African Brick Making Project: Part 2 – The Manual

Newly formed bricks drying on the outskirts of Morogoro

 

 

 The second part of this project is a guide for understanding the current practices used to make bricks in Morogoro, and how these practices affect the built and natural environments. The process is a long one, from the initial extraction of the soil and clay, to the forming, drying, building of the kiln, and finally firing of the bricks. Some of these kilns are made to fire over a hundred thousand bricks at once, a whole seasons worth of work. The length of the process proved a challenge to trying to document every stage in a limited time frame. My translator and guide Andrew Mjawa, was my champion in this regard. He’d grown up a brick maker, and then studied English in hopes of becoming a trekking guide for tourist excursions into the Uluguru Mountains. Having a guide who knew the process as well as the brick makers was invaluable, especially for the firing, which happens very quickly. We took many wild taxi and bicycle rides in chase of rumors some camp was about to light a kiln.  And so now to it….

Extraction and preparation of the soil

Among the major limiting factors in determining an area’s suitability to brick production is the soil conditions of that geographical region. According to a report prepared by S.A. Hathout in 1973 which maps various soil conditions throughout Tanzania, the deposits around Morogoro consist of “sandy clay with excessive drainage.” Observations made at three different clay extraction sites reveal a consistent striation of the two soil types. The top layer consists of a reddish sandy soil with a layer of black clay beneath. This black clay layer was observed at depths ranging between one and two meters.

Both of these layers are required for the proper consistency. For this reason the workers responsible for the initial extraction of the soil either scrape vertically across the two layers to create a workable pile of semi-mixed soil with a relative consistency of one part sand to one part sand, or they pull down each soil type separately and work the mixture with the hoe until a rough mix of the proper consistency is achieved.

Although this rough mixture of one part sand to one part clay was consistent across the three observed sites in Morogoro, the initial determination of the proper ratio is made in small scale production by several simple on-site tests. The first of these is to test the black clay striation for plasticity. This may be done simply by extracting a small amount of the material and adding water until the sample can be molded into various shapes with the hands. If the soil will not hold a shape, or consistently breaks apart, the sample contains too much sand or silt. If the sample is easily molded and retains its shape then the plasticity is suitable.

The second on-site test determines the proper amount of sand which must be added to achieve a suitable mix. This test consists of rolling a series of small balls composed of a variety of mixtures, clay to sand. These balls- about the size of a small egg-  are left out in the sun to dry. The balls which retain their shape and become hard, but crack apart, do not contain enough sand in the mix.  The balls which crumble easily in the hand after drying contain too much sand or silt. The balls which retain their shape, become hard after drying, and contain no cracks are composed of the proper mixture.

As the critical factor in the proper mix is the ratio of sand to soil, the amount of sand or silt already embedded in the black clay striation will determine the amount which must be added, if any additional sand or silt is required. The consistency of the clay layer is solely a factor of the geological soil conditions of the region. For this reason it must not be assumed that the one to one ratio of sand to clay observed across the various sites in Morogoro determines this as a suitable ratio for other geographical regions containing soil with clay content. It must be assumed, rather, that each region with soil suitable for making bricks will require a slightly different ratio, and therefore independent testing.

Brick maker extracting and mixing soil

Post Extraction Concerns

After the material is extracted or dug from the exposed cross-section of  soil and roughly mixed in a pile, another worker will haul the material up to a wide open level area where the forms of the bricks may be made. This part of the process was done at each venue by a team of two. The first team member more thoroughly mixes the soil, adding water from a plastic bucket while also using a hoe to work the soil together. This is a lengthy process depending where the soil is derived.

From the two brick production sites in and directly outside of Morogro on the road to Dar es Salaam, the mixing of the soil carried up from the extraction site took about a half an hour. At the brick making site in Keebwa the preparation did not take as long as the material was brought up from the banks of a nearby stream flowing down from the Uluguru mountains and was already fairly saturated. Also important in the process of mixing both at the initial extraction point and where the material was heaped for brick forming is the removal of stones and other lumpy material.

The soil which has been properly mixed will be stiff without being too damp. According to the brick makers in Keebwa, soil which is too wet will not make good bricks while soil which does not contain enough water will be difficult to mold and thus slow the process of making.

One of the main issues from an environmental and sustainable standpoint is the post extraction practices.  In Keebwa, which is almost exclusively an agricultural community, they are unable to use land for planting after the layer of soil suitable for brick making is removed. As the landscape in Keebwa is rugged and mountainous, soil for brick making is sometimes extracted from the area leveled for the building itself, essentially in these cases, the building material came from the ground beneath it. In these communities, it is generally the farmer, or villager, who makes his own bricks for his own house. Transportation of building materials is almost is very difficult in these areas. The village of Keebwa for instance is a half an hour hike from the nearest road.

This is not the case, however, in the areas down the mountain in Morogoro where brick burning is a commercial enterprise. The effect of the extraction on these lower areas is the creation of large wet and swampy areas.  At the brick making site along the Dar es Salaam Road, these areas were scattered all over the fields.

Low Swampy areas left behind after Brick Making Activities can lead to the proliferation of Waterborne diseases

The site closer to town was similarly affected. Here, however, the area was planted with rice, the land at that location being suitable for rice planting both before and after the extraction process.  The harvested rice was used by the workers for making Ugali and the husks were used for firing small scale kilns using the slow burning technique.

The key issue highlighted in the Environmental Impact Assessment prepared by SUMO and the Morogoro Municipality, however, linked these wet areas to the proliferation of waterborne diseases in areas of the Municipality.

Brick Forming

After the soil is properly watered and mixed, the material is set into a form which determines the dimensions of the brick. There are several types of forms, and several methods used to achieve the bricks shape prior to firing. The type of form observed in Morogoro was a simple wooden frame.

Before beginning the process of placing the material into this form, the form is soaked in water. In the brickmaking sites in Morogoro the form was kept in a large plastic bucket until ready for use. Although the sand and soil were already watered and mixed prior to the forming process, additional water was added with a few splashes each time a new brick was made, the water coming from the large plastic bucket. In Keebwa, the wooden mold was kept in the bed of the stream from which the clay and silt was extracted, remaining there until after the mixing process was completed.

After the wooden form was removed from the water it is placed on level ground near the mound of mixed soil. There were two different techniques observed. At the sites in Morogoro the form was placed on a small wooden pallet of the same size. This allowed the newly formed brick to be moved and placed elsewhere to dry after the forming process was complete. By using this method the brickmaker lessens the distance between the mound of soil and the place where the brick is formed. In Keebwa, however, the brickmakers did not use the pallet.

After the form is placed on level ground, additional water is splashed or wiped on the inside of the form with a rag to ensure a good release. In Keebwa the soil was placed into the mold with a shovel. Each of the two cells in the mold takes about a shovel full and a half, requiring three trips from the soil mound to the forming site.

Forming bricks at a site in Morogoro


In Morogoro, where the brickmakers used the wooden pallet, the brickmaker works directly with the soil mound, pulling the material from it with his hands and throwing it in, working the material into the corners, and packing it down with several slaps before the final scraping and smoothing over with the palm of the hand.

After the brick is packed and smoothed the form is picked up by the wooden pallet set beneath it and taken over to where the bricks are drying, flipped over and placed in a row. Afterward the brickmaker returns to the mixed soil mound with the form and the pallet and starts the process over again from the beginning.

At the Morogoro site, one man working alone was able to form 400 bricks a day using this method.

Villagers in Keebwa forming bricks near the river bank.

Brick maker forming bricks at a commercial operation in Morogoro

Brick Drying

After the bricks are formed they must be left to dry for a period of at least three days. The exact time is determined largely by whether the atmospheric conditions favor drying. Bricks are typically laid out in rows on a level surface for the initial drying. Drying bricks on surfaces which are uneven can contribute to cracking either during the drying process or during the firing. After several days the bricks are turned on their side and then finally stacked in a loose arrangement along the edge of the level areas. This stacking serves two purposes. First, it allows a more even drying of the brick, especially that part which was in contact with the ground and it also clears the flat areas for more freshly made bricks to be laid for drying.

 

 

Stacking and turning bricks for drying

 

Bricks stacked for drying

Firing

At the brick making sites in Morogoro two different firing techniques and kiln types were observed, both operating at the small scale, but of differing capacities. The smaller capacity, of these two types handles burning at the scale of about six hundred to two thousand bricks, six hundred incidentally being the average amount two workers produced in a day at the Morogoro sites. The large capacity kiln and firing type observed handles upwards of 300,000 bricks at a time and can take nearly half a year to prepare for firing.

The smaller capacity kilns are structured and engineered much the way smoke kilns are built and fired in the realm of smoke fired pottery. The basic principle is hinged on properties of the available fuel source. Typical smoke kilns use material such as sawdust, or pine needles; materials which will burn slow and hot. In the case of the small scale kilns used to burn brick in Morogoro the material used is rice husks, a renewable fuel source typically discarded if not used for fuel.

Small Scale Rice Husk Fired Kiln

Small Kiln Construction

The construction of the kiln is made on a level section of ground typically built up on a plinth to limit exposure to wet conditions if it rains. On the subject of rain, these kilns are generally fired during the dry season to limit exposure to rain, as rain can ruin a kiln especially during the firing process. Once a proper foundation has been prepared two courses of unburned bricks are stacked loosely in long rows, leaving one or two centimeters of space between adjacent bricks to facilitate an adequate circulation of air and therefore draft during the firing.  Around these two rows at a distance of approximately 15 centimeters, a perimeter wall is constructed using either under fired or broken bricks. This wall is used primarily to hold and provide adequate ventilation for the fuel source. For this reason the perimeter wall, much like the two unburned courses in the interior, are stacked loosely.

Rice Husks

The basic structure of the kiln, after assembled, consists of two rows of unburned bricks with approximately 15 centimeters in between and a perimeter of under fired and broken bricks occurring at a similar distance. The rice husks are poured within this void, filling the kiln just past the brim of the perimeter wall in a convex mound rising towards the center.

This mound is lit from the top and burns down slow and steadily with additional fuel added at various points in the process, after layers of ash are scraped away. The ash layer created on top by the initial burning, and re-burning functions as an insulator to retain the heat of the inner unburned husks towards the interior, these being ventilated by the loose arrangement of the perimeter wall.

This firing process takes about six days in total, three days to reach the temperature required for adequate burning, between approximately 950 degrees centigrade and 1,150 degrees centigrade according to the pamphlet “Better Burnt Bricks” prepared by T. I. Svare and published by the National Housing and Building Research Agency, and three days to cool. If the bricks are removed without proper cooling the bricks are likely to crack from the temperature differential caused by the exposure of the outer brick to the air and the more insulated inner brick.

The entire timeline of the process of making and firing bricks using this method is as follows: 1 day to form the 600 unburned bricks, 4 days to dry, three days to fire, 3 days to cool, for a total of 11 days from initial extraction to final usable product.

Small Scale Rice Husk Kiln Firing

Large Scale Kilns

The main difference between the small and large capacity kilns relative the way they function is the fuel source. While the rice husks are suitable for the small scale it does not generate enough heat in the manner required to be used in kilns of larger capacities. The larger capacity field kilns in the most basic sense consist of a large stack of individual bricks spaced well apart to allow for ventilation and even distribution of heat. The base of this stack is constructed with an initial layer of fired brick to ensure a solid and even base that will not deform and settle during the firing, due to either heat or at worse case water intrusion.

Large scale wood fired kiln of over 300,000 bricks

These stacks occur at various sizes, anywhere from between 10,000 to 300,000 thousand bricks. As the capacity increases the kiln size increases in the lateral dimension as opposed to in height. The height of these kilns maxes out at around five to six meters. From there, if more capacity is required, the volume will be added to the length of the kiln with additional burning ports.

Burner ports in a wood fired kiln

The burning ports, run in only one direction forming a tunnel which runs all the way through the base of the kiln. For this reason they are constructed as a small archway, each brick on the interior of the tunnel staggered inward until a single unburned brick spans the top. The basic dimension of the tunnel should correspond this with length of about two bricks and the height of between five and seven courses. These specifications where described in the phamplet by T.I. Svare as well as observed as approximate values in the field.

Stacking the Bricks

The stacking of these kilns is accomplished simultaneous with the process of forming, especially in the case of the larger kilns. Some may take as many as five months to prepare before firing. As the stacking process becomes well underway, the brickmakers will begin to climb the sides of the kiln, standing at various points to toss individual bricks to a worker on the top who will place them.

Building a Kiln

After this process is complete a layer of mud-plaster is added all along the outside at a thickness of approximately 15cm to 20cm in thickness. This layer serves as the outer skin for the kiln and helps retain the heat to ensure the bricks on the outer layers will mature in the burning process. The mud plaster is prepared in a small plastic bucket and thrown onto the kiln in handfuls. The upper portions are reached in some cases with the use of a makeshift ladder, but more commonly all the plaster is applied by throwing these handfuls from the ground level.

 

 

In several kilns observed within the Morogoro Municipality a layer of broken burned bricks was stacked loosely around the lower portion of the kiln for the retention of heat. When this method was applied, a thinner plastering still occurred on the face of these bricks, but was focused more particularly in the cracks as opposed to a uniform covering.

Fuel

As previously mentioned the fuel source for this type of firing differs from the smaller scale kilns. Rice husks are not a sufficient fuel source for this firing method. Here the most efficient fuel is dry wooden logs cut to approximately one meter in length. This, of course, raises the issue of sustainability from the stand point of harvesting and has become an issue of contention between the brickmakers and local agencies who are sponsored by governmental and non-governmental organizations mandated to curb deforestation in the area.

In order to find sufficient fuel for the firing they must either harvest the wood from a private source, or they must apply for permits from the local authority to harvest from publicly managed land. In Keebwa, which is in close proximity to the forest reserve, and therefore closely regulated, the resident brickmakers use refuse and biomass from their agricultural activities as the fuel source to avoid dealing with the governmental agencies.

Wood fuel stacked and readied for a firing

Logs placed in the burning ports prior to firing

 Ventilation

Once the wood has been harvested and properly prepared, it is loaded into the tunnels so that the tunnel is nearly half full. The material is then lit from the leeward side so the flame moves through the tunnel and thus creates a chimney draft effect which thereby pulls the flame upward into the upper courses of the stack. This chimney effect is fed by air drawn from the large ends of the tunnel where the wood logs are inserted.

In several kilns observed around the Morogoro Municipality, additional vents were added on the side of the kiln perpendicular to the firewood tunnels through a specific method of stacking the blocks in the first course above the platform of previously fired brick.

 

The firing from this stage onward must be constantly monitored morning and night as additional fuel must be added when the logs burn down.

The process of firing these kilns does not take as long as the process to fire the smoke kilns as the required temperature is achieved more rapidly. This is true, with the exception of the extremely large kilns of 300,000 bricks which may take up to four days to fire. The typical kiln of around 20,000 to 100,000 bricks takes approximately one and a half to two days to reach temperature.

video adding fuel

 

Cooling Down

After reaching temperature the kiln is sealed and will continue to burn for another day, cooling very slowly over another two. The cooling of the kiln is critical to the process of brickmaking. If the brick is cooled too rapidly, it is prone to cracking and deformation. To slow the cooling process, the firewood tunnels are sealed with previously burned bricks and plastered over after temperature is reached. After up to three days, the plaster is scraped from the sides and the kiln is disassembled. The bricks are then sorted according to quality, the better quality bricks coming from the inside middle of the kiln where the highest temperatures were reached.

 




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