EXPLORING THE WORLD OF CLAY

Linda Xu and Kathleen Ross

Introduction to Ceramics......................................................2
Ceramics in Civilizations.....................................................3
All About Clay......................................................................6
Wedging................................................................................8
Tools of the Trade..................................................................9
Basic Techniques.................................................................11
Throwing.............................................................................13
Firing..................................................................................15
Glazes..................................................................................18
Toxicology............................................................................21
Glossary..............................................................................23
About the Authors...............................................................25
Illustration Credits.............................................................26
Pottery is sculpted artwork made from clay.
Statues, vases, cups, bowls, and plates are just a
few of the objects you can make, and potters
make these for a living. After they sculpt a piece,
they bake it in an oven called a kiln and paint
it with special mixtures called glaze. Then, they
bake it again so the glaze melts and turns
glassy. But this is only whiteware. Other types
of ceramics include structural ceramics, which
include bricks, pipes, and tiles; refractories like
kiln linings and crucibles; and technical ceram-
ics like gas burner nozzles, biomedical implants,
and missile nose cones. The main focus of this
book, however, will be on whiteware.
 Pottery survives for hundreds of years, unlike
paper, which eventually deteriorates. Even if it
breaks, it can be glued back together. This is
why archeologists have been able to learn about
past cultures through the ceramics they created.
A few of the civilizations that we know about
thanks to ceramics are the ancient Greeks,
Egyptians, indigenous people of Mesoamerica
and South America, Chinese, and Indians, al-
though there are many more groups with much
more pottery that we have yet to find.

The ancient Greeks often depicted mytho-

logical scenes in clay and are most
known for two styles: black-figure and
red-figure. In the black-figure style, the
people in the scenes are painted black on
red backgrounds. In the red-figure style,
the background was painted black, and
the figures were left red. The high tem-
perature of the kiln caused the iron to
combust and oxidize, which formed rust
and colored the clay red.
 The Chinese used ceramics to display differ-
ent aspects of their culture. They raised the
creation of ceramic from a necessity to an
art form of the highest degree. Later, they
discovered glazes, minerals that became
glassy when melted and added more colors
and detail to their pottery than paint; a very
common style was porcelain with blue glaze.
They utilized ceramics for many purposes,
from guarding tombstones, decorating tem-
ples and palaces, and warding off evil, like
the figurines performing the moon dance on
the left are doing.

The ancient Egyptians used ceramics in

everyday life as decorations, as eating
utensils, and as household containers.
Though these comprise the majority of
their uses, not all pottery had such mun-
dane purposes. The canopic jars, for exam-
ple, played a key part in the sacred ritual
of mummification. During this process,
the subjects main organs were removed
and placed in decorative jars to preserve
them for the journey into the afterlife. A
very distinctive feature was that each jar
had a different cover that depicted either a
man, a god, or an animal, each of which
would protect a different organ.
Ceramics created by the indigenous peoples of Central
South America and Mesoamerica are important in the
study of the culture of those regions. Mesoamerica has
a rich geological history comprised mainly from a
volcanic past. The metamorphic and igneous rock, as
well as the sand and ash, provided many types of
tempering. Mayans made polychrome pottery with de-
signs in red and black on cream or orange clay.
Zapotecs made many urns shaped like their gods,
whereas the Toltecs made orange pottery in a variety
of styles and glossy, dark pottery. Likewise, much
Aztec pottery uses orange-burning clay painted in
black designs. None used pottery wheels; they made
coils and slabs instead. At first, the designs had cur-
vilinear geometric motifs, but they later became more
naturalistic, and included bird, fish, and plant

India was an agricultural country that used pots

for storing water and grains. Pottery in India be-
gan in the Indus Valley Civilization because of the
high demand for earthenware and the availability
of clay. Potters shaped seals, decorations, and
figurines in addition to the more practical pots.
Over time, the simple style of Indian pottery was
developed and refined, resulting in many unique
forms. They had both glazed and unglazed pot-
tery, of both white clay and terracotta, which was
used when sculpting figurines to worship their
gods and goddesses during festivals. As you can
see, the ancient pottery of a country reveals much
about its people. It is a medium through which all
civilizations have always expressed their emotions
will continue to do so in the future.
Clay is a mineral compound of silicate
(silicon and oxygen) and aluminum that
becomes malleable when wet and becomes
hard when fired or dried. It usually forms
when rock that has at least some silicate is
dissolved by solvents, usually acids.
Sometimes, water heated by the core of the
Earth can form clay, but erosion produces
most of it. Most deposits are found in or by
large lakes instead of where they were
formed because of erosion transports the
clay deposits downstream.

The clay commonly used for pottery is

a mixture of a few of the thirty pure
clays and corroded minerals. These dif-
ferent compositions give the different
types of clays their unique properties
and special characteristics. An example
would be terracotta, a very recogniz-
able red-brown clay that many potters
use. Most clays can be categorized into
three groups: earthenware, stoneware,
and porcelain. The aluminum silicate
that they all include is kaolin, the
chemical structure of which is depicted
above with an image of dry kaolin.
Earthenware is the cheapest, most common type of
clay and comprises kaolin, mica, quartz, and feld-
spar. It can be red or white, like the plate below, and is
the most easily chipped and most porous of the clay
family. Porcelain is mostly kaolin and is fired at a
much higher temperature than earthenware. This
causes glass and a strong, heat-resistant silicate
called mullite to form, which makes porcelain mostly
impermeable, resonant, tough, translucent and resis-
tant to chemicals and thermal changes. Because of its
strength, artists are able to use very thing porcelain,
like in the flowers below. Stoneware mostly comprises
fire clay, a very heat-resistant substance used for fire
bricks, and is very dense, impermeable, and opaque.
Only a little glass forms, and impurities give it a grey
or brown color, making it resemble stone, as seen on the
left. Each type of clay usually has grog, fired clay
that has been ground down, inside. This lessens
shrinking and provides stability.
Wedging is a technique used to mix the clay
and push the air out of it. The two main types
are cone wedging and cylinder wedging. Both
are very similar to kneading dough and work
best with grapefruit-sized balls of clay. For
the cylinder method, you should put your
hands on the sides, at the two-oclock and ten-
oclock positions. Then, use both hands to
push down and away from you for a few
inches. Then, turn the clay so that its stand-
ing on the edge that was closest to you and
repeat twenty to fifty times. If the cylinder is
getting too long at any time, just push the
ends in a bit.

With the cone wedging method, you

put your hands on the sides of the clay
again and push down and away from
you with one hand while keeping the
other hand perpendicular to the table.
Then, tilt the clay onto the side that
you pushed down on and repeat this
cycle another fifteen to twenty times.
This method is best for throwing be-
cause the cone shape makes centering
easy. For any type of pottery, you
should wedge the clay before using it so
that your sculpture doesnt explode in
the kiln.
 In ceramics, we use tools like knives,
metal ribs, sponges, and wires. But, al-
most anything can be used as a tool. If
you want to burnish, or polish the sur-
face of something to smooth it out, for
example, you can use a spoon, a smooth
stone, or even a light bulb. For slicing
the clay, like when you make slabs,
you can use wooden knives, needle
tools, or scalpels. There are tools to scoop
out clay and rollers that let you flatten
it.

In throwing, we use a few other tools.

Sponges let us smooth the surface of
the clay and wipe away extra water that
accumulates on the sides and inside the
vessel. If water pools inside, the bottom
of the vessel stays too moist, weakens,
and ruptures. So, we make sure to
sponge it out. Any clay that spreads
out where you dont want it to can be
removed with either a wooden or metal
rib. After you finish molding the clay,
you use the wire tool to cut it off of the
wheel.
 Then, we use lifters like the ones on the left to lift
the piece off of the wheel to dry. We might use a
hair dryer to speed it up or just let it dry slowly.
After that, we trim the piece using the loop tools
on the previous page. We might use the needle
tools or the wooden knife first to mark up where
we want to cut. Then, we might add a few addi-
tions like a handle to a mug or a decoration on a
vase. To attach those, we score the surface, perhaps
with a comb or knife, and then rub slip on with
our fingers or a paintbrush. Afterwards, we fire
the piece and paint the glaze on. Ceramics tools
can be anything and everything; it all depends
on how you use them.

Mosaics
Mosaics are images made from piecing
together small pieces of colored glass,
stone, or clay. Their uses range from the
decorative to the religious and have
been featured in many cultures, both
ancient and modern. Today, many
mathematicians see mosaics as not only
an art form, but also as complex
tessellation problems for them to solve.
Also, we now have photomosaics. These
are fascinating pictures where a large
picture is composed of small tiles that
are each their own picture. You must
look closely to see the individual images.
Possibly the most vital technique of pottery is
slipping-and-scoring. We use this when we want
to join two separate pieces of unfired clay together.
First, you make scratches, or scoring, along the
surfaces of the two pieces of clay where you want
to join them together. Then, you lightly smear a
layer of slip, liquid clay, over both scratched ar-
eas and put the pieces together so that these areas
overlap. Press them firmly together and smooth
away the excess slip. The slip enters the scratches
and softens the surface of the clay so that it
hardens in time with the slip, joining the pieces
together as if there was originally a layer of clay
connecting the surfaces.

Another basic technique of ceramics is

coiling. With this, you take a good
handful of clay and roll it into a long
tube. Its best to use the palms of both
hands to make sure that the coil is
smooth and even. Then, you layer these
coils into patterns and slip-and-score
them together. You can include little
balls of clay to decorate, like on the left,
or make the coils look like something
else, such as snakes or vines.
 Slabs are also quite common. They
are, as the name suggests, flat blocks
of clay with even thickness. You make
them by taking leather hard clay and
using a roller, usually with rods of
equal height to help you maintain con-
stant height. Then, you cut the edges
at angles and slip-and-score them to-
gether. The jar below is a square slab
jar burnished with a spoon and deco-
rated with sgraffito, a technique in
which you paint underglaze or an en-
gobe over leather hard clay and then
Trompe l oeil carve away a layer to reveal the origi-
Trompe loeil refer to artwork so nal color of the clay in a design.
realistic that people actually mistake it
for the real object. The name is a
French word meaning to deceive the
eye, which happens through a careful
manipulation of perspective. These
pieces can be paintings (which use
shadows to create perspective), pottery
(which would use texture and special
glazes), architecture, and other
mediums. Now, tromp loeil are used
when designing sets for plays, movies,
and television programs, like Star
Wars. But theyve been fooling people
for much longer; George Washington
once bowed to a painting before
realizing that it was a picture when he
visited the studio of Charles Willson
Peale.
Throwing is one of the most well-known
pottery techniques practiced. This is the
process of sculpting pieces on the pottery
wheel, and anything made on it should
have perfect rotational symmetry, al-
though some potters purposely add im-
perfections later. Then, after the pieces
become leather hard, potters trim their
bases to give each of them a foot, the
small rings on the undersides of bowls
and vases. Wheels can also be used for
circular decorations and painting.
First, you take a ball of very well-wedged clay.
There must be absolutely no air bubbles because
the piece will be fired in a kiln, which reaches over
1100 C. When heated, gases expand rapidly,
unless there is a strong container around them,
which would cause the pressure to increase instead.
The pressure would grow until the clay shattered
and cause an explosion, which would destroy eve-
rything in the kiln. After wedging, wet the ball of
clay and the wheel very slightly and then firmly
throw the clay onto the center of the wheel. The wa-
ter creates suction so that the clay will stay there
when the wheel starts to move. Start the wheel.
Next, place your hands in the same positions as in
the picture to the left, the dominant hand at the 5
oclock position and the other on top.
 Apply pressure with both hands, while keeping
them as steady as possible. Soon, the clay should
begin to form into a symmetrical mound. Youll
know youre done if you can look at it from the
side and top and not see any changes as it spins.
Then, open the clay. Move your thumb to the top
and find the center, where theres almost no fric-
tion. Then, push your thumb in, making sure not
to reach the wheel itself or make the clay too thin.
After that, put your dominant hand on the same
position it had when centering and put your other
hand on the same location, but inside the vessel.
Slowly, move your hands up together, never leav-
ing either the inside or outside unsupported. The
vessel will narrow as you move your hand toward
the center and widen as you move away.

After shaping the piece, stop the wheel, use a wire

tool to separate the clay from the wheel, and use
lifters to move it onto a place where it can dry.
Once its leather hard, mark the approximate lo-
cation of the bottom of the vessel on the outside.
Then, make a mark below that to show the
thickness of the base. Place the piece upside
down on the wheel and press soft clay down
around it to stop it from moving or flying off
the wheel. Use a loop tool to cut away excess clay
and shape the foot. This process is called trim-
ming. After that, you can take a wet sponge
and lightly smooth the surface. Thats all it
takes to throw everything but plates. Cylinders
have an extra step, but the general process is the
same.
After sculpting pottery, you have to fire it to
make sure that it wont get soft the next time
it gets wet or shatter easily. Clay that has been
fired once but not glazed is called bisque. In
general, earthenware is fired at 1000-1200C,
stoneware is fired at 1100-1300C, and porce-
lain is fired at 1200-1400C. Firing causes
sintering, which fuses particles of powders.
This makes the pores of the clay smaller,
which shrinks the piece. Also, glass and other
minerals form and react to give each type of
material its unique characteristics.

Firing is done in kilns, and kilns have

changed greatly throughout history. The an-
cients just filled ditches with pots and fuel for
the fire, a process called pit firing, which gave
the pots a special texture due to carbonization
and ash. Eventually, potters added firing
chambers and stoking holes to conserve heat
and protect the pottery from soot. The holes also
improved the air flow and caused the fuel to be
burned more completely. Then, during the In-
dustrial Age, inventors designed kilns using
electricity, natural gas, and propane. Modern
kilns mostly use natural gas, and they have
computerized interfaces that let the potter con-
trol the temperature with more finesse and ex-
actness than before.
 There are two different types of kilns that are
normally used today, but both are insulated
boxes that have controlled inner atmospheres
and temperatures. There is the intermittent
kiln, which has the piece placed into the kiln.
The kiln is sealed, and the temperature is
raised. Afterwards, both the pieces and the kiln
need to be left to cool. There is also the continu-
ous, or tunnel, kiln (on left), in which the
greenware is moved through the kiln, towards
the center, which is the hottest part, and then
away from it. This slowly increases the tem-
perature the piece experiences as it moves for-
ward, instead of continuing to heat the kiln,
which wastes energy. As the name suggests,
this type of kiln greatly resembles a train tun-

A few examples of kilns in history include the Ana-

gama kiln, the electric kilns, the Feller kiln, the micro-
wave assisted firing technique, the Noborigama kiln,
the Sevres kiln, and the top-hat kiln. The Anagama
kiln is the oldest one and was brought to Japan from
China in the fifth century. It is like a tunnel kiln, but
the fire is on one side, the air chimney is on the other,
and the pottery stays in one place. The Noborigama
kiln is a modified Anagama one with multiple cham-
bers. Exemplary wood-burning kilns were the Sevres
kiln (able to reach the high temperature of 1280C) and
the Feller kiln (which used unburned gas previous fir-
ings to pre-heat air and reduce the firing cycle). The
electric kilns are used for small scale operation like
schools and hobby centers. The most recent design is
microwave assisted firing, which is highly economical.
Bisque firing is used to strengthen the pieces
enough to be able to actually use them. It is also used
as a more low-risk method of getting rid of pieces
with air bubbles or too much water. Never fire any
greenware that is not bone-dry unless you want an
explosion. A few potters glaze their greenware, but
this is a very bad idea with amateurs because, while
bisque can explode and break other pieces, glazed
pieces do that and stick to the sides of the other
works and the kiln. During the firing process for
earthenware, the temperature of the kiln should in-
crease 60C every hours for ten hours and then rise
75-100C until the temperature is 1050C. Stoneware
can be fired at 1100-1300C, but 1180-1280C is
more usually used. The temperature to fire porcelain
is between 1200-1400C.

When glaze firing, make sure that the kiln is at the

correct temperature. Overfired glaze is too runny,
and underfired glaze looks chalky. Start by increas-
ing 150C every hour and then, during the last hour,
only increase by 100C. Make sure that none of the
pieces are touching each other. If they are, the glazes
will fuse together when they liquefy, and you wont
be able to separate them easily. When the kiln reaches
the final temperature, let it stay that way for at least
10 minutes so that the heat can distribute itself
evenly. Also, make sure never to open the kiln when
the inside temperature is over 100C. The cold air will
solidify the glaze too quickly. Except for water, liq-
uids become smaller when they cool (because density
increases and mass doesnt, volume must decrease),
and the rapid shrinking makes the glaze crack.
Glaze is a vitreous layer of a substance which has
been fused to pottery to color, decorate, strengthen,
or waterproof it. Glazes are usually classified by
temperature and the type of clay. For example there
are Earthenware, Mid-temperature or High-firing
Earthenware, and Stoneware glazes. Normally,
glazes are glassy in appearance, but some can look
metallic or dull, depending on the ingredients. All
glazes have three basic components to them: a glass
former, which determines how shiny the glaze will
be; a flux, which determines the temperature at
which the glass will melt; and small amounts of
clay, which help the glaze bind with the pottery.

Glazes come in many different colors as

result of the minerals and inorganic com-
pounds contained in them. Commonly used
colorants include iron oxides, cobalt oxide,
chromium oxide, copper oxide, and copper
carbonate. Transparent glazes are clear
glass that might have small amounts of
these colorants, which allow any decora-
tions or underglazes that are under the
glaze to show, possibly tinted. This type of
glaze is particularly suitable for ceramics
or carvings that involve intricate detail,
such as those using sgraffitto or those with
paintings on them, like on the left.
 Newcomb Pottery
In 1894, the H. Sophie Newcomb Memorial College
brought together the fine arts curriculum with a
commercial enterprise to provide employment for
graduates when there were few work opportunities for
women. Approximately 90 women artists produced
about 70,000 pieces between 1895 and 1940. Within this
business, women were always the designers, whereas
men were always the potters. What makes the pieces so
unique is that no two pieces are exactly alike. This
pottery has nature motifs and, while early pieces were
monochromatic blues and greens, later pieces added
strong cobalt blue and even pinks. Newcomb Pottery is
exceedingly valuable as a collectible item. A four-color
Newcomb pottery vase sold in 1996 for $29,700, and few
Newcomb pieces are valued less than $1000.

Opaque glazes are made by adding an

opacifier, such as tin or zircon, to a
clear glaze to reduce transparency when
fired. Tin is typically the best opacifier
for earthenware glazes. Certain opaque
glazes, particularly Stoneware ones,
can show different colors underneath
them depending on the thickness of ap-
plication. This causes the glaze to seem
to break over edges and detail in sculp-
tures, adding an additional attractive
aspect to the pieces.
Matt glazes are opaque mixtures that are made of
tiny crystals that form in the glass. Smaller
crystals are used to achieve smoother textures,
and Vellum or Satin Matts are the smoothest.
Matt glazes especially dry ash glazes, which
are applied very thinly are good choices for
sculptural work because of this quality. There is
also another type of glaze which, instead of hav-
ing one consistent color, have mineral crystals
added to produce bursts of color. These are known
as Amaco glaze crystals. They can be applied
where desired on the piece while the glaze is still
wet. This way, the crystals will flow into the base
glaze when the ceramic piece is fired. Quite a few
glazes have these crystals already added.

Underglazes are used for adding pig-

ments such as stains to watered
down clay. They can be painted onto
fired or unfired ware. These glazes
tend to be highly pigmented for
strong color and are applied to ceram-
ics underneath another type of glaze,
but the ceramics have to be fired be-
fore the vitreous glaze is applied. Un-
derglazes are preferred when it comes
to painting ceramics because they do
not run of flow into each other like
many other glazes.
When dealing with ceramics, many substances
can be harmful through touch or inhalation. This
is especially true of glazes. For example, there is
lead in some of the glazes that we paint onto pot-
tery. To produce bright, clear glaze, particularly
for use on red clay, a small amount of lead is
added. This lead functions as a stable compound
known as a frit, so it is not harmful to the potter
or anyone else. However, acids attacking the
glazed surface of the finished piece may cause
the lead to become soluble again. These glazes are
known as low-sol and should never be used on
eating utensils or in homes or schools.

Some ceramics contain high levels of natu-

rally occurring radionuclides, especially
the radioactive members of uranium and
thorium compounds. Because of this, people
can detect higher radiation levels can be
when they test the area over pottery with
devices like Geiger counters. Sometimes,
the higher readings are due to uranium in
the glaze, whereas other times, these read-
ings result from the radionuclides in the
clay itself. Glazes that use compounds of
uranium, such as uranium oxide or so-
dium uranate, to impart color such as or-
ange-red, green, yellow, or black tend to
have the most radiation.
 The use of uranium in glazes stopped for a
time during World War II, but it resumed
again in 1959. Many people know that it
was used in the glaze of the extremely
popular, orange-red Fiesta dinnerware ce-
ramics that were produced from the 1940s
until 1973. However, this is not the only
case of radioactive ceramics; uranium
glazes have also been used in other types
of ceramics, such as wall and floor tiles,
pottery, and laboratory ceramics. In 1987,
Dedham Pottery the National Council on Radiation Protec-
Dedham Pottery was a pottery tion and Measurements Report 95 stated
company opened by the Robertson that no manufacturers were using ura-
Family in Dedham, Massachusetts nium-glaze in dinnerware, but previously
during the American arts & crafts produced works may still have radioactive
movement between 1896 and 1943. It glazes.
was known for its high-fire stoneware,
its fine crackle glaze, and thick, cobalt
boarder designs. This pottery comprises
mostly household china dishes that are
blue and white in color, and the most
common and recognizable design is a
repeating, crouching rabbit referred to
as "the Dedham rabbit." The rabbits
crouch on the ground with their ears
back, and in-between each rabbit stands
a vegetable stock. Other designs
featured elephants, dolphins, polar
bears, chicks, swans, turtles, ducks,
butterflies, lilies, clover, and
mushrooms.
Bisque (or Bisqueware) clay that has been fired but not glazed
Burnish to polish by rubbing
Greenware unfired clay
Glass Former an ingredient in glaze which determines how much luster it
has
Glazea thin coating of minerals that produces a glassy transparent or col-
ored coating on bisqueware
Firing applying heat to harden pottery in a kiln
Fluxan ingredient in glaze which determines the temperature at which it
must be fired to melt
Foot a small, short ring on the bottom of bowls, vases, plates, etc. that helps
to support the weight of the piece and anything placed on it
Frit a ceramic composition that has been fused in a special fusing oven,
formed into glass, and crushed to mix any toxic components in glazes with ox-
ides, which neutralizes the toxicity
Grogfired clay that has been powdered and added to unfired clay to add sta-
bility
Kaolinsoft white clay that is a part of most mixtures of clay that are sold
commerically
Malleable able to be shaped
Opacifier a substance added to a material to make it more opaque
Oxidize to combine with oxygen to form an oxide
Radionuclide a radioactive atom

Refractorya material that retains its strength despite very high temperatures
(often used to line kilns)
Scoring the making of scratches or creases on separate pieces of clay that are
to be joined together
Sgraffitoa technique in which you paint contrasting layers of slip over
leather hard clay and then carve away parts of a layer to reveal the other color
in a design
Sinterheating the powdered material below its melting point until its parti-
cles adhere to each other
Slip an opaque, creamy liquid that is made by mixing finely ground clay
and water and used to cement together pieces that have been made separately
and for decoration
Throwing sculpting clay on a pottery wheel
Trimmingremoving excess clay from the bottom of a thrown piece
Underglaze usually mixtures of water, clay, and colorants that can be ap-
plied to greenware (very similar to engobes and colored slips)
Vitreous glassy
Whiteware type of ceramics including tableware, tiles, sanitary products,
and pottery products
Linda Xu is a student of Massachusetts
Academy of Math and Science in Worcester,
MA. She is 16 years old, and was born in
Scranton, PA. Her ethnicity is Chinese, and
she is able to speak the language reasonably
fluently, as well as being able to speak Span-
ish. In her sophomore year of high school, this
author took a class in ceramics and pottery,
which sparked a future love of the subject. In
that same year, she also took a course in
chemistry and found it fascinating. She
hopes to continue to study both chemistry and
ceramics in her senior year of high school.

Kathleen Ross is a 17 year old junior at

the Massachusetts Academy of Math
and Science. She lives in West Boylston
with her parents and three of her eight
siblings. This author has a great interest
chemistry and chemical reactions, such
as the ones that are responsible for the
mineral glazes that have been used in
ceramics throughout the ages. She is also
interested in the toxins that have ap-
peared in popular pottery like fiesta ware
and various glazes.
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http://nikkilin.com/ClaySculptures/album/slides/ClaySculpture11.jpg
Page 3, http://students.ou.edu/L/Diondra.D.Lindquist-1/greek-vase%201.jpg
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Page 9, http://www.axner.com/
http://www.baileypottery.com/images/090-003.jpg
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Page 12, http://01varvara.files.wordpress.com/2008/09/irina-barkovskaya-cat-1999-fired-clay-
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Page 19, http://www.antiques-bible.com/ppf/term/newcomb+pottery/definition.asp
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Glossary, http://1.bp.blogspot.com/_lfveUheQ0aY/SPp_CyPJQyI/AAAAAAAAArE/ecWJbUl5O9w/s400/
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