GEOLOGIC ORIGIN OF CLAY
Clay is a mineral 'stew' that is the result of the erosion of the earth's
crust over vast spans of time. What was originally the mineral feldspar in
igneous rocks, primarily granite, breaks down over time and becomes the
microscopically fine-particled clay that we form with our bare hands. How this
transformation takes place is a matter of geology and time. The effects of
erosion over enormous spans of time cause igneous rocks to disintegrate, and
the feldspar content is altered to kaolinite, which is the identifying
substance in clay. Those clay deposits which remain at or near the site of the
parent material (granite) are called residual or primary clays. These so-called
residual clays are grainy and lack the smoothness necessary for workability.
These clays are said to be non-plastic because they do not shape easily. Those
clays which have been transported by water, wind, and ice and deposited in
locations distant from the source material are called sedimentary or secondary
clays.Compared to residual clays, sedimentary clays are more plastic, and the
particles are smaller, more uniform, and more mixed with other materials. Under
the microscope, clay particles resemble playing cards in form. They are flat,
hexagonal, and thin, like cards. When wet, the particles can 'slip' across each
other, as in a deck of cards. This ability to 'slip' is what gives a clay its
workability, called plasticity. So, to summarize, potters need plastic clays
for wheel throwing and hand building. Mining companies explore the world to
find natural deposits of clays to mine and blend for sale to industry and to
studio potters.
http://seco.glendale.edu/~rkibler/clay.html
crust over vast spans of time. What was originally the mineral feldspar in
igneous rocks, primarily granite, breaks down over time and becomes the
microscopically fine-particled clay that we form with our bare hands. How this
transformation takes place is a matter of geology and time. The effects of
erosion over enormous spans of time cause igneous rocks to disintegrate, and
the feldspar content is altered to kaolinite, which is the identifying
substance in clay. Those clay deposits which remain at or near the site of the
parent material (granite) are called residual or primary clays. These so-called
residual clays are grainy and lack the smoothness necessary for workability.
These clays are said to be non-plastic because they do not shape easily. Those
clays which have been transported by water, wind, and ice and deposited in
locations distant from the source material are called sedimentary or secondary
clays.Compared to residual clays, sedimentary clays are more plastic, and the
particles are smaller, more uniform, and more mixed with other materials. Under
the microscope, clay particles resemble playing cards in form. They are flat,
hexagonal, and thin, like cards. When wet, the particles can 'slip' across each
other, as in a deck of cards. This ability to 'slip' is what gives a clay its
workability, called plasticity. So, to summarize, potters need plastic clays
for wheel throwing and hand building. Mining companies explore the world to
find natural deposits of clays to mine and blend for sale to industry and to
studio potters.
http://seco.glendale.edu/~rkibler/clay.html