Chemistry Reference
In-Depth Information
transported by such natural agencies as water streams and winds and en
route mixes with other mineral particles. When it is finally deposited, at
more or less distant places from its site of origin, it does so as secondary clay,
generally containing over 50% of nonclay matter such as sand, limestone,
iron oxides, and organic matter (the latter is derived from vegetable and
animal wastes). The iron oxides impart to most secondary clays a yellow,
red, brown, and, occasionally, even a green color; organic matter darkens any
clay, whatever its color.
The particles of secondary clay are usually smaller than those of primary
clay; the fineness of the particles is the result of the mechanical friction
undergone during transportation from their place of origin to where they
become settled. The small particle size makes wet secondary clay more
plastic and pliable than wet primary clay and therefore highly appreciated
by potters for making ceramic objects. Two particular types of secondary clay
that have been extensively used for making pottery are
ball clay
, which is
rich in organic matter and
red clay
, rich in iron oxides. When extracted from
the earth the actual color of these clays is usually yellow, brown, gray, or
black, but during firing they turn red; the change in color is due to iron ions
in the clay being oxidized and turning a rich red at high temperatures.
7.3.
CLAY AND CERAMIC MATERIALS
Three main properties render clay suitable for making ceramic materials: its
plasticity
when wet, its hardness when dry, and the toughness, increased
hardness, and stability that it acquires when fired. The addition of water to
dry clay produces a clay-water mixture that, within a narrow range of water
content, has
plastic properties
: it is deformed, without breaking or cracking,
by the application of an external stress, and it retains the acquired shape
when the deforming stress is removed. Wet clay mixtures can, therefore, be
modeled, molded, or otherwise made to acquire a shape that will be retained
after the forming operations. Water-poor mixtures are not plastic, however,
and excess water results in mixtures, known as
slips
, that are too fluid to
retain a shape, as shown in Table 56.
The plasticity of clay-water mixtures is due principally to two factors:
the flat platelet shape of the clay particles and their small size. When clay is
wet, the water, which under such conditions is known as
water of plasticity
,
envelops each particle, acts as a lubricant between the particles, and allows
them, when an external force is applied to the mixture, to readily slide along
each other, as shown in Figure 51.
As for the drying properties of clay-water mixtures: when wet clay dries
at ambient temperature, the water of plasticity that surrounds the particles
