There are mainly two methods to determine the parameters of the soil for brick making: Practical field low cost methods and complex laboratory analysis. Low cost field methods test for the soil are mainly performed to obtain a first impression of its properties to produce good quality bricks. For confirmative tests it is always recommended to get the soil tested for its constituents from an accredited laboratory.
There are a few basic field tests that can be performed in the field to find out suitability of soil for brick making. Although indicative, a good professional will always derive the correct conclusion out of the field based tests. Few Prevailing Field test are:
Take some loose soil from the possible brick making soil and put an appropriate amount of water into it in order to make a sticky paste. After the soil is saturated with water, mix the soil paste by hand. Try to make a ball with this soil. Roll the moist ball in the hand enough so that the ball is dried out a little bit. Pinch out a little bit of the soil with the thumb and the index finger and smear on the thumb by the index finger at one go. The smearing should be done as fine as possible. During this process, feel for any coarse particles. It will be felt by the fingers. After the smearing, if the soil does not form a smooth and thin layer, then the soil is sandy. If the thin soil layer is shiny and evenly spread out over the thumb then the soil is plastic in nature. Let the thumb dry out. After drying, if the soil layer falls off easily or can be removed then the soil is sandy or silty in nature with probably low plasticity. However if the soil sticks to the thumb and index finger after drying, then it is plastic in nature.
Ball test is mainly for getting a first feeling about the soils sand and clay proportion. Take a handful of soil and put some water in it. Water should be enough to make the soil moist and make dough by hand. With the hand and fingers mix the soil and water thoroughly. After uniform mixing try to make a ball out of the soil. This activity might take some time depending on the amount of water added. If the water content is too much then add more dry soil. Observe the smoothness of the surface of the ball. For plastic soils the surface will be shiny and uniform. For sandy soils the surface will be dull and rough. Also with sandy soils it will be difficult to make a round shaped ball.
Wet Ball test
Immediately after the ball is reasonably well formed, drop the ball from a height of at least 1 meter. Alternatively the ball can be dropped from shoulder height with hands straight. Take care that the surface on which the ball is dropped is leveled and clean – preferably a concrete surface or a hard surface. Never do the test on a wet or a loose surface. Observe the ball on the floor. If the ball retains its shape with little amount of deformation at the bottom only, then the soil is plastic clayey in nature. However if the ball flattens out upon hitting the floor, then the soil is sandy in nature.
Dry ball test
Repeat the tests by making balls and dry them under atmosphere or under a small open fire. Cool the balls and repeat the test. If the ball cracks into many pieces after contact with the floor then the soil is sandy in nature. However, if the ball breaks into two to three pieces then the soil is clayey and plastic in nature.
This process is also known as ‘Bottle Test’ for determining the proportion of clay and sand particles in the soil. It also gives the idea of percentage mixture of sand and clay in the soil. Fill one-fourth quantity of the glass beaker with the required soil. Add half teaspoonful of salt (to accelerate the deflocculation process) into the soil. Add water to about 50% above the soil level. Wait for a few minutes till the water percolates to the bottom (there will be a distinct colour difference between dry and wet soil). Stir the soil and water mixture vigorously with a spoon for at least 2 minutes. The colour of the stirred material should be uniform. Pour the stirred slurry into a measuring cylinder. Add some more water in the beaker and drain off the entire soil into the measuring cylinder. Repeat the process of adding more water in the beaker until it is totally empty. While pouring the soil into the measuring cylinder ensure that no soil is sticking to the sides of the measuring cylinder to avoid distortion of proportions. Place the measuring cylinder on a level platform and allow it to stand for at least 12 hours or until the water becomes clear at the top. As soon as the water is clear, there will be distinct granulation layers which represent the fineness/plasticity or coarseness/non-plasticity of the taken sample. Firstly measure the height of the bottom most visible layer which is classified as sand and calculate the percentage of sand. Secondly measure the topmost visible layer which is classified as clay and calculate the percentage of clay. The layers between the top (clay) and bottom (sand) represents the silt content of the soil sample. A representative soil mix of clay, silt and sand for good brick production is mentioned in the following table.
20 to 45
25 to 45
20 to 35
25 to 38
7 to 16
15 to 25
Too much clay content in any soil reduces the workability during brick moulding and increases the shrinkage rate thus forming cracks during drying of green bricks.
Too much of sand distorts the plasticity of the soil and results in high rate of breakage.
SOIL SHAPE TEST
Take some loose soil from the possible brick making soil and put an appropriate amount of water into it in order to make a soil ball. Try to form the soil mixed with water into a smooth and uniform ball with one hand only. If after repeated attempts the soil daub does not form into a round ball, then the soil is sandy. During this process, if water is released out of the ball then the soil is silty/sandy. However if after a few attempts, a good, smooth and round ball is formed, then the soil is semi plastic to plastic in nature. Wash hands with water. If washing is easy then the soil is silty/sandy with low clay content. However if after repeated washing, soil is sticking into the palms then the soil is clayey in nature. After washing, if the soil still sticks to the corners of the finger nails then the soil has probably very high clay content.
Lime is one of the most dangerous mineral for brick making. The presence of lime in soil defines whether to start a brick business or not if no other alternative soil is available. The presence of lime needs to be detected during the initial soil testing stage only. It does not affect the green brick quality. However after firing if the lime nodules containing fired bricks are kept in the open, it absorbs water from the atmosphere and expands, thereby bursting the solid brick. The more lime that is present in a soil the more bricks will be damaged. Hence profitability of the business is questioned. Therefore determination of lime is of utmost importance for making a GO or NO-GO decision in the brick business. The following method describes a simple field test of determining whether the soil contains lime or not. Take a lump of soil from the required area or depth from the selected brick making soil. Try to avoid loose soil collection since the lime nodules (if any) might not be contained within a small amount of loose soil. Look for soil lumps which have white spots in them. Take a representative soil sample. Ground the soil into a loose form by hand. Place a small amount of soil in the Petri dish. Do not put water into the soil, since it will delay the process of reaction with the acid. Take a small amount of acid (any acid found in the common market e.g. toilet cleaner) by a pipette and put it over the soil sample in the Petri dish. Observe the reaction it creates. Lime is present if the reaction causes effervescent (type of melting) or bubbling action.
Soil that contains any lime traces must never be used for brick making.