TYPES OF CLAYS: GEOLOGIC ORIGINS, WORKING PROPERTIES
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 handbuilding.
Mining companies explore the world to find natural deposits of clays to mine and blend
for sale to industry and to studio potters.
TO SUMMARIZE: FELDSPAR IN IGNEOUS ROCKS breaks down to RESIDUAL CLAYS, then over time
into SEDIMENTARY CLAYS which are mixed with other materials such as colorants and
grog and are blended into CLAY BODIES.
CLAY BODIES
Rarely do potters use a single, sedimentary clay as a working clay. Experience has
taught that even better results are obtained when several different clays are blended
together. Such a blended clay is called a clay body. By blending, potters could vary
the color and texture of their clays as well. There are two general categories of
clay bodies:
EARTHENWARE CLAY BODIES
- Firing temperature rarely
exceeds 2050º F
-
Clay color ranges from white to
terra cotta (brick red)
-
Texture varies from smooth to
rough
STONEWARE/PORCELAIN CLAY BODIES
-
Firing temperature is from 2050-2400º F
- Clay color ranges from white (Porcelain) to brown (Stoneware)
-
Texture varies from smooth (Porcelain, no GROG) to coarse (Stoneware, contains GROG)
NOTE that the primary difference between the categories is the maximum firing temperature
possible. Earthenware clays will MELT if fired to the higher temperatures of stoneware
and porcelain. This is extremely important to know when purchasing clay for use. A
wrong choice will result in all your work melting in the kiln, ruining what you have
made. But that's only the beginning of the disaster. Your pieces will melt onto our
kiln shelves, fusing into them at extremely high temperatures. You will destroy the
kiln shelf at a cost of $100.00 each. You will be responsible for this damage. Your
pieces will also melt onto people's work in the kiln, ruining them as well. It is
impossible to put a price tag on this loss. Avoid the problem by only using the clays
sold by the bookstore. Note that another difference in clays is color. Clays that
are tan, brown or brick in color contain iron oxide (terra cotta and stoneware) as
the coloring agent. Clays that lack iron oxide are gray to white in color (porcelain).
Note that another difference in clays is texture. Clays vary in particle size, and
some are much coarser than others. Frequently coarser clay bodies contain a particulate
additive called
grog which gives the body roughness. Porcelain clays have little or no grog. Stoneware
clays usually have some. Earthenware clays may or may not have grog, so this difference
alone does not help us distinguish low- from high temperature clays. Grog is commonly
either sand or fired clay which has been crushed and sized. Lacking the microscopic
size and shape of clay particles, grog decreases the plasticity of the clay body,
but it does have a beneficial effect on shrinkage. Since it is not clay, grog does
not shrink as clay does. Therefore, its presence in clay reduces the overall shrinkage
rate of the clay; more grog = less shrinkage, less grog = more shrinkage. So, the
presence of a small amount of grog in a clay body may be a good thing. It reduces
the shrinkage,yet if not used in too great amounts, it will not significantly reduce
plasticity. Porcelain clays lack grog, and consequently have the highest shrinkage
rate, making them extremely difficult for inexperienced potters to use. Most of your
work will crack in drying. I do not recommend beginning with porcelain for this reason.
DIFFERENCES IN CLAY BODIES AFTER FIRING
Fired EARTHENWARE clay bodies are porous, fragile and often lead glazed (especially
historically). Fired STONEWARE and PORCELAIN clay bodies are non-porous, much less
fragile, and never lead glazed.
POROSITY refers to the ability of a material to absorb water. Earthenware pieces, having been
fired to a lower temperature, do not fully mature, or vitrify, and as such, allow
water to slowly pass through the wall of the pot. AT the higher the firing temperature
of stoneware and porcelain, less water can pass through, so these clay bodies exhibit
practically no porosity. Likewise, earthenware fired near the 2000 degree temperature
will exhibit little porosity, especially if they are glazed. That's why pieces fired
very low, such as red clay planters, fired around1200 º F will exhibit pronounced
porosity. At the extremely high temperatures that Stoneware and Porcelain are fired,
little, if any porosity is noticed, even if the pot is unglazed.Just as higher temperatures
yield greater water retention, pots fired at stoneware/porcelain temperatures are
much stronger and durable for everyday usage. Additionally, as temperatures increase,
colors are driven from the glaze, so that fewer colors are possible at higher temperatures.
In this class, we will be using two clay bodies, both stoneware:
VEGAS BUFF is a stoneware that is unusual in that it contains little iron oxide, yielding a
stoneware that is uncharacteristically lighter in color. This clay does contain some
grog, which is helpful in reducing the cracking common in light colored clays. Vegas
Buff is a good clay for wheel throwing or hand-building.
RUSSIAN RIVER STONEWARE is a darker stoneware, containing iron oxide and grog. It is an excellent wheel and
slab clay
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