Glass and Ceramics 177
this process depends not only on the crystal structure of the mineral but also
on the grain size. Various dehydration reactions lead to the formation of high
temperature alumino–silicate phases, such as mullite (3Al 2 O 3 2SiO 2 ) or
crystobalite (a form of silica, which is stable above 1470°C). To simplify the
composition of ceramics for classification, the different phases present are
summarised in triangular diagrams (e.g.CaOAl 2 O 3 SiO 2 for earthenware).
Ceramics can be classified by considering the firing temperature and the
resulting porosity: thus high-fired stoneware (produced at above 1000°C) has
porosities less than 2% and low-fired earthenware (firing between 600 and
900°C) with far more than 10% porosity are at the upper and lower ends of the
scale. Porcelain (defined as white and translucent ceramic, fired up to 1400°C)
can exhibit an extremely low porosity, whereas terracotta or raku (both fired
below 1000°C) would be examples of high porosity.
Various types of kilns evolved during the history of ceramic production:
fired brick walls were constructed for field kilns in contrast to periodic kilns,
serving only for a limited production campaign; round or tunnel kilns provide
different features concerning the duration of the firing cycle or the temperature
uniformity. A kiln with an oxygen-deficient atmosphere produces harder
ceramics (and a different colour) than a kiln with an oxidising atmosphere.
3.4 Colouration and Glazes
The choice of raw materials (and additives) as well as the firing technology
influences the colour of the final product, resulting in cream, yellow, grey, red
or brown bodies. Additional colouration of the body can be achieved by coat-
ings, including sprinkling of sand or salts, through mineral slurries or coloured
glazes, which are solidified in a second firing process.
Glossy surfaces are obtained by dipping or painting the ceramic body with
a slip, which is actually a diluted clay mixture with a similar composition to
the body clay. Ancient Greek and Roman pottery are well-known examples
for this technique.
Glazes are basically a thin layer of glass (around 0.2–0.4 mm thickness)
fired onto a ceramic body. They can be applied by mixing directly the raw
ingredients in water or via a frit which is a suspension of ground glass powder.
In “lead glazes” the major flux is a lead compound, whereas “tin glazes” refer
to lead-containing glazes, opacified with tin oxide. “Alkaline glazes” are very
similar to medieval window glass (with Na 2 O or K 2 O, CaO and SiO 2 as the
main components) (see earlier).
Glazes are not only used to decorate art objects or to create a dense surface
for storage vessels. Glazes are also popular for colouring of architectural elem-
ents and facades (Figure 14). As an example, a glaze used to obtain a brown–
green glossy surface on a brick would contain (all numbers in wt%): 48 PbO,