Bricks are burnt in kilns after moulding and drying so as to impart hardness and strength and to increase the density of the brick so that it absorbs less quantity of water.
Physical and chemical changes take place in burning of bricks. Heating brick earth up to about 640°C produces only physical changes. At this temperature moisture and water of crystallization are driven out, and the organic matter is burnt. Such a brick that can absorb moisture from the air can get back hydrated. Such bricks are said to be poorly burnt and disintegrate when subjected to moisture contact.
If brick earth is heated up to 700–1,000°C, it undergoes chemical changes. During chemical action alumina and silica in brick earth fuse together resulting in a compound which is strong and stable. After this chemical transformation, it does not turn back to break earth on cooling. Further, it does not crumble when immersed in water. Thus, the break earth burnt above 700°C is different from the original brick earth. Hence, to get a good quality brick it has to be heated to the required temperature.
On heating the brick earth beyond 1,300°C, the above materials get vitrified. The bricks begin to lose their shape.
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