1
Ceramic Crucibles
At least 20,000 years ago clay materials were shaped into figurines and then hardened by
baking in a fire. About 10,000 years ago humans began to form utilitarian vessel out of clay and
harden them in a fire. Baking clay alters both its chemical composition, structure, and properties. A
kiln fired ceramic is genuinely a different substance from clay, one that does not occur in nature.
The clay you will use in this experiment is an earthenware clay called kaolin that consists
primarily of water and the mineral kaolinite. The water provides the “plasticity” that allows you to
shape the clay. To turn clay into ceramics requires heating through three distinct phases. The first
phase does not require a kiln at all (temperatures below 100˚C) and involves merely the evaporation
of the water of plasticity to give a dry clay that retains its shape. This state however is not useful as
pottery because if you add water to the “leather dry” clay, it will soften and lose its shape (however,
it can be still re-worked into a different shape). Thus, this first phase is reversible.
The second and third phase of ceramic formation require temperatures that can only be
obtained in a kiln. When your piece is fired in the kiln irreversible changes take place in its
chemical structure. Although the dry kaolinite has no water molecules in it per se, it loses water by
combining some of the oxygen and hydrogen atoms to form water. The water released is really just a
by-product as the kaolinite is converted into mullite and silica:
3 Al2Si2O5(OH)4
kaolinite
→
3 Al2O3•2SiO2 + 4 SiO2 + 6 H2O
mullite
silica
water
The crystalline structure of kaolinite shows two layers: a silica layer and an alumina layer with
additional hydroxyl groups (–OH) attached to the aluminum atoms. Mullite is a complex material
that comes in many forms. For simplicity, we show it here as a 3:2 aluminum silicate. Kaolin that
has been fired to 1000˚C is said to be calcined, which means to expel by-products, typically water or
carbon dioxide, during roasting with strong heat. The calcinations binds the two “slippery” layers of
kaolinite into one rigid material. Thus, in phase two we have carried out a chemical reaction that
turned clay into stone.
The transformation to pottery is not yet complete. In stage three we fire the ceramics to the
vitrification temperature. To vitrify means to melt and turn into a glass-like substance. The
melting point of mullite (1920˚C, ~3500˚F) and silica (1650˚C, ~3000˚F) are too high to be attained in
ancient kilns, and the even the modern one used in this experiment. It is not the mullite or silica
that melts, but rather the impurities in the kaolinite. One such impurity is called “grog” and
includes materials sand (again SiO2) and/or crushed pottery that give the ceramic internal strength.
Grog also gives earthenware ceramics a rough surface. Certain kinds of clay, like porcelain clay,
have little or no grog. Grog also do not melt at kiln temperatures. However, kaolin is actually a
mixture of kaolinite and ball clay, an earthy material that contains impurities like iron oxide that
give it color. The impurities in earthenware clay vitrify at about 900˚C (~1650˚F), while those of
porcelain clay vitrify at 1300˚C (~2375˚F). When the impurities melt they soak into the crystal
lattice of mullite and silica. When the ceramic cools the impurities coat the minerals, making the
ceramic impervious to cooking fire (260˚C, 500˚F).
The earliest ceramic vessels were presumably formed by hand-shaping lumps of clay or
snakes (ropes) of clay. With the invention of the potter’s wheel, a lump of clay placed at the center of
a spinning disk could more evenly and smooth formed to give a more uniform product. The vessel
you will make does not involve the familiar potter’s wheel, but will instead be molded by hand.
FYEC 175
Crucible
2
Time Management
This experiment will be spread out over several days. On day 1 you will make your crucibles – at
least four of them. Make the lids first let them dry for as long as possible. Ideally you should wait 4
to 5 hours, but you don’t have that long, so try to let them dry for up to an hour. Then make your
lids. We will let them dry until the next class (day 3). They will be weighed, then fired and ready for
you in the next class (day 5).
Overview
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Hand form crucibles
Dry crucibles
Form lids on dried crucibles
Weigh crucibles
Fire crucibles
Re-weigh crucibles
A. Shaping the wet (plastic) clay into a “pinch pot”
Materials
low fire earthenware clay
beaker
1. Before you start have a bowl or beaker of water ready.
paper
drying board
Note: If you work clay with dry
hands, it is likely to crack.
2. Procure a 6 cm x 6 cm x 6 cm lump of earthenware clay
(white or red) from your instructor.
3. Place a piece of regular 8.5" x 11" paper on your bench.
This will be your work surface.
Hint – because the clay is sticky, it
is easier to turn clay on paper than
on clay on the bench.
4. Place the lump of clay onto the work surface and press
your thumb into the middle to create a dimple.
5. Rotate the clay and continue to work deeper into the pot
until wall of the crucible begin to take shape.
Hint – if your hand or the clay
starts to get dry, wet your fingers in
a bowl of water.
6. Be sure to stop every once and a while to check the shape
of the pot. Strive to for wall of uniform thickness (~ 8
mm) as you rotate the pot. The pot should be conical or
parabolic in shape: rounded at the bottom and sloping
upward and outward on the sides.
Hint – You success at this operation
depends on your ability to stop and
analyze the developing shape and
make necessary corrections.
7. Once you are satisfied with the shape of the bowl, start to
for the base. To begin gently pinch just below the bottom
of the bowl.
8. Flare the base to the pot will have a will, firm base to
stand on.
Hint – A common mistake at the
point is form the pedestal off-center
resulting in a pot that tips. Be sure
to check and correct as you go.
9. Make any final adjustments to the shape of the crucible.
Again your goal is uniformity and symmetry.
FYEC 175
Crucible
3
10. When you are satisfied with the shape place the pot on
the paper and hold a knife blade against the pot at a
constant height and turn the pot to mark a line that
indicates a uniform height. Then gently cut the clay until
even all the way round.
11. Make 3 more crucibles.
12. Scratch you initials and a unique number unique on the
bottom of the pedestals.
13. Place your crucibles on a board to dry for an hour or so.
B. Final shaping and making the lid
1. Procure a fresh lump of clay about 2 cm x 6 cm x 6 cm.
2. Either by hand or with a rolling pin, roll the clay flat to a
thickness of about 8 mm.
3. Place your partially dried crucible upside down on the
flattened lid. Use a knife to cut a round circle that is
larger than your crucible by 1 cm all the way around.
4. Turn your crucible right side up and set it on its pedestal.
Gently press the soft lid into the harden crucible until the
lid has a rim that folds over the edge of the crucible. The
lid will look like a shallow dish.
Gentle – if you press too hard you
will deform the crucible.
6. Turn the crucible and lid over again so the crucible is
again resting on the upside-down lid. Gently remove the
crucible from the lid and set it right side up.
7. Make lids for your other crucibles.
Hint – Each crucible has its own
unique shape and therefore its own
unique lid. Don’t get them mixed
up.
8. Label each lid with your initials and the number that
matches the one on the pedestal.
9. Transfer the crucibles to a board. Let them dry until next
class.
C. Weighing crucible
1. Weigh each crucible and record the weight in your lab
notebook.
2. Turn in your crucible and lids for firing.
D. Firing your crucible
1. Your crucible will be fired for you in an electric kiln to cone
05 (1063˚C, 1945˚F). Your fired crucible is now bone dry
and called “bisque” or “bisqueware”.
FYEC 175
Crucible
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2. Record the weight of the fired crucibles.
3. Calculate the amount of water lost from each crucible (as a
percentage), and then average the values.
Questions – How much water was
lost? If your clay were pure, dry
kaolinite, how much water should
be lost?
Adapted from Caveman Chemistry by Kevin M. Dunn (Universal Press, 2003, p. 60)
FYEC 175
Crucible