List of commonly used ceramic products.
Ceramic Product # 1. Clay Products:
The clay occurs plenty in nature. The clay is a distinct product of chemical weathering of igneous rocks. The felspar is predominant in igneous rocks. One of the variety of felspar is orthoclase felspar. It is whitish, greyish or pinkish in colour. The rocks disintegrate easily, if orthoclase felspar is in abundance in their structure. Thus the orthoclase felspar is mainly responsible for the production of clays in nature. This mineral, on decomposition, gives kaolinite which is free from iron oxide and alkalis.
The term kaolin is used to indicate the product having composition of pure kaolinite. In kaolin, the alumina and silica compounds are held in a colloidal state and these compounds form the basic constituents of all clays. In addition to these compounds, the various other materials such as silicates of calcium and magnesium, iron oxide, free sand, sodium, manganese, chromium, etc. also occur in clays in small proportions.
The clay, when made wet with water, possesses a high degree of tenacity and plasticity. Such plastic clay can be moulded in desired shapes. It is then dried and burnt.
The clay contains water in two forms, namely,
(i) Free water,
(ii) Combined water.
The free water is removed during drying. To remove chemically combined water, the clay is heated to a high temperature. At this stage, the chemical changes occur among the constituents of clay and new products are formed which are hard and compact. Beyond a certain limit of temperature, the clay becomes soft and products lose their shapes. This limit of temperature will depend on the quality of clay.
The clay products which are employed in building industry are tiles, terra-cotta, earthen-wares, stone-wares, porcelain and bricks.
Ceramic Product # 2. Tiles:
The tiles may be defined as thin slabs of brick which are burnt in kiln. They are thinner than bricks and hence they should be carefully handled to avoid any damage to them.
The tiles can be classified in the following two groups:
(I) Common tiles – These tiles have different shapes and sizes. They are mainly used for paving, flooring and roofing.
(II) Encaustic tiles – These tiles are used for decorative purposes in floors, walls, ceilings and roofs.
The encaustic tiles are manufactured from carefully prepared ordinary clays, colouring materials and sometimes with finer clays. Depending upon the colouring pigment added in the clay, these tiles obtain the desired print or colour after manufacture.
An encaustic tile usually consists of the following three layers:
(1) Body – It is made of coarser clay.
(2) Face – It comprises of a 6 mm coat of finer clay and the colouring matter for making the ground of the pattern.
(3) Back – It is a thin coat of clay to prevent the tile from warping.
The manufacturing process of these tiles is as follows:
(i) The face is moulded to the desired pattern.
(ii) The coarser clay body is put on the back of the face and also a thin coat to form the back.
(iii) The maker’s name is stamped on the back.
(iv) A few holes are kept for joining with cement during laying.
(v) The clay with different colours is poured into the different portions of the pattern so as to obtain the desired design of colours.
(vi) When the green tiles become dry enough for handling, the excess earth is removed or scraped off.
(vii) The green tiles are then trimmed, dressed, dried and burnt in the dome kiln.
(viii) If required, the burnt tiles may be glazed by dipping them into a mixture of powdered glass and water and then reheating them.
Ceramic Product # 3. Terra-Cotta:
The terra means earth and cotta means baked. Hence the terra-cotta means the baked earth. It is thus a type of earthenware or porous pottery made from local clays and glazed with glazes containing galena. It is soft enough to be scratched by a knife.
Manufacture of Terra-Cotta:
Following four distinct operations are involved in the manufacture of terra-cotta:
(i) Preparation of clay
Each of these operation will now be briefly described.
(i) Preparation of Clay:
For terra-cotta, the selected clay is taken. The clay should contain a slightly higher percentage of iron oxide, about 5% to 8% and proportion of lime should be less, about 1 per cent or so. Sometimes several varieties of clay with high alumina content are taken and then to this mixture is added sand, ground glass, old terra-cotta or pottery. The addition of such materials gives strength and rigidity to the terra-cotta products and it prevents shrinking while drying.
Such clay is made free from any impurity such as grit, pebbles, organic matter, etc. It is then finely crushed and pulverized. The water is added in required quantity and the ingredients are thoroughly mixed with spades. Such wet clay is kept for several days in a damp condition for weathering and tempering.
It is then pressed or kneaded in a pug mill and it is made ready for the next operation of moulding. The required quantity of colouring substance is added at this stage to obtain the desired shade of colour in the final product of terra-cotta.
The clay is placed in moulds which represent the pattern or shape in which the product is to be formed. For terra-cotta work, special moulds of plaster of Paris or templates of zinc are used. The size of moulds is determined by keeping due allowance for shrinkage. The fine sand is sprinkled on the inside surface of moulds and clay is then pressed in moulds with hand.
The moulds filled in with clay are kept for some days for drying. After this period, the articles of the terra-cotta are taken out from the moulds and they are allowed to dry further in a room or under a shed. The drying should be done carefully and slowly with proper control of temperature. The gradual drying helps in retaining the correct shape and size of the blocks.
The dried products are then burnt in special muffle furnaces. Fig. 3-2 shows a typical muffle furnace.
A muffle indicates a box or a compartment of a furnace in which things can be heated without contact with the fuel and its products. A damper indicates a metal plate which is provided in an opening to regulate the draught. The dried articles are arranged in muffle and temperature of kiln is raised to about 1200°C.
This temperature is maintained for about four days and the burnt products are then allowed to cool down in kiln for a period of about five days. For getting the glazed products, the glazed materials should be applied by brush on terra-cotta products before they are burnt.
Varieties of Terra-Cotta:
The terra-cotta articles are of the following two types:
(i) Porous terra-cotta
(ii) Polished terra-cotta.
(i) Porous Terra-Cotta:
To prepare porous terra-cotta, the saw dust or ground cork is added in clay before the stage of moulding. When articles from such clay are burnt in a kiln, the organic particles are burnt and they leave pores in the articles. The porous terra-cotta is a fire-proof and a sound-proof material. It can be chiselled, sawn and nailed easily with nails, screws, etc. It is light in weight, but it is structurally weak.
(ii) Polished Terra-Cotta:
This is also known as the fine terra-cotta or faience. To obtain this variety of terra-cotta, the articles are burnt at a lower temperature of about 650°C. The first burning is known as the biscuiting. The articles brought to biscuit stage are removed from kiln and are allowed to cool down.
They are then coated with glazing compound and burnt again in the kiln at a temperature of about 1200°C. The faience is available in a variety of colours and it indicates superior quality of terra-cotta. It is used for ornamental purposes and in industrial areas since it is ordinarily unaffected by the adverse atmospheric conditions.
Advantages of Terra-Cotta:
Following are the advantages of terra-cotta:
(i) It is strong and durable material.
(ii) It is available in different colours.
(iii) It is cheaper than ordinary finely dressed stones.
(iv) It is easily cleaned.
(v) It is easily moulded in desired shapes.
(vi) It is fire-proof and can therefore be conveniently used with R.C.C. work.
(vii) It is light in weight.
(viii) It is not affected by atmospheric agencies and acids and is capable of withstanding weathering actions better than most kinds of stone.
Disadvantages of Terra-Cotta:
Following are the disadvantages of terra-cotta:
(i) It cannot be fixed during the progress of work. But it is to be fixed when the work is in final stage of completion.
(ii) It is twisted due to unequal shrinkage in drying and burning.
Uses of Terra-Cotta:
Following are the uses of terra-cotta:
(i) The hollow terra-cotta blocks are used for various ornamental purposes such as facing work, arches, cornices, casing for columns, etc.
(ii) It is adopted for all sorts of ornamental work.
(iii) It is used as a decorative material in place of stones for ornamental parts of buildings such as cornices, string courses, sills, copings, bases of pillars, fire places, etc.
Ceramic Product # 4. Earthenware:
The term earthenware is used to indicate wares or articles prepared from clay which is burnt at low temperature and cooled down slowly. The clay is mixed with required quantity of sand, crushed pottery, etc. The addition of such materials prevents the shrinkage during drying and burning.
The earthen-wares are generally soft and porous. When glazed, the earthen-wares become impervious to the water and they are not affected by acids or atmospheric agencies. The terra-cotta is a kind of earthenware. The earthenware is used for making ordinary drain pipes, electrical cable conduits, partition blocks, etc.
Ceramic Product # 5. Stoneware:
The term stoneware is used to indicate the wares or articles prepared from refractory clays which are mixed with stone and crushed pottery. Such a mixture is then burnt at a high temperature and cooled down slowly.
The stoneware is more compact and dense than earthenware. When glazed, the stone-wares become impervious to the water and they are not affected by acids or atmospheric agencies. The sound stone-wares give clear ringing sound when struck with each other.
The stone-wares are strong, impervious, durable and resistant to corrosive fluids and they resemble fire bricks. The stone-wares can be kept clean easily and hence they have become very popular as the sanitary articles such as wash basins, sewer pipes, glazed tiles, water closets, gully traps, etc. They are also used as jars to store the chemicals.
The term porcelain is used to indicate fine earthenware which is white, thin and semi-transparent. Since the colour of porcelain is white, it is also referred to as the white-ware.
The clay of sufficient purity and possessing high degree of tenacity and plasticity is used in preparing porcelains. It is hard, brittle and non-porous. It is prepared from clay, felspar, quartz and minerals. The constituents are finely ground and then they are thoroughly mixed in liquid state. The mixture is given the desired shape and it is burnt at high temperature.
The various types of porcelains are available and they are adopted for various uses such as sanitary wares, electric insulators, storage vessels, reactor chambers, crucibles, etc.
The porcelains are of two types, namely low voltage porcelain and high voltage porcelain. The low voltage porcelain is prepared by dry process and it is mainly used for switch block, insulating tubes, lamp sockets, etc. If some quantity of alumina or silicate of magnesia is added, it can resist high temperature to a certain extent.
The high voltage porcelain is prepared by wet process. Table 3-3 shows the varieties of high voltage porcelain.
Ceramic Product # 7. Clay Blocks:
The blocks can be prepared from clay and they are used in the construction of partitions. Such blocks may either be solid or hollow. Fig. 3-3 shows a typical hollow clay block.
The blocks are usually of section 300 mm X 200 mm and the thickness of hollow blocks varies from 50 mm to 150 mm. The thickness, in case of solid block, is about 40 mm. The blocks are provided with grooves on top, bottom and sides. These grooves help in making the joints rigid and they serve as a key to the plaster. Sometimes the surfaces of blocks are made glazed in a Hollow clay block variety of colours.
It is found that partitions of clay blocks are efficient in preventing fire and passage of sound. They are light in weight and are non-shrinkable.