.
Early in the firing about one piece of wood is put into the firebox every 5 – 10 minutes depending on its size. Towards the end of the firing this rate increases to about one piece of wood every minute.
The temperature rise for the first 8 – 10 hours is quite rapid and often reaches 1000 centigrade after 6-8 hours. The temperature rise after this is slower and generally the temperature is maintained above 1250 degrees for more than 6 hours. Usually the kiln will be maintained above 1300 centigrade for at least one hour to allow the glazes to mature and melt properly.
The rate of feeding the firebox is determined by the need to use the fuel most efficiently. That means that only light smoke comes out of the chimney. Smoke is unburnt fuel. Too much smoke also indicates an inefficient burn and produces less heat.
Wood kilns require attentive stoking to maintain temperature rise and to maintain a ‘reduction atmosphere’ (slightly smoky) which produces more interesting colour in the melted glazes.