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The firing of the field kiln is the last step in making the bricks and is the most important. If the bricks are fired well, they will be of a good quality and an ideal building material for building permanent structures such as schools, homes and clinics. If the kiln is not fired well or if done without sufficient care and caution, you can lose the majority of the bricks in a kiln and all the time, effort, energy, firewood and resources that went into it will have been wasted.
Other fuels, such as coal or oil, can also be used to fire a brick kiln. Information on building and firing a field kiln using coal can be found on pages 91 to 99.
The firing of the field kiln is the most important phase in the production of bricks and to obtain a good firing, it is essential that the correct size and quantity of firewood is cut and brought to the kiln site. It is also very important that there is a sufficient quantity of firewood to supply the brick site. Therefore, it is highly recommended that all brick production units establish woodlots to supply their fuel needs. Information on establishing woodlots can be found in Appendix 5.
Quantity of Firewood
The general rule for estimating the quantity of firewood is that for every 1,000 bricks which you wish to fire, one ton or three cubic metres of cut and stacked firewood will be needed.
Figure
If less than this quantity is used to fire the bricks, the bricks will be underfired and therefore weak and will probably not be suitable for use in construction.
One cubic metre is a quantity or volume with exact dimensions. It is almost impossible for the average person to estimate correctly the number of cubic metres in a randomly stacked pile of firewood. For this reason, it is preferable to measure the quantities of firewood by stacking it in rows of cubic metres.
Figure
One cubic metre is a volume with a length of 1 metre, a width of 1 metre and a height of 1 metre.
It is easy to measure one cubic metre of stacked firewood and using the following method, it is possible to accurately measure the quantity of firewood that will be necessary to fire the kiln.
First, cut four sticks or bamboo poles to a length of 1.2 metres. Mark a square on the ground with one metre sides and then drive one of the sticks or bamboo poles into each corner of the square leaving one metre sticking out of the ground. These four sticks should form a cube that has a length, width and height of one metre. If the space or volume between the sticks is filled with wood, you will have one cubic metre of firewood. Rows of cubic metres are marked out near the kiln where the firewood will be piled.
Figure
An average person can cut and stack approximately one cubic metre of firewood per day. To assist the woodcutters, rows of cubic metres should also be marked out in the bush were they are cutting. They should always pile their cut wood in these stacks in order to verify their daily output and to ensure that the wood cutting goals are being met.
Cutting the firewood is a time-consuming task. Always begin cutting and transporting the firewood to the kiln site as soon as you begin to make bricks. Do not underestimate this task and do not leave it to the last moment; you should always have workers cutting firewood.
One final important rule that cannot be overemphasized is to never begin firing a field kiln before all the firewood needed to fire the bricks has been delivered to the kiln site!
Note: To give you some idea of the quantities required, you will need 10,000 bricks to build just the walls of a house measuring 7 metres by 6 metres.
|
Cubic Metres of Firewood Needed to Fire Bricks |
|
|
Number of Bricks |
Cubic Metres of Firewood |
|
1,000 |
3 |
|
5,000 |
15 |
|
10,000 |
30 |
|
20,000 |
60 |
|
25,000 |
75 |
|
30,000 |
90 |
Size
The best sized wood for firing a kiln is one metre long, with a diameter of 100 to 200 mm. This size of log is the easiest to throw into the fire tunnels and burns fairly slowly.
Firewood with a diameter less than 100 trim is too small and tends to bum too quickly in the kiln. If the diameter of the log is greater than 200 mm, it is difficult to throw into the fire tunnels because of its weight and size.
Figure
If the kiln site is well organized, it will make the work much easier and safer for those who tend the fires. It will also be much easier to control the quantity of firewood used during the firing process.
A suggested layout is shown in the drawing below. The firewood is neatly stacked in rows of cubic metres with a space of one meter between each row. Because the kiln becomes so hot, it's important to leave an open space of three meters between the kiln and firewood. Stacking the firewood in this way makes it is easy to verify that the correct amount of firewood is on the site and ready to be used.
Figure
Do not haphazardly stack firewood close to the kiln. This situation is dangerous for the workers who tend the fires, especially at night, and makes it difficult to accurately estimate the amount of firewood.
Some kiln sites are plagued by strong seasonal winds that are constantly changing direction. One solution is to build a temporary high grass wall around the kiln site. This will help prevent winds from blowing down into the fire tunnels which would cool the kiln and cause more firewood to be used.
Number: Depending on the size of the field kiln, you will need from 12 to 20 people to tend the fires during the firing stage which lasts from one to two weeks. The workers should be divided into two groups of 6 to 10 people. The first shift will work during the day from 06:00 hours until 18:00 hours and the second will work during the night from 18:00 hours to 06:00 hours. At no point during the firing should the kiln be left unattended. A general rule is that you will need at least one person per tunnel at all times during the firing of the kiln.
Figure
Responsibilities: The workers chosen to work in the night shift must be the most responsible and dependable. Tending the kiln is hard work. The workers must be constantly feeding firewood into the kiln both day and night. Though tempting, it is essential that the night shift do not sleep and allow the fires to go down. If poorly tended, the temperature in the kiln will drop and you will need many more cubic metres of firewood to bring the temperature back up to where it should be. This wastes time and firewood, and could even completely ruin the firing of the kiln and destroy the bricks inside.
If the temperature does drop, there will likely not be enough firewood to bring it back up to temperature. If this should happen, you will have to stop the firing, collect more firewood and start the whole firing process again from the beginning.
Worker Protection and Safety
Firing a field kiln is very hard work and can be dangerous to the worker because of the high temperatures involved.
A combination of the heat and hard work causes the worker to perspire excessively and lose essential salts and fluids which can cause exhaustion, cramps or even heat stroke. To prevent this, have an abundant supply of fresh clean drinking water available at all times to the workers and encourage them to eat more salt with their food during this period.
Figure
All workers tending the fires should also be equipped with leather boots, gloves and aprons to prevent burns to the body.
When clay bricks are heated to a high temperature, a chemical reaction occurs in the clay which makes the brick permanently hard, durable and resistant to weather and water.
When a brick is heated to a temperature between 20°C and 150°C, it loses most of the water added to the clay during the preparation phase.
When heated from 150°C to 600°C, the clay brick loses its remaining water. When firing a kiln, you will see a white vapour or steam coming from the top of the kiln during these first two phases. If you were to stop the firing process at 600°C, the bricks in the kiln would be useless for building purposes since they would not be stabilized and would easily be worn down by wind and rain.
When the temperature starts to rise over 600°C, chemical changes begin to occur in the clay which give the brick colour, hardness and durability. Temperatures of 900°C and above cause vitrification to occur. This means that a small quantity of glass-like material forms which helps glue all of elements in the clay together. It is after this point of vitrification that the brick will be at its hardest and most resistant and will be ideal for construction.
The final quality of the brick will depend on the amount of vitrification which occurs. It therefore is essential that the temperature of the kiln rises to well above 900°C in order to obtain a well-fired brick.
Figure
When firing a field kiln, it is important to raise the temperature of the bricks to at least 950°C. Therefore it is necessary to have a means to accurately measure this temperature. There are three methods that can be used:
1. Pyrometer
2. Pyrometric Cones
3. Colour
1. Pyrometer: A pyrometer is a sophisticated thermometer used to measure high temperaturesin brick kilns.
Figure
2. Pyrometric Cones: A pyrometric cone is made of a ceramic material that bends and deforms at a predetermined temperature. For example:
- cone No. 015 (60°C/hour) will bend at 790°C
-
cone No. 08 (60°C/hour) will bend at 950°C
- cone No. 04
(60°C/hour) will bend at 1,060°C
Figure
When building the field kiln, the cones are placed in a visible location where they can be seen through "peep" or "spy" holes. Therefore, when the cones begin to bend, the temperature in that spot is known. For more information about these cones see Appendix 4.
Pyrometers and pyrometric cones are accurate but tend to be expensive or difficult to obtain for a small rural producer of bricks. For this reason, it may be better to use the following method:
3. Colour: It is possible to estimate the temperature of the kiln by observing the colour of the bricks in the interior of the fire tunnels. As the temperature of the bricks change, so will the colour. The following table gives the various colours which can be seen during a firing and the corresponding temperature.
Figure
In order to bring a field kiln to the correct temperature of at least 950°C, the bricks in the fire tunnels should have a yellow-white glow before shutting down the kiln. If this is done, you can be quite sure that the rest of the kiln has reached the correct temperature and that the bricks will be well fired.
|
Temperature Colour Chart | |
|
Colour of Brick |
Approximate |
|
Dark Red to Red |
600°C to 750°C |
|
Red to Brilliant Red |
750°C to 800°C |
|
Brilliant Red to Orange |
800°C to 900°C |
|
Orange to Yellow |
900°C to 1,100°C |
A wood fire needs air to burn well but too much air can cool the fire, reduce the temperature of the bricks and increase the consumption of firewood.
To help control the air flow into the tunnels, place metal sheets in front of the tunnel openings. The metal sheets can be old roofing sheets or old 200 litre fuel drums cut open and bent flat. These sheets are placed in front of the tunnels when the fire is burning and are removed temporarily when you need to add more firewood to the fire.
Figure
If metal sheets are not available, use loosely stacked bricks in the opening. You may find though that building this temporary door with bricks is time consuming since it needs to be built up and taken down each time firewood is fed into the fire tunnels. It can also be rather hazardous for the workers when the kiln reaches very high temperatures.
Too much air can cool the fire, but closing the tunnel opening completely will cut off all air and the fire will go out. It is necessary to experiment with the position of the sheets. One method is to place two or three bricks in the tunnel opening and rest the metal sheet on top of these bricks. Usually enough air can flow underneath the sheet and between the bricks to maintain the fire.
1. Preheating
2. Main Firing
3. Soaking Stage
4.
Closing the Field kiln
5. Cooling
6. Opening of the Field Kiln
1. Preheating
You begin firing the kiln with a small gentle fire with low heat in the centre of each tunnel. The objective of this phase is to drive off all the water or moisture in the bricks. During this stage you will see a white vapour or steam rising from the top of the kiln; this is the water being released from the bricks.
The small fires are maintained until the white vapour or steam is no longer seen coming from the top of the kiln. If the vapour is difficult to see, climb to the top of the kiln using a ladder and simply feel with your hand whether the heat coming off the top of the kiln is moist or not. This stage of firing lasts for two days or more, depending on the size of the kiln and the moisture content of the bricks. At the end of this phase, the temperature of the kiln will . have reached approximately 150°C.
Figure
During the preheating phase, never cover the top of the kiln with sand or soil. It must be left open to allow the steam to leave. All tunnels are open (ie. unblocked) at both ends during this period but metal sheets are used to control the air flow into the tunnels.
Important: If the top of the kiln is sealed or if the fires in the tunnels are too large and the temperature is brought up too quickly, the bricks will break.
2. Main Firing
Once the preheating phase has been completed, the main firing begins and will take a total of four to six days to complete. The objective of this stage is to bring all parts of the kiln up to the correct firing temperature of approximately 950°C.
On one side of the kiln, all tunnel openings are blocked up completely with bricks and mud mortar. On the other side of the kiln, firewood is fed into the tunnels to build up a large hot fire. The firewood is pushed down as far as possible into the tunnels. This first Dart of the main firing will last about two to three days or until the bricks in the tunnels begin to glow red. (This colour indicates a temperature of between 600°C and 800°C.)
When the bricks in the tunnels are glowing red, the tunnels which were open during the first two or three days of the main firing are blocked up with brick and mud mortar and the ones which were initially blocked are broken open. Firewood now continues to be fed in from the open side until the bricks in the tunnels are a glowing yellow-orange colour. (This colour indicates a temperature between 900°C and 1,100°C). This second half of the main firing stage will last another two or three days depending on how well the fires are fed and the size of the kiln.
Figure
To confirm that the correct maximum temperature has been reached, climb to the top of the kiln using a ladder and throw a handful of dry grass onto the top of the kiln. If the grass catches fire instantly, the maximum desired temperature has been reached.
3. Soaking Stage
It is important to maintain the fire and temperature of 900°C to 1,100°C inside the kiln for another 12 to 24 hours. This period is called the soaking stage and is very important because the heat enters into the centre of the brick and the brick gains its greatest strength. The fires should not be allowed to go down at this point.
If the bricks have been counted correctly, the firewood has been calculated and measured accurately, and the kiln has been fired well, there should be sufficient firewood left to just finish this phase.
4. Closing the Kiln
In this phase the top of the kiln is covered with 100 to 200 mm of sand or soil and the tunnels are filled one last time with firewood before being blocked up with brick and mud mortar. This is done to keep the heat in the kiln from escaping and to permit the kiln to cool as slowly as possible
Figure
The sealing off of the top of the kiln is done by a number of people on ladders forming "bucket brigades" and passing soil or sand up to the top of the kiln. The people on top of the kiln who spread the sand or soil stand on wooden boards temporarily laid on top of the kiln to protect their feet from the heat. As the one group is covering the top of the kiln, another should be filling the tunnels with firewood and sealing the tunnel openings.
5. Cooling
It is important that the field kiln is allowed to cool down as slowly as possible. This means that the kiln should not be opened for at least two weeks and preferably four weeks after the kiln has been completely sealed. If it is opened too soon, some of the the bricks may be cracked by the cool air rushing into the kiln. The bricks will obtain a higher strength if the kiln was fired and sealed well and allowed to cool slowly and naturally.
6. Opening of the Kiln
Once the kiln has cooled, it can be opened and dismantled. You will find that up to 10% (or 10 bricks out of every 100) of the bricks removed from the kiln are either cracked, broken, underfired, or unsaleable. This is normal for any field kiln of this type. If however more than 20% are cracked, broken, or underfired, this indicates that the type of clay used to make the bricks was not the correct type or that the kiln was not fired properly.
Figure
Note: broken bricks should be saved for insulating the next field kiln.
When dismantling the kiln and before using or selling the bricks, they should be sorted into piles of good quality, "seconds", under fired, and over fired bricks. Underfired bricks (these are usually found near the outside walls) can be used to build your next field kiln and re-fired again. To help keep track of the quantity of usable bricks, stack them in piles of 1,000.
After opening and dismantling the kiln, you may find that there are a number of unacceptable bricks.
Over or Under Sized Bricks
Although the bricks were made using the same size mould, you may find that the bricks coming from the same kiln can be a variety of sizes. This can be caused by:
- temperature differences within the kiln, or
- different types of clay used to make the bricks.
All clays will shrink when fired and the amount of shrinkage will depend on the type of clay and/or the temperature to which it is fired. To reduce the variations, use the same clay type to make all your bricks and tend the fires well so that equal heat distribution throughout the kiln is maintained.
It is advisable to check the length of the bricks as they are
removed from the kiln and sorted. The variation in dimensions should be kept
within certain limits (see chapter on standardization of brick sizes) and if
they do not conform, the backs should not be sold as first quality.
Glazed
Bricks
Some bricks will have melted together or have a glass-like or glazed appearance. This a result of:
- firing these particular bricks at too high a temperature (over 1,200°C).
Over fired bricks are usually found in the firing tunnels or sometimes even in the middle of the kiln just above the tunnels. Depending on the soil composition, they may be dark blue to purple in colour.
Over fired bricks are very hard and can still be used in construction. They are usually used in foundations or sometimes laid into a wall if a darker coloured band or design is desired.
Low Strength Bricks
Bricks that are low in strength, break easily and dissolve when placed under water indicate that:
- an incorrect type of clay was used for the bricks;- the bricks were underfired, that is, not brought up to and maintained at the minimum temperature of 950°C during the firing of the kiln;
- not enough firewood was used (for every 1,000 bricks you need three cubic metres of firewood);
- the firing stages were cut short; or
- the temperature was allowed to drop very low at some point during the firing.
Cracked or Broken Bricks
If many of the bricks are cracked or broken, this may have been caused by:
- using the wrong type of clay;- plastering or sealing the top of the kiln before the firing, which prevented the steam from escaping;
- having a fire which was too large and hot during the preheating phase which caused the moisture or water in the bricks to turn to steam too quickly;
- opening the kiln before it had cooled properly; or
- stacking and firing the bricks in the kiln without first drying them sufficiently.
The following is an example of how to fire a 20,000 brick field kiln day by day. It describes the work, the tasks of each group of workers, the observations they should make, and the possible problems they might encounter. It should be noted that the time period for each phase is not fixed since much depends on local conditions.
Day 1: Preheating Stage
Objective:
The objective of this phase is to gently remove the water that is trapped within the bricks.
Day Shift (06:00 -18:00)
Tasks:
- bring enough dry grass to cover the floor of each of the tunnels to a height of 10 to 15 cm. Place the dry grass in the middle of the tunnel, one metre from each mouth of the tunnel.- cover the dry grass in the tunnels with small sticks up to about half the height of the tunnel.
- cover the sticks with firewood about 10 cm in diameter. Leave a space of about 30 cm from the top of this pile to the roof of the tunnel to allow space for the passage of air.
- set fire to the grass in all the tunnels at the same time.
- maintain the low heat fires throughout the day.
- use metal sheets as doors for the tunnel openings to help control the rate of burning.
Observations:
- after a few hours you should begin to see a white smoke or steam vapour coming from the top of the kiln. This is the water being driven out of the bricks.
Problems to Avoid:
- make sure the fires are kept small and "cool". A large hot fire at this stage could cause many bricks to crack and split.
Night Shift (18:00-06:00)
Tasks:
- maintain the small tires throughout the night.
Observations:
- you should be able to see the "white smoke" or water vapour coming from the top of the kiln.- with a ladder, climb to the top of the kiln and with your bare hand feel the smoke or steam coming off of the kiln; it should feel moist.
Problems to Avoid:
- the workers may fall asleep during the night allowing the fires to go out.
Figure
Day 2: Preheating Stage
Objective:
The objective of the small fires is to slowly and gently raise the temperature of the kiln to 150°C. During this period all free water in the brick will be driven out.
Day Shift (06:00 - 18:00)
Tasks:
- maintain the small fires.
- at 14:00 hrs., begin closing all the openings on one side of the kiln with bricks and mud.
- after closing one side, continue to maintain the small fires.
Observations:
- you will probably see less "white smoke" coming off the top of the kiln.
Problems to Avoid:
- do not build hot fires in the tunnels yet.
Night Shift (18:00-06.00)
Tasks:
- continue to maintain the small fires.
Observations:
- if you climb to the top of the kiln you will feel less moisture on your hand than the night before. In fact, depending on how well the bricks were dried before being placed in the kiln, it may feel dry.
Problems to Avoid:
- the workers may fall asleep allowing the fires to diminish. This will cause a drop in temperature inside the kiln. Firewood would be wasted bringing the temperature back up to where it was.
Day 3: Main Firing
Objective:
The objective of the main firing is to raise the temperature of the kiln up to 950°C.
Day Shift (06:00 -18:00)
Tasks:
- at 06:00 hrs., begin stoking the fires with more firewood, creating a hot fire.
- after stoking the fires, use metal sheets to cover each tunnel opening to control the air flow into the kiln.
- during the day maintain the hot fires by stoking the fires when necessary.
- repair cracks with mud that will begin to form in the kiln's plastered exterior.
- when stoking the fire, you need to push the logs as deep as possible into the tunnel.
Observations:
- after about 8 hours from starting the main burning, you may begin to see the bricks in the tunnels glowing a dark red colour.
Problems to Avoid:
- to prevent the workers from being seriously burnt, each must have leather gloves, aprons and boots.
- to avoid smothering the fire, add only one log at a time, waiting until it catches fire before adding the next.
- to prevent dehydration of the workers, an ample supply of fresh clean water must always be available to drink.
Night Shift (18:00 - 06:00)
Tasks:
- continue to stoke the fires keeping them hot. Push all firewood as far back into the tunnel as possible.- the tunnels will begin filling up with ashes and it may be necessary to remove them from time to time. An easy way of doing this is to fix a hoe head to a long stick, or even better, weld it to a 2 to 3 metre metal pipe. It is then used to pull the ashes out of the tunnels. The glowing coals should be pushed back deep into the tunnels.
Observations:
- the bricks in the tunnel may change from a dark red to bright red glow.
Problems to Avoid:
- during the night the temperature of the kiln will drop if the workers fall asleep. To prevent this, supervisors should be encouraged to visit the kiln site throughout the night.- as the fires get hotter, the chance of serious burns increases.
Day 4: Main Firing (continued)
Objective:
The objective is to continue to raise the kiln temperature to 950°C.
Day Shift (06:00 - 18:00)
Tasks:
- continue to stoke the fires, keeping them hot.- push all firewood deep into the fire tunnels.
- remove excess ash.
- at 14:00 hrs., close the tunnel openings with bricks and mud on one side of the kiln and open them on the other.
- repair any cracks in the plaster with more mud.
Observations:
- by 14:00 hrs., about half of all the firewood would have been consumed, and for this kiln of 20,000 bricks, 30 cubic meters would be left.- the bricks in the tunnels will glow a bright red colour.
Problems to Avoid:
- the work of closing off the tunnel openings on one side of the kiln and opening them on the other is very hard work. It would be advisable to have extra workers to help with this task.
Night Shift (18:00 - 06:00)
Tasks:
- continue to stoke the fires keeping them all equally hot.
- push the firewood deep into the tunnels.
- remove excess ashes.
Observations:
- the bricks in the tunnels will be glowing a bright red to reddish orange colour.
Problems to Avoid:
- do not allow the fires to go down.
Day 5: Main Firing and Soaking Stage
Objective:
The objective of the main firing is to raise the temperature of the kiln to 950°C. When this temperature is reached, it is necessary to maintain it for as long as possible to allow the heat to enter into the centre of the brick and for the clay to undergo a critical chemical change. This period during which the heat is maintained is called the soaking stage.
Day Shift (06:00 -18:00)
Tasks:
- continue to stoke the fires with firewood to bring the temperature up to 950°C.
- remove ashes when necessary and always push the firewood as deep into the fire tunnels as possible.
Observations:
- at 06:00 hrs., the bricks in the tunnels will be glowing bright orange in colour.- by 18:00 hrs., the bricks in the tunnels will be glowing yellow-orange in colour.
- when you climb to the top of the kiln and look down into the kiln between the spaces of the top course of bricks, you will see the bricks glowing red.
- when you throw dry grass onto the bricks in top of the kiln, it will take about three seconds before it will ignite.
Problems to Avoid:
- at no point allow the fires to go down.
- do not allow cool air to enter the kiln. Always use the metal sheets to close down the tunnel openings.
Night Shift (18:00 - 06:00)
Tasks:
- continue to stoke the fires with firewood, maintaining the high temperature of the kiln.
- always push the firewood as deep into the fire tunnels as possible; remove ashes when necessary.
Observations:
- at 18:00 hrs., the glow of the bricks within the tunnels should be orange/ yellow in colour.- at 06:00 hrs., the glow of the bricks within the tunnels should be yellow.
- if you climb to the top of the kiln and look down between the spaces of the bricks, you should see a bright red glow.
- during the night you will see a red glow coming off of the top of the kiln.
- at 06:00 hrs., if you throw dry grass onto the bricks on top of the kiln, it will take about 1 to 1 1/2 seconds to ignite and burn.
Problems to Avoid:
- at this point in the firing the fires are consuming firewood very rapidly. Control the firing by using metal sheets on the openings. Do not leave the doors wide open; just the minimum of air should enter.- do not allow the fires to go down at this point as you may not have enough firewood on hand to bring the kiln back up to maximum temperature and maintain the soaking stage.
Day 6: Soaking Stage (continued) and Closing The Field Kiln
Objective:
When the temperatures in the firing tunnels reach 950°C, it is necessary to maintain this heat for as long as possible. This stage is called the soaking stage. During this period the temperature evens out through the kiln and the heat penetrates to the centre of each brick. Various chemical reactions will be taking place within the back which will give it its hardness and durability. When the soaking stage is over, the tunnel openings will be sealed and the top of the kiln covered with sand or soil to prevent the rapid lost of heat.
Day Shift (06:00 -18:00)
Tasks:
- continue feeding firewood into the tunnels maintaining the yellow glow of the bricks in the tunnels.- keep the metal sheets in front of the tunnel openings when not stoking the fires. Leave just the minimum of space under the doors to allow the air in.
- keep pushing the firewood as deep as possible into the tunnels.
- when the stock of firewood is almost finished, begin filling each tunnel with firewood and seal off each opening with bricks and mud.
- as the tunnels are being sealed off, begin covering the top of the kiln with a 10 to 15 cm layer of sand or soil.
This is hard work but should be done as fast as possible. It is advisable to call in as many people as possible to assist with this task.
Observations:
- the glow of the bricks within the tunnels should be a yellow colour. If the colour is red you know that the temperature has gone down during the firing either because of poor stoking methods or leaving the tunnel mouths open.- if you were to toss dry grass on to the top of the kiln it should ignite and burn almost instantly.
Problems to Avoid:
- remember to prepare in advance the soil or sand needed to seal off the top of the kiln. A number of buckets, ladders and walking boards will also be necessary. Also remember to prepare and have on hand the bricks and mud needed to seal off the tunnel openings.
Day 7 to 21: Cooling
Leave the sealed kiln to cool down as slowly as possible. The longer you leave the bricks to cool, the stronger the bricks will be. If the kiln is still hot after two weeks let the kiln cool for another week or two.
Day 22: Opening The Field Kiln
Once the kiln has cooled down, you can begin to open it. If you have followed the directions in this manual correctly, and your clay is of a good brickmaking quality, you will find that your bricks are fired well and will be suitable for construction.
Sort the bricks into good quality, "seconds", underfired and overfired bricks. The underfired bricks can be fired again in the next kiln. Stack the bricks into piles of 1,000.
The following is an estimate of the time it should take to build and fire a field kiln of 10 000. 30 000. and 50 000 bricks.
|
Task |
Field Kiln of |
Field Kiln of |
Field Kiln of |
|
Kiln Building | |||
|
Building the Kiln* |
1 to 2 weeks |
2 weeks |
3 weeks |
|
Plastering the Sides* |
3 days |
2 days |
5 days |
|
Stacking Firewood* |
1 day |
2 days |
3 days |
|
Kiln Firing | |||
|
1. Pre-heating Stage** |
2 days |
3 days |
3 to 4 days |
|
2. Main Firing** |
4 days |
4 to 6 days |
5 to 8 days |
|
3. Soaking Stage** |
12 hours |
12 to 24 hours |
12 to 24 hours |
|
4. Closing the Kiln* |
3 hours |
4 hours |
5 hours |
|
5. Cooling** |
1 week |
2 weeks |
2 to 3 weeks |
|
Total |
25 to 31 days |
41 to 44 days |
51 to 63 days |
*The time wilt depend of the number of people available and their willingness to work.
** The duration of this phase will depend on the:
- size of the kiln
- amount of wind
- size, type and humidity of firewood
- thickness of the insulation
- heat of the fire
1. Plaster kiln walls with two layers of mud, allowing the first layer to dry before applying the second. Throw the mud with force so it fills the cracks well. The mud should have sand added to it to prevent it from cracking when the kiln starts to heat up.
2. Have ready at the kiln site 3 cubic metres of wood for each 1,000 bricks you wish to fire. For example if you plan to fire 20,000 bricks, you will need 60 cubic metres of firewood.
3. Firewood should be cut one metre long for easy handling and should be between 10 and 20 cm in diameter.
4. Firewood that is crooked or forked is unacceptable because it cannot be thrown easily through the narrow tunnel openings.
5. All wood that is brought in should be carefully stacked, measured, and recorded. This is necessary not only for correct payment of labour, and verifying quantities but also for computing kiln costs and efficiency.
6 Warm the bricks slowly for several days to remove any moisture in the bricks; this will prevent the bricks from cracking.
7. Heat should be distributed evenly throughout the kiln. This is done best by controlling the fires in the tunnels and not allowing one tunnel to become hotter than an other.
8. Either increase the kiln heat or hold it steady, but never allow it to decrease until the firing is complete.
9. To increase the fire and heat inside the kiln, use smaller and drier pieces of wood. Freshly cut logs and large logs make a cooler, slow burning fire.
10. Add wood to the fire several sticks at a time, allowing them to catch fire before adding more. Filling the fire tunnels with too many logs at once actually reduces the fire by reducing the air flow and suffocating the fire.
11. The heat from the fires should circulate through all the bricks as it rises and exits out the top. Heat that exits through the wall or kiln doors is wasted because it is not heating the upper bricks. Therefore fill wall cracks with mud and close or lower the kiln doors if too much heat is leaving.
12. Use metal sheets to control the air flow into each tunnel mouth. The advantage metal sheets over brick doors is that they can be easily raised, lowered, or removed. This allows better control of air flow, and therefore of heat.
13. Long poles are needed to push the logs deep into the fire. Nail a shovel or a hoe head to the pole end and wrap the end with tin to help protect it from the fire.
14. During the firing there will be a buildup of ash and burning coals. Use the shovel or a long handled hoe to even out the coals. If allowed to build up too high, the bed of coals will interfere with the entry of wood and air. When there are excessive ashes, pull them out of the kiln with the same tool.
15. The last day of firing, the soaking stage, is the most important because it is during this stage that the chemical changes occur, making the bricks strong and durable. The kiln must be carefully supervised at this time to see that the kiln holds its maximum heat for at least 12 hours.
16. The kiln heat can be measured by observing the colour of the bricks at the top of the tunnel arches. Stand at the side of the tunnel opening and stoop to the ground. Look upwards at the tunnel roof towards the kiln centre. At first these upper arch bricks will glow a dull red; then change to a reddish-orange; then orange-yellow; then yellow white. The objective is to try to reach the yellow-white colour and hold it for 12 to 24 hours.
17. Kiln fires must be tended day and night. One worker per tunnel is recommended. Workers should be selected for strength and coordination because they must be able to cast large heavy logs accurately into the fire. They should also have a strong sense of responsibility to come to work at the proper time and to be vigilant that the fires are not neglected or allowed to die down. The workers must understand that their work must continue at the kiln site until they are replaced by workers from the next shift. They must never leave the fires unattended for any reason.
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