ECE-302: BUILDING MATERIALS & CONSTRUCTION UNIT -1 LECTURE - 7: BRICKS: ECE-302: BUILDING MATERIALS & CONSTRUCTION UNIT -1 LECTURE - 7: BRICKS By NAURAS SAIYED Assistant Professor Civil Engineering Department This lecture covers the following:: This lecture covers the following: Bricks Manufacture of clay bricks Classification of clay bricks Properties of clay bricks Testing of clay bricks Problems of efflorescence and lime bursting in bricks and tiles. Bricks: Bricks Bricks – advantages over Stone: Bricks have greater fire resistance than stone or concrete masonry Its size enables easy handling and placement in walls It can be easily adapted to small-scale and large-scale structures to give pleasing appearance and texture Bricks: Bricks Constituents Brick clays are produced by blending together various clays (surface clays, shales , and fire clays) to produce the desired chemical composition and physical properties. Clays can be divided into calcareous clays containing 15% Calcium Carbonate which gives yellow color when burnt noncalcareous clays containing silicate of alumina, feldspar, and iron oxide Iron oxide gives buff, red or salmon color, when burnt Composition of good brick earth: Composition of good brick earth Alumina Chief constituent of all kinds of clay Imparts plasticity to clay so that it can be moulded If in excess, it makes the brick warp and crack on drying and burning Silica/sand It imparts uniform shape to bricks Its presence prevents cracking, shrinking and warping Its excess destroys cohesion between particles and makes the brick brittle Lime It should be present in very small quantities not exceeding 5% It helps in fusing sand(alone infusible) which then acts as cementing material for brick particles. It should be very finely powdered else on burning the lumps will convert to quicklime which expands in presence of moisture resulting in splitting of bricks. The excess of lime causes the brick to melt and loose its shape Oxide of iron It is added in small quantity – 5-6% It helps fuse sand Imparts red color on burning Magnesia A small amount of MgO imparts yellow color to the brick It decreases shrinkage Its excess leads to decay of brick Manufacture of clay bricks: Manufacture of clay bricks Preparation of brick clay Unsoiling , Digging , Cleaning, Weathering , Blending , Tempering Moulding the bricks Hand Moulding , Machine Moulding Drying the bricks Natural , Artificial Burning the bricks Burning in clamps, Burning in klins PowerPoint Presentation: SITE FOR OBTAINING CLAY/EARTH DIGGING Preparation of clay: Preparation of clay Unsoiling :- Top layer of 20cm depth is removed as it contains impurities. Digging: - Clay dug out from ground is spread on levelled ground ( just a little deeper than the general level) in about 60cm to 120cm heaps. Cleaning: -Stones, pebbles, vegetable matter, etc. are removed and lumps of clay are converted into powder form. Weathering:- Clay is exposed to atmosphere from few weeks to full season for softening and mellowing. (Preferably dug before monsoon) Blending:- Clay is made loose and any ingredient to be added to it is spread out at top and blended by turning it up and down in vertical direction. Tempering:- Clay is brought to a proper degree of hardness, then water is added to clay and whole mass is kneaded or pressed under the feet of men or cattle. For large scale, tempering is usually done in pug mill as shown in the figure. Pug mill used for tempering of clay: Pug mill used for tempering of clay Tempering/Kneading of clay with the help of cattle or men: Tempering/Kneading of clay with the help of cattle or men Process of tempering : Process of tempering Clay with water is placed in pug mill from the top. When the vertical staff is rotated by using electric pair, steam or diesel or turned by pair of bullocks. Clay is thoroughly mixed up by the actions of horizontal arms and knives when clay has been sufficiently pugged , hole at the bottom of tub, is opened out and the pugged earth is taken out from ramp for the next operation of moulding . Moulding: Moulding Clay, which is prepared from pug mill, is sent for the next operation of moulding . Following are the two ways of moulding . Hand Moulding Machine Moulding . Hand moulding: Hand moulding Moulds are rectangular boxes of wood or steel, which are open at top and bottom. Steel moulds are more durable and used for manufacturing bricks on large scale as shown in figure. Bricks prepared by hand moulding are of two types. a) Ground moulded bricks b) Table moulded bricks PowerPoint Presentation: (a) Ground moulded bricks: ground is first made level and fine sand is sprinkled over it. Mould is dipped in water and placed over the ground to fill the clay. Extra clay is removed by wooden or metal strike after the mould is filled forced mould is then lifted up and raw brick is left on the ground. Mould is then dipped in water every time lower faces of ground moulded bricks are rough and it is not possible to place frog on such bricks. Ground moulded bricks of better quality and with frogs on their surface are made by using a pair of pallet boards and a wooden block (b) Table- moulded bricks: Process of moulding these bricks is just similar to ground bricks on a table of size about 2m x 1m. PowerPoint Presentation: GROUND MOULDING DIFFERENT MOULDS Machine moulding: Machine moulding This method proves to be economical when bricks in huge quantity are to be manufactured at the same spot. It is also helpful for moulding hard and string clay. These machines are broadly classified in two categories (a) Plastic clay machines (b) Dry clay machines a) Plastic clay machines: This machine containing rectangular opening of size equal to length and width of a brick. Pugged clay is placed in the machine and as it comes out through the opening, it is cut into strips by wires fixed in frames, so there bricks are called wire cut bricks. b) Dry clay machines: In these machines, strong clay is first converted into powder form and then water is added to form a stiff plastic paste. Such paste is placed in mould and pressed by machine to form hard and well shaped bricks. They carry distinct frogs and exhibit uniform texture. Drying: Drying The damp bricks, if burnt, are likely to be cracked and distorted. Hence moulded bricks are dried before they are taken for the next operation of burning. Bricks are laid along and across the stock in alternate layers. The drying of brick is by the following means ( i ) Artificial drying – drying by tunnels usually 1200C about 1 to 3 days (ii) Circulation of air - Stacks are arranged in such a way that sufficient air space is left between them free circulation of air. (iii)Drying yard- special yards should be prepared slightly higher level prevent the accumulation of rain water (iv) Period for drying – usually about 3 to 10 days for bricks to become dry (v) Screens – screens are necessary, may be provided to avoid direct exposure to wind or sun. PowerPoint Presentation: Newly-formed bricks are dried under shelters in a natural way. When the bricks are almost dry, they are set vertically in order to make room for the next batch of bricks. Burning: Burning This is very important operation in the manufacturing of bricks to impart hardness, strength and make them dense and durable. Heating clay upto 640 degree C produces physical changes. If such clay is cooled back, it absorbs moisture from air and gets hydrated back to its original state. Such poorly burnt clay is unstable However, if clay is heated up to 700-1000 deg C chemical changes take place by which alumina and silica in clay fuse together resulting in a compound which is strong and stable. Burning of bricks is done either in clamps or in kilns. Clamps are temporary structures and they are adopted to manufacture bricks on small scale. Kilns are permanent structures and they are adopted to manufacture bricks on a large scale. PowerPoint Presentation: A typical clamp is as shown in figure Kilns: Kilns A kiln is a large oven, which is used to burn bricks by 1) Intermittent kilns 2) Continuous kilns Intermittent kilns: These are intermittent in operation, which means that they are loaded, fired, cooled and unloaded. a) Intermittent up-draught kilns b) Intermittent down-draught kilns Continuous kilns These kilns are continuous in operations. This means that loading, firing, cooling and unloading are carried out simultaneously in these kilns. There are three types of continuous kilns. a) Bull’s trench kiln b) Hoffman’s kiln c) Tunnel kiln PowerPoint Presentation: Intermittent kiln PowerPoint Presentation: Bulls Trench Kiln – Invented in England 1876 Commonly used in India, Bull Trench kiln has a permanent brick chimney over 30 meters high. The chimney requires skilled labors to construct and is costly to build. The kiln can only be operated in continuous mode. It has no roof and can only be used outside the monsoon season. PowerPoint Presentation: Bulls trench kiln seen from the loading end Design of a Bulls trench kiln Fired bricks being drawn from the kiln A canvas prevents air from entering the kiln from the wrong end A Bulls trench kiln with a fixed chimney BULL'S TRENCH KILN PowerPoint Presentation: HOFFMAN’S KILN PowerPoint Presentation: COMPARISON – CLAMP BURNING Vs KILN BURNING PowerPoint Presentation: Firing Process PowerPoint Presentation: BTK - Bull’s Trench Kiln VSBK – Vertical Shaft Brick Kiln PowerPoint Presentation: Traditional indian clay brick kiln Vertical Shaft Brick Kiln Bull’s trench brick kiln brick clamp Dimensions of bricks: Dimensions of bricks Prescribed size of bricks as per Indian Standards. Nominal size of bricks is the size including the thickness of mortar in brickwork Depression made on top of brick is known as frog. Size Ordinary brick (in cm) Tile brick (in cm) Actual 19 x 9 x 9 19 x 9 x 4 Nominal 20 x 10 x10 20 x 10 x 4 PowerPoint Presentation: Occasionally the brick is cut into various shapes to fill in spaces at corners and other locations where a full brick does not fit. Classification of clay bricks: Classification of clay bricks Bricks can broadly be divided into two categories. ( i ) Unburnt or sundried bricks (ii) Burnt bricks ( i ) Un burnt or Sun dried bricks- UN burnt or sun dried with the help of heat received from sun after the process of moulding . These bricks can only be used in the constructions of temporary and cheap structures. Such bricks should not be used at places exposed to heavy rains. (ii) Burnt Bricks: The bricks used in construction works are burnt bricks and they are classified into the following four categories. PowerPoint Presentation: a. First Class bricks: These bricks are table moulded and of standard shape. The surface and edges of the bricks are sharp, square, smooth and straight. They comply all the qualities of good bricks and used for superior work of permanent nature. b. Second class bricks: These bricks are ground moulded and they are burnt in kilns. The surface of bricks is some what rough and shape is also slightly irregular. These bricks are commonly used at places where brick work is to be provided with a coat of plaster. c. Third class bricks: These bricks are ground moulded and they burnt in clamps. These bricks are not hard and they have rough surfaces with irregular and distorted edges. These bricks give dull sound when struck together. They are used for unimportant and temporary structures and at places where rainfall is not heavy. d. Fourth class bricks: These are over burnt bricks with irregular shape and dark colour . These bricks are used as aggregate for concrete in foundation, floors, roads, etc because of the fact that the over burnt bricks have compacted structure and hence, they are some times found stronger than even first class bricks. PROPERTIES OF BRICK: PROPERTIES OF BRICK Color Texture Porosity Fire Resistance size variation compressive strength absorption. PowerPoint Presentation: Color: The color of fired clay depends upon its chemical composition, the firing temperatures and the method of firing control. Of all the oxides commonly found in clays, iron probably has the greatest effect on color. Regardless of its natural color, clay containing iron in practically any form will exhibit a shade of red when exposed to an oxidizing fire because of the formation of ferrous oxide. When fired in a reducing atmosphere, the same clay will assume a dark (or black) hue. Creating a reducing atmosphere in the kiln is known as flashing or reduction firing. Given the same raw material and manufacturing method, darker colors are associated with higher firing temperatures, lower absorption values and higher compressive strength values. However, for products made from different raw materials, there is no direct relationship between strength and color or absorption and color. PowerPoint Presentation: Texture: Coatings and Glazes : Many brick have smooth or sand-finished textures produced by the dies or molds used in forming. A smooth texture, commonly referred to as a die skin results from pressure exerted by the steel die as the clay passes through it in the extrusion process. Most extruded brick have the die skin removed and the surface further treated to produce other textures using devices that cut, scratch, roll, brush or otherwise roughen the surface as the clay column leaves the die Brick may be tumbled before or after firing to achieve an antique appearance. PowerPoint Presentation: Porosity Porosity is an important characteristic of brick. In contrast to other moulded or pre-cast building materials, the porosity of brick is attributed to its fine capillaries. By virtue of the capillary effect, the rate of moisture transport in the brick is ten times faster than in other building materials. Moisture is released during day-time and re-absorbed during night-time. The ability to release and re-absorb moisture (a "breathing" process) by capillary effect is one of the most useful properties of brick that helps to regulate the temperature and humidity of atmosphere in a house. This distinctive property makes brick an admirable building material, particularly suitable for houses in the tropics. On the other hand, all porous materials are susceptible to chemical attacks and liable to contamination from weathering agents like rain, running water and polluted air. Porosity of building material is an important factor to consider in respect its performance and applications. Experiment results show that bricks with water absorption rate at 8% is 10 times more durable in resisting salt attack than that with water absorption rate at 20%. Well burnt brick has a normal water absorption rate less than 10% in contrast to that of concrete block and cement mortar exceeding 15%. This explains why brick walls require comparatively minimum maintenance in the course of time PowerPoint Presentation: Fire Resistance Brick is inherent with excellent fire resistance. the non-combustibility of brick helps to promote its use in building houses against fire. PowerPoint Presentation: Size Variation Because clays shrink during both drying and firing, allowances are made in the forming process to achieve the desired size of the finished brick. Both drying shrinkage and firing shrinkage vary for different clays, usually falling within the following ranges: Drying shrinkage: 2 to 4 percent Firing shrinkage: 2.5 to 4 percent Firing shrinkage increases with higher temperatures, which produce darker shades. When a wide range of colors is desired, some variation between the sizes of the dark and light units is inevitable. To obtain products of uniform size, manufacturers control factors contributing to shrinkage. Because of normal variations in raw materials and temperature variations within kilns, absolute uniformity is impossible. Consequently, specifications for brick allow size variations. PowerPoint Presentation: Compressive Strength and Absorption Both compressive strength and absorption are affected by properties of the clay, method of manufacture and degree of firing. For a given clay and method of manufacture, higher compressive strength values and lower absorption values are associated with higher firing temperatures. Although absorption and compressive strength can be controlled by manufacturing and firing methods, these properties depend largely upon the properties of the raw materials. TESTS FOR BRICKS: TESTS FOR BRICKS A brick is generally subject to following tests to find out its suitability for the construction work. ABSORPTION CRUSHING STRENGTH HARDNESS PRESENCE OF SOLUBLE SALTS SHAPE & SIZE SOUNDNESS STRUCTURE PowerPoint Presentation: ABSORPTION Water Absorption in Bricks is carried out by immersing it in water for 24 hrs. It is again weighed and the diff is in wt. indicates the amount of water absorbed by Bricks. It should not in any case exceed 20% of wt. of dry Bricks. CRUSHING STRENGTH : CRUSHING STRENGTH Crushing strength of a brick is found out by placing it in a compression testing machine. It is pressed till it breaks. The min. crushing strength of bricks is 55 kg/cm 2 . HARDNESS : HARDNESS In this test, a scratch is made on brick surface with the help of finger nail. If no impression is left on the surface, the brick is treated to be sufficiently hard. SHAPE & SIZE : SHAPE & SIZE In this test, a brick is closely inspected. It should be of standard size as per Indian standards. Its shape should be truly rectangular with sharp edges. PRESENCE OF SALTS : PRESENCE OF SALTS The soluble salts, if present in bricks will cause efflorescence in the surface of bricks. For finding out the presence of soluble salts in a brick, it is immersed in water for 24 hrs. it is then taken out and allowed to dry in shed. The absence of grey or white deposits in its surface indicates absence of soluble salts. If the white deposit cover about 10% surface, the efflorescence are said to be as moderate; when deposits are to be more than 50%, the efflorescence become heavy and it is treated as serious when such deposits are converted into powdery mass. SOUNDNESS : SOUNDNESS In this test, two bricks are taken and then struck together with each other, the bricks should not break and a clear ringing sound should be produced. STRUCTURE : STRUCTURE A brick is broken and its structure is examined. It should be homogeneous, compact and free from any defects Problems of efflorescence and lime bursting in bricks and tiles: Problems of efflorescence and lime bursting in bricks and tiles Efflorescence is a phenomenon that soluble slats dissolved in water are carried, deposited and gradually accumulated on brick surfaces to form an unsightly scum. The soluble salts may be originated from the raw material of bricks. But in most cases, efflorescence is caused by salts from the external sources such as ground water, contaminated atmosphere, mortar ingredients and other materials in contacts with the bricks. Three conditions must exist before efflorescence will occur. First: There must be water-soluble salts present somewhere in the wall. Second: There must be sufficient moisture in the wall to render the salts into a soluble solution. Third: There must be a path for the soluble salts to migrate through to the surface where the moisture can evaporate, thus depositing the salts which then crystallize and cause efflorescence. PowerPoint Presentation: If these three conditions can be controlled, there should be no efflorescing of masonry walls. Reduce all soluble alkali sulfates. Use good details to prevent water from entering the masonry. Use good construction practices to eliminate migratory paths for moisture. Lime Bursts/pops” A phenomenon that sometimes occurs with new bricks is lime pops. Lime pops are caused by small particles of lime that are located near the surface of the brick. When the brick absorbs moisture, the lime particles swell and cause portions of the brick face to "pop" off. The result are small pits in the surface of the brick with a white spot in the center. The white spot is the lime particle. It is strictly an aesthetic issue and will not affect the structural integrity of the brick. The picture below is an example of lime popping. Qualities of Good Brick: Qualities of Good Brick Bricks should be table moulded , well burnt in kilns, copper coloured , free from cracks and with sharp and square edges. Bricks should be uniform shape and should be of standard size. Bricks should give clear ringing sound when struck with each other. Bricks, when broken, should show a bright homogeneous and compact structure free from voids. Bricks should not absorb water more than 20 percent by weight for first class bricks and 22 percent by weight for second class bricks, when soaked in coldwater for a period of 24 hours. Bricks should be sufficiently hard no impression should be left on brick surface when it is scratched with finger nail. Bricks should have low thermal conductivity and they should be sound proof. Bricks should not break when dropped flat on hard ground from a height of about one meter. Bricks, when soaked in water for 24hours, should not show deposits of white salts when allowed to dry in shade. No brick should have crushing strength below 55kg/cm2 PowerPoint Presentation: THANKS Reference: Reference The following slides may be referred for further understanding of concepts. TYPES OF BRICKS: TYPES OF BRICKS Clay brick Sand lime Cement brick Perforated bricks Hollow bricks Paving bricks Pressed bricks PowerPoint Presentation: Advantages of bricks : Brick will not burn, buckle or melt. Brick will not rot and allow Termites to invade. Brick will not rust and corrode. Brick will not dent. Brick will not fade from the Sun's UV Rays. Brick will not be damaged by high winds, rain or hail. Brick will not require constant maintenance. Brick will not devalue. Brick will not limit your design options. Burning: Burning This is very important operation in the manufacturing of bricks to impart hardness, strength and make them dense and durable. Burning of bricks is done either in clamps or in kilns. Clamps are temporary structures and they are adopted to manufacture bricks on small scale. Kilns are permanent structures and they are adopted to manufacture bricks on a large scale. PowerPoint Presentation: A typical clamp is as shown in figure PowerPoint Presentation: A trapezoidal shape in plan with shorter end slightly in excavation and wider end raised at an angle of 15 degree from ground level. A brick wall with mud is constructed on the short end and a layer of 70cm to 80cm thick fuel (grass, cow dung, ground nuts, wood or coal) laid on the floor. A layer consists of 4 or 5 courses of raw bricks laid on edges with small spaces between them for circulation of air. A second layer of fuel is then placed, and over it another layer of raw bricks is put up. The total height of clamp in alternate layers of brick is about 3 to 4 m. When clamp is completely constructed, it is plastered with mud on sides and top and filled with earth to prevent the escape of heat. The period of burning is about one to two months and allowed the same time for cooling. Burnt bricks are taken out from the clamp. Clamp burning: Clamp burning Advantages ( i ) The bricks produced are tough and strong because burning and cooling are gradual (ii) Burning in clamps proves to be cheap and economical. (iii) No skilled labour and supervision are required for the construction of clamps. Disadvantages ( i ) Bricks are not of required shape (ii) It is very slow process (iii) It is not possible to regulate fire in a clamp (iv) Quality of brick is not uniform Kilns: Kilns A kiln is a large oven, which is used to burn bricks by 1) Intermittent kilns 2) Continuous kilns 1) Intermittent kilns: These are intermittent in operation, which means that they are loaded, fired, cooled and unloaded. a) Intermittent up-draught kilns b) Intermittent down-draught kilns Intermittent up-draught kiln: This is in the form of rectangular with thick outside walls as shown in the figure wide doors are provided at each end for loading and unloading of kilns. A temporary roof may be installed to protect from rain and it is removed after kiln is fired. Flues are provided to carry flames or hot gases through the body of kiln. PowerPoint Presentation: Intermittent kiln PowerPoint Presentation: ( i ) Raw bricks are laid in row of thickness equal to 2 to 3 bricks and height 6 to 8 bricks with 2 bricks spacing between rows. (ii) Fuels are filled with brush wood which takes up a free easily. (iii) Loading of kiln with raw bricks with top course is finished with flat bricks and other courses are formed by placing bricks on edges. (iv) Each door is built up with dry bricks and are covered with mud or clay. (v) The kiln is then fired for a period of 48 to 60 hours draught rises in the upward direction from bottom of kiln and brings about the burning of bricks. (vi) Kiln is allowed to cool down and bricks are then token out. (vi) Same procedure is repeated for the next burning Bricks manufactured by intermittent up drought kilns are better than those prepared by clamps but bricks burnt by this process is not uniform, supply of bricks is not continuous and wastage of fuel heat. Disadvantages: Quality of brick is not uniform Over burnt near top and under burnt near bottom Supply of brick is not continuous Wastage of fuel as the kiln has to be cooled down every time after burning PowerPoint Presentation: Intermittent down-draught kilns: These kilns are rectangular or circular in shape. They are provided with permanent walls and closed tight roof. Floor of the kiln has opening which are connected to a common chimney stack through flues. Working is same as up-draught kiln. But it is so arranged in this kiln that hot gases are carried through vertical flues upto the level of roof and they are then released. These hot gases move down ward by the chimney draught and in doing so, they burn the bricks. Advantages: ( i ) Bricks are evenly burnt (ii) Performance of this kiln is better than that of up-draught kiln (iii) This kiln is suitable for burning of structural clay tiles, terra cota because of close control of heat. PowerPoint Presentation: Continuous kilns These kilns are continuous in operations. This means that loading, firing, cooling and unloading are carried out simultaneously in these kilns. There are three types of continuous kilns. a) Bull’s trench kiln b) Hoffman’s kiln c) Tunnel kiln a) Bull’s trench kiln: This kiln may be of rectangular, circular or oval shape in the plan as shown in figure. It is constructed in a trench excavated in ground either fully under ground partially projecting above ground openings is provided in the outer walls to act as flue holes. Dampers are in the form of iron plates and they are used to divide the kilns in suitable sections and most widely used kiln in India. PowerPoint Presentation: The bricks are arranged in such a way that flues are formed. Fuel is placed in flues and it is ignited through flue holes after covering top surface with earth and ashes to prevent the escape of heat usually two movable iron chimneys are employed to form draught. These chimneys are placed in advance of section being fired. Hence, hot gases leaving the chimney warm up the bricks in next section. Each section requires about one day to burn. The tentative arrangement for different sections may be as follows Section 1 – loading Section 2 – empty Section 3 – unloading Section 4 – cooling Section 5 – Burning Section 6 – Heating b) Hoffman’s kiln: This kiln is constructed over ground and hence, it is sometimes known as flame kiln. Its shape is circular to plan and it is divided into a number of compartments or chambers. A permanent roof is provided; the kiln can even function during rainy season. The following fugure shows plan and section of Hoffman’s kiln with 12 chambers PowerPoint Presentation: Chamber 1 - loading Chamber 2 to 5 – drying and pre-heating Chambers 6 and 7 - burning Chambers 8 to 11 - cooling Chamber 12 – unloading The initial cost of installing this kiln is high, the following advantages ( i ) Good quality of bricks are produced (ii) It is possible to regulate heat inside the chambers through fuel holes. (iii) Supply of bricks is continuous and regular (iv) There is considerable saving in fuel due to pre heating of raw bricks by flue gases. c) Tunnel kiln: This type of kiln is in the form of tunnel, which may be straight, circular or oval in the plan. Raw bricks are placed in trolleys which are then moved from one end too the other end of tunnel. Raw bricks get dried and pre-heated as they approach zone of fire. In zone of fire, bricks are burnt to the required degree and they are then pushed forward for cooling. When bricks are sufficiently cooled, they are unloaded. The kiln proves to be economical when the bricks are manufactured on a large scale. As temperature is under control, uniform bricks of better quality are produced. PowerPoint Presentation: COMPARISON – CLAMP BURNING Vs KILN BURNING Brick making explained in pictures: Brick making explained in pictures PowerPoint Presentation: To make the bricks, earth in situ is mixed with water (as seen above) allowed to sit, then put into molds (top right) and laid out for sun drying (right). They are then stacked and covered for baking as described in the next slide . PowerPoint Presentation: The bricks are then unearthed and stacked for loading onto trucks or wagons. There are tall smoke stacks (seen in figure on left), each in the center of a mound of sun dried mud bricks (shown in figure below) which are covered with fine ash and sand prior to baking. At one end of this 7 ft high mound of area 120x50 sq ft are holes for crushed charcoal which burns and produces heat to bake bricks.