NTS STUDY

Bricks: Suitable Earth, Preparation Process, and Types

The use of mud bricks by humans has been practiced since ancient times. Initially, they were used as sun-dried (unburnt) bricks. Burnt bricks were a common construction material among the Egyptians. Today, they are primarily made from specially selected and matured brick-earth consisting of Silica (35 to 70 percent) and Alumina (10 to 20 percent).

Too much silica makes the brick brittle, while too much alumina causes the brick to warp and crack during drying and burning. The presence of other agents like lime, magnesia, and iron oxides is also desirable, as they act as coloring agents and fluxes during the burning process. When clay is heated at low temperatures, it loses moisture and undergoes only physical changes; such under-burnt bricks crumble when placed in water. However, at high temperatures (700 to 1100°C), chemical changes occur, resulting in well-burnt bricks that do not break in water.

Suitable Earth for Bricks

A brick-earth quarry should contain soil suitable for brick making or soil that can be made suitable by mixing with others.

• Laboratory Tests: Laboratory tests such as liquid limit, plastic limit, and shrinkage limit can be used to determine suitability.

• Manual Check: Alternatively, an experienced person can judge suitability by kneading the soil between their fingers.

• Brick vs. Tile Clay: Clay requirements for tiles differ from bricks; tiles require more plastic clay because they are molded into thinner, impermeable sections.

Process of Preparing Bricks

The different stages of preparing general bricks are as follows:

1. Preparing the Clay:

   • After removing vegetation, the soil is excavated in steps rather than layers to ensure a better distribution of components found at different depths.

   • Weathering: For first-class bricks, the clay is left exposed to the open air for a considerable time so that lumps break down and mature.

   • Refining: For superior bricks, the clay is washed and processed before molding.

2. Molding of Bricks:

   • Bricks are molded based on the desired quality. They can be hand-molded, machine-molded (as wire-cut bricks), or pressed in molds.

3. Burning of Bricks:

   • Bricks are burnt in temporary clamps or permanent kilns.

   • Clamps: Batches of dried bricks are stacked with firewood or coal, sealed with mud, and cooked over several days.

   • Kilns: Modern kilns are permanent structures with multiple chambers (intermittent or continuous). The cycle of loading, drying, burning, and cooling can take up to two weeks. Kilns are preferred over clamps because heat can be better controlled, resulting in uniform quality and fuel savings.

Types of Manufactured Bricks

In India, hand-molded bricks are more common and less expensive than machine-molded ones.