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forming, the greater the shrinkage on drying and firing, which can bring
about cracking of the ware. For this reason, substances that do not
shrink on drying and firing are added to the plastic raw material. In
practice, they are sometimes called non-plastic raw materials, since they
do not form plastic paste with water.
From this aspect, the basic ceramic raw materials can be divided into
the following three groups, according to their main function.
1. Plastic raw materials, i.e. kaolin and clays, w'hich render
plastic forming possible. The clay minerals, which these raw materials
contain, are responsible for their properties.
2. Non-plastic ran' materials which reduce drying and firing
shrinkage, whilst simultanously reducing plasticity. In comparison with
plastic raw materials, they are coarse-grained, usually non-porous, or at
least so stable that they do not shrink on firing. Typical examples are
quartz, corundum, calcined (presintered) clay, etc.
3. Raw materials which, from the standpoint of plasticity, show a
behaviour similar to that of the previous group, but whose main purpose
is to produce a melt on firing (so-called fluxes). These substances
promote or accelerate sintering. Typical examples are alkali feldspars.
A number of new ceramics do not necessarily use clay raw materials as
plastic components. This function is met either by suitable organic
substances (the so-called plasticizers) or use is made of other forming
methods. With these materials, which include in particular the newer
types of technical ceramics, the traditional classification of raw
materials has lost its significance.
Raw materials and their properties are of prime significance in the
technology of ceramics. The manufacturing process and properties of the
product are affected by the chemical and mineral composition of the raw
materials, by their crystalline structure, particle size and their
surface condition. All these factors may vary over wide ranges, in
particular with natural raw materials. Detailed information on the raw
materials, obtained by suitable test methods, is essential for mastering
the manufacturing process.
Almost all the main ceramic raw materials belong to the groups of
oxides, silicates and carbonates. They were dealt with, especially as
regards their behaviour during technological operations, in Chapter I.
Further necessary data will be specified in connection with the
individual types of ceramics. The problems of ceramic raw materials have
been dealt with in detail in specific monographs, for example, by
Radczewski (1968) and Grimshaw (1971).
Raw materials are usually supplied to ceramic works in ground form,
with suitable grain sizes. The preparation of mixtures then only requires
measuring and blending.
With respect to the ratio of components and their grain sizes, the
mixtures should be composed in order to allow good forming, to bring
about strengthening and densification by subsequent firing, and to
produce the required phase composition of the product.
Traditional ceramics based on the utilization of clay raw materials
are shaped from mixtures containing specific amounts of water, so that
the main mixture forms are moist powder, plastic mass or casting slip.
Mixes free from clay raw materials neither acquire satisfactory
plasticity by mere addition of water, nor show satisfactory green
(unfired) strength. In this case, the function of the clay-water
combination is assumed by a suitable organic substance which acts as
plasticizer and temporary binder. The following general observations
concerning volume and granulometric relations hold, in principle, for
both types of ceramic mix.
1.1 Volume relations in mixtures
When water is added to a clay which has air trapped in the pores, the air
is gradually expelled and replaced by water (Fig. 152). This may bring
about better packing and reduction of volume, as rhe water acts as a
"lubricant". At point a, all the air has been replaced by water. At
higher content, the water penetrates into the points of contact and
separates the particles. Plastic bodies are obtained between points a and
b. At point o, the body has a high yield point and low deformability (cf.
below). In the direction towards b, the yield value decreases and
deformability increases, together with increased drying shrinkage because
the particles are more closely packed when water is removed. Further
addition of water brings the system into the region of fluid suspensions.
This concept of water distribution in a clay can also be employed for
mixtures in which the clay constitutes one of the main components. The
clay particles bind to
their hydrophilic surface comparatively thick water layers* which are
then responsible for the characteristic rheological properties of these
systems. In mixes which do not contain the clay component, mere addition