Chemistry Reference
In-Depth Information
CLAY
POTTERY AND OTHER
CERAMIC MATERIALS
7
Clay
is the generic name for very fine grained, unconsolidated earthy min-
erals that are supple and pliable when wet, become hard and retain their
shape when dried, and turn into a stony material, known as
fired clay
, when
heated to red heat. It is because of these unique properties that very early in
the development of humans, clay became a most important raw material,
used to make bricks, pottery, and later, other ceramic materials (Kingery
1986-2000). Clay absorbs and loses water easily, and the water absorbed pen-
etrates the surface of the grains, which swell accordingly and shrink when
they lose the water. The grains also
adsorb
gases: their surface takes up and
retains gases such as carbon dioxide and water vapor (in the soil, the gases
adsorbed on clay particles are used as nutrients by plants).
The meaning of the word clay differs among various fields of interest.
In geology, for example, the term is used to refer to very fine grained min-
erals formed as a result of the weathering of siliceous rocks (see Textbox 45);
in chemical mineralogy, a clay is an unconsolidated mineral belonging to the
group known as the clay minerals, and in soil science the word clay is used
to refer to the inorganic fraction of the soil that is made up of very small par-
ticles (below 0.002 mm in diameter) having a very large surface area per unit
weight (Murray 1986). Clay is of particular interest in archaeology and in
materials science, since, for a very long time, it has been used for making
ceramic materials
such as
pottery
and
fired brick
, which are among the most
ancient and most frequently encountered human-made materials in archae-
ological excavations (Newman 1987; Grimshaw 1980). In these fields clay is
defined as an earthy aggregate of mineral particles that when mixed with
water exhibits plastic properties, on drying becomes rigid, and when heated
to a sufficiently high temperature acquires hardness, strength, and chemical
and physical stability (Rice 1987).
