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More Technical Information
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Clay and Glazes for the Potter by Daniel Rhodes is the potter's bible of
technical information - a must read if you want to be a serious potter. If you
have questions about oxidation, reduction, how glazes work, want to
formulate your own clay and glazes and want to understand different kinds of
clays and glazes, then buy this book and read it!
Clay: Clay is found in sedimentary deposits and soil. There are many types of
clay. Different clays have different physical and chemical properties. The
properties potters are concerned with are color, shrinkage plasticity maturing
temperature and porosity. They vary with each kind of clay. Ball clay is very
plastic and matures at cone 7 to 10. It is too gummy and has too much
shrinkage to be a good throwing body but makes a good addition if you need
more plasticity in your clay. Red earthenware clay matures at much lower
temperatures of cone 1 to 4. It is porous when fired to maturity and gets its
warm terracotta color from lots of iron in the clay. Stoneware clay is less plastic
than ball clay, has less iron than earthenware clay and matures at
temperatures of cone 7-10. It has little porosity and fires to a warm tan or brown
color. It can be a good throwing body straight out of the ground. Porcelain is
white because it has almost no iron it in. It matures at cone 12-16. It is not
plastic and requires the addition of ball clay of some other plasticizer to be a
good throwing body. It also requires the addition of about 25% flint and 25%
feldspar make it mature at cone 10 or 11 and to make glazes fit well.
There are many other types of clay. Most clay bodies potters work with are
actually blends of various clays, feldspars, flint and plasticizers. There is much
to learn about digging clay, and formulating clay bodies. Clays are formulated to
have the desired color, firing temperature, plasticity, shrinkage and porosity the
potter requires for the type of work s/he wants to produce.
Glazes: Glazes are similar to clays in their chemical composition but have more
fluxes. A flux is something that causes things to melt at a lower temperature.
The fluxes in glazes are things like calcium, sodium,and potassium and are
sourced from things like lime, whiting, feldspar and talc. They flux the alumina
and silica in the glazes and cause the glaze to melt into a glass and fuse to the
pot's surface. Metallic oxides like red iron oxide, copper carbonate, cobalt
carbonate, rutile and titanium dioxide give glazes their color. When the glaze
gets hot in the kiln and melts, chemical reactions take place. The heat,
atmosphere in the kiln, metallic oxides and other glaze materials all affect the
color and look of the fired glaze.
Firing: Firing can be divided into high or low temperature firing and oxidation
or reduction. Energy sources are wood, gas, oil and electricity. Low temperature
firings are done from 1200 to 1900 degrees Fahrenheit. High temperature firing
is between 2200 and 2500 degrees F. Electric kilns produce an oxidizing
atmosphere. Kilns that burn fuel may be fired in oxidation or reduction. To fire a
kiln in reduction, you need to starve the fire of oxygen so you get a smoky fire.
A smoky fire has incomplete combustion and the smoke contains carbon
monoxide and unburned carbon. Reduction firing affects the colors of the
glazes.
The temperature and atmosphere you fire in will have a big effect on the color
and feel of your glazes and clays.
Low fired pots are porous and will absorb liquid and foods. They can be used
on an open flame for cooking and were used that way by primitive people. High
fired pots are strong, hard and nonabsorbent. They are subject to thermal
shock and will crack if used on a stove top, but are good for oven use and food
storage.
technical information - a must read if you want to be a serious potter. If you
have questions about oxidation, reduction, how glazes work, want to
formulate your own clay and glazes and want to understand different kinds of
clays and glazes, then buy this book and read it!
Clay: Clay is found in sedimentary deposits and soil. There are many types of
clay. Different clays have different physical and chemical properties. The
properties potters are concerned with are color, shrinkage plasticity maturing
temperature and porosity. They vary with each kind of clay. Ball clay is very
plastic and matures at cone 7 to 10. It is too gummy and has too much
shrinkage to be a good throwing body but makes a good addition if you need
more plasticity in your clay. Red earthenware clay matures at much lower
temperatures of cone 1 to 4. It is porous when fired to maturity and gets its
warm terracotta color from lots of iron in the clay. Stoneware clay is less plastic
than ball clay, has less iron than earthenware clay and matures at
temperatures of cone 7-10. It has little porosity and fires to a warm tan or brown
color. It can be a good throwing body straight out of the ground. Porcelain is
white because it has almost no iron it in. It matures at cone 12-16. It is not
plastic and requires the addition of ball clay of some other plasticizer to be a
good throwing body. It also requires the addition of about 25% flint and 25%
feldspar make it mature at cone 10 or 11 and to make glazes fit well.
There are many other types of clay. Most clay bodies potters work with are
actually blends of various clays, feldspars, flint and plasticizers. There is much
to learn about digging clay, and formulating clay bodies. Clays are formulated to
have the desired color, firing temperature, plasticity, shrinkage and porosity the
potter requires for the type of work s/he wants to produce.
Glazes: Glazes are similar to clays in their chemical composition but have more
fluxes. A flux is something that causes things to melt at a lower temperature.
The fluxes in glazes are things like calcium, sodium,and potassium and are
sourced from things like lime, whiting, feldspar and talc. They flux the alumina
and silica in the glazes and cause the glaze to melt into a glass and fuse to the
pot's surface. Metallic oxides like red iron oxide, copper carbonate, cobalt
carbonate, rutile and titanium dioxide give glazes their color. When the glaze
gets hot in the kiln and melts, chemical reactions take place. The heat,
atmosphere in the kiln, metallic oxides and other glaze materials all affect the
color and look of the fired glaze.
Firing: Firing can be divided into high or low temperature firing and oxidation
or reduction. Energy sources are wood, gas, oil and electricity. Low temperature
firings are done from 1200 to 1900 degrees Fahrenheit. High temperature firing
is between 2200 and 2500 degrees F. Electric kilns produce an oxidizing
atmosphere. Kilns that burn fuel may be fired in oxidation or reduction. To fire a
kiln in reduction, you need to starve the fire of oxygen so you get a smoky fire.
A smoky fire has incomplete combustion and the smoke contains carbon
monoxide and unburned carbon. Reduction firing affects the colors of the
glazes.
The temperature and atmosphere you fire in will have a big effect on the color
and feel of your glazes and clays.
Low fired pots are porous and will absorb liquid and foods. They can be used
on an open flame for cooking and were used that way by primitive people. High
fired pots are strong, hard and nonabsorbent. They are subject to thermal
shock and will crack if used on a stove top, but are good for oven use and food
storage.