CLAYS
CLAYS AND CLAY BODIES
The concept of a clay body has evolved in order to give the ceramic artist certain
advantages over the problems and limitations of using single clays for all kinds of
wares. Historically potters have always tended to settle where there was a good
deposit of useful clay. The wares that were made reflected the character and limitations
of that clay. Much of the world’s most beautiful pottery was made in just that way. If
properly used, contemporary potters have the opportunity to utilize technology and the
cheap and widely available selection of different clays and other materials to expand
their possibilities and the expression of their character in pottery. There is no longer the
imperative of setting up shop near ideal clay deposits. Adding sand or grog to a plastic
clay in order to reduce cracking and shrinkage has been practiced for thousands of
years and might be thought of as the first clay body. EGYPTIAN PASTE is another
example of a very early type of clay body, as was the discovery of the Islamic potters
that ground glass (FRIT) could be added to whitish clays in order to made them hard
and translucent at low temperature, seeming to reproduce the “real” porcelains being
made by the Tang Dynasty potters in China and much admired in the Middle Eat and
Europe. The examples given above lead directly to what has become the first principle
of clay bodies, which is that clay bodies are usually composed of three types of
materials:
Clays-
Lend plasticity, color, texture and are of course, the main ingredient in clay
bodies. Main types or groups of clays – KAOLINS, BALL CLAYS,
STONEWARES, FIRE CLAYS and EARTHENWARES. Within each
group are all sorts of properties. These properties must be studied in
order to understand their proper and logical use in clay bodies.
Flux-
A flux is a material (or materials) that causes melting and fusion of the
clays, giving hardness, density and durability to the body. The main clay
body fluxes are FELDSPARS, FRITS and TALC with the occasional use of
WHITING, DOLOMITE, BONE ASH, GERSTLEY BORATE and a few
others to supplement the main ones.
Fillers-
Fillers are materials that reduce shrinkage, both wet to dry and in firing
and impart other special properties to specific types of clay bodies. Mainly
flint, grog, sand, wollastonite and alumina, with the less frequent and
special uses of things like fiberglass, nylon fibers, sawdust, vermiculite,
pearlite, pyrophlyte, barium carbonate, petalite, etc.
The study of main clay bodies is a study of an informed combination of the three groups
listed above. How do these materials interact with each other to give specific results at
specific temperatures?
CLAY BODIES:
CLAY BODY considerations:
Considerations for Body Development
Color
Defines material selection of clays and fluxes
Significant influence on glaze color response
May alter color with body stain (see Mason) or oxide
Firing Temperature
A function of the firing capabilities available to you
Direct influence upon color palette
Influences material selection to address thermal shock or other stresses on thick ware
Greatly influences fired strength and body/glaze integration
Density
Will determine overall strength, lightness, permeability, durability, resistance to staining
and acids, translucency and glaze fit.
Texture
Smooth or course textures are both visual and structural considerations
Influences both clay and filler material selections
Greatly influences suitability of various forming processes
Plasticity
Greatly influences suitability of body to various forming processes (esp. throwing)
High plasticity=high clay content=high shrinkage
Shrinkage (dry and fired)
Low shrinkage=low clay content=low plasticity
Greatly influences suitability for various forming processes
Determines how susceptible ware is to cracking and warping during drying
Greatly influences clay and filler selection
Direct impact upon finished size of work (proportional to scale of work)
Thermal Properties
Coefficient of Thermal Expansion (CTE)
Influences firing schedule/type (raku/quick fire)
Affects durability of flameware/ovenware
Influences glaze fit
Kiln Atmosphere
Vapor glazing will influence selection of clays (i.e. high free silica=more vapor glaze)
Oxidation or reduction may also influence materials selection
These considerations are inseparably integrated in composing a clay body. To
successfully discuss clay body composition, it is necessary to break down the individual
components of a body.
Clay bodies can be broken down into three basic categories:
Clays
Kaolins
Fire Clays
Ball Clays
Stoneware
Earthenware
Fillers
Grog
Sand
Pyrophyllite
Wollastonite
Flint
Fiber
Fluxes
Feldspars
Nepheline Syenite
Frits
Talc
Individual materials within the more general categories have distinctive characteristics.
Each clay and material has a rather specific color (at different temperatures), maturation
temperature, working properties and texture. These individual characteristics are of
ultimate importance when choosing materials for a clay body composition.
A basic description of some common materials and their characteristics follow.
CLAY DESCRIPTIONS:
KAOLINS -
Most refractory (highest temperature) and white.
Stocked at Metro:
EPK (Edgars Plastic Kaolin) -
Grolleg/China Clay -
Florida Kaolin, white 200 mesh, medium
plasticity. EPK is a water washed,
secondary kaolin mined near Edgar,
Florida. It is among the whitest-firing
and most plastic of the North American
kaolins, making it a valuable addition to
both porcelain bodies and glaze
suspensions. In glaze mixtures, EPK
provides suspension, gelling, and
application characteristics found in no
other clay. It is economical and has
been consistent over the years.
English Kaolin, pure white, 200 mesh,
high plasticity (higher cost). Grolleg is
moderately plastic, has low titania
content and relatively high flux content,
low shrinkage, and white fired color,
making it excellent for translucent
throwing or casting bodies. Note that it
has twice the iron oxide of #6 Tile kaolin
but 40 times less titania. This is the
primary reason for Grolleg’s
translucency. The high flux content also
allows one to formulate a porcelain with
Tile #6
Helmar Kaolin
Not stocked:
Kaopaque Kaolin -
FIRE CLAYS -
more clay and less feldspar, making up
for the lower plasticity of Grolleg.
6 Tile Kaolin is a selectively mined,
airfloated kaolin produced in Georgia. It
is offers high plasticity, white fired color,
and high green strength. It is not as
white as EPK; its higher titanium content
tends to make bodies high in Tile #6
rather ivory-colored in oxidation firings,
and grey in reduction.
Kaolin that works especially well as
flashing slip for salt, soda, and wood
firing, especially since Avery kaolin is no
longer being mined.
Commercially purified and plasticized
Kaolin, pure white, 200 mesh, good
plasticity, used a whitener.
High temperature, shale clays, coarse texture
Stocked at Metro:
Hawthorn Bond -
Lincoln Not stocked:
PBX or Valentine -
BALL CLAYS -
Missouri Fire Clay (A.P. Green &
Hawthorn Bond), flint clay, 20 mesh,
medium iron content, medium to low
plasticity
Pennsylvania/Ohio Fire Clay, 30 mesh,
low iron content, medium plasticity
Red Fireclay (A.P. Green), 20 mesh,
high iron content, red/brown, medium
plasticity
High plasticity, high shrinkage
Stocked at Metro:
Kentucky Ball (OM4) -
200 mesh, low iron content, high
plasticity, white to gray.
Tennessee Ball Clay(SGP) -
200 mesh, low iron content, high
plasticity, off white to gray. Excellent for
quick slip casting.
Gold Label.
XX Sagger -
High refractory ball type clay, 200 mesh
Medium iron content, medium to high
plasticity, warm white to orange, not as
plastic and more refractory than ball
clay.
PV(Plastic Vitrox)
STONEWARE -
A California feldspathic earth, usually
packaged under the label PVC for
“Plastic Vitrox Clay”, that is similar in
makeup to Cornwall Stone but fires
whiter. It is preferred by many for use in
white clay or glaze formulas where
kaolin or a “white clay” is called for.
Middle temperature
Stocked at Metro:
Goldart -
200 mesh, medium iron content,
medium plasticity, gray-brown
Foundry Hill Cream
A good plastic ball clay that was out of
production for awhile. Not as plastic as
Not Stocked:
Cedar Heights Bonding EARTHENWARE –
Semi plastic, same as composition as
Goldart - coarser grind
Low temperature, secondary clay
Stocked at Metro:
Barnard -
Redart -
Not stocked:
Ocmulgee -
FILLERS
Slip clay, 200 mesh, high iron content,
dark brown to black. This is not a basic
clay body, material, but a slip clay used
mainly for color.
(Cedar Heights Co.) 200 mesh, high iron
content, high plasticity and shrinkage,
low temperature, red color.
Coarse Georgia earthenware, 20 mesh,
medium to high iron content, medium
plasticity, wide temperature range, tan
to red color
At Metro:
Fiber -
Paper Fiber:
Flint/Silica/Quartz -
Grog and Sand -
Calcined Kaolin/Glomax
Pyrophyllite -
Sawdust
Wollanstonite -
Bentonite
Macaloid
Increases working and dry strength
primarily for hand building. Two kinds:
nylon fiber or glass fiber, the latter is
hard on the hands. Both types melt or
burn out in firing.
Great for building strength and reduced
shrinkage.
Used for increased hardness as a
glass former, aids in vitrification. Also
used for whiteness. 200 mesh for clay
bodies and most glazes, 325 mesh for
glaze and porcelain.
Opens clay bodies; decreases
shrinkage and warpage; increases
temperature, tooth and the ability to
withstand thermal shock. May increase
silica levels in vitrified bodies (quartz
inversion) Grog is burned fireclay. In
various mesh grinds. Molochite is a
white, porcelain grog. Grogs and sand
available in many different mesh sizes.
A white filler that reduces shrinkage but
does not contribute plasticity.
Used for thermal shock resistance,
Primarily in ovenware. Increases Silica
levels in vitrified bodies (quartz
inversion)
Opens clay bodies, decreases
shrinkage. Primarily used for
excessively thick clay forms, decreases
workability when coarse. Burns out in
firing. Mixed in wet as a water additive.
Used for thermal shock resistance in
ovenware, Raku, etc. Available as both
a powder and a fiber. Wollastonite
reduces drying and fired shrinkage and
adds fired strength to clay bodies (use
fibrous form as raw material). Also
helps make lovely “satin” type glazes
(use powder form).
plasticizer and suspension agent
A very fine-grained plastic clay of
volcanic origin containing colloidal
matter. Used in small amounts as an
additive to clays to aid in plasticity and
in glazes as a suspending and binding
agent.
FLUXES -
Encourage minerals to melt, aiding in vitrification
Feldspars -
Potash Feldspars best for clay bodies,
especially high temperature, limit 25%
approximately.
Nepheline Syenite -
Lower temperature Feldspar,
used
Frits
Talc
primarily for lower temperature clay
bodies (6), limit 25% approximately.
Lowfire flux. Limit 20%. For most clays
use Frit 3124.
Magnesium silicate, low temperature
fusion. Low temperature clay body flux,
inexpensive. Used in commercial low
fire white clays.
FELDPARS:
Custer -
potash spar
Cornwall Stone -
calcium/soda/potash spar
Minspar (Kona) -
soda spar, mid range
Nepheline Syenite -
soda spar, low range melt
Spodumene -
lithium spar