Qualities/States of Clay • Clay is a product from the earth that when heated becomes hard.
Qualities/States of Clay • Geologically, clay comes from decomposed rock. It is typically carried by water and settles together in a particular area where it is mined.
Qualities/States of Clay • Chemically, clay is a combination of Alumina, Silica, and water along with other minerals.
Qualities/States of Clay • Physically, clay’s crystal structure is that of tiny sheets with water between them. The sheets are held together but can slide past each other like a wet deck of playing cards.
Qualities/States of Clay • Heat causes the clay to harden. At 600°c the water is driven off and leaves a bonded alumina silicate structure. Further heat, 800°c, causes melting of the silica and other materials into a vitrified, or glass like, substance.
Qualities/States of Clay • The clay that we use in this room is called Earthenware. • Other types of clay are stoneware, typically used today to make dinnerware, and kaolin, a main ingredient in porcelain.
Handling the Clay • There are many procedures and techniques that you will learn throughout Ceramics. • Each procedure serves a purpose and is meant to aid you in reaching success in your ceramic endeavors.
Handbuilding Methods • There are four basic handbuilding methods you will learn this semester. • Slab • Coil • Pinch Pot • Wheel
Slab Building • A method of handbuilding in which flat slabs are created and then formed and adhered together at a leather hard state. • Soft slabs can be draped over or into forms, rolled around cylinders or folded and curled.
Coil Pots • Made from ropes or coils of clay. Layered one upon another to create the walls of the pot. Smooth coils or leave some coils exposed.
Pinch Pot A method of handbuilding involving inserting your thumb into the center of the clay body and using your thumb and fingers to pinch out the form.
Throwing Using the potter's wheel to make forms by hand from plastic clay
Wedging • Working clay on a surface with the palms of the hands in order to remove air from it and obtain a uniform consistency (evening out the particles) making it easier to work it.
Scoring • Scoring clay is the method potters use to adhere two pieces of clay together. Simply use any pointed clay tool to make X shaped marks into the clay where you will be adhering another piece.
Slip • Slip is another name for clay glue. Slip is simply wet sticky clay. Slip can be applied by a paint brush or simply your finger. Do not use just water.
Grog • Fired clay that has been crushed into granules and added to a clay body to increase strength, control crying and reduce shrinking.
Stages of Clay • As clay dries out, it transitions through different stages. • Each stage is equally important in that different techniques can be applied during different stages. • It is extremely important to know and be able to determine the stage that your clay is in.
Stages of Clay • Plastic – wet, raw clay. • In this state it is best to wedge and prepare your clay for building and construction. • This is the softest form of clay and the easiest to manipulate. • The clay feels wet and is a dark gray.
Stages of Clay • Leatherhard • In this stage the clay has begun to lose most of it’s moisture. It can stand on it’s own and is no longer malleable, soft or easily molded. • It is best to complete most carving and surface design techniques at this stage. • Best stage to build 3-dimensionally. • The clay feels damp and cold and is a dark gray.
Leatherhard- can not continue to remold but effective carving at this stage
Stages of Clay • Bone Dry • In this stage, the clay is completely dry but has not yet gone through the kiln. • The clay is in it’s most fragile form in this state and can easily break. • The clay may feel powdery and will look very light gray.
Stages of Clay • Bisque • In this state, the clay has gone through it’s first firing in the kiln. • The clay is now vitrified and stronger. It is still fragile but can withstand pressure and handling. • The clay is white in color.
Stages of Clay • Glazed or Final • In this stage, the clay has gone through the second firing and is complete. • Glaze has been applied to all surfaces and the piece is ready for grading.
Greenware • Any pottery that has not been bisque fired. The clay could be wet, leather hard, or bone dry.
Firing the Clay There are two firings: Bisque Glaze
Kiln • Kilns are thermally insulated chambers, or ovens, in which controlled temperature regimes are produced. They are used to harden the clay body.
Kiln continued… Kilns can be electric, natural gas, wood, coal, fuel oil or propane. Each kind of kiln will give the glaze firing a different effect. The most basic kiln is the electric. Note: we use electric kilns.
Bisque Firing • The first firing, without over glaze.
Glaze Firing • The final firing, with glaze.
High Relief • A strongly raised or deeply carved pattern.
Low Relief • A slightly raised or shallow carved pattern.
Sgraffito • A decorative process by which a line is scratched through a layer of slip or glaze before firing to expose the clay body beneath. From the Italian, meaning “scratched out”.
Underglaze • A colored decoration applied on raw or bisque ware before the glaze is applied.
(Over)glaze • A glass-like surface coating that is used to decorate and seal the pores of the clay at a bisque state.
What exactly is glaze? • Geologically, the major components of glaze come from the same place as clay. However, some of the lesser materials may be mined from variety of sources.
What exactly is glaze? • Chemically, you’ll find 3 major compounds in a glaze: Silica, Alumina and a Flux. These occur in various proportions along with other subtances that give color.
What exactly is glaze? • Heat causes the glaze ingredients to melt and form a glass. The Silica melts to a glass. The Flux allows the Silica to melt at a lower temperature. The Alumina leeps the molten glass from flowing off the pot.