I.Lewis on thu 5 aug 99
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To: Subscribers clayart=40lsv.uky.edu
From: Ivor Lewis iandol=40pirie.mtx.net.au
Subject: Kaolin, Ball clay
I was pleased to read the posting by Paul Lewing about the nature of Kaolin =
and
Ball clay. To hear that the basic ingredient we use may not be as described =
in
the =22dictionaries=22 and =22text books=22 gladdened my heart. His =
description of all
regular plastic clays, presumably excluding the bentonites, as =85=22B-axis
disordered kaolinite, which means that the layers do not lay perfectly =
aligned
in the same direction with each other.=22=85 makes very interesting reading,=
but I
would like to see a picture to confirm what I believe it to mean. Well, I =
had a
look in the minerals book and there is a reference to Ralph E. Grim, Clay
Mineralogy, McGraw-Hill which I have not read so I will try to access a copy
through our travelling library. It may have more to say on the point.
My own understanding has always been that Ball Clays were originally derived
from Kaolin after metamorphic activity within granite intrusions. Such =
kaolin
deposits, when exposed, were weathered and transported to new locations, =
often
during periods of glaciation. The classic example is the relationship =
between
the granites of Dartmoor and Exmoor and the sedimentary ball clays of Dorset=
in
England. Transported during erosion from its original site Kaolin was =
reduced in
particle size and diluted with other substances, which eventually included
organic material. Also, since it was removed without the intervention of =
man, it
contained the residual materials which existed in the metamorphosed =
granites=3B
remnants of unaltered felspars, mica, quartz, iron minerals, titanium =
minerals,
even gold. Depending on the way it was moved it was sorted and refined to =
give
us the variety of secondary clays which we can buy or prospect today.
I believe fireclays are refractory because most of the alkali and alkali =
earth
elements, the things we call fluxes, have been removed. They are soluble and
were either leached out or extracted by plants. In Britain, many fireclays =
are
mined from below coal measures. The plants which formed into coal depleted =
soil
in which they grew of essential elements. In some placed these shales, so =
called
=22seat earths=22 were consolidated into shale.
My own opinion is that plasticity of kaolin and ball clay are inadequately
described and explained. However, this is of little consequence when they =
are
used as ingredients in a glaze for their alumina and silica content. What =
has
been written ignores facts which are known about the nature of clay =
crystals. If
elementary physics, particularly what is known about electrostatics is used,=
a
new model emerges which gives a very different shape for colloidal clay
particles. Though clay may, as bought in the bag as a milled powder in the =
dry
state, be like ultra miniature soap flakes, when they are wet they must have=
a
different shape, one which forms infinitely deformable flocks. What I have
written about this has, so far, been rejected by magazine editors.
I may not answer Paul=92s question, because I don=92t quite understand what =
=22B axis
disordered=22 means, but I certainly think his mentor was right about them =
all
being derived from original kaolin. Should I find out more, I=92ll be in =
touch.
Best regards from South Australia, where the grass and the weeds are growing=
and
the almonds are in blossom.
Ivor Lewis.
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