Bricks, Terra-Cotta, Glazing of white product

Updated: Sep 9, 2020

Operation Involve in Manufactureof brick

1. Preparation of clay. Good brick earth, a mixture of pure clay and sand along with a small quantity of finely divided lime is first dug out, broken up, watered and kneaded well under feet till it becomes a homogeneous mass. The tempered earth is then covered up with mat pieces and allowed to dry gradually till it is just soft-enough for moulding. For manufacturing superior bricks, the clay

is generally prepared by pug mills.

2. Moulding. The well prepared clay is moulded in rectangular steel or wood moulds without top and bottom, their longer sides project a few centimeters to act as handles. Bricks are usually moulded on a block of wood having a projection 6 mm deep and same length and breadth as the inside dimensions of the mould. Moulding of bricks done on the stock board, is called table moulding.

3. Drying. The moulded bricks are then allowed to dry so that these are sufficiently hard to be handled. When the bricks become sufficiently hard, these are stacked. Eight or ten layers of bricks on edge with intervals of about one metre between them, are generally stacked.

4. Burning. Well dried bricks are burnt in clamps or kilns to attain desired crushing strength and also to impart red or yellowish colour.

Classification and Characteristics of Bricks as per Indian Standard Institution

First Class Brick

Properties These bricks are well burnt, having smooth and even surface, with perfect rectangular shape and uniform reddish colour. When struck with other brick, these give a metallic ringing sound. These should not leave any mark when scratched by finger nail. These should not absorb water more than 20% of its weight when immersed in cold water for 24 hours. When broken into two pieces these should show a uniform compact structure. These show slight efflorescence.

Used These are used for good structures such as outer walls and facing work when no plastering is done. These are also used in floors and reinforced bricks slabs. Such bricks should be laid in rich mortar.

Second Class Brick

Properties These are not perfectly rectangular in shape and are having rough surface, but are hard, slightly over-burnt and uniform in colour. These give ringing sound when struck with each other. Water absorption should not be more than 22% by weight, when immersed in water for 24 hours. These show slight efflorescence.

Used These are used for internal walls, not exposed to atmosphere, these are used in facing work, which should be plastered. These cannot be used for R.B, work. Such bricks may be laid in mud or lime mortar

Third Class Brick

Properties These bricks are not burnt properly in the kilns and may be slightly under/over-burnt, hence these are soft and can be easily broken. These are light red in colour, with yellowish tinge. On striking, these do not give a ringing sound. These should not absorb more than 25% of water by weight when placed in cold water for 24 hours. Efflorescence in these bricks is moderate

Used These are used for inferior construction works, or at places where there is less rainfall or presence of dampness.

Jhama or over-burnt Bricks

Properties Due to excess fusion and temperature bricks get over-burnt loose their shape and get twisted. These bricks yet dark bluish in colour.

Used These bricks are not used in building construction work. In the form of broken pieces, these may be used as road metal, also in foundations and floors as soling material

Tests for the Acceptance of Bricks for Building Construction

1. Dimensions and tolerances test

2. Compressive strength test

3. Water absorption test

4. Efflorescence test.

1. Dimensions and tolerances test -This test is performed to know the accuracy of the dimensions of the bricks.

Procedure. Proceed as under :

1. Take twenty bricks out of the given samples.

2. Remove loose particles of clay and small projections from the bricks.

3. Arrange them on a level surface in contact with each other and in a straight line.

4. Measure the overall length of the bricks having size 19 x 9 x 9 cm laid by means of a steel tape.

5. The dimensions of 20 bricks should be within the following limits :

Class Length Width Height

Class A 368 to 392 cm 174 to 186 cm 174 to 186 cm

Class B 350 to 410 cm 165 to 195 cm 165 to 195 cm

2. Compressive strength test-This test is performed to determine the crushing strength of bricks

Procedure. Proceed as under :

1, Take five bricks out of the sample at random.

2. Immerse the bricks in water at room temperature for 24 hours.

3, Take out the bricks from the water and wipe off surplus water from their surfaces.

4. Fill the frogs and all voids in the bed and face with cement mortar 1:1( 1 cement: 1 clean sand).

5. Store the bricks under damp gunny bags for 24 hours and there-after immerse them in water for 72 hours.

6. Take out the bricks from water, wipe off dry. Place the bricks with flat surfaces horizontal and mortar filled frog face upward between two or three thin ply sheets and centre them between the plates of a compression testing machine.

Apply the load at a uniform rate of 140 kg/cm² per minute till the brick fails.

8. Take the average value of the compressive strengths of the five bricks.

9- The compressive strength of a common brick should be 50 Kg/cm²

3. Water absorption test -This test is performed to determine water absorption of the bricks. If the water absorption capacity of a brick is more, its strength will be comparatively low.

1. Select five bricks at random out of the given sample.

2. Dry them in a ventilated oven at 105° to 110° till they attain practically constant weight.

3. Remove the bricks from the oven and cool them to room temperature.

4. Weigh the bricks in a balance. Let it be W₁ kg.

5. Immerse the five bricks in water completely at 27° ± 2C for 24 hours.

6. Remove one brick from water and wipe off its surfaces with a damp cloth.

7. Weight the brick within three minutes after its removal from water. Let its weight be W₂ kg.

8. Water absorption capacity = (W₂-W₁)/W₁ X 100

9. Take the average value of the water absorption capacities of the fi ve bricks.

10. For 1st Class bricks, the water absorption capacity should not be more than 20% by weight.

4. Efflorescence test.

Procedure. Proceed as under :

1. Take five bricks at random from the given sample.

2. Place each brick on end in a dish containing distilled water ensuring depth immersion at least 2.5 cm.

3. Keep the dish in a ventilated room (Temp. 20° to 30°C) till the whole of distilled water in the dish evaporates.

4. Again pour 2.5 cm depth of distilled water in the dish and keep it till the whole of water gets evaporated.

5. Now, examine the bricks for efflorescence as detailed below :

(i) No perceptible deposit Nil - efflorescence

(ii) 10% area covered with thin deposit of salts

(iii) 50% area covered with deposit of salts Slight - efflorescence

(without any powdering or flaking surface)

(iv) 50% area covered with deposit of salts Moderate - efflorescence

(accompanied by flaking of surface)

(v) Heavy deposits of salts Serious - efflorescence

accompanied by flaking of the surface.


Terra-cotta which is a baked clay or baked earth is a superior variety of clay products and is usually moulded in the same manner as bricks. It is made from a mixture of fine clay (60%), crushed pottery (20%), white sand (14%) and powdered glass (6%), with a quantity of desired colouring substance.

For making a porous and sand proof terra-cotta, either sawdust or ground cork may be mixed with clay before moulding. Organic particles burn away during the burning of the moulded and dried terra-cotta and thus leaving behind small pores. Terra-cotta is used for architectural and ornamental parts of superior buildings as a substitute for stones. It is used as sound proof material and its hollow blocks prevent dampness in the structure

Glazing Of White Ware Products

Surfaces of white ware products are generally glazed to improve their appearance and also to protect them from the action of atmosphere, sewage and strong chemical agents. For providing transparent glazing, self glazing is the most important method. In this method a solution of sodium chloride is thrown in the kiln when the product is well burnt at a temperature 1200° to 1300°C. Due to high temperature, the sodium chloride evaporates and combines with silica of soil to make soda silicate. Soda silicate again combines with alumina, lime or iron of the clay to form a thin transparent layer. Vapours of volatilised salt get into every pore of the product and thus make it impermeable.

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