 |  | Village Level Brickmaking (GTZ, 1989, 124 p.) | ||
 |  | Building And Firing A Coal-Fired Brick Kiln | ||
|  | (introduction...) | ||
| | How To Lay Out The Kiln | ||
| | Construction Of The Kiln | ||
| | Firing The Kiln | ||
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Good quality bricks can also be made by firing the kiln with coal instead of firewood. If coal is available in your area, use it, since the use of firewood for brickmaking can lead to rapid deforestation.
The method of building the field kiln and firing the bricks using coal is quite different than that used with firewood. The number of bricks one can fire in a kiln of this type is from 10,000 to 100,000.
The following is a step-by step description for building a 20,000 brick field kiln (sometimes called a scove kiln) using coal as the fuel. The approximate length and width of the kiln is 4 metres. The overall height of the kiln is about 3.5 metres.
Level the Site
Step 1: Select a level area of ground approximately 6 metres by 6 metres and remove all rocks and surface vegetation. Make sure the area is smooth and level.
If you have underfired bricks from the last kiln, use them to build a strong base on which you can build the kiln. Simply lay the bricks frog side down with no spaces between them and make a brick floor slightly larger than the overall size of the kiln. The new kiln can now be built on this base.
If you don't have under fired bricks, make sure the ground is well compacted and level. The kiln is then built directly on top of the ground.
Lay Out the Boundaries of the Kiln
The length and width of the kiln will be equal. To measure the length and width, bricks will be used as the unit of measurement. For a 20,000 brick kiln, the length will be 16 brick lengths and the width will also be 16 brick lengths. If the unfired bricks are 250 × 125 × 80 mm, the length and width of the kiln will be approximately 4 metres by 4 metres.
Step 2: Drive a stake close to the corner of the area which has just been cleaned and levered. Tie a cord to the stake and stretch the cord 5 metres in the direction of where you wish to place one of the walls of the kiln. Drive another stake into the ground and tie the cord to it. This cord will mark the length of the kiln and the actual length of the kiln will be measured by using bricks.
Step 3: Beginning about 1/2 metre from the first stake, place a row of 16 bricks along the outside of the cord. These bricks are placed on edge with their ends touching. (These bricks are placed temporarily and are used only to mark the boundaries of the kiln. They will be removed later.)
Step 4: With another cord and stake, lay out the second side of the kiln. The cord is stretched for 5 metres, perpendicular to the first cord and should just touch the end of the last brick. Make sure the corner is a right angle (90°) by either using a large builder's square or using the 3:4:5 rule. (See Appendix 3 for more information on the 3:4:5 rule).
Figure
Step 5: When the corner is square, begin to measure the width of the kiln with backs. Starting at the point where the cords cross, lay 16 backs end to end along the outside of the cord.
Step 6: With another cord and stakes, lay out the third side of the field kiln. The cord is stretched perpendicular to the second cord and should just touch the end of the last brick. Make sure the corner is a right angle (90 ) by either using a large builder's square or using the 3:4:5 rule.
Step 7: Measure out the length of the kiln as you did for the first side, placing 16 bricks end to end.
Step 8: With another cord, lay out the fourth and final side of the kiln, forming a large square 16 bricks long by 16 bricks wide. The cord should just touch the ends of the bricks on the first and third sides.
Step 9: Check that all the comers are right angles (90°) by measuring the diagonals. If the length of the two diagonals are equal, this indicates that all the corners are right angles. If the two diagonals are not the same length, go back and check each comer individually, using the 3:4:5 rule.
Also check that all the sides are equal in length. Even though they may have the same number of brick lengths, there could be some slight differences in the lengths of the individual bricks.
To ensure that the kiln is strong and will not collapse during the firing, it is important to use good quality (no cracks and regular in shape) well-dried unfired bricks in layers 1 through 8.
All bricks used to build the kiln are always placed on their side because they are stronger in this position. Never place the bricks on their flat side.
Quantity of Coal
You will need to have your supply of coal ready before starting to build the kiln as the coal is placed within the kiln at the same time the bricks are. It is estimated that you will need approximately 1 1/2 sacks of coal and 1/2 sack of selected ashes (cinders) for every 1,000 bricks to be fired. Therefore, a 20,000 brick kiln will need approximately 30 sacks of coal and 10 sacks of selected ashes or cinders.
The coal can be waste ashes from a coal fired power station or a coal fired locomotive. The coal should not be too fine as this will prevent good air flow through the kiln. Nor should the coal be larger than fist size as this will require labour to break the coal down into smaller bits to fit in the coal firing layers.
Layer 1
Begin to place the bricks in a row (2 bricks wide) along one of the cords inside the boundaries that mark the kiln. All bricks are placed on their sides.
Leave a space of about 50 mm beside it and place another double row in the same direction.
Continue placing double rows with 50 mm space between them until the area marked by the cords is filled. If your bricks are 250 × 125 × 80 mm, you should have placed about 14 rows of bricks. It may be necessary to adjust the spaces slightly to fill the area completely up to the cord boundary. If adjustment is necessary, the spaces should not be larger than 60 mm or smaller than 40 mm.
Figure
When the first layer of bricks has been completed, remove all the bricks, cords and stakes which were used to mark the boundaries of the kiln.
Layer 2
The bricks in the second layer are placed perpendicularly to those in the first layer. Note that there are no spaces left between the bricks and they touch another brick on all 4 sides. You should now have a solid floor of unfired bricks.
Figure
Layer 3
Place a row (3 bricks wide) of unfired bricks around the outside of the kiln. Remove the bricks as shown in the drawing leaving an opening. This space will be used to start the kindling fires for igniting the kiln. This space is about the length of a brick. Depending on your brick size, you may have to adjust slightly the placement of the bricks to get this opening correct.
Once the outer walls and openings are established begin placing a single row of bricks, touching end to end, from one corner diagonally across the kiln to the opposite corner.
Figure
Leave a 50 mm space and place another single row of bricks parallel to the first. Continue placing single rows of bricks, leaving about 50 mm between them until the area is filled with diagonal rows.
During the placement of the diagonal rows, the 50 mm spaces are filled to a depth of 25 mm with small wood chips and shavings. The space is then filled to the top with cubes of coal. The coal should not be too fine or packed too tightly since this would restrict the draught during the firing of the kiln.
Figure
Layer 4
This layer is very similar to layer 3 except that all bricks are laid perpendicular to those below it.
Place bricks perpendicular to the bricks which form the outside wall. There are no spaces left between these bricks. Also leave gaps for the fire openings as was done in the third layer.
Diagonal rows of bricks are laid in the same manner but in the opposite direction to those below.
All spaces between the diagonal rows and the outside walls are completely filled with coal cubes.
Figure
Layer 5
Place a triple row of bricks around the outside of the kiln, exactly as you did in Layer 3. Continue to leave a gap where the kindling fires will be started.
Place rows of single bricks parallel to two opposite sides as shown in the diagram. Leave a 25 mm space between these rows.
These spaces are completely filled with cubes of coal which are smaller than those used in the previous layers.
Figure
Layer 6
Place the bricks, touching on all four sides, perpendicularly to those below it. Any small spaces that may occur are filled with coal.
On the outside walls, bricks are placed so that they span and close the gap for the kindling fires. Note in the drawing how they are placed.
A small space is left running in a straight line across the middle of the kiln, forming a cross as shown in the diagram. This space is filled with coal.
The outer dimensions of layer 6 are 50 mm smaller than the layer below (25 mm on each side). This is repeated in each subsequent layer so that the kiln tapers on all sides towards the top, and which makes it more stable.
Figure
Layer 7
All the bricks are laid perpendicular to those below them. They are close packed with the exception of the 2 or 3 rows just inside the exterior wall as shown in the diagram. Here the 25 mm spaces are filled with small coal or good cinders. The purpose of this arrangement is to increase the amount of heat near the outside surfaces of the kiln since the bricks in this area often suffer from under firing.
The outside dimensions of this layer are 50 mm smaller (25 mm on each side) than the lower level.
Figure
Layers 8, 9,10
These layers are arranged in the same manner as layer 7 but each level is laid perpendicular to the one below it. The outside dimensions of these layers are 50 mm smaller (25 mm on each side) than the layer below.
Figure
Layers 11,12, 13, etc.
The kiln is built to the required height by repeating layers 5 through 10. Coal is always used in layers 5 and 6, but good cinders may be used in layers 7, 8, 9, and 10.
Final Layer
The final layer is a protective one and no fuel is added. It is laid perpendicularly to the layer below it. On top of this are one or 2 layers of broken, misshaped unfired bricks or old fired bricks.
A heap of ashes is placed centrally on top of the kiln so that it is available when needed. When the kiln is well alight, the ashes may be spread over the top of the kiln to retard combustion, if necessary, and prevent the loss of valuable heat.
Figure
Insulation
The sides of the kiln are plastered with clay to reduce heat loss. The plaster also prevents more air than necessary from entering the kiln. If the kiln is well sealed with plaster, the only place that air can enter for combustion is through the lower spaces left for the kindling fires.
If the plastering mud contains a lot of clay, it will crack and fall off during firing; mix a little sand into the mixture to prevent this from happening. The plaster should be applied in two layers, allowing the first to dry before applying the second. Throw the mud with force so that it fills the cracks well.
The lower five layers are left unplastered until the kiln is well alight. Once alight the fire openings are closed with bricks and layers 3, 4, and 5 can be plastered. Do not plaster layers 1 and 2 as this is where the air for the kiln will enter.
Figure
The advantage of firing with coal is that it is much less work than firing a kiln with firewood since the fuel is built into the kiln and no more is added during the firing. However, the disadvantage is that you have very little control over the firing process. You can only build the kiln as carefully as possible, following the directions, and with the correct amount of coal. Once the fire is started, with the exception of spreading ashes on the top of the kiln, there is little you can do, other than wait until the kiln has finished firing and the fires die out themselves.
To begin firing the kiln, light all 8 fire openings located in layers 3, 4, and 5 of the kiln. Once the kiln is burning briskly, the 8 fire openings are closed with bricks and mud plaster. All air entering the kiln should now be coming through layers 1 and 2. Layers 3, 4, and 5 can now be plastered, but remember not to plaster layers 1 and 2.
During the firing of the kiln, have some mud plaster ready to repair the cracks that will form in the plaster. As the heat of the kiln increases, it is not unusual for the plaster to crack and fall off. Be prepared to repair this or heat will escape and the kiln will not reach the correct temperature.
Do not cover the top of the kiln during the first three days of firing since steam will be given off. (If the kiln top is covered before all the steam can escape, the bricks may be destroyed.) Once the top layer is so hot that a handful of grass tossed on top instantly catches on fire, you can spread the ashes (cinders) until the top of the kiln is completely covered.
The kiln is allowed to fire until the fuel inside the kiln is finished. The length of time for this will depend on the following:
1. The quantity and quality of the fuel used and how it was packed in the kiln.
2. The regulation of the air supply through the fire and air openings.
3. Rains and heavy winds that cool the kiln.
4. Covering the top of the clamp with ash.
5. The size of the kiln.
Like kilns using firewood, the bricks must reach a temperature of at least 950°C in order to produce good quality bricks, and this should take several days depending on the size of the kiln. Because it is difficult to control the heat of the fires, it is important to have the bricks as dry as possible before building and firing the kiln. When all the fuel has been burnt, the kiln must be left to cool as slowly as possible, before being opened. This period should take 2 to 4 weeks.
Like a wood fired kiln, the coal kiln should not have more than 10% of unacceptable bricks (bricks that are over or under-sized, glazed, broken, or underfired). The causes of poor quality bricks fired by coal will be similar to those fired by firewood. Therefore, if necessary, review the problems, reasons and solutions described on pages 80 to 81.
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