Glaze & Clay Tutorial - 2
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BASIC NOTES ON CLAYS AND CLAY BODIES
by Robert Fromme
Why is a basic understanding of clay important when we are learning
about ceramics, surfaces and glazes?
* Sometimes the art object requires no glaze and the organic unit
depends upon a natural or manipulated clay surface as part of
* Many problems in forming and drying the objects can be traced
back to the composition of the clay.
* The color and character of many glazes depend upon its
relationship to the clay body under it. (bleeding iron,
* The fit of the glaze on the clay body (expansion and contraction
of both) is an important consideration for anyone working with
glazes or clays.
* The composition of the clay related to problems with the
inversion of quartz and other forms of silica in the heating
and cooling kiln which may cause major problems for anyone
trying to fire their work.
* An understanding of the nature of clay is critical for most
forming and firing techniques and if the crafts person doesn't
understand what is going on throughout the total process, the
greatest glaze in the world will not insure the efforts.
* Clays such as the kaolins, ball clays and slip clays are used
in glazes as one of the raw materials. The Alumina and Silica
which can be found in clay are two critical ingredients in most
* I am sure some of you can come up with some other reasons for
taking a little of our time for notes on clays.
WHERE DOES IT COME FROM?
Scientists tell us that our world was once a body of molten
lava. The outside cooled and a variety of chemical elements
blended and stratified into a crust made of mostly igneous
rock. The rock surfaces of our earth began to erode as our
atmosphere began to form moisture along with the other forces
we associate with weathering. In time, the rock began to
disintegrate into clay. The gasses and pressure from below
worked up through the crust while the expansion, shrinkage and
squeezing of the rigid exterior continued to work erode the
surface stone. Of course, the procedure continues even today.
THE CLAY PARTICLE
When we look at clay through an electron microscope, the clay
particles seen as a thin hexagonal plate, approximately 100
times longer than it is thick. (Note: Richard Burkett has
placed an image file of clay particles as seen under an electron
microscope in another directory of the Ceramics Gopher at SDSU.)
When we add the water of plasticity to the dry clay, moisture
between the flat plates creates a surface tension attraction so
that the particles do not easily pull apart, but they slide
easily over one another. The flat shape of the clay particle
and the surface tension when water is added gives the strength
and plasticity which we associate with clay in its workable state.
When the clay object is formed and the material is dry, the
tiny, flat, packed particles seem to lock closely together
giving the greenware its structural soundness. No doubt, the
same forms created from inorganic earthen materials which have
little clay in the mixture would crumble even before the
drying process was finished.
Clay vs. Clay Body
At this point, let us make a distinction between the terms
'clay' and 'clay body'. We will use the term clay to refer
to those materials of a plastic quality which are formed by
natural forces and which are to be found in nature. The term
'clay body' will be used to indicate a mixture of clay like
materials with other inclusions for a specific ceramic
technique. In other words, a 'clay body' may have several
different kinds of clay, fluxes, silica, grog, and other
ingredients for color,plasticity, warping, cracking,
shrinkage, porosity, firing temperature,texture and etc. A
single clay from the natural world will seldom have all of the
characteristics which the potter will need for a particular
ceramic technique. The principles of forming a body are the
same regardless of whether it is earthenware, stoneware, or
Plasticity (workability or elasticity)
The basic clay may be overly plastic or not plastic enough for
a particular use. Clays that are too plastic (tiny clay
particle size) may not hold their shape when throwing. The
mix will be very sticky and in the wet state the mixture may
not have the strength to stand while being thrown. While
throwing a cylinder with this kind of clay, the form will
appear to squat near the bottom after each pull up the form.
Clay that is not plastic enough will not throw or move with
ease when it is under pressure. It may tear or rip when thrown
or crack when bent, even when very wet. Many Handbuilding
processes do not need a clay body that is as plastic as the
typical throwing body. Handbuilding clays are often made to be
less plastic to reduce problems with fabrication and drying.
Clay that is very plastic will almost always shrink more than
a less plastic clay, often leading to drying problems such as
warpage and cracks.
The old standard test for a clay with enough plasticity for
throwing is to wrap a 1/2 inch thick clay coil around your
finger and if it cracks exorbitantly, it is not plastic enough
for the requirements of the forming technique.
Ancient Chinese potters discovered that they could increase a
clay's plasticity by aging it, thus developing a more intimate
relationship between particles, water, and fillers. There is
the old story of organic inclusions such as milk being added
to the clay as it is prepared and stored for the coming
generation while using clay which had been prepared years
earlier by their ancestors. Such organic additions can cause
problems in studio clays as they spoil and smell bad in the
The potter can reduce plasticity by adding fillers such as
grog, sand, or flint if it is not possible to use a less
plastic clay in the body mixture. The inclusion of at least
a small amount of fine-particled, plastic clay will usually
improve the dry (green) strength of a clay body.
Clay Body Color
When your locate a clay in nature, the fired color may be
lighter or darker than desired. Metallic inclusions such as
iron or manganese often 'bleed' out from the body and into the
glazes on the surface.
For a light or near-white clay body, a relatively pure clay is
needed to start the mixture and only small amounts of colorant
are necessary to alter it to a tan or buff color. One can not
work backwards and lighten or bleach a darker clay to make it
If the potter wants to darken a body, the most common addition
is iron. Some additional colorants are: cobalt (blue), copper
(green), chrome (green), and manganese (brown). Be warned that
many metal oxides and compounds are considered hazardous. A better solution
may be to add natural dark clays such as Barnard clay
to the clay body instead of metal oxides. Iron oxide is usually
a safe addition. In addition, oxides like copper, manganese and cobalt may
add unwanted flux to the clay body. Most fired
clays range in color from buff to black, depending on how
much iron is present-the more iron,the darker the fired clay
Often, a clay as it is found in nature may fire too open or
too tight and contain more or fewer visual impurities than
desired. When the clay is too rough or visually coarse, you
can add clay of a finer texture.
Keep in mind that the non-plastic material you add will affect
a clay's plasticity. If the clay needs to be adjusted the
other way, grog, Kyanite,sand, and other aggregates (such as
vermiculite) can be added. A number of organic inclusions
have been tried throughout history, rice, sawdust, hair,
cattail fuzz, paper pulp, nylon fiber, fiberglass and may
other materials can be added to the clay body. The organic
inclusions will burn out in the firing and while the more
traditional things like grog and sand will survive the heat.
Other ideas for visual texture, have included red brick
grog, or white porcelain grog, shale, iron shavings and
Shrinkage (Warping or Cracking)
Clay body shrinkage refers to the loss of size that occurs
throughout the total drying and firing process. There are
three distinct periods of shrinkage.
The first is when the piece air-dries to become bone dry, the
second is during bisque firing, and the third is during
As a general rule, we can assume that the more plastic a clay
body the more shrinkage. Clay with finer particles absorbs
more water, expands more, and when the water is forced out by
drying or firing, one can expect more shrinkage.
Less plastic clays and fillers such as grog or sand can be
added to "open" the body and help with a shrinkage problem.
Such additions in large amounts can also reduce plasticity.
Spodumene or wollastonite can be added instead of flint or
fluxes in a clay body in order to lower the shrinkage even
When the clay artist reduces shrinkage, other problems such as
warping, and cracking during drying will also improve.
Warping results from the shrinkage of clay which is highly
absorptive or it may also result from poor technique which
creates uneven walls, uneven drying, unsupported weight,
uneven moisture consistency of the clay,uneven firing support
and the plastic memory of the clay.
Firing a clay body at too high a temperature can also lead to
warping as the body is heated beyond its maturation
temperature and begins to loose structure in the melt.
Cracking may also result from poor technique. It can be caused
by having clay which is too wet, excessive shrinkage, or uneven
drying. Cracks may appear when moisture is not consistent
between different parts of the object, when the wall thickness
is not even, when force-drying causes uneven drying, when you
use thick glaze applications, or thick slip applications.
One must also remember that the expansion and contraction of
materials as they are heated and cooled and chemical changes
such as the quartz inversion during firing also has an effect
upon the warping and cracking during and after firing. (Quartz
inversion is a transformation of crystalline silica that
occurs at about 1060 deg. F /571 deg. C and that causes the body
to shrink considerably as it cools.)
Clays can shrink as little as 4% or as much as 25%. A clay
body that meets all a potter's needs and shrinks no more than
about 12% is acceptable for most techniques.
Maturation Temperature (Porosity)
After firing, some natural clays remain too porous or become
too dense at the chosen firing temperature. The higher a
particular clay body is fired, the more vitreous it becomes.
However, other factors in formulating a body will affect the
maturation. Color, plasticity, texture, or shrinkage desired
in a clay body may call for the addition of materials or clays
which affect the final maturing temperature. Vitreous clay
bodies can be made at any temperature, but are most often
found at the higher firing ranges such as stoneware or
Adding lower melting ingredients lowers the maturation
temperature of the clay body and decreases the potential for
water absorption in the fired ware, while at the same time
increasing firing shrinkage. Typically, fired porcelain
absorbs from 0-3%, stoneware 1-5% and earthenware 4-10%
liquid. In general, we add ingredients to lower the maturation
temperature if we want to make a clay body less porous,more
vitreous and with a better ring when tapped. However other
factors like thermal shock or freeze resistance may also affect
how vitreous one would want the clay to be when fired.
RESIDUAL AND SEDIMENTARY
(or Primary and Secondary Clays)
Clay types are identified by the way they geologically formed.
We have two main classifications, primary and secondary. These
are sometimes called residual and sedimentary. In other words,
the primary or residual clays remained at their original
location and the secondary or sedimentary clays were moved from
the primary site to a new location by wind, rain,or ice. Natural
weathering has relocated most of the surface of our world and
primary clay deposits are relatively rare. Of course, the
amount of mineral impurities and organic matter would be
greater in those clays which had been transported from their
original location and subjected to a mix of miscellaneous
other inclusions as thy were being moved and relocated. The
grinding action of clay particles in water,wind, and ice
created, as a consequence, very fine particle sizes, making
secondary clay extremely plastic. As the materials settled in
river and lake beds, the weighty, coarser particles settled
first, leaving the minute,more plastic particles on the
Kaolin (China Clay)
Deposits of kaolin occur in Europe, England, North America,
and Asia. Scholars suggest the name 'kaolin' comes from the
Chinese words 'Kao' and 'Ling' which means 'high hill',
giving reference to the location of original deposits of the
Seldom in nature will we find any materials which is pure,
however, the primary kaolins are often 95% pure (free of
impurities such as iron, manganese, alkalies and other
inclusions which may be found in the darker sedimentary
clays). The kaolins have been exposed to less weathering so
the particles size is usually larger. This means that the
kaolins can be thought as generally less plastic than some of
the other types of clays.
The lower melting alkalies are among the original inclusions
in the igneous rock. These and other lower melting inclusions
are water soluble and end up being leached out as the rock is
reduced to clay. The loss of the lower melting inclusions
results in a clean, near-white firing clay that matures at
elevated temperatures (Approx. 3275 deg. F / 1805 deg. C).
Some locations have been commercially developed for the
extraction of deposits of secondary kaolins. These clays are
those that have been transported at some time in history by a
mode that did not cause the inclusion of a large amount of
impurities. However, because of the additional physical
activities involved with the development of these clays, the
particle sizes of secondary kaolins are usually finer,making
the secondary kaolin easier to work with (more plastic) than
kaolin from a primary location. Secondary kaolins usually
loose some of the purity as they are relocated to the second
deposit. The typical formula for kaolin is the same formula
that we associate with kaolinite, the crystalline form in most
clay like materials, Al203.2SiO2.2H2O.
After the discussion of secondary or sedimentary clays, the
origins of Ball clay will be easy to understand. Thousands
of years past, in the swamps and low flat lands, these clays
seem to have been laid down along with organic layers which
would later be mined as coal. The name 'ball clays' comes
from the practice in the mines involving the removal of
layers of clay from the coal deposits. It seems that the
clay was rolled into balls, loaded onto the back of the pack
animals or carts, and transported out of the area where the
coal was being mined. Like most sedimentary clays, the
ball clays have fine particles. They are very plastic
and are important additions for clay bodies used in throwing
and other forming techniques.
Although, the ball clays are often good additions to other
clay bodies to add plasticity, there are some problems which
we should keep in mind. With the fine particle size, the
shrinkage and tightness of the body create forms which often
shrink a great deal, dry slowly and do not allow for the
passage of steam out of the ware in the early stages of the
fire. Ball clays in excess may add a sticky quality to a clay
body. Also, the amounts of organic material (coal dust, etc.)
and extra sulfur present in ball clay account for its brown
color and early firing odors. In spite of the dark raw color,
the clays are usually fairly low in iron and other impurities
and fires to a light buff color. Alkalies and other lower
melting impurities, as well as the small particle size of the
ball clays render the materials lower in maturation
temperature than the cleaner kaolins (china clay). Ball clays
mature at around(2345 deg. F / 1285 deg. C).
Ball clays can be found in many locations and the commercial
locations of extensive mining are in Tennessee and Kentucky.
Kentucky ball clay,Old Mine no. 4 is quite pure and may be
fond in many raw batch weight glaze recipes as well as in some
porcelain formulas to help increase plasticity.
Earthenware (Red Clay)
Natural earthenware deposits are usually found in outcroppings
or in sedimentary patches along streams or rivers. You may
also discover these clays along highways that cut through
layers of secondary earth.
This sedimentary clay is probably the most common clay in
nature where numerous lower melting inclusions (impurities
such as alkalies and iron) lower its maturation temperature.
We use the term 'flux' to refer to the lowering of melting
temperatures. Earthenware clays melt at such low temperatures
that they seldom become vitreous and the ware continues to be
porous after firing (1850 deg. F/1010 deg. C). For this reason,
the work is usually glazed if it is to contain liquids. The
body usually fires a dark red because of the high iron content
in the clay. Seldom will a naturally deposited earthenware
fill all of the requirements of a particular forming or firing
technique and characteristics such as particle size and
plasticity often vary a great deal. Like the other clays in
nature, earthenware also require other materials to meet the
full set of potter's requirements (maturing point, texture,
color, and plasticity).
Earthenware Clay (Low-fire White or Buff Clay)
We should note that recent interest in low firing techniques
have given rise to the use of the term earthenware when
talking about any clay body which has been formulated to mature
at the earthenware temperature range. The origin of these
white clays can be traced to Europe and the early efforts to
duplicate the imported porcelain which was being traded from
the Orient. These European clay bodies (Soft-Paste Porcelain,
bone china, etc.) were formulated using large quantities of
fluxing inclusions, lowering the melting temperature for the
relatively clean mixtures of kaolin and ball clays. The best
descriptive term for this kind of clay body is 'low-fire clay'
or 'low-fire whiteware' in order to distinguish it from red
earthenware in nature.
Another term, 'white earthenware', is also used to distinguish
the man-made clay bodies from the red earthenware of nature.
Stoneware clays are even less pure than the ball clays. Some
of the impurities, calcium, alkalies, iron, and feldspar drop
the maturation temperature a bit lower (2300 deg. F / 1262 deg. C).
The fired color for these clays becomes much darker because of
the additions of metals like iron in the sedimentary mix. One
additional difference between the ball clays and stoneware
clays involves the size of their particles. Here, ball clay
usually beats the stoneware in plasticity and finer particle
In the raw form, stoneware clays usually require the addition
of other clays and chemicals to adjust their properties to
match the demands of specific forming techniques. The
adjustments usually involve lowering shrinkage, improving
plasticity fired color, texture, and adjusting for a specific
maturation temperature. There are always the old potter's
stories of looking for the clay nests of 'mud dauber wasps'
and running them though a firing in order to see if a usable
stoneware clay deposit might be in the vicinity. As one would
expect, local deposits of stoneware clay will seldom match all
of the technical requirements of maturation temperature,
color, plasticity etc.
Fireclays are characterized by large particle size (low
plasticity),elevated maturation temperature (2650 deg. F / 1454 deg. C)
and large inclusions of impurities such as iron. Aside from
their inclusion with other clays to raise the maturing point
for the clay body, lesson drying shrinkage,open the body so it
will dry quicker, add color, add texture and to lesson
The clays which have been given the name 'Sagger Clays' are
usually fireclays or coarse stoneware clays which are of
fairly large particle size and which mature a high
temperatures. The clays get their name from the practice of
using clay containers, saggers, in the kiln to protect the
ware and allow for stacking the work. With new developments in
fuel, kiln building and the manufacture of refractory kiln
shelves and posts, saggers are seldom used today. Sagger clay
had to withstand many firings and support the weight of
additional saggers and ware in the loaded kiln.
In addition to sagger clay for throwing the containers to
stack the kiln, a cheap source of low grade clay was needed in
the kiln stack to level the stack and seal potential cracks in
the kiln walls and around bricks of kiln doors. Like sagger
clay, this 'wad clay' was usually a coarse stoneware or
fireclay with good strength and low shrinkage. It does not have
to be plastic and it does not require the traditional quality
standards of preparing clay for specific forming techniques.
Throughout history, slip clays have provided the basis for
very good glazes for stoneware and porcelain. Fine particles
and large quantities of impurities (flux) cause slip clays to
melt around 2250 deg. F/1235 deg. C. The common slip clay
formerly used by potters is Albany clay with its high iron
content and beautiful rich brown to black shiny surface. With
little or no alteration, an acceptable surface can be obtained anywhere between
2235 deg. F/1225 deg. C and 2380 deg. F/1305
deg. C. As Albany slip is no longer mined, other substitutes
include Alberta slip from Canada and various recalculations of
recipes using locally available earthenware clays.
We must make a distinction between 'slip clay' which is found
in nature and 'casting slip' which is a man-made mixture of
clay, water and various chemical deflocculants (electrolytes)
such as sodium silicate or soda ash which makes the mixture
into a liquid, but with less water and subsequent shrinkage.
This makes a slip which is useful for pouring and casting in
Clay particles have a natural attraction to one
another(positive and negative charges) that brings them
together or to 'flock'. The attraction requires large amounts
of water to break up the particles and render the clay into a
slip under normal conditions. To alter the charges so the
particle will act like the backs of magnets and repel each
other, an electrolyte is used. The use of these chemicals
reduces the amount of water needed to disperse the particles
and make a fluid,cartable mixture.
We have various kinds of clays of volcanic origin and all of
them ensue from the weathering of volcanic glass or deposits
of volcanic ash. Their particles size ranges from that of most
clays to some of the finest of all clay particles.
Bentonite, the most common volcanic clay (Al2O3.4SiO2.H2O), is
often used as a plasticizer for other clay bodies. At four to
five times more plasticity than other clays, only a small
percentage (2-3%) is needed to add workability to another clay
body which is in need of plasticity.
Bentonite should always be added to the dry ingredients of a
glaze or clay (body) before adding water, or it should be
thoroughly slaked (or blended in a blender) with a portion of
the water to be added to the clay. If you do not do this,
you will be forced to struggle with a gummy mass of bentonite
which will not mix into the other ingredients easily.
Bentonite in a glaze (1-3%) will help hold most glazes in
suspension, assist glaze adhesion, and harden the dry surface
without noticeably affecting the fired glaze.
The clay we call 'adobe clay' is usually clay from near the earth's
surface and which has proven acceptable for making the sun-dried
bricks. It is usually quite sandy and not very plastic. Some clay
artists have experimented with the addition of petroleum products
in the adobe mixtures for their sun dried sculptures.
This is a common term for surface deposits of clay which show up in
farmer's fields and other locations where the plastic material can
cause problems for vehicles and farm implements.
You will want to consult the many fine ceramics texts which are
available. This file is intended to help the student establish a
very basic understanding of clays and clay bodies as they relate to
forming techniques, glazing and other forms of surface decoration.
The temperature ranges and percentages in this file are approximate
and based on experience in the craft. I can not control the
conditions of your clay and glaze testing so I am not responsible
for any problems or damage which may result from your use of this
basic information. I hope you will continue your research into the
specific areas of your interest using texts and additional on-line
educational materials. I hope these notes will help you in your
search for mastery of ceramics.
(c) 1994 Robert Fromme / For educational purposes only.
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