Glass and Ceramics 175
from 2 mm down to 2m. Clay minerals exhibit a very large surface area
per unit weight, which explains their high attraction for water and other
substances.
Whereas soil scientists would classify clay by considering the particle size,
a chemist describes it as part of the family of silicates and more precisely as
hydrated silicates of aluminium. However, the classification of minerals with
a stoichiometric formula does not reflect the complexity of their structure. In
order to explain the properties of clay minerals in an adequate way, the
coordinationof silicon has to be described first, before looking at a long-
range order in the crystals.
The basic structural unit of silicates is the SiO 4 tetrahedron that may be
bonded to other cations only (e.g. as orthosilicates). However, SiO 4 units may
also share one or more oxygen atoms at the corner of the tetrahedron
to form rings or chains. Most of the clay minerals consist of infinite sheets
of SiO 4 tetrahedra, which explains their plasticity and absorption capacity
for water between the two-dimensional network. In contrast to this, a three-
dimensional network of SiO 4 units leads to stable and hard materials, with no
attraction to water. Crystalline quartz is the simplest example for such a com-
pound, buildingthe bridge to glass (see above).
The SiO 4 tetrahedon can be regarded as an ion charged as SiO 44 . Tetrahedra
are linked by sharing corners through SiOSi bridges. Clay, based on a two-
dimensionalnetwork connects tetrahedra that share three corners, thus reflect-
ing the general formula [(SiO 5 )n]2n. The stacked sheets are held together
by cations and hydroxyl groups, in order to obtain electrical neutrality in
the mineral. One point to note is that aluminium can be incorporated in the
mineral for partly replacing silicon in the plate-like laminae, but it can also
act as interstitial cation, which further complicates the general formula. As a
relatively simple example, kaolinite should be mentioned, with the chemi-
cal formula Al 2 (SiO 5 )(OH) 4 (Al is bonded here only as cation between the
flakes).
When using the term “clay” as raw material for ceramics, this usually refers
to a mixture of different clay minerals, with minor amounts of other non-clay
minerals, such as quartz. In terms of their future potential use, these clays are
classified, e.g. as “white-burning clays” (used for whitewares), stoneware
clays (containing fluxes) or brick clays (containing iron oxide). Fillers and
fluxes can be added to improve plasticity, to reduce shrinkage during drying
or to reduce the firing temperature.
The water content of the clay has to be gradually reduced before firing to
avoid fracture. Evaporation at room temperature leads to a dry clay consisting
of a porous open structure. This is the best moment to clean the pottery, to
join pieces together or, to decorate the surface. By heating up dried clay to
temperatures above 800°C, one obtains ceramics or pottery.