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Physical properties of clay
Plasticity
Clay has one unusual physical property called plasticity,
that makes it very useful for making objects. Clay is one of
a very few materials that have this quality. A plastic material
is simultaneously very malleable and yet strongly adhesive to
itself. In the case of clay, plasticity results from a.
the extremely small size of the particles, b. the flat
plate shape of the particles, c. and the film of water
that forms between these plates when the material is wetted.
Because the particles are flat, many adjacent particles will
lie parallel to each other with a thin film of moisture between
them. The moisture film lubricates the plates allowing them to
slide back and forth over each other making the material very
malleable. At the same time, the moisture film creates suction
between the plates that causes them to cling strongly together
which gives the material strength to hold what ever shape it
has been given.
Plasticity varies greatly from one clay type to another. Clay
that is short (low plasticity) either has relatively large particles
or has been contaminated with a non plastic material such as
sand. Highly plastic clays such as Ball Clay, are made of a high
percentage of pure clay with small particles. Plasticity is also
effected by the presence of organic matter that develops over
time. Freshly mixed clay from a dry bagged form will be difficult
to work with until it has had a chance to age for at least a
couple of days.
Shrinkage
All clays shrink when they dry and again in the glaze fire.
Shrinkage, (about 7% in drying and another 7% in the glaze fire),
can be the cause of considerable hassle. If Clay dries unevenly
different parts of a piece will shrink at different rates. This
frequently causes stresses that result in cracks. This is particularly
a problem with large irregular shapes. It is also a problem in
low humidity conditions that cause very fast drying. Drying shrinkage
is also a function of particle size with large particle short
clays shrinking less than small particle plastic clays.
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