INTRODUCTION:
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 (in line with the birth of agriculture) 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 (Al2Si2O5(OH)5). 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.
The additional water provides plasticity to the clay allowing it to be shaped. To turn clay into ceramics
requires heating through three distinct phases. The first phase does not require a kiln at all; atmospheric
dry (at temperatures below 100 °C) involves merely the evaporation of the water of plasticity to give a
“leather dry” clay that retains its shape. This state however is not useful as pottery. If you add water to
leather dry clay, it will soften and lose its shape. Thus, this first phase is reversible.
The second and third phases of ceramic formation are irreversible and require temperatures that can
only be obtained in a kiln. The second phase is calcination. Occurring between 350 oC (662 oF) to 800 oC
(1472 oF), calcination is a thermal decomposition, which in the case of kaolinite, results in an atomic
rearrangement with the release of water molecules. Although the dry kaolinite has no water molecules
in it per se, it loses water by combining some of the oxygen and hydrogen atoms present 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 → 3 Al2O3∙2SiO2 + 4 SiO2 + 6 H2O
kaolinite
mullite
silica
water
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, the ceramics are fired 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. Instead, impurities in the clay, notably iron oxide
(Fe2O3, melting point ), melt at lower temperatures. The temperatures at which this happens will
depend on the impurities which happen to be present in the original clay. The impurities in earthenware
clay vitrify at about 900°C (~1650°F). When the impurities melt they soak into the crystal lattice of
mullite and silica. When the ceramic cools the impurities coat the minerals and solidify, in effect gluing
the crystals together and making the ceramic impervious to cooking fire (260°C, 500°F). It has become
pottery.
PROCEDURE:
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.
To be useful for later experiments, our crucibles must be structurally sound, lest it crack and lose the
work you have so carefully prepared. To maintain its integrity, the crucible should be all of a single piece
of clay, not pressed together from several lumps. It should be free of trapped air bubbles, which might
cause it to shatter in the kiln. For this reason, it is imperative that the clay not be rolled out and pressed
back together, that it not be flattened and folded on itself.
1. Before you start have a bowl or beaker of water ready.Start with a lump of clay approximately 6
cm x 6 cm x 6 cm and place it onto a piece of paper to prevent its sticking to the table top.
2. Place the lump of clay onto the work surface and press your thumb into the middle to create a
dimple.
Rotate the clay and continue to work deeper into the pot until wall of the crucible begin to take
shape. 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. Your success at
this operation depends on your ability to stop and analyze the developing shape and make
necessary corrections.
a. If your hand or the clay starts to get dry, wet your fingers in a bowl of water.
b. There should be no seams or cracks, which would weaker the vessel and interfere with
the uses to which you will put it.
3. Once you are satisfied with the shape of the bowl, start to form the base. To begin gently
squeeze the base from the outside to form a pedestal. Use your thumb to open up the mouth of
the crucible, pressing a dimple into the pedestal at the bottom.
a. The thickness of the walls—even the walls of the pedestal—should be no more than 1
cm thick.
b. The crucible should be 8-10 cm in diameter (3-4 inches) & 10-12 cm tall (4-4.7 inches).
c. A common mistake is to form the pedestal off-center resulting in a pot that tips. Be sure
to check and correct as you go.
4. Use a knife to trim the lip of the crucible opening so that they are parallel to the table top.
5. Set the first crucible aside and make a second one through the same process.
6. Procure a fresh lump of clay about 2 cm x 6 cm x 6 cm. Either by hand or with a rolling pin, roll
the clay flat to a thickness of about 8 mm.
7. 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.
8. 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.
a. Gentle – if you press too hard you will deform the crucible.
9. 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.
10. Make lids for your other crucibles.
a. Hint – Each crucible has its own unique shape and therefore its own unique lid. Don’t
get them mixed up, label the set with your initials & a code for matching purposes.
11. Your crucible will now be fired for you in an electric kiln to cone 05 (1063 °C, 1945 °F).
Adapted from Caveman Chemistry by Kevin M. Dunn (Universal Press, 2003, p. 60-72)