|Introduction|| Disadvantages of brick clamps
The use of brick clamps is by far the oldest and most rudimentary method of firing bricks. When "scoved" (that is, plastered on the outside for greater efficiency), they become scove clamps or kilns. If the clamp is enclosed within four permanent walls, it becomes a rectangular Scotch kiln, but in its most basic form it is a carefully constructed stack or clamp of bricks.
As a method of firing bricks it is still used all over the world because it still has several advantages over more modern and sophisticated methods. This is despite the clamp being potentially the most energy inefficient method of firing, because so much heat is allowed into the atmosphere during both firing and cooling, and fuel combustion is both uncontrollable and inefficient. Brick clamps can be made moderately efficient, but they cannot compare with updraught continuous kilns, as far as fuel used per fired brick and brick wastage is concerned.
A basic brick clamp
Brick clamps come in several shapes and sizes, depending on the number of bricks to be fired at one time and the fuel or fuels available. They are usually constructed on a level surface of pre-fired bricks, laid side by side for a wood fired clamp, or with spaces between them for a coal, agri-waste or fuel inclusion fired clamp. These spaces around each brick are finger width, that is, 2 to 2.5 cm wide. This is designed to facilitate the passage of air under the brick clamp to allow for the even combustion of the fuel. Bricks are "set" (loaded onto the base) up to 40 layers high. The bricks are placed on top of each other for the first 8 to 12 layers, above which each layer is inset by 2 to 3 cm, so that the clamp tapers towards the top. The tapering produces greater clamp stability, which is important as the whole structure will move with the expansion and contraction of the bricks during firing. For greater stacking stability, clamps can be built on ground that has been "saucered", this means the centre of the site has been scooped out to a depth of 30 cm. Having a floor with gently sloping sides means the bricks of the clamp are set sloping towards the centre, and the whole clamp can then be built sloping continuously from the ground upwards. Less firing cracks will result with a clamp of this shape, and the external surface area is reduced to help minimize heat loss.
Wood burning clamps have large firing tunnels placed at regular intervals, passing through the base of the clamp. These have to be large enough to allow for the size of woodfuei available, and are usually two bricks wide by eight brick layers high (50 x 100 cm). They are two bricks apart (50 cm), and have corbelled arches at the top, where two layers of brick come in to create the top of the arch. All the bricks are placed in a clamp on their sides, and these figures are for a 25 x 12.5 x 8 cm brick. The tunnels can be split in two by being bricked across in the centre of the clamp. This is done if the clamp is large, and therefore has a wide base, or if the fuel is provided in cut lengths. This prevents cross-draughts blowing through the clamp, and improves the central drawing capabilities of the clamp, all better for fuel combustion. Narrow low tunnels are built if the clamp is to be fired with portable gas or oil burners, which are moved down the clamp from tunnel to tunnel as firing progresses.
Firebars can be built into the firing tunnels, two thirds of the way down from the top, to improve wood combustion, which is especially useful towards the end of the firing, when maximum temperature input is required. To avoid the expense of firebars and the added complication of building them into the clamp, a perforated wall of fired bricks, three layers high, may be built across the lower part of the firing tunnel's entrance. This simple modification helps to heat the primary air as it goes into the tunnel, and pieces of wood can be placed on the wall at one end, to allow the air to pass underneath. This improves combustion and reduces the build up of embers.
A small woodfuelled brick clamp in
Wood and dunkey dung fuelled clamp (Sudan)
West African scoved brick clamp with saucered foundation and fired brick wood rests
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Coal fired clamps are constructed on a spaced brick floor, so that air can be drawn in under the whole clamp area. A band of coal, 20 to 30 cm thick, is laid right across the base of the clamp, being contained within walls of pre-fired brick, with the green bricks laid on top. This does result in the slumping of the clamp during firing, but makes the kiln easier and faster to construct. Depending on whether coal is added to the clay in the bricks and the quality of the coal, another thinner layer of coal is added higher up the clamp. The clamp is lit with gas or kerosene burners, applied to small ignition tunnels at regular intervals down both sides of the kiln, or with small wood fires, placed in larger ignition tunnels. These larger ignition tunnels have to be partially blocked off once the clamp is alight, otherwise too much air will pass through the clamp, effectively cooling it.
Another method used is to place a layer of wood chips across the base of the kiln, amongst widely spaced bricks (4 to 5 cm spacing), and add more coal between the bricks for the first ten layers, gradually reducing the brick spacing and amount of coal towards the top of the clamp. Coal fines can also be added between the bricks as each layer of the clamp is built, though this depends on how good the coal is and if fuel has already been mixed into the bricks.
Clamps are very suitable for firing bricks where fuel is included in the mixture used in the manufacture of the bricks. The addi- tion of between 5 to 10% of sieved rice husk, sawdust, coal fines, coke dust or animal dung
means that the clamp once started burns itself, with the bricks being fired from both internal and external combustion. The addition of animal dung to the clay increases the workability of the mixture when the bricks are formed, as well as acting as fuel when they are fired.
As the bricks shrink during firing and the fuel turns to ash, the spaces between the bricks get larger, and too much cold air will be drawn through the clamp. This has to be reduced in the later stages of firing, or an uneven firing will occur, with hot and cold spots developing within the clamp. This is controlled by reducing the size of the vents at the top of the clamp, by adding more bricks to close them down, or by covering them with steel sheets. Another method is to cover the top of the kiln with a layer of ash and brick rubble, or dry soil, which still allows for the combustion gases to escape slowly and evenly. The air that is allowed to enter the kiln is reduced by partially closing the ignition tunnels with fired bricks or steel sheets.
The air spaces in the clamp floor tend to be self-regulating, as they tend to become blocked with ash, when the clamp has been alight for a while.
To contain the available heat within the clamp, fired bricks are used to line the outside walls and top. These bricks are placed tightly together to create an outer skin, reducing the cooling effect of wind and possibly keeping out rain during firing. Regularly spaced openings are left in the fired brick lining on the top of the clamp, which act as vents. These vents allow the steam and combustion gases to escape from the clamp during firing. They can be reduced in size during the later stages, to contain the heat during soaking, when the clamp has reached top temperature. The vents are closed off altogether and sealed, when firing has finished and cooling starts.
Any steps that can be taken to increase the efficiency of a brick clamp, to both reduce the amount of fuel used and to obtain a higher percentage of well fired bricks, is a good investment.
A coal fired scove clamp/kiln
Brick clamp firing tunnels
Because of their versatility and low cost, especially where labour is cheap, brick clamps will always remain in use. Coal fired clamps are still used in the northern hemisphere to produce high value bricks of special colouring and textures. The main problem with clamps is the very low energy efficiency, coupled with the high percentage of brick wastage. This poor efficiency can be offset by using any available waste products to replace some of the fuel during firing.
The first hours of the firing, when the bricks are being thoroughly dried, can be fuelled with agri-waste or combustible rubbish. If fuel is incorporated into the bricks, the clamp will then fire itself with the minimal addition of a prime fuel, such as wood.
The high percentage of brick wastage can be kept to a minimum by constructing the brick clamp accurately and providing the maximum amount of insulation around the outside. The more the clamp is sealed and insulated on the outside, the less underfired bricks will be produced, and a lower amount of fuel will be required. This insulation and improved performance is obtained by using the partially fired bricks from a previous firing, to surround the green bricks in the centre of the clamp, and surrounding these in turn with closely fitting layers of fully fired bricks. The whole of the outside of the clamp can then be smeared (scoved) with a thick layer of clay, combined with ash, rice husk or dried grass, to obtain the maximum insulation.
The encasing of abrickclamp in aplaster of clay is called scoving. Clamps sealed and insulated in this way are therefore called scove clamps or kilns.
Controlling the airflow into the combustion tunnels under the clamp, helps improve fuel combustion and cut down on heat wastage. This can be achieved by covering the tunnel openings with pieces of sheet metal between stoking with fuel. Fired bricks may also be used to control airflow into the clamp more accurately, the firing tunnels being opened and closed gradually, by the addition and removal of a couple of bricks at a time.
Maintaining a steady rise in temperature through the clean and efficient combustion of fuel is the ideal. Stoking at regular intervals with pieces of dry wood, that has been cut and split to size, or the addition of small charges of fuel, is preferable to over-filling the combustion tunnels with large amounts of damp fuel at widely spaced irregular intervals.
The maximum amount of air should be allowed in after fuel is added, then the air should be reduced once the fuel is burning well. At the end of the firing, the tunnels are sealed off completely, along with the vents at the top of the clamp to start the cooling process slowly. After a day or so, the tunnels and vents can be opened again to allow for accelerated cooling to take place.
The next stage towards improved fuel efficiency is to fit fire bars and firedoors to the combustion tunnels. This effectively converts them into much more efficient fuel burning fireboxes. This is not often done on brick clamps, because of their temporary nature, but is a more common feature of Scotch kilns, which represent the next stage of brick kiln development.
Controlling the exhaust gases, coming from the vents at the top of the clamp with pieces of scrap metal during the firing, holds the maximum amount of heat in the kiln and controls combustion. At the start of the firing, there need to be enough vents that are fully open, to allow all the water vapour out of the clamp quickly. Later, the size of the vents can be reduced to slow down the draft through the clamp, but still allow enough fresh air to enter the firing tunnels for effective combustion. A balance needs to be obtained between allowing enough air through the clamp for combustion and keeping excess air down to a minimum to prevent cooling.
By building the clamp so that the firing tunnels are at 90 to the prevailing wind, using only dry fuel in small quantities at a time, and using bricks that are as atmospherically dry as possible, makes a lot of difference in reducing the energy consumption and improving the efficiency of what is a very practical, and versatile, but basically fuel hungry system for firing bricks.
A scoved brick clamp
|Text, drawings and Figure 3 by
Appropriate Development Consultants
Cumbria LA23 3LD
German Appropriate Technology