Civil Engineering Reference
In-Depth Information
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 asso-
ciated ceramic plumbing 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 used in the porcelain industry. Finely-ground waste
glass can be added to increase the volume of the mix. 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: cooking salt glaze,
lead glaze and earth glaze. Earth glaze is mainly a lime glaze, which can also
have pigments added in the form of metal oxides or salts. Many of these are envi-
ronmental poisons, and there are very 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 a particularly high-quality clay.
Lead glaze and earth glaze are applied to the 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. Pigments used in glazes (see Table
8.5) can be mixed to achieve other colours.
Production of light expanded clay
All clays can be expanded, though some expand more easily than others. The
ideal clay is very fine, with a low lime and high iron content. Smelting must not
occur before the clay has expanded - this mainly depends upon the minerals in
the clay.
Clay used for the production of expanded clay pellets needs to air for about a
year before being used. It is then ground, mixed with water and made into pel-
lets. Medium-quality clay can have chemicals added, mostly ammonia sulphite
in a proportion of 3 per cent volume of the dry clay, and sodium phosphate in a
proportion of 0.1 per cent. The lower the iron content in the clay, the lower the
use of energy in the kiln.
Expansion can occur in a vitrifying kiln where sawdust, oil or coal can be
mixed with the clay and then fired. Alternatively, the more efficient rotating kiln
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