Page 29 - Curriculum Visions Dynamic Book

 Bricks are among the most important products made with sheet silicates.
The chemistry of firing
Ceramics are produced by firing (heating) clays in a kiln (oven). The air-dried clay objects put into the kiln may look and feel dry, but they
still contain water molecules locked between the sheets of the clay minerals. This water is only lost when the clay is heated to a very high temperature.
At the same time, the high temperature causes an irreversible chemical reaction to occur. The clay changes into a new mineral (mullite) which has tiny needle-shaped crystals. This is what makes the ceramic strong. Also, minute grains of silica form and fuse together, a process called sintering.
These chemical changes may also cause
the colour of the clay to change. If pure kaolinite is used, the ceramic becomes white. However, if the kaolinite contains or is mixed with impurities – compounds of metals such as chromium, iron or manganese – a coloured ceramic will be produced. This is because, during the heating process, the metal becomes oxidised. The oxides absorb some of the wavelengths of natural light, thus making the object appear coloured.
ceramic: a material based on clay minerals which has been heated so that it has chemically hardened.
magma: the molten rock that forms a balloon- shaped chamber in the rock below
a volcano. It is fed by rock moving upwards from below the crust.
oxidation: a reaction in which the oxidising agent loses electrons. (Note that oxidising agents do not have to contain oxygen.)
sintering: a process that happens at moderately high temperatures in some compounds. Grains begin to fuse together even through they do not melt. The most widespread example of sintering happens during the firing of clays to make ceramics.
 A kiln-fired porcelain pot.
EQUATION: Chemical change during the firing of a ceramic
Kaolinite (clay) ➪ mullite (fired clay) + silica + water vapour 3Al2Si2O5(OH)4(s) ➪ Al6Si2O13(s) + 4SiO2(s) + 6H2O(g)
 Coloured ceramic tiles.
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