Brick Clamps Index Introduction Disadvantages of brick clamps Clamp size and shape Increasing the efficiency of brick clamps Wood fired clamps Further reading Coal fired clamps Advantages of brick clampsIntroductionThe 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 orkilns. 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 severaladvantages over more modern and sophisticated methods. This is despite the clamp beingpotentially the most energy inefficient method of firing, because so much heat is allowed into theatmosphere during both firing and cooling, and fuel combustion is both uncontrollable andinefficient. Brick clamps can be made moderately efficient, but they cannot compare withupdraught continuous kilns, as far as fuel used per fired brick and brick wastage is concerned.
Figure 1 A basic brick clampBack to indexClamp size and shapeBrick clamps come in several shapes and sizes, depending on the number of bricks to be fired atone 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 allowfor 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 isinset by 2 to 3 cm, so that the clamp tapers towards the top. The tapering produces greater clampstability, which is important as the whole structure will move with the expansion and contractionof the bricks during firing. For greater stacking stability, clamps can be built on ground that hasbeen "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 towardsthe 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 reducedto help minimize heat loss.Back to indexWood fired clampsWood 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, andare 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 topof the arch. All the bricks are placed in a clamp on their sides, and these figures are for a 25 x12.5 x 8 cm brick. The tunnels can be split in two by being bricked across in the centre of theclamp. This is done if the clamp is large, and therefore has a wide base, or if the fuel is providedin cut lengths. This prevents cross-draughts blowing through the clamp, and improves the centraldrawing capabilities of the clamp, all better for fuel combustion. Narrow low tunnels are built ifthe clamp is to be fired with portable gas or oil burners, which are moved down the clamp fromtunnel to tunnel as firing progresses.Firebars can be built into the firing tunnels, two thirds of the way down from the top, to improvewood combustion, which is especially useful towards the end of the firing, when maximumtemperature input is required. To avoid the expense of firebars and the added complication ofbuilding them into the clamp, a perforated wall of fired bricks, three layers high, may be builtacross the lower part of the firing tunnels entrance. This simple modification helps to heat theprimary air as it goes into the tunnel, and pieces of wood can be placed on the wall at one end, toallow the air to pass underneath. This improves combustion and reduces the build up of embers. Figure 2 A small woodfuelled brick clamp in Africa (Photo: H. Norsker) Figure 3 Figure 4
Wood and dunkey dung fuelled clamp West African scoved brick clamp with saucered (Sudan) foundation and fired brick wood restsBack to indexCoal fired clampsCoal fired clamps are constructed on a spaced brick floor, so that air can be drawn in under thewhole 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 resultin 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, anotherthinner 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 smallwood fires, placed in larger ignition tunnels. These larger ignition tunnels have to be partiallyblocked 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, amongstwidely spaced bricks (4 to 5 cm spacing), and add more coal between the bricks for the first tenlayers, 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 thisdepends 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 themanufacture of the bricks. The addi- tion of between 5 to 10% of sieved rice husk, sawdust, coalfines, coke dust or animal dungmeans that the clamp once started burns itself, with the bricks being fired from both internal andexternal combustion. The addition of animal dung to the clay increases the workability of themixture 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 getlarger, and too much cold air will be drawn through the clamp. This has to be reduced in the laterstages of firing, or an uneven firing will occur, with hot and cold spots developing within theclamp. This is controlled by reducing the size of the vents at the top of the clamp, by addingmore bricks to close them down, or by covering them with steel sheets. Another method is tocover the top of the kiln with a layer of ash and brick rubble, or dry soil, which still allows forthe combustion gases to escape slowly and evenly. The air that is allowed to enter the kiln isreduced 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 withash, 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 andtop. 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 firedbrick lining on the top of the clamp, which act as vents. These vents allow the steam andcombustion gases to escape from the clamp during firing. They can be reduced in size during thelater stages, to contain the heat during soaking, when the clamp has reached top temperature. Thevents 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 amountof fuel used and to obtain a higher percentage of well fired bricks, is a good investment. Figure 5 A coal fired scove clamp/kilnBack to indexAdvantages of brick clamps They are cheap and straightforward to build. There is no permanent structure to install and maintain. A level area of ground and a good supply of fuel and green bricks is all that is required. They can be built next to the supply of clay and fuel, so that transport costs are kept to a minimum. They can be of any size ranging from 5,000 to 100,000 bricks at a time, so they can accommodate fluctuations in brick production. Once lit, they do not require much at- tention, especially if the fuel is included in or amongst the bricks in the clamp. Very large brick clamps can be fired continuously, with fired bricks being unloaded at one end of the clamp and green bricks loaded at the other, while the fire moves through the middle. The result is a continuously firing clamp.
Clamps can be fired with a large variety of fuels, including agri-waste, such as rice husk, coffee husk, sawdust, coconut husk, dung, etc, as well as fossil fuels. Different fuels can be used at different stages of the firing and in a variety of ways. They can be mixed into the green bricks, sprinkled around them, placed in layers between them, or burnt in the tunnels under the clamp. It depends on fuel price and availability, as well as its calorific value, which determines the amount needed. As woodfuel becomes scarcer and more expensive, agri-waste or any suitable combustible rubbish can be used, at least for the first heating and drying stages of the clamp firing. For example, the clamp can be started with slow burning rice husk, to provide a gentle heat that needs little or no attention overnight. Then coconut husk is used to move the temperature through the middle ranges. And finally, split rubber wood is used to provide the top temperature and soaking period. Figure 6 Brick clamp firing tunnelsBack to indexDisadvantages of brick clamps Basic brick clamps are the least energy efficient method of firing bricks, with a lot of heat being lost by radiation through the walls, and convection from the top of the clamp. The fuel is not consumed efficiently as there is little or no control over its combustion once the clamp is lit. Fuel consumption of 2,800 to 3,500 kJ/kg fired brick is to be expected, depending on the size and design of the clamp, plus the fuel and method of combustion. This low figure is partially due to the high percentage of broken and over or under fired bricks produced.
They are very labour intensive, being assembled and disassembled by hand, and if not built correctly and fired badly, can result in a very high percentage of incompletely fired bricks. Up to 20% of the bricks produced by a basic brick clamp can normally be expected to be over or under fired by this method. They are very slow to fire, taking several days to heat up and cool down, and there is little or no control over the firing once it has started. They are highly susceptible to the prevailing weather conditions, especially strong winds, which will result in a very uneven firing, with many more underfired and overfired bricks.Back to indexIncreasing the efficiency of brick clampsBecause of their versatility and low cost, especially where labour is cheap, brick clamps willalways remain in use. Coal fired clamps are still used in the northern hemisphere to produce highvalue bricks of special colouring and textures. The main problem with clamps is the very lowenergy efficiency, coupled with the high percentage of brick wastage. This poor efficiency canbe 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 itselfwith 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 clampaccurately and providing the maximum amount of insulation around the outside. The more theclamp is sealed and insulated on the outside, the less underfired bricks will be produced, and alower amount of fuel will be required. This insulation and improved performance is obtained byusing the partially fired bricks from a previous firing, to surround the green bricks in the centreof the clamp, and surrounding these in turn with closely fitting layers of fully fired bricks. Thewhole 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 inthis way are therefore called scove clamps or kilns.Controlling the airflow into the combustion tunnels under the clamp, helps improve fuelcombustion and cut down on heat wastage. This can be achieved by covering the tunnel openingswith pieces of sheet metal between stoking with fuel. Fired bricks may also be used to controlairflow into the clamp more accurately, the firing tunnels being opened and closed gradually, bythe 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 theideal. 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 withlarge 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 bereduced once the fuel is burning well. At the end of the firing, the tunnels are sealed offcompletely, along with the vents at the top of the clamp to start the cooling process slowly. Aftera day or so, the tunnels and vents can be opened again to allow for accelerated cooling to takeplace.The next stage towards improved fuel efficiency is to fit fire bars and firedoors to the combustiontunnels. This effectively converts them into much more efficient fuel burning fireboxes. This isnot often done on brick clamps, because of their temporary nature, but is a more common featureof 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 scrapmetal 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 watervapour out of the clamp quickly. Later, the size of the vents can be reduced to slow down thedraft through the clamp, but still allow enough fresh air to enter the firing tunnels for effectivecombustion. A balance needs to be obtained between allowing enough air through the clamp forcombustion 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 dryfuel 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 ofwhat is a very practical, and versatile, but basically fuel hungry system for firing bricks. Figure 7 A scoved brick clampBack to index
Further reading The Basics of Brick Kiln Technology, Jones, Tim, Aus der Arbeit von GATE, Vieweg, Braunschweig, 1995 Brickmaking in Developing Countries, Parry, J.P.M., Review prepared for the Building Research Establishment, Garston, U.K., 1979 Brick and Lime Kilns in Ecuador, En- ergy, Environment and Development Se- ries No. 13, The Stockholm Environmental Institute, Stockholm, 1992 Small Scale Brickmaking, ILO/ UNIDO, Technical memorandum No. 6, International Labour Office, Geneva, 1984 Village Level Brickmaking, Beamish, Anne; Donovan, Will, Aus der Arbeit von GATE, Vieweg, Braunschweig, 1989Back to index Published byText, drawings and Figure 3 byTim Jones German Appropriate Technology ExchangeAppropriate Development Consultants Dag-Hammarskjld-Weg 1"Cannon Hey" Postfach 51 80Storrs Park D - 65726 EschbornBowness-on-Windermere Federal Republic of GermanyCumbria LA23 3LD Phone +49 - 6196 - 79-3190United Kingdom Fax +49 - 6196 - 79-7352 (1995)