TERRACOTTA

It is an Italian word, Terra means clay and Cotta means burnt. Terracotta is refractory clay product and is used in ornamental parts of buildings. The clay used for its manufacture should be of superior quality and should have sufficient iron and alkaline matters. By varying iron oxide in clay, desired colour can be obtained. The clay is mixed with powdered glasses, pottery and sand ground to fine powder and pugged several times till it gets uniform and soft for moulding. Terracotta is impervious, hard and cheap. When properly made the material weathers well and becuse of its glazed surface can be cleaned easily. The product is burnt in special kilns (Muffle furnance).

Preparation of Clay :

The clay is mixed thoroughly with water in a tub. Powdered pottery, glass and white sand are added to it in sufficient proportions. It is then intimately mixed with spades. The intimate mix is then placed in wooden boxes with joints. This allows the surplus water to drain off. Thereafter the mix is passed several times through pug mills.

Moulding and Drying :

Special porous moulds are made of Plaster of Paris or of zinc. The pugged clay is pressed into moulds. The dried articles are taken out of the moulds after a few days and then dried slowly.

Burning :

Terracotta is burned with care to get uniform colour in muffle furnance between 1100– 1200°C.

Composition :

Uses :

1. Hollow blocks of terracotta are used for masonry.

2. Cornices and arches.

3. Statuettes.

4. Ornamental works.

5. Being fire proof, terracotta is most suitable as casing for steel columns and beams.

6. Porous terracotta is used for sound insulation.

Classification :

Terracotta is of two types, the porous and the polished (Faience).

Porous Terracotta :

It is manufactured by mixing sawdust or finely fragmented cork in the clay and has the following characteristics.

1. Light weight.

2. Resistant to weathering action.

3. Fire resistant.

4. Can be nailed and sawn to various shapes.

5. Sound proof.

6. Poor strength—used only for ornamental works.

Polished Terracotta :

Is highly glazed architectural terracotta with relatively coarse body. These are made from refractory clays with addition of quartz sand and fusing agents such as chalk.The polished terracotta is also called terracotta twice burnt. The 1st burning is called biscutting and is done at 650°C. Then, this product is coaled with glazed solution which imparts texture and colour. Thereafter it is dried and fired at 1200°C. The material

1. is hard, strong and durable.

2. can be given different colours.

3. is leak proof (water absorption < 12 per cent) and can be easily cleaned.

4. is resistant to chemical action.

5. is resistant to weathering action of atmosphere.