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CLOSE THIS BOOKSmall-Scale Brickmaking (ILO - WEP, 1984, 228 p.)
VIEW THE DOCUMENTI. Objectives of firing
VIEW THE DOCUMENTII. Techniques of firing
VIEW THE DOCUMENTIV. Auxiliary equipment

Small-Scale Brickmaking (ILO - WEP, 1984, 228 p.)


I. Objectives of firing

The firing of green bricks changes their physical structure and gives them good mechanical properties and resistance to slaking by water. If carried out properly, the firing process should minimise the occurrence of the following problems:

- the splitting of bricks due to the incomplete removal of moisture before firing;

- Low strength bricks due to insufficiently hard firing;

- Slaking by water, due to inadequate control of the firing temperature;

- Bricks fused together, melted on one face, or distorted by the load imposed by other bricks on top; these problems are caused by too high a temperature;

- Variety of sizes of fired bricks although the green bricks were of the same size; this is caused by temperature variations between different parts of the kiln;

- Fine cracking over brick surfaces resulting from a too rapid temperature change, either during heating or cooling, or from condensation of water vapour from heated bricks on to cooler bricks(47);

- Local cracking over hard lumps or stones mixed in the clay. These inclusions should have been removed during clay preparation, although rapid changes of temperature may have aggravated the problem;

- Black cores in bricks: these are not necessarily detrimental if they are due to the presence of carbon although the latter ought to have been burnt up as a contributory fuel in the interest of cost reduction (48). Black cores may also be due to iron in the reduced, low valency ferrous form. Both causes may be remedied by providing sufficient oxygen (e.g. by leaving adequate paths for the flow of air between the bricks and through the kiln(47)). An opening material may also need to be mixed in the body;

- Bloating: cracked blisters appear on the surface of the bricks as a result of the pressure of gases produced after vitrification has commenced; holding the temperature steady at an earlier stage could allow the gases to diffuse out while the body is still permeable. The problem may also be alleviated by incorporating an opening material in the clay;

- Limebursting: this problem may be solved by removal or grinding of pieces of limestone, or in some cases, by adding salt(49). Alternatively, the quicklime formed within the brick may be dead burnt by heating to approximately 1,100 °C (50);

- Efflorescence and sulphate attack of cement-based mortars and renderings: this problem may be reduced by harder firing, as an alternative to other methods and precautions (see Chapter IV);

- Scum on brick surfaces may be minimised by preventing condensation of products of combustion on cold, green bricks.

II. Techniques of firing

Whatever system of firing is used, it is recommended that a low heat be first applied to the green bricks in order to drive off any residual moisture. This should continue until no more steam is evolved. This part of the heating is known as water-smoking. The completion of this firing stage may be simply tested by inserting a cold iron bar into a space purposely left between the bricks in the kiln, and by withdrawing it after only a few seconds. Condensation on the bar indicates that steam is still being evolved, and low heat should be continued until no condensation is found on the cold bar after re-insertion. This could take a whole week in some cases and needs plenty of air through the kiln.

Once water-smoking is complete, a rate of rise of temperature of 50 °C per hour may be safe in fully - controlled kilns, which are outside the scope of this memorandum. At the critical temperatures (e.g. quartz inversion) the rate can be temporarily reduced to avoid problems. In more simple kilns, the heating rate should be slower partly because of the lack of precise temperature control and partly because of the impossibility of getting enough fuel burning in some kiln designs. Although a slow rate of heating is safer, faster rates involve less heating time, lower heat losses and, therefore, lower costs. The optimal heating rate is that which requires the shortest heating schedule while yielding a product of satisfactory quality. A maximum of two weeks may be needed for the whole firing process.

Maximum temperatures with little air should be held for at least several hours. A whole day may be needed with some kilns with poor heat distribution in order to ensure a maximum yield of good quality bricks. During this firing stage, known as the “soaking” stage, the heat diffuses through the kiln, various chemical reactions take place and a glassy material is formed. Once the soaking is complete, the heat source may be removed.

The cooling rate should not be too rapid. In practice, natural cooling within the large mass of thousands of bricks, with limited air flow, is satisfactory. Cooling may take a whole week. More air may be allowed in once lower temperatures are reached in order to speed up cooling.

III. Kiln designs

In general, large kilns are more economical on the use of fuel than small kilns as less heat is lost through the proportionally smaller outside area of the kiln. Thus, separate teams of brickmakers may use the same large kiln on a co-operative basis, and thus benefit from lower firing costs.

There is a wide variety of kiln types and sizes. These may be split into two major groups: the intermittent kilns and the continuous kilns.

Intermittent kilns are filled with green bricks which are first heated up to the maximum temperature and then cooled before they are drawn out from the kiln. Thus, the kiln structure is also heated during the process.

Consequently, all the heat within the bricks and kiln is lost into the atmosphere during cooling. Intermittent kilns are very adaptable to changing market demands, but are not the most fuel efficient. They include the clamp, scove, scotch and downdraught kilns.

The continuous kilns have fires alight in some part of them all the time. Fired bricks are continuously removed and replaced by green bricks in another part of the kiln which is then heated. Consequently, the rate of output is approximately constant. The continuous group of kilns includes various versions of the Hoffmann kiln, including the Bull’s trench, zig-zag and high draft kilns, and the tunnel kiln. The latter is a capital-intensive, large-scale continuous kiln, which is outside the scope of this memorandum.1 Continuous kilns utilise heat from the cooling bricks to pre-heat green bricks and combustion air, or to dry bricks before they are put into the kiln. Consequently, continuous kilns are economical in the use of fuel.

1 In the tunnel kiln, bricks stacked on heat-resistant trolleys or cars are subjected to increasingly hotter temperatures, then cool off before leaving the tunnel.

III.1 The clamp

The clamp is the most basic type of kiln since no permanent kiln structure is built. It consists essentially of a pile of green bricks interspersed with combustible material. Normally, the clay from which the bricks are moulded also includes fuel material. The clamp kilns were commonly used in the United Kingdom. Some of these, containing one and a quarter million bricks, are still used for the production of bricks of various colours and textures.

It is possible to use a variety of burnable waste materials in brick clays (e.g. sifted rubbish, small particles of coke, coal dust with ashes, breeze). In countries where timber is produced, large quantities of sawdust may be mixed with clay before firing. This will reduce the expenditure on the main fuel for burning the bricks. Waste materials should be of relatively small size and should not exceed in weight 5 to 10 per cent of the total mixture. Otherwise, the clay will become difficult to mould or the finished product may become too weak or too porous. Furthermore, the added fuel material should be thoroughly mixed with the clay.

A flat, dry area of land is first chosen, and a checkerwork pattern of spaced out, already burnt bricks laid down over an area of approximately 15 m by 12 m. Fuel in the form of coke, breeze or small coal2 is then spread between the checkerwork bricks, covering the latter with a layer at least 20 cm thick. Dry, green bricks are next closed-laid on edge upon this fuel bed.

2 Coke or breeze is generally used in clamp kilns in the United Kingdom. Small coal is used in a number of developing countries, such as Zambia (51).

A clamp is generally made up of approximately 28 layers of bricks. Its sides are sloped for stability (see figures VII.1 and VII.2). Three or four holes1 at the base of one of the clamp walls are formed in order to allow the initial ignition of the fuel bed. Two courses of already fired bricks are next laid on top of the green bricks for insulation purposes. Fired bricks are also laid against the sloping sides of the clamp as it progresses. Sometimes, a second thin bed of fuel is laid at a higher level in the clamp(51).

1 These holes are known as the “eyes” of the clamp.

Once several metres of the length of the clamp have been built up, the fuel bed may be ignited with wood stuffed into the eyes of the clamp. The latter are bricked up with loosely placed burnt bricks once the fuel bed is alight. As the fire advances, more green bricks are built into the clamp.

During burning, the heat rises through the bricks above, fumes and sometimes smoke leaving the top of the clamp. The rate of burning is not easily controlled and depends upon several factors, including wind strength and direction. Some wind protection with screens can help control the temperature. Ventilation, and hence burning rate, can also be controlled, to some extent by an adjustment of the burnt brick covering the top of the clamp. For example, these bricks may be spaced out or removed in order to speed up the firing of the bricks in a given area. Conversely, they may be tightened up or covered with ashes in order to slow down the burning rate. It is desirable to have the fire advancing with a straight front at a steady rate. Bricks close to the edges of the clamp will tend to be underfired as a result of higher heat losses. This may be partially rectified by placing a little more fuel near the edges of the clamp. Extra fuel may also be spread between the top bricks during firing.

The firing process is indicated by the sinking of the top of the clamp. Under the right circumstances, the latter will settle evenly. Once the fire has passed through a particular point, the bricks start to cool. They may then be withdrawn, sorted into various grades, and sold. Thus, bricks within one clamp are set and drawn simultaneously. After a number of weeks, the fire reaches the end of the clamp. Before then, construction and lighting of a new clamp may be started as previously, if market demand requires it.

Figure VII.1 - Clamp kiln - schematic drawing

Figure VII.2 - Clamp kiln (United Kingdom)

If enough air flows through the bricks during firing, the oxidising process will give them a red colour. Where air is scarce, reducing conditions due to the gases from the burnt fuel will yield orange or yellow bricks, especially if a limy clay is used for moulding. Variations in colours will be normal even on a single brick face.

As the fuel is in close contact with the green bricks, the fuel efficiency of a large clamp of 100,000 to 1 million bricks can be fairly high (e.g. about 7,000 MJ per 1,000 bricks). Smaller clamps will be less efficient, as a result of the greater proportion of outer cooling area for a given volume. However, they may be operated successfully with only 10,000 bricks. For low production rates, it is only necessary to fire a clamp occasionally and have the bricks in place until they are sold.

The bricks near the centre of the clamp will be the hardest. Others should be sufficiently good for many uses. They should be sorted for sale as best, “seconds” and soft-burnt bricks. However, 20 per cent of the bricks may still not be saleable. Fortunately, many of these rejects can be put into the next clamp for refiring, or used in the clamp base, sides or top.

III.2 The scove kiln

A widely used adaptation of the clamp is the scove kiln, also mistakenly called a clamp. If the fuel available is of a type which cannot be spread as a thin bed at the base of the kiln and/or is not in sufficient quantity to burn all the bricks without the need for replenishment, tunnels can be built through the base of the pile in order to feed additional fuel (figure VII.3). This is a suitable method of burning wood, the latter being one of the most frequently used fuels for small-scale brickmaking in developing countries. Usually, the outer surface of the piled-up bricks is scoved, that is to say plastered all over its sides, with mud (figure VII.4). Thus, the name of the scove kiln.

The construction of a scove requires a level, dry area of land. Previously fired bricks, if available, lay bed face down to form a good, flat surface. Three or four layers of bricks are used to form the bottom of the tunnels. The width of each tunnel is approximately equal to that of two brick lengths. Three lengths of bricks separate the tunnels. Alternate courses are laid at right angles to each other (i.e. a course of headers, followed by a course of stretchers). Two short tunnels (e.g. approximately 2 m long) may be sufficient for a small number of bricks. For large numbers of bricks, tunnels cannot be longer than approximately 6 m. Otherwise, fuel inserted from both ends will not reach the centre of the tunnel. Large numbers of bricks are dealt with by extending the number of tunnels to cope with the requirement. Figure VII.5 illustrates the construction of a four-tunnel scove.

Figure VII.3 - Scove (Madagascar)

Figure VII.4 - Scoving face of kiln (Madagascar)

The fourth and successive courses of bricks are laid in such a way that rows of brickwork finally meet, and tunnels are thus completed. The progression of the early stages of construction of a scove is shown in figure VII.6. In the foreground, a few courses of fired bricks are set, marking out the tunnel positions. In the middle of the picture, the first corbelled-out course of green bricks is partly set, while further back several courses are laid.

Green bricks are set above tunnel level, in alternate courses of headers and stretchers up to a height of at least 3 m above the ground. At the edge of the scove, each course is stepped in a centimetre or so, to give a sloping side. Small spaces are left between the bricks to allow the hot gases from the fires to rise. The required maximum spacing between bricks is a ‘finger width’. This is easy to achieve although a narrower spacing may be satisfactory. As the scove is built up, an outer layer of previously burnt bricks is laid, to provide insulation. This will also allow the proper firing of the outer layers of green bricks.

On the top of the green bricks, two or three courses of previously fired bricks should be laid, bed face down and closely packed. The whole structure should then be scoved with wet mud to seal air gaps. Turves are sometimes laid on top to reduce heat losses. The wet mud should not contain a high fraction of clay if cracks are to be avoided during firing.

Some of the top bricks half-way between the tunnels must not be scoved so that they may be lifted out to increase air flow through the kiln as required. The provision of this adjustable ventilation can be most useful in controlling the rate of burning.

Firewood is set into the tunnels (figure VII.7) for firing. It should preferably be at least 10 cm across, in pieces about 1 m in length. Kindling should be set in the mouth and bottom of the tunnel. Since the heat of the fire is to rise up into the bricks, it is essential that strong winds do not blow through the tunnels, cooling bricks down, and wasting heat. Such winds may increase fuel consumption by 25 per cent. A number of measures may be taken to avoid this waste of heat, including the blocking of the centre of the tunnel during construction, or the temporary blocking of tunnel mouths with bricks. In the latter case, one end may be bricked up and fire set at the other end. Once the fire is well alight, that end may be bricked up while the previous one is opened and lighted. Thereafter, the fires may be controlled by bricking up tunnel mouths with loose bricks and adjusting the vents on top. As fuel burns away, it must be replenished.

Figure VII.5 - Scove: schematic drawing

Figure VII.6 - Corbelling scove tunnel (Sudan)

Figure VII.7 - Wood in scove tunnel (Sudan)

As with all kilns, heat must be gentle at first until all the water in the bricks is driven off. Adequate air flow is therefore essential to remove the steam produced. Thus, the vents should be open, and the fires kept low so long as steam is seen to rise from the top of the scove. This water smoking period may last several days.

Once the water smoking stage is completed, the fires may be built up gradually to increase temperatures up to a maximum over a period of a few days. A maximum temperature is indicated by the charring of dry grass or paper thrown on top of the scove, or the appearance of a red glow by night. The vents should be closed with fired bricks well before the maximum temperature is attained in order to regulate the burning rate and, thus, help to even out the temperature amongst the bricks. The maintaining of this temperature for several hours (i.e. soaking stage) requires a last charge of fuel, the closure of the tunnel mouths and the sealing of closed vents with mud.

The scove should be left to cool naturally for at least three to four days. Then if necessary some bricks may be removed from the outside to speed the later stages of cooling. Subsequently, the bricks may be left in position until sold. Before collection or despatch, under- and over-fired bricks must be discarded and the remainder, if of variable quality, should be sorted out into good quality, ‘seconds’ and soft-fired bricks. Rejects may be incorporated in the next scove.

Although wood is generally the fuel used in scoves, oil-burners are used in some countries. Coal, which is also an alternative fuel for scoves, requires a special grate at each end of the tunnel mouth, and is therefore more appropriate for firing in permanent kilns.

The fuel efficiency of scoves is low, 16000 MJ of heat being required per 1,000 bricks for a typical African scove (10, 52). A square scove has a smaller cooling area than a rectangular scove, for a given number of bricks. However, it will require a relatively longer tunnel which may exceed the allowed length for proper lighting of the scove. Thus, small kilns could be square while larger ones may need to be rectangular.

In order to increase the heat efficiency, the height of a scove should be as great as possible, so long as saleable bricks are obtained from the top. Safety must be borne in mind, however, as high scoves tend to be unstable as a result of shrinkage of bricks during firing. Moreover, a high setting complicates the placing of green bricks on top courses, and increases the risk of accidents. Figure VII.8 shows a high scove of approximately 60,000 bricks, after firing and stripping of the outer bricks.

A scove may be built for firing a few thousand bricks only, but will be less fuel efficient than larger scoves.

III.3 The Scotch kiln

The Scotch kiln is similar to the scove, except that the base, the fire tunnels and the outer walls are permanently built with bricks set in mortar (figure VII.9). The kiln itself has no permanent top, green bricks being set inside the kiln, as shown on the extreme left of figure VII.9. Much basic construction work is thus saved. A plane and section of a seven-tunnel Scotch kiln is shown in figure VII.10. Walls on either side are buttressed, and corners are massively constructed. Access into the kiln is through a doorway in the end walls. This doorway is filled temporarily with closely laid bricks (without mortar) during kiln operations. In some kilns the whole end wall is temporarily erected (40).

Wood is often used for firing the kiln, although oil burners or coal grates may also be installed. The sink of the bricks - after shrinkage - is more easily measured than in the clamp or scove kiln, since the fixed position of the permanent side walls may be used as a reference point. The sink gives an indication of the firing process within the kiln.

The advantages of the Scotch kiln over other permanent kiln structures are its simple design and easy erection. Setting and drawing of bricks are also simple.

The Scotch kilns, like the clamp and scove, are updraught kilns. They have been widely used in developing countries. Their chief failing is the irregular heating and consequent large proportion of under- and over-burnt bricks. This is especially true for clays with a short vitrification range as they cannot be fired without a good temperature control.

Fuel consumption of the order of 16,000 MJ per 1,000 bricks is generally the norm for Scotch kilns (8, 42).

III.4 The down-draught kiln

In the down-draught kiln, hot gases from burning fuel are deflected to the top of the kiln which must have a permanent roof. They then flow down between the green bricks to warm and fire them. The green bricks rest either upon an open-work support of previously fired bricks (figure VII.12) or upon a perforated floor through which the warm gases flow. These gases are then exhausted through a chimney outside the area of the kiln after passage through a flue linking the kiln floor to the chimney. The warm gases rising through the height of the chimney provide sufficient draught to pull the hot gases down continually through the stack of green bricks.

Figure VII.8 - A high six-tunnel scove (Madagascar)

Figure VII.9 - Small Scotch kiln (Madagascar)

Figure VII.10 - Scotch kiln - Schematic drawing

The down-draught kiln is more heat efficient than the up-draught kiln described earlier. It can be used for various ceramic products (e.g. drainage pipes and tiles of various types) in addition to the firing of bricks. The kiln can be operated at high temperatures and may then be used for the production of refractory ware.

Circular down-draught kilns may be built in place of rectangular kilns. They are stronger than the latter, but require reinforcement with steel bands to keep the brickwork from deteriorating through periodic cooling and heating. Rectangular down-draught kilns are more simple to build, although they require also steel tie-bars as a reinforcement. They however have the advantage of being easier to set with green bricks than circular kilns. Figure VII.11 shows the ground level plan of a rectangular down-draught kiln of massive construction, with 14 grates for burning fuel. A number of grates stocked with lighted wood are shown in figure VII.13. The grates may be prefabricated from iron bars, as indicated in figure VII.11. A “flash” wall is built behind the grates to keep the flames off the nearby green bricks. The wall in the figure is continuous. Alternatively, separate “bag” walls can be built around the back of each fire (see figure VII.12 right-hand side). The continuous wall tends to even out the heating effect.

Hot gases rise to the arched crown of the kiln and are drawn down between open set bricks (figure VII.12) by the chimney “suction”, through the perforated floor (shown in the figure) along its centre line. There should be a few small holes at the base of the flash wall, in the underground flue in order to ensure the burning of bricks near the bottom of the wall. A metal sheet damper is available near the bottom of the chimney in order to vary the flow of gases and exercise control over the operation of the kiln. The control of air flow is achieved by the use of metal doors. These should be thick enough to avoid distortions (figure VII.14).

Entrance to the kiln is through small arched doorways (figure VII. 15) referred to as “wickets”. These are bricked up temporarily during firing.

Figure VII.11 - Rectangular down-draught kiln

Figure VII.12 - Setting and bag walls in down-draught kiln (Ghana)

Figure VII.13 - Fires in kiln grates (West Africa)

Figure VII.14 - Metal damper door for kiln grate (West Africa)

Figure VII.15 - Rectangular down-draught kiln with covering (Ghana)

The height of downdraught kilns should not be too great, as it is difficult and time consuming to set bricks at heights that may not be easily reached by workers.

Down-draught kilns may hold from 10,000 to 100,000 bricks. The one shown in figure VII.15 takes 40,000 bricks.

Fuel consumption depends greatly upon the condition of the kiln, the manner of setting the bricks and the control of the firing process. For example, damp foundations absorb heat, and a badly fitting damper may waste fuel. The loss of heat varies from one type of kiln to another. Large kilns consume less fuel than small kilns, for a given number of bricks. An under-filled kiln loses as much heat as one properly filled. An over-filled kiln prevents the passage of hot gases, and this requires a longer burning cycle. Too much draught allows more heat to be wasted up the chimney. Given the above varying circumstances, the heat required by downdraught kilns varies from 12,000 to 19,000 MJ per 1,000 bricks. An exact estimate of heat consumption requires an in-depth study of the characteristics of the kiln (5, 24, 33).

III.5 Original circular Hoffmann kiln

The Hoffmann kiln is a multi-chamber kiln where the air warmed by cooling bricks in some chambers pre-heats the combustion air for the fire, and exhaust gases from combustion pre-heat the green bricks. The main advantage of this kiln is its particularly low fuel consumption rate.

The original Hoffmann kiln was circular (see figure VII.16) and built around a central chimney. An arched-top tunnel surrounds the chimney at a distance of a few metres, and is connected to it by 12 flues passing through the brickwork between the tunnel and the chimney. Each flue can be closed off by dropping a damper. Entrance into the tunnel is through any one of 12 wickets. During operation most of the kiln’s tunnel is full of bricks either warming, being fired or cooling.

Figure VII.16 - Original Hoffmann kiln

A typical condition of the kiln is shown in figure VII.16. All but two neighbouring wickets are closed. Cold fired bricks are drawn from one part of the tunnel adjacent to one of the open wickets and dry green bricks are set by the other wicket. Cold air flows to the warm chimney through both wickets. This air cannot pass through the recently set bricks as they are sealed off with a paper damper across the whole width of the annular tunnel. The air flows through the bricks which are drawn, into warm bricks further down the kiln (counter-clockwise in the figure) close to the fire. As the air flows counter-clockwise, its temperature rises through contact with increasingly hot bricks. The air is thus pre-heated and ready for efficient combustion in the firing zone of the kiln where fuel is fed in through closeable holes in the tunnel roof. Thus, little fuel is consumed for heating the combustion air. The latter also performs the useful task of cooling bricks for drawing, thus making kiln space available on a relatively short time. The hot products of combustion cannot be vented straight to the chimney through the nearest flue, as the latter is closed (this is indicated by a dot in the circle centre in the figure). Instead, the hot gases pre-heat unburnt bricks. Thus, less fuel is required at the firing stage in order to get the bricks to the maximum temperature. Next, the cooled gases flow through recently set green bricks, bringing the latter to the water smoking stage. These bricks are sufficiently warm to exclude the forming of condensation. Figure VII.16 shows the gases leaving from the open damper (no dot in the circle). Subsequently, this damper is closed, the next one (counter-clockwise) is opened and the bricks marked “set” start the water smoking stage. The fuel feed, and the drawing and setting operations, are also moved counter-clockwise at this stage1. Once the part of the tunnel marked “setting” has been filled with green bricks up to the next flue, a paper damper is pasted over the bricks and the wicket (counter-clockwise) is then broken down and cooled fired bricks are withdrawn. The paper dampers can be torn open by reaching through the fueling points with a metal rod.

1 The Hoffmann kiln described in this memorandum is operated counter-clockwise. Other kilns may, however, be operated in a clockwise fashion.

Figure VII.16 also shows a sectional drawing of the Hoffmann kiln. Bricks in the firing zone are on the left-hand side of the figure, and the empty part of the kiln - between drawing and re-setting - including the closed flue damper, is on the right-hand side. A roof covering protects the kiln from adverse weather. Wickets are shown as two thin walls, separated by an air gap. Thus, heat is kept in the kiln without the need for expensive building work at the wickets.

In the original Hoffmann kiln, fuel fed through the roof falls into hollow pillars formed by bricks set for firing. Ash from the fuel causes some discoloration of the bricks.

The tunnel is subdivided into 12 notional chambers which are identified by the flue positions. Each chamber is approximately 3.5 m long and 5 m wide. The height of each chamber is restricted to about 2.5 m for easy working conditions.

Daily rate of production from such a continuous kiln is at least 10,000 bricks.

The advantages of the original Hoffmann design include the identical chambers, the fairly short flues and low fuel consumption (2,000 MJ per 1,000 bricks(8)).

III.6 Modern Hoffmann kilns

Increased demand for bricks in industrialised countries require the erection of substantially larger kilns than those originally designed by Hoffmann. Consequently, the original circular kiln has been modified for the following reasons:

- increases in the floor area of the chambers require considerably more building work between the chambers and the chimney;

- larger diameter kilns and longer flues increased costs considerably and greatly complicated the operation of the kiln;

- the circular shape of the kiln is inconvenient for some sites;

- curved walls make the setting of bricks a difficult operation;

- a circular kiln does not allow the construction of a long tunnel unless the diameter is to be increased considerably. Yet a long tunnel is more appropriate for the transfer of waste heat.

Under these circumstances, the original design was modified into the so-called elliptical Hoffmann kiln, which has long straight walls and a few curved chambers at the end (see figure VII.17). The operating principle is exactly the same as that of the original design. The main difference relates to the larger number of chambers available in the elliptical design.

The operation of the modern elliptical Hoffmann kiln may be summarised with reference to figure VII.17. It includes the following sequence of events1:

- open wickets allow fresh air into chambers 16, 1 and 2 while bricks are drawn from chamber 2;

- other bricks are cooled and air heated in chambers 3 to 6;

- the hot air is used for graduated combustion in the next three chambers 7 to 9;

- exhaust combustion gases are pulled by the action of the chimney through chambers 10 to 13, thus preheating the bricks;

- gases leave the kiln at the end of chamber 13.

1 The dotted lines in figure VII.17 indicate the boundaries between chambers, and the position of paper cross dampers is shown at inter-chamber boundaries by continuous thick lines.

Figure VII.17 - Scheme for operating a modern elliptical Hoffmann kiln

In this type of kiln, gases are too cool for water smoking. As they carry much water vapour, there is a risk of spoiling green bricks by condensation (e.g. softening of bricks, surface cracking, and scumming by salts deposited from the products of combustion). Overcoming these problems - which may arise with exhaust gases at less than 120 °C and which are present to some extent in the original circular Hoffmann kiln - requires a second flue which connects all the chambers. Any of the latter may be connected or disconnected to this so-called hot air flue by opening or closing dampers in the same manner as for the main flue connection. In figure VII.17, the hot air flue is regarded as being in the central island of the kiln. Some of the warmed fresh air is taken off by the chimney suction applied to the flue. It is provided by chambers 3 and 4 where bricks are still hot, passed down the hot air flue, then into chambers 14 and 15 where drying, or water smoking takes place. Hence the drying is done with clean warm air, containing no moisture or products of combustion. This air then flows from chambers 14 to 15 into flues (where damper are open) and is exhausted by the chimney. Meanwhile, fresh green bricks are set in chamber 16.

The production rate of most Hoffmann kilns is approximately 25,000 or more per day, a too large output for the type of brickworks considered in this memorandum. However, small kilns can be built to produce only 2,000 bricks per day(8). Figure VII.18 shows hollow clay blocks set within an elliptical Hoffmann kiln with a capacity of about 10,000 ordinary size bricks per day. This is an interesting kiln since wood is used for firing in place of coal (figure VII. 19). A wide variety of agricultural wastes may also be used in place of wood in the top-fed kiln shown in figure VII. 19. For example, sawdust has been used in Honduras (10).

Fuel consumption of elliptical Hoffmann kilns vary according to the kiln condition and method of operation, as mentioned in the previous section. It is estimated at approximately 5,000 MJ per 1,000 bricks.

Figure VII.18 - Blocks in small Hoffmann kiln (Madagascar)

Figure VII.19 - Feeding of Hoffmann kiln with wood (Madagascar)

VII.7 Bull’s Trench kiln

A large fraction of the cost of construction of the Hoffmann kiln is in the building of the arch of the long tunnel, and in the provision of a chimney, with connecting flues and dampers. Thus, the idea behind the design of an archless kiln by a British engineer (W. Bull) in 1876.

As with the two types of Hoffmann kiln, the Bull’s trench kiln may be circular or elliptical. Both forms have been widely used throughout the Indian subcontinent. Construction of this type of kiln is briefly described below.

A trench is dug in a dry soil area which is not subject to flooding. It is approximately 6 m wide and 2 to 2.5 m deep.1 Alternatively, especially if the soil is not sufficiently dry, the trench may be dug to only half of this depth, while excavated material is piled up on the trench side, and held out off the trench by a brick wall starting at the bottom of the trench (figure VII.20). The total length of the trench is approximately 120 m. It is so constructed as to constitute a continuous trench.

1 Excavated soil may be suitable for brickmaking.

When in operation, the Bull’s Trench is full of bricks warming, being fired or cooling. Cooled bricks are drawn and new green bricks are set, while the fire is moved progressively around the kiln. The exhaust gases are drawn off through 16 m high moveable metal chimneys with wide bases, which fit over the openable vent holes set in the brick and ash top of the kiln. These chimneys are guyed with ropes to protect them from strong winds. The type of chimneys shown in figure VII.21 require six men to move them. This figure also shows the method of fueling whereby small shovelfuls of less than 1.5 cm size coals are transferred from storage bins on top of the kiln, and sprinkled in amongst the hot bricks through the removable cast-iron feed holes. Metal sheet dampers are used within the set bricks to control draught.

Figure VII.22 shows the sequence of events diagramatically. The setting of the bricks within the kiln must be such as to allow sufficient air flow between the bricks and wide enough spaces for the insertion and burning of fuel and the accumulation of ashes. However, the whole setting must be sufficiently strong and stable to ensure safe operation of the kiln. The setting in figure VII.20 shows occasional cross link bricks, between the separate bungs or pillars of bricks, tying bungs together. Information on the firing of these kilns is available (54) and kiln designs are standardised(55).

Modifications to this type of kiln have involved the provision of flues from the trench so that chimneys can be moved on rails located on the centre island rather than over the setting bricks in the kiln. A major problem is the corrosion of the mild steel chimneys normally used. They may rust through within only a few months. Accordingly, some kilns have been redesigned with dampers opening to flues connected to a permanent brick chimney.

Figure VII.20 - Cross-link bricks between the separate bungs or pillars of bricks in Bull’s Trench kiln

Figure VII.21 - Bull’s Trench kiln: chimney and feeding (India)

Figure VII.22 - Bull’s Trench kiln - Firing sequence

The whole Bull’s Trench kiln is very large, a normal output being 28,000 bricks per day. With a narrow trench output could be reduced to 14,000 bricks per day. It is not possible to shorten the trench as this will affect the heat transfer efficiency. The depth of the trench cannot be reduced either without impairing the firing behaviour. The kiln would be very big to roof over, and is most suited to dry weather conditions. The chief advantage of this type of kiln is its low initial construction costs.

Fuel consumption is much better than in intermittent kilns, 4,500 MJ being required for firing 1,000 bricks (10, 22, 56). About 70 per cent first-class bricks can be obtained, the remaining bricks being of poorer quality.

III.8 Habla kiln

The effective tunnel length of the Hoffmann type kiln may be increased by the building of zigzagged chambers. The resulting kilns, known as the zigzag kilns, have a faster firing schedule than the Hoffmann kiln. However, they require a fan - and therefore electrical power - as air must travel a longer path and a simple chimney does not provide sufficient draught for air circulation. Fans provide a more steady draught than chimneys and can be better controlled. They allow a larger transfer of heat to the water-smoking stage, thus saving fuel. However, it is best to avoid condensation on fan blades and subsequent corrosion of the latter by having gases extracted at 120 °C. This is especially important if fuel or clay contain sulphur compounds such as pyrites which are transformed into sulphuric acid in the kiln gases.

The zigzag kiln developed by A. Habla is an archless kiln. One additional simplifying feature of this kiln is that the zigzagging walls are temporary structures of green bricks which may be sold after firing. Habla kilns are of various designs: in some kilns the flues are returned from all chambers to the central island while in others, some of the flues are returned to the outer walls. Figure VII.23 illustrates the former type of kiln. For simplicity, it omits the hot air flue which can be carried above the main flue for providing clean drying air.

The Habla kiln is rectangular, but close to a square. The one illustrated in figure VII.23 has chambers numbered 1 to 20, every second chambers being accessible through a wicket. Partition walls of dried green bricks, with a thickness of only one brick length, are alternatively built out from the central island and the outer wall. These partition walls deflect the gases from the island to the outer wall, through the wide-set bricks between the partitions. As the temperature of any particular chamber rises, the wide-set bricks are first heated. Then, as bricks in the partition shrink a little, draught through the partitions increases, and the bricks in that partition as well as the wide-set ones become fired.

Figure VII.23 - Habla type kiln - Firing sequence

In figure VII.23, chambers 1 and 2 are empty. Bricks are drawn from chamber 3. Air passing through the open wickets is warmed as it cools bricks in chambers 4 and the following ones. After the firing zone, the exhaust gases preheat bricks, flow out of chamber 15 through the chamber flue and open damper (shown in the section drawing), and enter the main flue from where they are expelled through a short chimney by the fan. Bricks in chambers 16 to 19 are water smoked by clean warm air from chambers 4 to 6. Paper dampers are used as in the case of the Hoffmann kiln.

An interesting modification to the layout shown in figure VII.23 is to build a pair of short partitions in line with each other, approaching from the island and outer wall, but leaving a gap in the middle. Secondly, a wall may be built in the middle of the kiln, separated by two gaps from the island and outer walls. The fire can then travel along two paths simultaneously, around both ends of the second partition, through the central gap of the third, then around both sides of the fourth and so on. This modification should help speed the rate of firing.

Large Habla kilns, producing 25,000 bricks per day (57), have been built in a number of countries. Recently, in India, a 24-chamber high-draught kiln of this type has been developed (58) for an output of 30,000 bricks per day. It is fired with coal and wood. Figure VII.24 is based on published information on this Indian kiln. The latter may be reduced in size for a production of 15,000 bricks per day. Roofed zigzag kilns may also be built for as few as 3,000 bricks per day (8).

The Habla kiln is economical to construct and operate. It has a larger capacity relative to its area than other continuous kilns. This feature reduces the costs of land and construction. Furthermore, the kiln has a long firing zone, allowing difficult clays to be fired more easily. The long-firing path assists heat exchange between gases and bricks, thus improving fuel efficiency. Because partitions are of green bricks, less permanent brickwork has to be heated and cooled, thus adding to fuel efficiency. The shrinkage of bricks in the partitions and consequent leakage of hot gases shortens the distance travelled by the latter. Thus, less power is needed to drive the fan. As a result of partition leakage, the kiln has relatively few “dead spaces” where heat is insufficient to fire bricks properly. The building of partitions of green bricks at the start of each operation does not increase labour costs since the bricks are removed for sale and may thus be regarded as part of the whole setting. Another advantage of the Habla kiln is the easy access to the structure.

Figure VII.24: Central Building Research Institute high-draught kiln (India)

Fuel consumption of a zigzag kiln is estimated at 3,000 MJ per 1,000 bricks(57). Consumption in the high-draught Indian archless kiln is also approximately 3,000 MJ per 1,000 bricks.

IV. Auxiliary equipment

IV.1 Kiln control

Temperature control, including control of the temperature attained and of the rates of increase and decrease, is an essential activity in brickmaking. Such control may be carried out in a number of ways, including the following:

Analysis of the colour of gases coming off the kiln: During the initial stage of heating (water-smoking) white gases indicate that the bricks are not thoroughly dry. Thus, the fires must be kept low. The presence of wetness on an iron bar withdrawn from the kiln after only a few seconds’ insertion is indicative of the water-smoking stage. From approximately 500 °C upwards, the colour of the hot kiln provides clues to the temperature attained. The interpretation of kiln colours should be based on previous temperature readings and colour examination. For example, the colour of smoke of a scove kiln varies as the whole kiln reaches top temperatures. The sink of the bricks is also an indication of the process of vitrification.

Use of instruments: Instruments are also available to assist the kiln operator. At the water-smoking stage, a thermometer within a protective metal sheath may be pushed in or lowered on a chain amongst the bricks. The presence of moisture on the metal (after withdrawal of the thermometer) is indicative of the water-smoking stage. A reading of the temperature may be obtained by keeping the thermometer amongst the bricks for a longer period. While moisture is still being driven off, the temperature remains at about 100 °C. Once it rises to 120 °C, fires may be increased as moisture in the bricks will have been removed. Thermocouples may be used in place of thermometers especially if a continuous reading of the temperature is required. A chart recorder, which displays the temperatures measured by several thermocouples, may be used for this purpose. In a continuous kiln, thermocouples could be placed in the preheating zone, the maximum temperature firing zone, in the coolest part of the cooling chamber from which air is being taken to the chimney, in the base of the chimney and in the hot air flue if it exists (48). A reading of the chart will indicate the state of the kiln and the need to adjust firing in order to obtain better quality bricks or to improve fuel efficiency. It also gives the manager an indication of the quality of the kiln operation. Thermocouple probes must be of a corrosion-resistant metal or in protective sheaths.

Pyrometric cones, which better measure the effect of both time and temperature upon clay, may be used in place of thermocouples. They are made from carefully controlled mixes of clays and fluxes, and will therefore deform, or squat at different stages in a heating schedule. For example, a cone which squats at 1,140 °C with a rise of 60 °C per hour, will squat at 1,230 °C with a temperature rise of 300 °C per hour. Thus, pyrometric cones indicate the way in which fired clay products perform in a kiln. The temperature at which cones squat, when heated at a standard rate, is known as the pyrometric cone equivalent (PCE). Cones may be used in laboratory experiments and in full-size production kilns, and are useful in applying the results of laboratory investigations. In practice, three cones are used to control the firing temperatures within a kiln at a given PCE: one cone of the adopted PCE value, one of 20 °C less, and one of 20 °C more. These cones are set in a fireclay base and placed between bricks well in from the kiln wall but in line with a plugged spy hole, sealed with clay. The cones are checked periodically until the first cone, with the lowest PCE rating, begins to bend over. The fire should then be stocked less as the cone with the highest PCE rating should not be allowed to squat. In other words, only the first and second cones should squat. Squatting of the third cone indicates that the kiln has been heated beyond the intended temperature. Sets of cones can be placed amongst the bricks remote from the holes in order to check temperatures attained in any part of the kiln. Alternative systems, using rings or bars, are also used.

Draught gauges, consisting essentially of water-filled, open-ended U-tubes of glass, may be used to measure the draught or suction at any given point of the kiln by connecting one end of the tube to a chamber or chimney and measuring the difference in water levels in the tube. This gives a quick check on the kiln’s functioning, and indicates whether dampers should be adjusted.

It is good practice to keep a record book of all kiln operations, including temperatures, draught, numbers and sizes of bricks set, and the number and size of saleable bricks of various grades. The quantity of fuel, operators on duty and any incidental remarks about wind, rain, etc. should be noted in the record book.

IV.2 Brick handling

Labour costs may be substantially lowered if the handling of bricks (e.g. green, dried, and fired bricks) can be rationalised, and if efficient transport devices are used. Furthermore, these devices can be of considerable assistance in relieving the burden of workers. A simple litter borne by two people can be used for the transport of bricks as in the case of clay. The crowding barrow (figure VII.25) is extremely useful, especially in confined spaces such as in the circular downdraught kiln. The barrow is short, with its wheel well placed under the load which may be stacked high. The balance of the barrow is adjusted by placing the bricks according to the workers’ height. Thus, little weight is taken on his hands and the load does not tip forward.

V. Fuel

Given the high prices and scarcity of many fuels, achievement of fuel efficiency becomes an important factor in brickmaking. Table VII.1 compares typical requirements of the different kilns described in Section III. It must be appreciated that actual amounts could vary widely from those given, depending upon the size of kiln, size of bricks, nature of clay, firing temperature, the condition of the kiln, and the skill of the operators. Calorific values of wood, coal and oil have been taken as 16,000, 27,000 and 44,000 MJ per tonne respectively. Figures in brackets indicate that the fuel is not suitable for a particular kiln.

Figure VII.25 - Crowding barrow

Table VII.1

Typical fuel requirements of kilns

Type of kiln

Heat requirement
(MJ/1,000 bricks)

Quantity of fuel required
(Tonnes/1,000 bricks)


























Original Hoffmann





Modern Hoffmann





Bull’s Trench















Source: 5, 8, 10, 22, 24, 33, 44, 52, 56, 57, 58.

Where wood is used as a fuel, trees should be replanted to replenish supplies. Sometimes, commercial ventures provide an incidental supply of wood (e.g. in the rubber estates, where trees fulfil their latex-yielding life within 30 years or so). The practice of coppicing is worthwhile: small-size wood is cut from a low level, thus allowing the trees to continue to grow. Coppicing may yield 125 tonnes of wood every year from a square kilometre (22).

Many other materials can be utilised to assist in the firing of bricks, in addition to wood, coal and oil. Gas, either naturally occurring such as Sui gas in Pakistan, or made from plant wastes by gasification or by bio degradation processes (biogas), may be burnt in simple burners, preferably set low down in the kiln.

Agricultural and plant wastes may also be used for firing, including sawdust (e.g. in Honduras (10)) and rice husk (e.g. in experimental kilns and in a Bull’s Trench kiln in Pakistan). A large quantity of ash, some 18 to 22 per cent of the weight of the husk, is produced on burning. As this ash may block the bottom of the Bull’s Trench kiln, coal is used without husk on every few blades. Groundnut husks, coffee husks, chaff, straw and coconut husks have also been used on an experimental basis. They may be burnt in the grate of an intermittent kiln, or fed in through the top of Hoffmann, Bull’s or Habla kilns. However, the volume required for sufficient heating is very large in most cases. These wastes may not be easily burnt in kilns equipped with grates. They may therefore constitute only part of the fuel source. Dung is a traditional fuel in Sudan.

Waste materials may also be mixed with clay (e.g. 5 to 10 per cent by weight) instead of being directly used as a fuel source. While this method is technically feasible, it may produce more porous and weaker bricks.

Other wastes have been tried in a number of kilns. These include old rubber tyres, waste engine oil, ashes, clinker, pulverised fuel ash from coal-fired power stations, and coal washery wastes.

The optimum air flow for highest fuel efficiency in a kiln can be determined by experiment, especially if good records are kept. Adequate air is necessary to obtain full combustion of fuel. However, too much air will have a detrimental cooling effect.

The best use of fuel is generally obtained in continuous kilns, properly maintained and run, using a fraction of waste materials. Economy of scale favours the larger kilns, but fuel for transportation of bricks from a large-scale plant may negate the fuel savings from the operation of the kilns.

VI. Productivity

The range of outputs of the various kilns described in Section III is shown in table VII.2 Although smaller units might be built than those quoted, lack of data on these units precluded their inclusion in the table.

Table VII.2

Range of outputs from various kiln types

Type of kiln

Capacity (bricks, ‘000s)

Capacity (bricks per day, ‘000s)



10 - 1,000


5 - 100


15 - 25


10 - 50


Original Hoffmann

10 - 15

Modern Hoffmann

2 - 24

Bull’s Trench

14 - 28


15 - 30

Many factors affect the obtained outputs. In general, good maintenance and provision of roofs over kilns will improve the quantity of bricks produced, but will add to costs. Quantities produced will also depend upon the required quality standard and the skill of the operators. The market demand, weather, infrastructure and supply of raw materials will govern the rate at which production may proceed.

Capital-intensive equipment is available for transportation and the setting of bricks, but all the kilns described in Section III may be operated on a labour-intensive basis. All but the downdraught and Hoffmann kilns involve the extra task of covering the green bricks with previously burnt ones and ashes.

Most of the kilns described in this memorandum are operated on a fairly labour-intensive basis. Table VII.3 shows the difference in labour requirements between these kilns and more automated kilns used in industrialised countries.

Table VII.3

Labour requirements



Labour requirements for firing, including drying
(man-hours/1,000 bricks)

Traditional plant with clamp



Traditional plant with coal-fired clamp



Moderately mechanised plant

United Kingdom


Highly automated plant

United States


Source: 10

VII. Brick testing

VII.1 Purpose of testing

The purpose of testing is to check the production process; to remedy faults to ensure a saleable product; and to guarantee the quality and performance of marketed bricks. Given the intended use of bricks in housing construction, they must resist local weather conditions and should not contain materials which will damage them or the applied finishes such as renderings, plaster or paint. They must be strong enough to withstand both the dead load of the building itself, and the live load imposed by occupancy or wind. The thermal or moisture movements should not also be so large as to build up unduly large stresses. Testing should therefore consist of ensuring that bricks have all the required characteristics for efficient use in building.

VII.2 Initial checks on quality

A quick check of quality consists in striking two hand-held bricks. A high-noted ring indicates that they have been thoroughly fired. On the other hand, a dull sound indicates either cracked or soft-fired material. Similarly, bricks which cannot be scratched and rubbed away with the edge of a coin are hard and of good quality. Nevertheless, bricks without a good ring or which can be marked may still be perfectly suitable for many construction purposes. These two tests only serve to identify some of the best materials.

The general appearance of bricks may give a quick indication of quality. Regular shapes and sizes, sharp arrises, unblemished surfaces and freedom from cracks are signs of good bricks. Colour is difficult to interpret. However, within a batch of bricks from any one brickworks, the darker colours are likely to relate to the harder fired, stronger and more durable bricks.

VII.3 Standard specifications

Many countries have published their own standard specifications for bricks, and reference should be made to these where available. The methods of testing described in the British Standard(37) have been devised after much research and many years of experience, and are of wide interest. However, they may not necessarily be appropriate for other countries. Actual numerical limits must be decided locally according to what is required and what can be made.

VII.4 Sampling

The testing of bricks should be made on a representative sample of the latter since variations in preparation of raw materials, drying and firing produce bricks of variable characteristics. The testing sample should preferably contain 40 bricks picked up at random while they are being unloaded from the kiln. It is more difficult to obtain random samples once they have been placed in a large stack.

VII.5 Dimensions

Since bricks are used in fairly long runs, the testing of brick dimensions need not be carried out on individual bricks. Instead, 24 bricks should be chosen at random from the sample 40 bricks and small blisters or bumps knocked off. The 24 bricks should then be placed against an end stop touching end to end on a long bench. If the nominal length of each brick must be, for example, 21.5 cm, the far end of the row of 24 bricks should be 516 cm from the end stop. The permissible variations (for example 508.5 to 523.5 cm as in the British Standard for 21.5 cm bricks) should be clearly marked above the level of the bricks on a board behind the bench. Similarly, the width and height of the 24 bricks should be checked as bricks which comply with the standards for one dimension do not necessarily comply in other dimensions. Table VII.4 gives the dimensions calculated from batches of African bricks and the requirements of a relevant Standard: none complies in all three dimensions.

Table VII.4

Overall dimensions of 24 bricks

Brick source

Length (cm)

Width (cm)

Height (cm)


























*Dimension complies

Source: 51

VII.6 Compressive strength

The compressive strength of harder fired bricks is greater than that of other bricks although the strength of most bricks is likely to be adequate for simple buildings. Even a fairly weak brick with a compressive strength of only 7 MN/m2 may support a 300 m column of bricks without crushing. However, loadings in actual buildings are increased in supporting pillars and around wall openings.

As the strength of dry bricks is higher than that of wet bricks, the former should be immersed in water for 24 hours before testing. Although a simple wooden beam, used as a lever, can be sufficient to crush green bricks or adobe, it is unlikely to be successful for higher strength burnt bricks. Instead, a compression testing machine is commonly used in public works departments, universities, and technical and research institutes. To avoid high local loadings due to small irregularities on the bed faces of bricks in contact with the steel plattens of the machine, thin plywood plattens should be placed on both bed faces. Bricks with frogs or large perforations should be bedded in mortar which is allowed to set prior to testing. Ten bricks should be tested, and the mean strength calculated.

Table VII.5 gives test results obtained on batches of bricks from two different works in East Africa. They were tested by the British Standard method. Although a few individual bricks from one works are weaker than required for a particular grading (not the lowest), the mean value complies with the standard requirement. Similarly, a few bricks from the other works would not meet a requirement of the Indian Standard(36), but the mean value is satisfactory.

Table VII.5

Compressive strength of 10 bricks

Brick source

Compressive strength MN/m2

Individual bricks


Minimum requirement


Class II


4.1, 4.0, 8.1, 8.1,




11.1, 10.9, 7.7,

4.0, 6.1, 7.1


6.2, 6.5, 6.5, 8.8


11.4, 7.7, 11.5,

6.3, 10.3, 9.9

Source: 61

For calculated load-bearing structures and civil engineering works, the strength of the bricks should be determined and used in the design calculations.

For non-load-bearing partitions, bricks of only 1.4 MN/m2 may be used(37).

A low-cost impact testing method, claimed to give useful information on brick strength, consists in dropping a weight several times on the pieces of a broken brick in a containing cylinder(62).

The density of dry bricks is easily determined, and is sometimes quoted instead of strength.

VII.7 Resistance to erosion by water

Although compressive strengths may be more than adequate, dampness and flowing water can cause severe deterioration in buildings. Unfired earth bricks or adobes are eroded or slaked by water. On the other hand, firing yields bricks with excellent resistance to water erosion. The testing of resistance to erosion by water requires the soaking of brick samples in water for 24 hours. Good bricks should show no sign of softening or slaking.

A more severe test consists in spraying water on bricks for several weeks. This test allows the separation of bricks into various quality groups as low-quality bricks get eroded while high-quality ones remain unaffected.

VII.8. Water absorption

Hard fired bricks will absorb less water than other types of bricks. The quantity of water absorbed by a dry sample of bricks, when immersed in water for 24 hours or eight days(7), or boiled for five hours(37) is often estimated in testing laboratories. However, the results of this simple test are not easy to interpret. Generally, absorptions of less than 15 per cent by weight in the cold tests would be indicative of satisfactory brick strength and durability. Exceptionally, higher absorptions may be found with some types of clay although the bricks may still be satisfactory.

VII.9 Rain penetration

In practice, moderately high water absorption is acceptable in a brick wall, since rain water dries out once good weather conditions return. On the other hand, under some circumstances, rain running on walls made of low absorption bricks may enter the wall surface through small cracks between bricks and mortar. Rain penetration tests may be set up by building walls exposed to either rain or a water spray on one side, and observing the other sheltered side of the wall. The quality of not only the bricks but also of the workmanship in laying them, is of significance in determining the performance of the brickwork.

VII.10 Efflorescence, soluble salts and sulphate attack

Appearance of efflorescence on brickwork is considered unsightly and in extreme cases may cause spalling of faces of bricks. It may occur on initial drying or after subsequent wetting of the bricks. One test for determining the presence of efflorescence consists in half immersing bricks in distilled water for two weeks. Soluble salts, if present, will dry out on the top corners of the bricks as water is being soaked up. If only slight efflorescence occurs, it may be assumed that no problems are likely to arise in practice(7). Alternatively, sample bricks can be covered with a polythene sheet to prevent evaporation from the non-visible face in the finished brickwork, while exposing the other face upwards. A bottle of distilled water is then inverted on this latter face and the water allowed to soak in. Subsequently, the water dries out, and salts present in the bricks are carried to the surface where the amount of such salts may be estimated. Generally, if more than half of the exposed area is covered with salts, or if the surface flakes, the bricks would be regarded as efflorescent(37).

The nature of soluble salts can be determined in the laboratory by standard methods of chemical analysis carried out on powder obtained by drilling or fine grinding of brick samples. More than 3 per cent by weight of salts is considered a high concentration. For special quality brickwork, acid soluble sulphates should not exceed 0.5 per cent, calcium 0.3 per cent, magnesium 0.03 per cent, potassium 0.03 per cent and sodium 0.03 per cent(37).

VII.11 Lime blowing

Hydration of quicklime particles derived from limestone in brickmaking clays can cause pitting on brick faces. When, in spite of precautions in manufacture, a problem persists, it may be possible to alleviate it by docking the bricks (i.e. by immersing them in water(50)). Docking apparently slakes the lime to a softer form which may extrude into neighbouring brick pores. Bricks should be soaked for approximately 10 minutes so that water penetrates at least 15 mm. Otherwise, the problem may worsen. Large quantities of water are required, and the increased weight of the bricks may add to transportation costs. Efflorescence may appear as the water dries out.

Testing for lime blowing can be done by immersing brick samples into boiling water for 3 minutes(50), or preferably into a steamy oven(63).

VII.12 Frost

Frost is one of the most destructive natural agents but only when brickwork is frozen while in a very wet condition. In climates where frost occurs, outdoor test walls or laboratory freezing and thawing tests can be used to assess frost resistance (10, 64). Harder fired, less porous bricks are generally more resistant to frost.

VII.13 Moisture and thermal movement

Reversible and irreversible moisture movements are likely to be small and of little consequence for small buildings. Tests can be carried out to determine these movements if bricks are to be used for long runs of brickwork, or in tall structures. Thermal expansion is of similar significance and may be measured: it is likely to be approximately 4 x 10-6 m per °C (i.e. a change in temperature of 20 °C will cause a 12 m run of brickwork to change 1 mm in length).

VII.14 Durability and abrasion resistance

To investigate performance, including resistance to abrasion by wind-blown sand, small test walls should be constructed outdoors from the various types of bricks made at any works. Changes in the brick surface over various periods of time will indicate the degree of resistance of bricks to abrasion.

VII.15 Use of substandard bricks

Bricks that do not meet the required standards may be used for other purposes, and not be entirely wasted.

Underburnt bricks may be returned for refiring, or may be useful in kiln construction. Overburnt bricks may also be used in kiln construction or can be broken up and used as concrete aggregate.

Reject bricks may be broken up and used for road building or soakaways. If finely ground, they may be used as grogs in brickmaking.

Reject, underfired bricks exhibit pozzolanic properties when ground down to powder. The latter may thus be mixed with lime for the production of a cement substitute. Alternatively, the lime released when ordinary Portland cement sets will react with the brick powder. Thus, crushed soft-fired clay could constitute a useful mortar ingredient.