Joseph Herbert on thu 16 oct 97
Mr. McCoy was wondering if there was some lurking sand fraction in his
backyard clay. This is a question that can be partly answered with the
application of gravity, aided by water and a glass vessel. Air dry some
small pieces of the clay in question really well and then slake about a cup
of clay pieces in a quart of water until it falls apart. Stir well. Let sit
undisturbed in a glass container for a few hours or days. Observe the bands
of different colors that formed as the different constituents settled from
the water. The relative thickness of each band is an indication of the
relative amount of that material. This will work in a quart mayonnaise jar
but the effect can be accentuated by using a tall, narrow tube like a
graduated cylinder or the clear plastic tubular cover for fluorescent tubes
in food service applications (got to plug one end). In any event, the time
it takes a particle to settle through a water column is related to the size
of the particle and the shape of the particle. Large round particles settle
fast, large flat particles settle next fastest, small round particles settle
slowly and small flat particles settle VERY slowly. In the material in
question, this should mean the sand particles settle fastest and will bet to
the bottom before any of the clay particles get there. In fact, the falling
of the sand particles stirs the water enough to keep the clay suspended. So
the lowest layer is biggest, heaviest sand, the top layer is smallest
flattest, lightest clay.
The only fly in this ointment is the tendency of clay particles to clump
together and make larger particles of them selves. The defloculants that are
used to make terra sigulotta (SP?) separate these clumps into their minute
individuals. It is an unsettling experience for them, but is for the best.
So, add defloculating agent, use a blender, stir for days, whatever. Once
the water is clear, the stacked layers in the tube or jar represents the
constituents of your clay arranged in order by size and density from bottom
to top.
If you really want to know what each layer is composed of, a little powered
sample X-ray diffraction will answer most questions. Most studio potteries
don t have this capability but many college geology, chemistry, or physics
departments do. A properly chosen graduate student might attempt to identify
one of the layers. This only works with a sample that is composed of one
mineral, mixtures are really difficult to work with.
By the way, the do-it-yourself instructions for building an X-ray diffraction
unit start with "get a 50,000 volt transformer " which may explain their
absence in pottery studios. Just a guess.
Joseph Herbert
JJHerb@aol.com
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