Burning of Bricks
The burning of clay may be divided into three main stages.
Dehydration (400-650 o C): This is also known as water smoking stage. During dehydration,
Oxidation Period (650-900 o C): During the oxidation period, (1) remainder of carbon is eliminated and,
(2) the ferrous iron is oxidized to the ferric form. The removal of sulphur is completed only after the carbon has been eliminated. Sulphur on account of its affinity for oxygen, also holds back the oxidation of iron. Consequently, in order to avoid black or spongy cores, oxidation must proceed at such a rate which will allow these changes to occur before the heat becomes sufficient to soften the clay and close its pore. Sand is often added to the raw clay to produce a more open structure and thus provide escape of gases generated in burning.
Vitrification-To convert the mass into glass like substance - the temperature ranges from 900- 1100 o C for low melting clay and 1000-1250 o C for high melting clay. Great care is required in cooling the bricks below the cherry red heat in order to avoid checking and cracking. Vitrification period may further be divided into (a) incipient vitrification, at which the clay has softened sufficiently to cause adherence but not enough to close the pores or cause loss of space-on cooling the material cannot be scratched by the knife; (b) complete vitrification, more or less well-marked by maximum shrinkage; (c) viscous vitrification, produced by a further increase in temperature which results in a soft molten mass, a gradual loss in shape, and a glassy structure after cooling. Generally, clay products are vitrified to the point of viscosity. However, paving bricks are burnt to the stage of complete vitrification to achieve maximum hardness as well as toughness.
Burning of bricks is done in a clamp or kiln. A clamp is a temporary structure whereas kiln is a permanent one.
Burning in Clamp or Pazawah: A typical clamp is shown in Fig. 2.9. The bricks and fuel are placed in alternate layers. The amount of fuel is reduced successively in the top layers. Each brick tier consists of 4-5 layers of bricks. Some space is left between bricks for free circulation of hot gasses. After 30 per cent loading of the clamp, the fuel in the lowest layer is fired and the remaining loading of bricks and fuel is carried out hurriedly. The top and sides of the clamp are plastered with mud. Then a coat of cowdung is given, which prevents the escape of heat. The production of bricks is 2-3 lacs and the process is completed in six months. This process yields about 60 per cent first class bricks.
Kiln Burning: The kiln used for burning bricks may be underground, e.g. Bull's trench kiln or overground, e.g. Hoffman's kiln. These may be rectangular, circular or oval in shape. When the process of burning bricks is continuous, the kiln is known as continuous kiln, e.g. Bull's trench and Hoffman's kilns.
On the other hand if the process of burning bricks is discontinuous, the kiln is known as intermittent kiln. Intermittent Kiln: The example of this type of an over ground, rectangular kiln is shown in Fig. 2.10. After loading the kiln, it is fired, cooled and
unloaded and then the next loading is done. Since the walls and sides get cooled during reloading and are to be heated again during next firing, there is wastage of fuel.
Continuous Kiln: The examples of continuous kiln are Hoffman's kiln (Fig. 2.11) and Bull's trench kiln (Fig. 2.12). In a continuous kiln, bricks are stacked in various chambers wherein the bricks undergo different treatments at the same time. When the bricks in one of the chambers is fired, the bricks in the next set of chambers are dried and preheated while bricks in the other set of chambers are loaded and in the last are cooled.
Note: In the areas where black cotton soil occur, a more elaborate method of processing is
followed. The clay, which may be black or a mixture of black and yellow, is first washed free of the lime kankar in the 'GHOL' tanks. The slurry is then run off to the setting tanks. After 3-4
days when the clay has settled down, the supernatant water is bucketed off. Opening material like powdered grog of fine coal ash (passing 2.00 mm sieve), which opens up the texture of clay mass, is then added in predetermined proportions. This is usually 30 to 40 per cent of the mass of clay. A solution of 0.5 per cent sodium chloride may also be added at this stage to prevent lime bursting. The clay is then thoroughly mixed with the opening materials added and allowed to dry further for a period of 3-4 days till the mix attains the correct moulding consistency. Grog is prepared by lightly calcining lumps of black cotton soil (about 10 to 15 cm dia.) in a clamp at about 700 o to 750 o C. Coal ash, fire wood, brambles, etc. may be used as fuel. The fuel and clay lumps are arranged in alternate layers in the clamp. After calcination the clay is pulverized in a machine, such as disintegrator, a hammer mill or a pan-mill to a fineness of less than 2.0 mm.
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