More copious thanks to Reader's Digest Crafts and Hobbies for the material for this entry.
Geology: Dirt, as in the dirt you walk on, is divided into three types of particles: from biggest to smallest: sand, silt, and clay. Clay particles are very very small and not visible to the naked eye. I know this from earth science. What follows is what I did not know before I read this excellent book. Billions of years ago the earth was a swirling mass of primordial fire and molten metals, which gradually settled, pulled toward the earth's core by gravity, with heavier particles like metals and larger pieces sinking deeper than lighter elements, which rose to the surface. This cooling and settling also combined elements to form minerals, the most common of which (almost 60% of the earth's crust) is feldspar, and feldspar is what decomposed, over uncounted years of weathering by wind and rain, to make clay. Here is clay in the earth.
Chemistry: Feldspar, being a mineral, contains most of the same stuff as the earth's crust, and in about the same proportions. This "stuff" includes many metal oxides: SO2, Al2O3, MgO TiO2, CaO (silica, alumina, magnesia, titania, lime) - as well as sodium, potassium, and iron oxides, which don't have extra names ending in -a. Potter's clay is mainly silica (55-60%). Silica, alumina, and water are the only compounds common in large quantities to all clays.
Clay particles, as mentioned, are really tiny - <.001mm - and have thin, flat, approximately hexagonal shapes. The particles have slight negative charges, so they are attracted to water, and once they are wet, they cling to each other, in a phenomenon known as plasticity, the most important physical property of clay.
Here are some clay particles magnified approximately 40,000x.
Physical properties: Plasticity refers to how well clay particles cling to each other. Smaller particle sizes, have greater the overall surface area, so clay can cling to more water. The more water clay holds, the tighter the particles cling, and the more plastic the clay is. So in short, the smaller the particle size, the greater the plasticity of the clay. And more plasticity is good. (White stoneware is described as "fairly plastic").
Another important thing to note is that when one is throwing, the clay is simply saturated with water - water already in the clay and water added on the wheel. Water takes up space. As the piece dries, the water evaporates, and the piece shrinks accordingly. Shrinkage rate varies with different types of clay. After bisque firing, the clay I use has shrunk by about 25%.
Geology: Dirt, as in the dirt you walk on, is divided into three types of particles: from biggest to smallest: sand, silt, and clay. Clay particles are very very small and not visible to the naked eye. I know this from earth science. What follows is what I did not know before I read this excellent book. Billions of years ago the earth was a swirling mass of primordial fire and molten metals, which gradually settled, pulled toward the earth's core by gravity, with heavier particles like metals and larger pieces sinking deeper than lighter elements, which rose to the surface. This cooling and settling also combined elements to form minerals, the most common of which (almost 60% of the earth's crust) is feldspar, and feldspar is what decomposed, over uncounted years of weathering by wind and rain, to make clay. Here is clay in the earth.
http://weewebwonders.pbworks.com/f/Clay-ss-2005.jpg
Chemistry: Feldspar, being a mineral, contains most of the same stuff as the earth's crust, and in about the same proportions. This "stuff" includes many metal oxides: SO2, Al2O3, MgO TiO2, CaO (silica, alumina, magnesia, titania, lime) - as well as sodium, potassium, and iron oxides, which don't have extra names ending in -a. Potter's clay is mainly silica (55-60%). Silica, alumina, and water are the only compounds common in large quantities to all clays.
Clay particles, as mentioned, are really tiny - <.001mm - and have thin, flat, approximately hexagonal shapes. The particles have slight negative charges, so they are attracted to water, and once they are wet, they cling to each other, in a phenomenon known as plasticity, the most important physical property of clay.
Here are some clay particles magnified approximately 40,000x.
Physical properties: Plasticity refers to how well clay particles cling to each other. Smaller particle sizes, have greater the overall surface area, so clay can cling to more water. The more water clay holds, the tighter the particles cling, and the more plastic the clay is. So in short, the smaller the particle size, the greater the plasticity of the clay. And more plasticity is good. (White stoneware is described as "fairly plastic").
Another important thing to note is that when one is throwing, the clay is simply saturated with water - water already in the clay and water added on the wheel. Water takes up space. As the piece dries, the water evaporates, and the piece shrinks accordingly. Shrinkage rate varies with different types of clay. After bisque firing, the clay I use has shrunk by about 25%.
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