| Introductory Science Concepts | Hands-On Activities |
Overview
Students are introduced to the origins, chemical composition, structure, and properties of clay. They learn some of the techniques of working with clay to create one or more objects of their own design. In a later clay session, they will glaze and fire their objects.
Students are first introduced to the overall structure of the earth as in the following diagram:
Through the process of weathering, i.e., exposure to water, air, elevated temperatures, organic acids released by decay of vegetation, and the melting and movement of glaciers, silicates undergo chemical and physical changes that produce clay.
Feldspar is the most abundant silicate mineral. It's chemical formula
may be generalized as KAlSi3O8. One of the chemical reactions that feldspar
may undergo during weathering is:
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[Physical
Science 6.02]
[Chemistry
1.03]
[Chemistry
2.03]
Kaolin may be thought of as pure clay. In kaolin, the atoms of oxygen (or the hydroxyl group, OH), silicon, and aluminum are arranged in a layered structure as depicted in the following diagram.
These tiny plates have polar or charged surfaces which tend to attract polar water molecules. There can be various numbers of water molecule layers between the plates, depending on the availability of water in the clay environment. The water layers between the plates act as a lubricant and give clay its characteristic plasticity, or ability to be molded. As water is removed from between the plates by evaporation, the clay becomes more rigid and brittle.
Pure Kaolin is white and is usually found near the feldspar deposit
from which it was formed. Most clay derived from the earth, however,
is colored. The color is due to impurities which become mixed with the
kaolin during the weathering process. A very common impurity is iron(III)
oxide which may be yellow, orange, or red in color. Besides lending color
to the clay, the iron(III) oxide serves as a flux. A flux is a substance
that lowers the maturing temperature of clay. The characteristic red clay
of the Carolina Piedmont contains red iron(III) oxide. As an iron rich
earthenware or "low-temperature firing" clay, it remains porous after firing.
Red clays are commonly used to make bricks, tiles, and flower pots and
is the clay used by the students in this session.
Having been introduced to the chemical and physical nature of clay, students obtain chunks of clay to work with. At first, they are encouraged to simply get a feel for clay by squeezing, rolling, flattening, and pinching it in their hands. They discover the role that water plays in making clay plastic. If too much water is added to clay, it becomes limp and won't hold a shape. If the clays dries out, it loses its plasticity and begins to crack and break.
Students learn some traditional techniques used by potters. They learn to flatten a piece of clay with a slab roller or a rolling pin. They may also "throw out" a chunk of clay until it forms a slab of consistent thickness. They learn to make clay coils by rolling a piece of clay back and forth between their hands and a flat surface. Clay coils can be used as ropes or can be flattened and attached together in a variety of ways to add height to a structure. Students learn to add smaller forms, made separately, to a main structure, making sure to dampen and roughen ("slip and score") the surfaces of the pieces that will be stuck together. They also learn how to use various wooden sticks and carving tools to add texture to and/or modify the clay surface. Students are reminded to avoid including both thick and thin regions of clay in their structures. On firing, uneven thickness causes tensions and such pieces are likely to crack. They also must not create a structure that contains trapped air. The pressure of trapped air increases significantly at the high temperatures in a kiln and may be great enough to crack or even blow up a clay piece and may ruin other pieces that are being fired at the same time.
[Chemistry
4.01]
[Physical
Science 2.01]
Students are then encouraged to create a clay object of their choosing. The first creations tend to be containers of some sort. Within a short period of time, the students begin to apply what they have learned to create objects that reflect their own personality and imagination. After making forms, students can decorate their pieces with slip. Slip is clay that has been softened with water to a creamy texture. A white or colored slip may be brushed or poured over a clay body of contrasting color. Students may scratch through or "sgraffito" the slip revealing the clay body color beneath. They may employ other variations of slip decorating, such as, slip trailing, inlaying, marbling, or use of stencils. When students have questions or problems, instructors are available to make suggestions and teach optional techniques.
The students' work is labeled and set aside to dry.
If time permits, students are introduced to the potter's wheel. The instructor first demonstrates the technique of centering a piece of clay on the wheel. Then the instructor shows how the rotating hunk of clay can be formed, with the use of both hands, into a symmetrical bowl or cup. Since centering is a technique that is difficult and time consuming to learn, the instructor centers pieces of clay on the wheel and students are encouraged to try to make a vessel by working the clay between their hands while keeping the wheel rotating with their feet. The potential of this lesson is somewhat limited due to the short period of time available.
Students do gain an appreciation of the coordination and concentration required for competence with the wheel. As the wheel spins, they can feel the forces acting on the clay through their hands.
Before the next clay session, the students' clay creations will be bisque
fired by the instructor. During the next session, students will learn about
glazing and raku firing and apply these processes to their clay objects.
Dr. Claire OIander, Appalachian State University
