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CLOSE THIS BOOKSmall-Scale Brickmaking (ILO - WEP, 1984, 228 p.)
VIEW THE DOCUMENTI. Origin and distribution of raw materials
VIEW THE DOCUMENTIII. Clay testing and significance of results

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


I. Origin and distribution of raw materials

Brickmaking requires sufficient supplies of suitable soil, sand, water and fuel. The purpose of this chapter is to describe the various types of clay which may be used in brickmaking.

The essential ingredient in the soil used for brickmaking is clay. The size of each clay particle is extremely small, generally less than 0.002 mm across. Various forces act between these fine particles in a moistened clay, allowing the latter to be formed into the desired shape, which must be retained on drying. Clayey materials can be readily identified by simple manipulation of moist samples with a view to checking the plasticity of the latter.

A wide variety of raw materials may be used for brickmaking, ranging from soft sticky muds to hard shales. However, all these materials must contain a moderate proportion of clay-size particles. Too high a proportion of such particles will result in excessive shrinkage of moulded bricks as they dry, with consequent risk of cracking. On the other hand, a soil with too low a proportion of clay particles will not be cohesive enough and will fall apart. The mineralogical nature of the clay must be suitable so that it is changed by heating in a kiln to a strong, water resistant vitrified form which can bind larger particles in the soil together.

Brickmaking clays may be found in most countries of the world. Geologically recent deposits are associated with existing valleys and rivers, and are often near the surface. Older deposits may be overlaid by other unsuitable material of varying depth, and may have been raised and inclined from their original positions. Thus, good deposits of clay may be found in gently rolling hills, but not mountains.

Information on clay deposits is available in many countries from National Geological Survey Departments, or may be obtained from Geological Institutes. Location of existing brickworks, pottery works or other ceramic production is evidence of workable deposits.

Prospecting for new clay deposits may be undertaken by first examining river banks, and the sides of any recent road or railway cuttings which give an instant section of the soil profile. Subsequently it is necessary to explore in more detail any newly-discovered deposits by taking samples from many points on a regular grid covering the ground area. The neatest and simplest means of obtaining a suitable sample is by using an earth auger. The latter can be powered by one or two people. As it is rotated, the auger drills its way down into the earth, providing samples of the cut out soil. Alternatively, a spade may be used to dig a narrow hole (figure II.2). However, it cannot go as deep as an auger. A pit may be dug instead in such a manner that a person with the spade can work on the floor of the pit. This will require the removal of a great volume of earth, and may not therefore constitute an efficient way of taking samples. For safety’s sake the pit should not be more than 2 metres deep.

It is wise to keep an accurate record of such investigations. A plan of the area should be drawn, and location of investigatory holes marked in and numbered. Samples taken out of the hole should be small enough to allow the identification of a change from one soil type to another. Usually, there is a top-soil in which plants grow, and which contains the decomposed products of plants. The top-soil depth should be measured and noted, as well as that of subsequent soil layers. As soon as clay is found, it will be recognised by the stickiness with which it adheres to the auger or spade. If a large stone is encountered when augering, it will have to be knocked out of the way, or broken, or a different type of auger used to cut a way past.

The survey will indicate the area covered by clay, its thickness and the depth at which it may be found, and the thickness of the top soil which must be removed during quarrying. If there is much top soil, it will not be worth the cost of removing it unless there is a good depth of clay beneath.

Simple testing of clay for suitability for brickmaking may be carried out on site. For more extensive testing, each soil type should be in a separate heap on boards or a large sheet, then reduced by quartering. Quartering is done by dividing the heap into four quarters of equal size and shape, discarding two diagonally-opposed quarters, and recombining the other two. This procedure is repeated until a small pile of a few kilograms remains. The latter should be placed in a strong plastic bag, labelled with the hole numbers and the depths from which the sample was extracted.

Figure II.1 - Bucket auger for sampling soil

II. Types of clay

It is essential that the raw material used for the production of bricks contains the following elements:

- sufficient clay fraction to ensure a good plasticity of the clay body, thus allowing the latter to be formed and retain its shape. The material is described as ‘lean’ or ‘short’ if the fine fraction is insufficient. The clay element should not exceed a certain limit which will render it too sticky for working. Furthermore, the dried bricks are liable to cracking due to high shrinkage if too much clay is present in the body. In this case, the material is described as ‘fat’. Some clay types with the above characteristic have high shrinkage rates;

- sufficient unreactive coarser grained material such as sand to mitigate the potential problem described above;

- proportions of silica and alumina in the clay from which the strong durable glassy material may be formed on heating to approximately 1000° C;

- alkalis or iron to assist in the formation of glassy compounds;

- constituents which do not produce excessive deformation or shrinkage at the firing temperature in the kiln;

- no impurities or inclusions which will disrupt the structure of the brick.

The size of particle present in the clay body affects the cohesiveness, forming characteristics, drying and firing properties of a clay.

II.1 Particles sizes in brickmaking soils

The various fractions of particles in soils are usually denoted by their size as given in Table II.1.

Table II.1

Definition of particle sizes in brickmaking soils


Size range (mm)



2 - 0.6


0.6 - 0.2


0.2 - 0.06



0.06 - 0.02


0-02 - 0.006


0.006 - 0.002


less than 0.002

In practice, a raw material for brickmaking should contain some clay fraction (say 10 to 50 per cent) together with some silt and some sand. Depending upon relative proportions of various elements in the raw material, the latter might be described, for example, as a silty clay or, if containing some clay and similar proportions of silt and sand, as a loam. Since the presence of both clay and a good range of other particle sizes is desirable, loams are particularly suitable for brickmaking.

II.2 Clay minerals

Materials for brickmaking range from soft muds through the partially compacted clays or muds and highly compressed shales. The fine particles in the clay fraction may consist of various mixtures of some 12 different groups of clay minerals. These groups are briefly described below.

The kaolin group is common and might be regarded as a typical clay mineral. In its molecular structure thousands of alternate flat layers of silica (silicon oxide) and gibbsite (aluminium oxide) occur, and give the particles their typical hexagonal plate-like structure. They are up to 0.002 mm across and can be seen under the electron microscope. This mineral presents no particular problems in brickmaking.

The montmorillonite group, which often occurs in the drier tropics, has two silica layers for every one gibbsite. This structure allows water molecules to enter in between the layers, forcing them apart. The resulting expansion of the clay may continue for several weeks under damp conditions(17) The layers close up again when the water is dried out. This has important consequences in brickmaking since montmorillonite-bearing clays have large drying shrinkages. The thin plates are generally smaller than kaolinite. The high specific surface area gives great plasticity, stickiness and strength to the montmorillonites (17).

The hydrous micas and illites, which have somewhat similar structures to the montmorillonites, are also frequently found in brickmaking materials. Chlorites, which are related to hydrous micas, are also found in various clay materials. The latter have magnesium and potassium within their structures.

Extremely small particles from a millionth to a thousandth of a millimetre across, termed colloids, are also present in clays. They carry electrical charges, so their movement in water and their properties are affected by the presence of salts. Thus, the physical properties of wet clays can be altered by additions of some chemicals which may, for example, increase their plasticity or reduce stickiness. An acidic addition flocculates the colloidal particles so they settle in water more readily whilst an alkaline addition deflocculates these particles and keeps them in suspension.

Mineralogical examination can help identify the substances present with a view to determining the likely suitability of a material for brickmaking.

II.3 Chemical analysis

Chemical analysis can help in the identification of the clay minerals present in the raw material. The relative proportions of silica and alumina are relevant, since the higher the proportion of alumina, the higher the temperature necessary to form the glassy ceramic bonding material which characterises ceramic products. Chemical analysis can also indicate the presence of water-soluble compounds such as the sulphates of potassium, sodium and magnesium. The drying out of the latter on the moulded bricks (before firing) produce unsightly scumming. If still present in the fired product, they may lead to efflorescence and, exceptionally, can spoil brick faces and lead to attack and expansion of cement-based mortars. Calcium sulphate can also produce this undesirable effect. With knowledge of these deleterious salts within the clay it might be possible to avoid problems with the bricks when finally built into walls, by choosing another clay deposit or allowing rain to wash salts out of the clay after it has been dug, or by firing the bricks to a higher temperature. Another solution to these problems is to add barium carbonate. This is, however, an expensive remedy which may not be feasible in many situations.

If potassium and sodium are found in the chemical analysis, but the compounds are not water soluble, they may indicate the presence of fluxes such as the felspars or micas. These are beneficial in reducing the temperature needed for formation of glassy material. Magnesium, calcium and iron (ferrous) compounds can also behave as fluxes.

Chemical analysis may be carried out on different size fractions of the soil. This is an important consideration since fluxes should be in the finest of particles sizes. Hence, their presence in only coarse fractions is of little significance.

Laterites occur as rock, gravel, sand, silt and clay in many tropical locations. They are high in alumina and low in silica. Thus, the use of laterite soils for brickmaking will require higher kiln temperatures. In practice, the presence of potassium and sodium-bearing compounds, and of iron compounds (which are often abundant and act as fluxes), should allow the production of bricks from laterites. The latter are defined in a number of ways, but the following definition is often accepted: The ratio of silica to sesquioxides (that is iron and aluminium oxides) must be less than 1.33 for the material to be a laterite. If the ratio is between 1.33 and 2, the material is lateritic, and if the ratio is greater than 2, it is non-lateritic.

Marls, which are clays with a high proportion of calcium carbonate (chalk, limestone, etc.), are identified by high calcium and high weight losses on heating in a full chemical analysis. They may have low vitrification temperatures which extend over only a narrow range. Thus, sudden fusion can occur in manufacture. If the calcium carbonate is present as large lumps, the latter will have a disruptive effect on the fired bricks after manufacture. These lumps should be removed or ground to less than 2 mm.

II.4 The drying process and drying shrinkage

A wet clay has the fine individual particles separated by films of water which are absorbed into the particle surfaces. In such a state the clay exhibits its typical plastic property which enables it to be shaped. On drying, the films are reduced and the particles get gradually closer. Thus, an overall shrinkage of the body is discernable. The shrinkage continues until the particles touch, but water still remains in voids between the particles. The clay then has a critical moisture content (CMC). As the water continues to dry out, no further significant shrinkage occurs. This is shown diagramatically in figure II.2. The practical significance of the process is that bricks must be dried slowly to the CMC, thus ensuring that all parts of the brick (top, bottom and inside) are shrinking at the same rate. If one face of a brick dries before the opposite face and becomes non-plastic, the latter face may crack as it dries while being held in position by the dried face. Different rates of shrinkage also cause bricks to become bowed, or banana-shaped by a similar process. Once the CMC is reached, faster drying may be used since there is no further shrinkage.

Clays for brickmaking should not have too high a shrinkage rate on drying if cracking is to be avoided. However, if the moulded bricks are dried very slowly, higher shrinkage material may be used. Montmorillonite has an exceptionally large drying shrinkage, so soils containing it (e.g. black cotton soils) would be best moulded from the driest possible mix, and then dried very slowly. In general, the greater the proportion of fine particles the greater the drying shrinkage, and the finer the particles the more the shrinkage. Hence, there should not be too much clay in brickmaking soil.

To reduce unacceptably high shrinkages, non-reactive coarse grained material may be mixed in with the soil. The additional materials frequently employed are sand, if it is available nearby, or ground-up reject bricks which are referred to as ‘grog’.

Drying should be as complete as possible before bricks are exposed to the heat of the kiln. Otherwise, steam may be produced in the bricks and develop enough pressure to blow them apart (other reasons are listed in Section I of Chapter VI).

II.5 The firing process and firing shrinkage

At a low temperature of 100 °C, any moisture remaining in the bricks is removed. The nature of the clay is not changed (i.e. the cooled and wetted clay retain its original characteristics - see figure II.3).

Figure II.2 - Drying curve


The first irreversible reactions start at approximately 450-500 °C, when dehydroxylation takes place. Part of the actual clay structure (the hydroxyl groups) is driven off as steam, resulting in a very small expansion of the brick.

Carbonaceous organic matter (derived from plants, etc.) in the soil will burn off in the temperature range of 400-700 °C, provided sufficient air is allowed in to convert it to carbon dioxide gas. Time is required for the brick to heat up, for oxygen to diffuse in, and for carbon dioxide to diffuse out. If this organic matter is not completely burnt off before the temperature rises to the point at which glassy material forms, the diffusion processes will not be possible, and carbon will remain within the bricks as undesirable black cores. The latter may also be caused by the lack of oxygen. An “opening material”, such as a burnt refractory clay, can be mixed in to aid gas diffusion.

Present carbonates and sulphides decompose at the top of the temperature range at which the organic matter is burnt, carbon dioxide and sulphur dioxide being given off.

Silica, which is a common constituent of brickmaking soils in the form of quartz, changes its crystal form at 573 °C. This so-called inversion is accompanied by an expansion. Consequently, the rate of rise of temperature must be slow if one is to obtain near-uniform temperature throughout the brick and thus avoid excessive stresses which could lead to cracking.

The glass formation, which is necessary to bond particles together and make the product strong and durable, commences at approximately 900 °C, depending upon the composition of the soil used. The process, known as vitrification, involves fluxes reacting with the various other minerals in the soil to form a liquid. The higher the temperature, the more the liquid formed, and the more the material shrinks. In practice, the heating must be restricted lest so much liquid forms that the whole brick starts to become distorted under the weight of the higher layers of bricks. In extreme cases, the bricks get fused together in the kiln. Gas formation can ‘bloat’ brick faces.

A few hours ‘soaking’ at the finishing temperature is recommended to ensure that the whole brick has attained uniformity. New materials, such as mullite, may crystallise from the liquid at temperatures which may reach approximately 1,100 °C for some brickmaking clays. In these ceramic reactions, a long firing time at a low temperature can have the same effect as a shorter firing-time at a high temperature. As cooling commences, the liquid solidifies to glass, bonding other particles together. The cooling rate should be slow to avoid excessive thermal stresses in the bricks, particularly once the quartz inversion temperature (573 °C) is reached, since shrinkage occurs in the presence of quartz.

The inevitable firing shrinkage should be fairly small, otherwise it would be difficult to maintain the stability of the bricks in the kiln.

II.6 Other basic requirements

High technology tends to limit the range of clay types acceptable for a particular process machine, and is less versatile as regards the type and grade of fuel. On the other hand, a wide range of materials and fuels can be used with less sophisticated technologies. Fuel, whether oil, gas, coal, wood, scrub or plant wastes, must be available for the brickmaking process and may be regarded as a raw material. Electricity may be advantageous for ancillary purposes. Water is also necessary and, for highly plastic clays, sand may also be required.

III. Clay testing and significance of results

Although highly sophisticated clay testing methods have been evolved, very simple tests can also give useful information. The former may be necessary for large turnkey projects, where equipment is often adjusted for specific raw materials characteristics. However, they require skilled staff not only to carry out tests, but also to interpret the results. On the other hand, simple tests may often be carried out on site, by less qualified personnel. Yet, the results may be more easily related to the use of the raw material than those obtained from more sophisticated tests.

The most direct test method used successfully for thousands of years involves visual inspection and the feel of the soil, and the carrying out of brickmaking trials.

Tests to investigate various aspects of a soil’s suitability for brickmaking are given below, starting with the most basic field test methods. Simple, intermediate technology tests are described next. Finally, a brief description of the more sophisticated tests which might be employed if adequate facilities exist, is provided at the end of this section.

III.1 Particle size

A visual inspection of the raw material will show whether the soil contains sand; a magnifying glass may assist in this operation.

The ‘feel’ of a soil in the hand will give an indication of the proportion of different particles sizes. When dry, a sand constituent gives a sharp gritty feel. A piece of the hard soil rubbed with the back of the finger nail cannot be polished. When wetted and broken down between the fingers, the sand particles become more readily visible.

If there is a high proportion of clay the dry soil will feel smooth and powder may be scratched off it. Furthermore, a surface of a small lump can be polished with the back of the finger nail. Damp soil can be worked into any shape, but will tend to stick to the fingers. The more clay in the soil, the more difficult it will be to remove it from the hands by wiping or washing.

A suitable brickmaking soil will have a high proportion of sand, so that it may not take a polish. High clay content soils may need addition of sand to make them suitable.

An estimate of the proportions of the various size fractions can be obtained using the sedimentation jar test. Any straight-sided, flat-bottomed, clear jar or bottle may be used. An approximately one litre capacity jar will be adequate (figure II.4). One-third of the jar is first filled with broken-up soil. Clean drinkable-quality water is then added until the jar is nearly full. The content of the jar is next mixed up, one hand covering its mouth to avoid spilling. The soil is then left to settle for an hour, shaken again and allowed to settle a second time. An hour later, the depth of the separate layers can be seen and measured. The bottom layer consists of sand and any coarser particles. The medium layer consists of silt and the top layer of clay. Often, the top two layers will merge together. The settlement of the clay fraction may be slow with some soils. The use of salty water for this test will flocculate the clay and help it to settle, thus giving a clearly defined level in the bottle which can be measured more easily.

Where laboratory facilities exist, a wet sieving process may be used to estimate the quantities of various sizes of sand. The soil is first washed through a nest of sieves of increasingly fine mesh, and the quantities retained on each sieve are dried and weighed. The difference between the weight of these fractions and that of the initial sample is then equal to the weight of silt and clay. Further information about the composition of these finer materials can be obtained using a sedimentation method (the Andreason pipette) or a hydrometer method. Details of these and other methods are described in British Standard Methods of Test for Soils for Engineering Purposes - BS 1377:1975 (18).

Figure II.4 - Jar test

Soil used for the production of bricks by traditional methods should contain the following:

- 25 per cent to 50 per cent of clay and silt; and

- 75 per cent to 50 per cent of sand and coarser material.

The soil should preferably contain particles of all sizes

III.2 Plasticity and cohesion

If the moistened soil is rolled by hand (on a flat surface) into a cylinder, a sharp break of the latter when pulled apart indicates a very sandy soil with low plasticity (7). On the other hand, the soil may be considered adequate for brickmaking if the cylinder elongates to the point of forming a neck before breaking

Another test consists of preparing a long cylinder of 10 mm diameter and letting it hang unsupported while holding it from one end. The length of cylinder which breaks off will provide fairly accurate information on the properties of the soil. The breaking-off of a piece of cylinder of 50 mm or less will indicate that the soil is too sandy for brickmaking. In this case, it will be necessary to add some fat clay or ant hill material to the soil. On the other hand, the breaking-off of a piece of 150 mm or more will indicate the presence of too high a proportion of clay, necessitating the addition of sand or grog to the soil. A soil adequate for brickmaking will require that the length of the broken-off piece of cylinder is between 50 mm and 150 mm (19).

The properties of the wetted soil will depend upon the moisture content. A ball of suitable soil containing the correct amount of water should break into a few pieces when dropped from the held-out arm on to hard ground. On the other hand, a flattening out of the ball will indicate that the soil is too wet, while the breaking of the ball into a large number of small pieces will indicate that the soil is too dry. Some more precise assessment of plastic properties can be obtained by simple laboratory tests. The soil should be mixed up with an excess of water to make a very runny paste or slip. The latter is then poured on a dry porous plastic plate, and mixed continuously with a spatula or knife. As water is absorbed by the plate, the soil will become less liquid and new incisions made with the knife will take longer to close. Once an incision remains open, approximately 5 g of material should be taken from its vicinity and weighed immediately. The sample is then weighed again after a few hours’ drying in an oven at 110 °C. The moisture content can thus be calculated as percentage of the dry weight of clay. This percentage is termed the liquid limit of the soil.

Some small pieces of the clay may be removed from the plate and rolled by hand of a flat-glass plate in order to make filaments approximately 3 mm in diameter (figure II.5). At first, long filaments may be fashioned easily. Then, as the soil dries out there will come a point when they just start to crack longitudinally and break up into pieces approximately 10 mm long. Once this occurs, approximately 5 g of such pieces should be weighed, oven dried, and weighed again to determine the moisture content as a percentage of the dry weight of clay. This percentage is termed the plastic limit of the soil.

The difference between plastic and liquid limits is the plasticity index.

When more advanced facilities are available the liquid limit should be determined with the cone penetrometer, described for example in BS1377 (see section III.1). In this test, the penetration of the point of an 80 g metal cone having an apex of 30° is measured as it rests for 5 seconds on the moistened soil. From a series of readings for different moisture contents the liquid limit is determined as the moisture content which gives a 20 mm penetration. The test for estimating the plastic limit is the same as that described above.

Several other testing methods are used in well-equipped laboratories (17).

Soils with a low plasticity index will be difficult to handle for brick-moulding: the green brick will distort after demoulding if the soil contains a small excess of water while the soil will be too stiff to mould if it lacks sufficient water. A high plasticity index is therefore preferred.

Soils with a high plastic limit will require a great deal of water before they can be ready for moulding. Long drying is then necessary prior to firing. A high plastic limit and very high liquid limit may indicate the presence of montmorillonite, with its attendant moisture movement problems. Thus, montmorillonitic soils may not be adequate for simple brick-moulding methods as the latter require a relatively high moisture content. They need either high compaction pressures on semi-dry mixes, or dilution with non-shrinking materials. Montmorillonites may give rise to size changes in the drying bricks as the humidity of the air varies naturally.

In a recently published book (20) reference has been made to an earlier suggestion (21) that, within the plasticity ranges indicated in table II.2, a soil may be adequate for the production of bricks by traditional methods. However, it may be possible to use materials with plasticity limits outside the ranges shown in the table.

Figure II.5 - Plastic limit test

Table II.2

Plasticity limits for good brickmaking soils

Plastic limit

12 to 22

Liquid limit

30 to 35

Plasticity index

7 to 18

III.3 Mineralogy and geology

The mineralogist recognises the presence of certain minerals in the field while the geologist identifies structures in the earth’s appearance that will assist in locating suitable raw materials sources.

The work of the mineralogist will consist largely of taking samples from the field and examining them under the microscope in a laboratory. On the basis of information from other tests, he may identify the components of a soil and thus determine their suitability for brickmaking by the various means available. In more advanced laboratories, the electron microscope (especially the scanning electron microscope) will be a useful tool. Identification of minerals will also be greatly assisted by X-ray diffraction analysis.

III.4 Chemical analysis

The colour of samples of materials obtained from field investigations gives some indication of the composition of the soil. Red soils may be high in iron, which can act as a flux. Very dark colours, or a musty smell in the damp soil, may indicate the presence of organic matter: it may be possible to use such soils, though their agricultural use should be given first priority. Dried out encrustations on the surface of the ground indicate the presence of soluble salts, which are best avoided for reasons given in Section II.3.

A simple laboratory test for the presence of sulphates consists of dissolving these salts and adding a solution of barium chloride. The forming of a white precipitate will indicate the presence of sulphates. On the other hand, chlorides can be detected by addition of silver nitrates. These chemical tests could be done on site, with a small portable test kit. The presence of calcium carbonate can be ascertained by the existence of lumps or nodules which are likely to be white, or by effervescence from gas produced by the addition of dilute hydrochloric acid to the soil. An estimate of the quantity of carbonate has been suggested(7): 1 to 8 per cent in case of slight bubbling; 8 to 16 per cent in case of pronounced bubbling; and 18 per cent in case of sudden foaming.

In a properly-equipped laboratory, a full chemical analysis may be undertaken, which, together with the mineralogical examination, can assist in identifying the constituents as mentioned in section II.3.

III.5 Drying shrinkage

High clay content (recognisable in wet conditions by the stickiness of the soil) is in dry weather, recognisable by the presence of shrinkage cracks in exposed soil, in either vertical or horizontal faces (see figures II.6 and II.7).

To obtain a measure of the shrinkage of a moist soil, which may seem suitable for brickmaking, the most simple method is to mould a few bricks from the soil and allow them to dry thoroughly. The length of the dried bricks and of the moulds are then measured in order to obtain an estimate of the linear drying shrinkage. The latter may be obtained from the following formula:

The appearance of the test bricks will give some indication of the suitability of the soil for brickmaking. It is suitable if no cracks appear on the surface. If some slight cracks appear it would be advisable to shorten the soil by adding 20 per cent sand or grog. In case of extensive cracks, 30 per cent might be mixed in. Soil too lean for moulding will have to be made more fat with other clays, or ant hill soil.

Generally, up to 7 per cent linear shrinkage may be tolerable, depending upon the nature of the material and the rate of drying. If linear shrinkage is more than 7 per cent shortening is advisable(22). In any case, it is necessary to know the linear shrinkage in order to determine the exact size of moulds needed for producing bricks of given dimensions.

If more organised test facilities are available, it would be advisable to prepare special shrinkage bars. For this test, an open-topped wooden mould, approximately 300 mm long by 50 mm deep and wide, should be made up by a carpenter or a sufficiently skilled handyman (figure II.8). The soil used in the test should be dried, if not already so, and broken down. Large stones should be removed. It is then mixed with just sufficient water to bring it near the liquid limit (i.e. pieces of the soil should be deformable yet retain their shape). If time permits the soil should be covered, left overnight, then mixed up again. The mould should be lightly greased inside to prevent the soil from sticking. Some moist soil is then laid in the bottom, and the mould tapped on the bench or ground to cause entrapped air bubbles to escape from the soil. The mould should be filled in the way described above in several stages, and excess soil struck off the top to leave a surface level with the surface of the mould. The soil should be dried slowly at first, at room temperature. Once shrinkage appears to have stopped, it may be tipped out the mould and dried in an oven at 110 °C. The linear shrinkage may then be calculated as indicated above.

Figure II.6 - Clay shrinkage on a vertical face

Figure II.7 - Shrinkage cracks in clay pit bottom

Figure II.8 - Shrinkage mould

III.6 Firing shrinkage

Some shrinkage during firing is inevitable. From 6 to 8 per cent linear shrinkage is desirable (5,7). The simplest field test to measure firing shrinkage is to burn a whole batch of bricks.

Measurements of firing shrinkage are more readily obtained in the laboratory than in the field. Small bars should be moulded, dried, measured, then fired to various temperatures in a laboratory furnace. They are then cooled and re-measured to calculate the linear firing shrinkage. A special furnace has been designed for this test. It requires only one sample, since it has a horizontal silica rod whose movement is measured outside the furnace as the temperature rises. A ‘gradient’ furnace of uneven temperature distribution can also give useful information.