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approximately as a produc* of the two values - the dependence of this on
water content shows a peak corresponding to optimum workability, The
strong dependence on particle size is shown in Fig. 157.
It should be noted that the various forming methods require somewhat
different rheological parameters, so that the way of determining the
optimum workability described above is just a rough approximation. In
addition, one should also consider plastic viscosity, which characterizes
the resistance to the acting stress offered by the body being formed.
The yield point is lowered by gradual increase in water content in the
raw material mix, until the range of liquid suspension is reached. If the
system contains so much liquid that the particles of solids are mutualy
separated by thick liquid layers, these suspensions behave like Newtonian
liquids. Deviations from this simple Theological behaviour arise when the
particles come into mutual contact. The yield point will arise or various
types of non-linear dependence of deformation rate on stress will occur.
The relation between yield point and volume concentration of clay in
aqueous suspension is shown in Fig. 158. On a logarithmic scale, the
dependence is linear
--- rh pastes
0.01 0.1 1.0 concentration
FIG. 158. Yield point of aqueous suspension versus volume concentration
of the clay component (from Norton, 1952).
showing different slopes for plastic masses and casting slips. Between
the straight lines, there is a region of mixtures unsuitable for current
The properties of plastic bodies dealt with so far have been
considered on the basis of using clay as the main component responsible
for favourable rheological properties. These properties can be
conveniently controlled by adjusting the content of clay in the mix.
However, the rheological properties depend to a large degree on the
respective type of clay and on the type of ions adsorbed on the panicle
surface. Even highly dilute suspensions of H+ kaolinite have a measurable
yield point. Generally, H+ and Ca2+ ions produce a high yield point in
clay-water systems, whereas a low yield point is brought about by
adsorbed Na+ ions. Substances that reduce the surface tension also reduce
the yield point significantly.
Satisfactory working properties of mixtures belonging to non-plastic
systems can be attained with the use of organic plasticizers. Frequent
use is made of methyl-cellulose or polyvinyl alcohol solutions, mixtures
of various waxes, etc. (refer to Chap. IV, Section 2.2).
In addition to the torsion plastograph mentioned above, a number of
other methods are used for appraising the properties of plastic bodies in
practice. Plastic strength (rigidity) is determined by means of the
Rebinder penetrometer which measures the depth of penetration of a cone
forced into the body under constant load. The respective quantity is
related to yield point. The commercial Brabender plastograph measures the
resistance of mixture to kneading. The extrusion viscometer, from which
the material is extruded under pressure, is analogous to flow-out
The Pfefferkorn method is based on measuring the deformation of a
cylinder (height 4.0, diameter 3.3 cm) due to impact of a 1.2 kg weight
falling from a height of 18.5 cm; it is used for determining the optimum
water content in clay-based bodies. The ratio of the original height to
that after deformation lies in the range 2.5 to 3.3 for easily worked
An apparatus and method of evaluating plastic behaviour by a cyclic
shear test was described recently by Hennicke and Kienow (1979); in this
publication, the reader will find up-to-date references to the above
methods and others of measuring plasticity and workability of ceramic
For practical purposes and technical inspection, it may be convenient
to use methods which do not provide well defined rheological quantities.
The main advantage is simplicity; complete determination of rheological
parameters is difficult and not always necessary because the relations
between rheological parameters and the practical working properties are
not always known.
1.4. The properties of casting slips
Ceramic mixes are frequently shaped by casting aqueous suspensions called
slips in gypsum moulds. The slips usually contain several solid
substances, of which the clay component has the critical influence on
casting behaviour. However, slips free from
the clay component can also be formed in this way. The slip consistency
depends on water (liquid) content, the size and shape of particles, and
on the nature of substances adsorbed on their surfaces. Particles of