This kiln is a hemisphere and the diameter ranges from 5 to 7.5 m. Capacity and cycle time vary with diameter as follows;
|
Diameter (m) |
Gross Volume (m³) |
Cycle time (day) |
|
5.0 |
32 |
7.8 |
|
6.0 |
56 |
8.10 |
|
7.0 |
90 |
15-17 |
|
7.5 |
110 |
17 |
A 6 m diameter kiln requires about 6 000 bricks of 0.24 x 0.12 x 0.06 m.
Actual charge capacity is less than gross volume due to the spherical form. A 7 m kiln has a practical charge capacity of about 60 steres of wood, or two-thirds of gross volume. The ratio is somewhat less for smaller kilns.
The commonest diameter formed is about 7 m. The yield percentage varies but typical yields are from 4.5 to 6 tons of fuelwood yielding one ton of charcoal.
1. Site selection and preparation
See paragraph 7.2.2 in main text of manual.
2. Brickwork
Drive in a stake in centre of kiln to be built and leave it projecting about 30 cm. To the top of stake attach by means of a leather strap a light wooden radius rod of equal length to dimension required for the diameter. In the end of the rod drive a stout nail to act as a precise measuring point to which each brick is laid (Fig. 23 photo 14).
The footing of the kiln is laid in a circular trench marked out to correspond with the radius rod. The trench is 0.3 m deep and wide enough to allow a footing the length of a brick wide and three courses thick to be laid. Make the footing 0.45 m wide beneath the two doors to provide a firm foundation.
Bricks are laid in mud mortar consisting of clay, sand and charcoal fines. The mortar must be firm and strong when dry, without peeling and shrinkage cracks. Keep the joints thin otherwise the kiln will not be strong and durable.
At ground level lay the first course of bricks around the kiln using the radius rod to maintain the correct internal diameter. The lower three courses above the footing are double thickness all around the wall.
Leave 12 air inlets at ground level evenly spaced, each hole being about 0.07 m square.
Then begin to leave out bricks in the outer ring from this point beginning at each end of a diameter at right angles to the axis of the doors. In this way a double thickness wall is built up to reinforce the kiln around each door (see fig. 23 photos. 15, 16, 17). The main kiln wall is single thickness.
Take care as the kiln wall nears the top to put each brick hard up to its neighbour, keeping joints as thin as possible so that each run of brickwork is tight and bricks cannot fall out through loss of mortar when the kiln is in use.
At the top of the kiln leave a circular hole or "eye" about 0.2 m diameter. This hole is for lighting and to allow emission of smoke and fumes during carbonization.
Before use the kiln must be allowed to dry out for about two to three weeks. The curing and drying out of the kiln walls and the earth floor is completed over the first three or four burns. The cracks and pores in the bricks become filled with tar and at the same time a lower yield of charcoal is obtained due to air leakage and the extra heat necessary to dry out each floor and the brick walls.
Any cracks which appear in the kiln walls must be immediately filled with clay slurry and any loose or faulty bricks replaced. This is important during the early life of the kiln but inspection and repair is also necessary after each burn to ensure long life for the kiln.
Reinforce the doorways of the kiln adequately as they are subject to shock loads when the kiln is being charged with wood. A steel or wooden post can be set each side of the door, clear of the kiln wall, to absorb accidental blows during loading operations.
