clay bodies that you will come into contact with are secondary clays
that have been adjusted, or completely formulated clay bodies. In
the formulated clays, the natural ingredients such as kaolins, ball
clays, bentonites and other ingredients are mixed up to create a
clay body of a particular type, e.g. a white earthenware. You can
make your own clay bodies according to available recipes, experiment
with your own variations or just buy a commercially manufactured
clay from a potters supply shop.
This is the most common type of clay. Terra-cotta is one type
of earthenware that is relatively coarse and red in color. Other
earthenware bodies may be finer and have various colors, ranging
from white to gray, buff and red. Earthenware clays are usually
fired between 1700o F (927o C) - 2100o F (1150o C). At this temperature
the clay body is still porous and needs to be glazed, e.g. if
it is to be used as dinnerware.
Raku bodies originated in Japan around the 16th C. They were commonly
used in the Japanese Tea Ceremony. They are a type of 'rough'
earthenware that harden at lower temperatures and must withstand
the thermal shock of a Raku firing.
To this end, they often contain a large amount of fire clay and
grog (clay sand).
Mid-fire clays are a type of hybrid earthenware/stoneware clay
that matures in between the two temperature ranges. This means
that it is possible to get the qualities of the higher fired stoneware,
while saving money and fuel in the firing. Mid-fire clays typically
mature around 2100o F (1150o C) - 2265o F (1240o C)
Stoneware clays are fired up to 2370o F (1300o C), where they
become quite hard and vitreous. Colors range from off white to
gray and dark brown. There are also various degrees of roughness
or fineness, depending on the formulation. This the hardest, most
durable type of clay.
Porcelain is a totally manufactured clay. While it is not synthetic
as such, various natural ingredients are refined and mixed up
according to formulas. A good porcelain must be very white, vitreous,
translucent while at the same time being plastic enough to work
with. Porcelain is fired from
2335o F (1280o C) to 2550o F (1400o C).
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