4

TILES AND CERAMIC MATERIALS

4.1 INTRODUCTION

Naturally available clay is used directly or in the processed form to make tiles and ceramic materials apart from making bricks.

Tiles are thin slabs made out of brick earth by moulding and then burning them. The best clay for tile making is generally found a few metres below the brick earth. Tiles have been used for roofs, floors, walls, etc.

Ceramics is the term used for the act of making objects with a special type of clay. This term is derived from the Greek word keramos meaning potter’s earth. Clay for ceramics is the product of chemical weathering of various types of rocks. Orthoclase feldspar is mainly responsible for the production of clays in nature. This mineral forms kaolinite during decomposition. Kaolin, a product having the composition of pure kaolinite, is fine white clay required to manufacture some ceramic articles.

Apart from bricks and tiles, other objects which are made using various types of clays and techniques are discussed below.

Earthenware is the term used for clay products burnt at low temperature. They have coarse surfaces and are opaque. General cooking vessels usually used in villages fall under this group.

Stoneware is a product of refractory clay, ground stone and pottery. These are burnt at high temperatures to vitrification. Stoneware jars are of great use. Earthenware and stoneware are together known as pottery.

Porcelain is a white translucent ceramic material of kaolin and feldspar. The objects are produced by firing at a very high temperature. Most crockery are made of porcelain and are glazed.

Terracotta is a hard brownish unglazed vitrified ceramic material used for ornamental purposes.

4.2 BUILDING TILES

As discussed earlier, building tiles are thinner than bricks and are susceptible to cracking and warping. Hence, special care has to be taken in the manufacture of tiles.

4.2.1 Characteristics of a Good Tile

A good-quality building tile should possess the following characteristics:

1. It should be uniformly burnt up to the requirement.
2. It should possess a uniform colour.
3. No cracks, flaws or bends must appear on the tiles.
4. The stipulated shape and size should be maintained.
5. It should produce a clear ringing sound when struck with another tile or by a light hammer.
6. It should have accurate measurement so as to fit well when placed in position.
7. When a tile is broken, the broken surface should exhibit even and compact surface.
8. It should be hard and durable.

4.2.2 Types of Tiles

Tiles may be grouped under two broad categories:

1. Common clay tiles
2. Ceramic tiles

1. Common Clay Tiles

Common clay tiles are further classified as clay roof tiles, clay floor tiles, clay terracing tiles and clay ceiling tiles.

(i) Clay Roof Tiles

Clay roof tiles are made of plastic clay, which is different from brick clay. There are three types of roof tiles that are used for covering sloped roofs, viz., Mangalore tiles, country tiles or pot tiles and Allahabad tiles.

Mangalore tiles are available in various shapes and dimensions (Fig. 4.1(a)). They overlap the tile beneath them and also lock with the adjacent tiles. These tiles are entirely popular in South India, particularly in the states of Kerala, Tamil Nadu and Pondicherry.

The full specification is given in IS: 654–1972. The gap provided at the corners should not be more than 6 mm. Two batten lugs are to be provided for the tile. Further, there are two eave lugs. In order to prevent from lifting off by high winds, at least one hole is provided at the crossribs near the eave end for securing the tile to the reaper or batten.

Figure 4.1 Common clay roof tiles

Country tiles are half round which are laid in pairs as undertiles and overtiles (Fig. 4.1(b)). This is achieved by placing the tiles on the sloped roof with the concave side up and with the longer end towards the ridge. Then another row of the same tiles with the convex side up and the small end towards the ridge is laid covering the adjoining edges of every pair of tiles below. These tiles are also sometimes used as ornamentation by placing the tiles over AC or GI sheets. These tiles are extensively used in villages.

Allahabad tiles consist of two shapes of tiles, one for bottom use and the other for top use (Fig. 4.1(c)). The overtiles are half round and are tapered, while the undertiles all are flat, tapered with upturned flanges at the sides. Specification are available in CPWD specification.

(ii) Clay Floor Tiles

These tiles are made from fine special blue clay which enables the tiles to be made thin and the surface smooth. In order to make the tiles hard and impervious, a mixture of ground glass and potteryware are added. In general, these materials are mixed together thoroughly and are extruded in the shape of a slab. These slabs are placed in moulds and are pressed to the required shape. They are burnt at about 1,300ºC of temperature. In order to give a good appearance they are salt glazed. These tiles are more suitable for walking on them barefooted.

(iii) Clay Terrace Tiles

These tiles are also flat tiles made in the same way as floor tiles. They may be hand made or machine made and burnt in a kiln. Large-size tiles are used nowadays. Detailed specifications are available in IS: 2690–1964.

2. Ceramic Tiles

(i) Glazed Ceramic Tiles

They are made from special ceramic clays in two operations. In the first operation the tiles are made and fired at 1,200–1,300ºC; then they are coated with glaze, decorations, etc. Again they are fired in ovens to get opaque glazing. Wear of the glazing is based on the temperature of firing and the thickness of glazing.

These tiles are used for walls or as floor tiles, where there is less usage. Large-size tiles are used for flooring in offices, airports, etc.

(ii) Vitrified Tiles

In this case special ceramic clay is mixed with oxides and burnt at a very high temperature. At a very high temperature, the clay becomes vitrified and we get coloured vitrified tiles. Compared to ordinary glazed tiles wherein the glazing is on the top surface, here the entire tile is made of vitrified clay.

The water absorption capacity is only 0.5%. Many types of fully vitrified tiles are now manufactured in India. These tiles are superior to ordinary ceramic tiles and inferior to marble tiles used for flooring.

These tiles after manufacturing are again ground to get smooth and straight edges so as to lay with very close joints. These tiles are preferred in kitchens compared to ceramic tiles.

(iii) Porcelain Tiles

Porcelain tiles are the latest addition to the ceramic tile family. These are prepared from clay, feldspar, quartz and minerals. They are available in different forms as plain, coloured and also with some decorative patterns, but are very brittle in behaviour. These tiles are very expensive.

4.2.3 Manufacture of Tiles

Manufacture of the common clay tiles involves the following processes:

1. Preparation of clay
2. Moulding of tiles
3. Drying of tiles
4. Burning of tiles

1. Preparation of Clay

As stated earlier, the best blue clay for tiles is generally found beneath brick earth. Impurities such as grit, pebbles, etc. are removed from the selected clay. The clay is ground into a powder form in crushing roller mills. It is then pugged.

For the preparation of superior tiles, the powdered clay is mixed with water in a tank and allowed to settle down. The coarser particles settle down, and the solution containing the finer particles is run off into a lower tank. The collected material in the lower tank is allowed to dry leaving the fine clay ready for further processing.

2. Moulding of Tiles

Depending on the pattern of tile to be moulded, the moulding method is selected. Wooden patterns are used for tiles which are not of uniform thickness throughout the length and are moulded in wooden plattens. The pugged clay is pressed into the mould and the tiles are taken out for drying.

When the pattern of tile is perfectly circular in shape, Potter’s wheel may be used. However, the diameter of the tile may vary along its length.

Mechanical method is resorted to for tiles having a uniform section throughout their length. The pugged clay is prepared through openings or orifices of the required shape and size. The moulded beam of clay coming out of the slot is received on a platform, and the tiles are cut to the required length by means of wire frames.

3. Drying of Tiles

After two days of moulding, they are slightly beaten with a flat wooden mallet to correct the irregularity in shape due to warping. They are then kept on edges for further drying under shade for 2 days. This is done to prevent warping and cracking.

4. Burning of Tiles

The tiles are burnt in a kiln which could accommodate 30,000–40,000 tiles. Such kilns are partly underground and are usually provided with a roof to protect the kiln from rain.

A layer of bricks is laid over the long narrow flues, and the flues are fed with wood. The bricks are so placed such that open spaces are left in between them so that flues can move.

Above the layer of bricks, the dried tiles are placed on edge layer by layer, until the kiln is fully filled with raw tiles. The doorways are closed.

The kiln is then fired initially with 800ºC, slaked for six hours and then increased to 1,300ºC for about 3 hours (Fig. 4.2). Once again it is slacked for 6 hours and then once more heated and kept steady for about 3–4 hours. Finally, the flues are filled with fuel and the doorways are closed by brickwork in mud. The kiln is cooled gradually over 72 hours. Then the tiles are removed.

Figure 4.2 Circular kiln for burning tiles

4.3 EARTHENWARE

It has been discussed that earthenware is manufactured from ordinary clay burnt at low temperatures. Sometimes the clay is mixed with sand, crushed pottery, etc. Earthenware are susceptible to damage by atmospheric action since they are soft and porous. In general, they should not absorb more than about 20% of water by weight. In order to make them impervious, they may be glazed.

Glazed earthenware tiles are used for floors and walls of kitchens, bathrooms, water closets, hospitals, etc.

4.3.1 Terra-cotta

Terra-cotta is earthenware of vitrified material. It is high-quality earthenware. It is used as a substitute for stone in the ornamentation of buildings.

The manufacture of terra-cotta involves the same four stages as that of tiles.

1. Preparation of Clay

The clay selected for terra-cotta should contain about 5–8% of iron oxide and about 1% of lime. Sometimes clay with high alumina content is used. To these selected clays, sand, ground glass, old terra-cotta or pottery are added. The addition of these materials provides strength and rigidity to the terra-cotta and also prevents shrinking while drying.

Such a mixed clay should not have grit, pebbles, organic matter, etc. It is mixed with water, shovelled and kept for several days under damp conditions. It is pugged in a pug mill before being taken to the moulding section. At this stage, the required quantity of colouring substance is added to get the desired shade of colour after burning.

2. Moulding

Apart from wooden moulds, special moulds of plaster of Paris or templates of zinc are used. Fine sand is sprinkled on the inside surface of the mould and the clay is then pressed in the mould with the hand.

3. Drying

The terra-cotta is allowed to dry in the mould for some days, and then the object in the terra-cotta mould is taken out and then allowed in a room or shed for drying. Gradual drying is to be done failing which the shape of the object may get changed or cracked.

4. Burning

The dried objects are then burnt in special muffle furnaces. A typical muffle furnace is shown in Fig. 4.3. A muffle furnace has the provision to heat the product without coming into contact with fuels. In order to regulate the drought a damper is provided at the side of the furnace.

Figure 4.3 Muffle furnace to burn terra-cotta

The dried products are arranged in the muffle furnace. The temperature of the kiln is raised to about 1,200ºC (Fig. 4.3). This temperature is maintained for 4 days. The burnt products are allowed then to cool for a period of about 5 days in the furnace itself. In order to get a glazed surface, the glazed material has to be applied by a brush on terra-cotta products before they are burnt.

4.3.2 Porcelainware

Porcelainware or merely porcelain indicates fine earthenware, which is white, thin and transparent. It is also sometimes called whiteware.

It is basically prepared from clay and mixed with feldspar, quartz and minerals. The ingredients are powdered and thoroughly mixed. The mixture is moulded to the required shape and then burnt. Porcelain has various uses. In buildings, they are used as sanitaryware, electrical insulators, etc.

4.4 STONEWARE

Stoneware are the products manufactured from refractory clay mixed with crushed pottery, powdered stone, etc.

The clay is prepared carefully and moulded to the required shape. The stoneware is then dried under the shade to prevent any cracking or deformation of the product. The dried material is burnt in a kiln, gradually heating to a high temperature. The high temperature is maintained for 1–3 days based on the size of the kiln and the material size. The stoneware are allowed to cool down gradually.

Stoneware are dense and compact when compared to earthenware. Stoneware are impervious to moisture, and can be kept clean easily. Glazed stoneware are highly resistant to acid or any atmospheric agencies.

1. Tiles are thin slabs made out of brick earth by moulding, drying and then burning.
2. Common clay tiles are: clay roof tiles, clay floor tiles and clay terrace tiles.
3. Clay roof tiles are: Mangalore tiles, country tiles and Allahabad tiles.
4. Ceramic is the term used for the act of making objects with a special type of clay.
5. Orthoclase feldspar is mainly responsible for the production of clay in nature. This mineral during decomposition forms kaolinite. Kaolin, a product composed of pure kaolinite, is a fine white clay required to manufacture some ceramic articles.
6. Ceramic tiles are classified as glazed ceramic tiles, vitrified tiles and porcelain tiles.
7. Manufacturing of common clay tiles involves the following operations: preparation of clay, moulding of tiles, drying of tiles and burning of tiles.
8. Earthenware is the term used for clay products burnt at low temperatures. They have a coarse surface and are opaque. Earthenware comprises terra-cotta and porcelainware.
9. Terra-cotta is a hard brownish unglazed vitrified ceramic material used for ornamental purposes.
10. Stoneware is a product of refractory clay, ground stone and pottery.

1. What is meant by the term “tile”? What are the characteristics of a good tile?
2. What are common tiles?
3. Explain different kinds of roof tiles.
4. How are clay floor tiles manufactured?
5. Discuss the properties of clay tiles of flooring and roofing purposes.
6. Enumerate different types of ceramic tiles.
7. Discuss the steps involved in the manufacture of tiles.
8. Why do you consider that the cement tiles are superior to clay tiles? Explain.
9. What are earthenware? Explain.
10. Explain the manufacturing and uses of glazed earthenware tiles.
11. What is terra-cotta? How is it manufactured?
12. What is whiteware? How is it prepared?
13. Explain stoneware production and their uses.