to complete, which makes it somewhat expensive, but should last 20 years of continuous
firing.
Except for the chimney sections above the roof, the entire kiln is assembled using a clay and
sand mortar mixture. The mix is approximately 2:1 sand: clay, with enough water to make it
workable. This is used exactly as cement mortar would be except that the joints are sealed. To
do this a very runny mixture of mortar is poured over each layer of brick work until all internal
cracks and joints are filled. A crew of three mortar mixers and carriers will keep the masons
quite well supplied.
The floor of the kiln begins 70cm below the ground. The hole is dug and carefully levelled and
completely compacted. Three layers of bricks are laid with clay and sand mortar mix.
Throughout the construction of this kiln, bricks on the interior of walls are bonded on crossed
diagonals to the walls. This technique increases the resistance of the walls to the outward
force of the arch, and in the floor helps to even the weight of the load. Figures 7 through 17
show most of the details of the kiln design. The kiln shown is used at sea level. The size of
the fireboxes and chimneys should be increased approximately two per cent in all linear
dimensions per 1,000 feet of elevation from sea level. So at 5,000 feet you will increase the
dimension 10 per cent; for instance the inside chimney dimension will go from 32cmx32cm
to 35cmx35cm. Also, the height of the chimney must be increased by 6 per cent per 1,000
feet of elevation. This must be done to account for the thinner air found at high elevation.
SELECTED DETAILS
THE BAG WALL The firebox leads to a wall on the inside of the kiln called the bag wall.
This wall is attached to the body of the kiln except where the firebox meets it. There
the openings flair open to let the gases out as evenly as possible, the bag wall rises to
the arch roof itself (see figures 11 and 17).
THE FIREBOX (figures 9, 10, 11 and 12) The firebox is made with a 5cm ledge 35cm from
its floor. This holds the grate. The grate can be made in three ways. The least expensive
grate is made by simply butting two bricks together. These bricks can be u3ed in only
one or two firings, then must be replaced. A more durable grate can be made from
ceramic arches made to fit the firebox. These will last for many firings. They are made in
a three piece plaster mould, or in a wooden brick type mould. A final method is to use
sections of railway track. Here the spacing must increase to allow ash to fall over the
bottom web of the rail. The arch of the firebox is built over a wooden from.
THE ARCH CEILING The arch is built using a special tapered brick (described earlier). The
key to a successful arch is a proper support during construction. Supporting formwork
should be placed at least on 75cm centres and be built from 4x16cm timbers with
reinforcements on both sides, where joints are made (figure 8). Stringers (or slats) from
2x8cm timber with 7cm gaps between each stringer. Each form is supported at either
end on the bag walls (before it is built to its final height) and on four or five poles going
to the kiln floor. This kiln uses a circular arch thus when the arch is laid the bricks line
up with the centre of the arch. A second layer of standard building bricks are then
layered over the arch bricks in line with the centre of the arch. It is crucial that the
mortar joints be completely sealed in each layer with the runny mortar mix so no heat
escapes through the arch during firing. The arch supports may be removed when the
mortar is quite solid.