1. Removal of Top Soil
The preparation of soil involves first renewal of loose materials at the top of the ground for a depth of about 200 mm. As it contains a lot of impurities, this material is not used for the preparation of bricks.
2. Digging and Spreading
The earth is dug out 200 mm from the ground. The soil is spread on the level ground, and heaps of clay are about 600 to 1200 mm.
3. Cleaning
The soil after being spread on the ground should be cleaned of stones, pebbles, vegetable matter, etc. If non-clay materials are in excess, the clay should be washed and screened. Such a process will be expensive and cumbersome. All the lumps of soil should be broken into a powder form.
4. Weathering
The soil is then exposed to the atmosphere for softening or mellowing. The period of exposure may last for a few weeks depending on the nature of soil. This imparts plasticity and strength to the soil.
5. Blending
To increase the quality of soil, additional soil such as sandy or calcareous clays may be added in suitable proportions with a small quantity of coal, ash, etc. The whole mass is mixed uniformly and water is added.
6. Tempering
It is the process of kneading the soil under the feet of men or cattle after adding the necessary quantity of water in order to make the soil stiff and homogeneous. In general, a soft plastic clay could be prepared by using about 25 to 30% of water. This procedure is adopted for the majority of common handmade bricks. For making superior bricks on a large scale, the earth is tempered in a pug mill.
A pug mill comprises of a truncated conical tub of 120 cm diameter at the top and 75 cm diameter at the bottom with a height of about 180 cm. A long vertical shaft is placed at the centre (Fig. 3.1). The central shaft is provided with a number of horizontal arms attached with cutting knives for breaking clay lumps, if any. A long horizontal arm is fitted at the top of the vertical shaft. The shaft is rotated with the help of bullocks or sometimes by electric power. Clay and water in the ratio of 1:3/4 are fed into the vessel from the top. The tempered clay is collected from the hole provided at the bottom of the vessel. This type of pug mill, as shown in Fig. 3.1, can be used to get sufficient soil for a daily output of about 20,000 bricks.
Figure 3.1 Pug mill
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