Gavin Stairs on tue 27 jul 99
>Alison Hamilton wrote:
...
>> Bell Dark is a type of ball clay - I like to describe ball clays as poor
>> cousins of kaolins, since not only does ball clay have kaolinite (pure clay
>> mineral), it also has free silica and some other oxides in it (which act as
>> fluxes). ...
an Paul Lewing replied:
...
>Anyway, we found out that, to a mineralogist, all the kaolins, all the
>ball clays, and even all the fire clays were the same thing! They were
>all kaolinite, specifically, B-axis disordered kaolinite, which means
>that the layers do not lay perfectly aligned in the same direction with
>each other. We were trying to find out the mineralogical basis for
>plasticity, maturing temperature, and fired color. Turned out that what
>made them act different was the impurities and the particle size (to
>greatly simplify the matter). ...
So you are basically saying the same thing. Ball clay properties derive
from those of kaolinite with the impurities like silica, organics, iron
oxide, titania, etc., and other clay minerals, like montmorillonites. The
statement that all clays are kaolinite is just a reference to the dominant
mineral present. Like saying that quartzite is made of quartz. It is, but
it is often impure, having significant quantities of other stuff, like
iron, or gold, even! The terms kaolin and kaolinite refer respectively to
the naturally occurring material, the clay (as a rock), and the mineral of
which it is chiefly composed. So kaolin, ball clay and fire clay are all
chiefly composed of kaolinite, a mineral.
The basic plastic and rheological nature of clay comes from its kaolinite,
in the form in which it is developed into clay body. Pure kaolinite tends
to be rather unlike the clay we are used to. It doesn't throw very well,
etc. Consolidated clay of all types is rather stiff, hard stuff. In order
to make it into a clay body, we do two things: we add stuff, and we
process it in water. Both alter the properties.
Gavin
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