Manufacture of ceramic tiles

In the third dynasty in Egypt, small glazed tiles in light blue, green and black were used to decorate the steps of the Saqqara pyramid. Nowadays ceramic tiles are widely used in both public buildings and dwellings. Their increased use in housing is largely a result of the development of the private bathroom with associated ceramic fixtures.

Quarry tiles and terracotta are produced from damp pressed clay in the same way as bricks, using the same raw materials. It is normal to fire the clay until it sinters, at up to 1000 °C.

Vitrified ceramic tiles and faience are fired from dry pressed clay, often with ground kaolin; a white clay also used in the porcelain industry. Finely ground waste glass can be added to increase the volume of the mix in proportions up to 75% (Lippiatt, 2007).The product is fired until vitrified, and the resulting tile is much more exact and smooth than products made from damp pressed clay.

All tiles can be glazed. There are three forms of glazing: salt glaze, lead glaze and earth glaze, which can also have pigments added in the form of metal oxides or salts (see Table 8.7). Many of these are environmental toxins, and there are strict rules as to how these materials are disposed of as waste products. Salt glaze is pure sodium chloride (NaCl) which is sprinkled on during firing and reacts with clay to produce a silicate glass. This process needs high temperatures and requires

Table 8.7 Examples of pigments used for glazing ceramics

Colour

Alternative pigments

Percentage

Yellow

Antimony trioxide

10-20

Chrome chloride

0.5-1

Ferric oxide

1-2

Vanadium oxide

2-10

Uranium oxide (rare)

4-10

Red

Cadmium oxide

1-4

Chrome oxide

1-2

Manganese carbonate

2-4

Green

Copper carbonate

1-3

Chrome oxide

1-3

Blue

Cobalt carbonate

1-3

Nickel oxide

2-4

very high quality clay. Lead glaze and earth glaze are applied to ready-fired products, which are then fired again.

Tiles that are coloured all the way through are usually vitrified and the added pigments are the same as those used in glazes.

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