Chemistry Reference
In-Depth Information
is impossible to prove. There is a way, however, to distinguish between such
accidentally formed “preceramic” dishes and intentionally made ones. The
outer surface of human-made pottery is always relatively smooth, while that
of naturally formed soil crusts is normally coarse. If ceramic dishes with
coarse outer surfaces were to be found in ancient habitation sites, or if food
or other organic residues were to be detected in their inner surface, they
would provide evidence of “preceramic pottery” having been used by
humans (de Korosy 1975).
Components of Ceramic Materials
Clay is the essential, although not the sole, constituent of a
clay paste
(or
clay
body
), as the mixture of raw materials used for making pottery is known. The
other essential component of most wet clay mixtures are the
nonclay materi-
als
, which are also known by a variety of other names, such as
fillers
,
non-
plastic fillers
,
nonplastic materials
,
inclusions
,
tempers
, and
additives
; these are
added to the clay so as to obtain a mixture with good working and drying
properties, which on firing acquires toughness and strength. A vast range of
materials, differing in their composition, their effect on pliability, the drying
and firing properties of the clay mixture, and the coherence and strength of
finished objects made from it, have been used as fillers. A short list includes
such varied materials as sand, volcanic ash, crushed rocks, shards and
seashells, twigs, straw, and dung (Bronitsky and Hamer 1986).
The method used in antiquity to shape a clay paste into objects can often
be established by observing the orientation of the particles of filler within
finished pottery: in ring- or coil-shaped vessels, for example, the particles
are randomly oriented, whereas in “wheel-thrown pottery” they are oriented
in a particular direction, the long axes of the particles lying parallel to the
turning plane of the wheel on which the vessels were turned. Some fillers,
such as sand and crushed igneous stone, are stable to heat changes and
remain unaltered during firing. Others, however, undergo radical changes
when heated to high temperatures; organic matter, for example, is oxidized
and partly or entirely removed from fired pottery and sedimentary stone or
shell (which are composed of calcium and/or magnesium carbonate) are
cal-
cinated
(see Textbox 34).
7.5.
MAKING CERAMICS
The generic name used to refer to ceramic objects shaped from a wet mixture
of clay and fillers that is then dried and subsequently fired at high temper-
atures is
pottery
. Making
pottery
involves a number of working stages:
