Archive for September, 2005

Storing clay

Any container with a tight-fitting cover can be used for storing moist clay. If you buy your clay in tubs you can keep it moist and plastic by covering it with wet burlap. When working on a clay project over a period of days, remember to cover the piece with damp cloths, rewetting them as necessary. This will help to keep the clay in good working condition until you have finished the project.

Slip is usually stored in a bucket, jar or similar container covered with a lid, but if your slip tends to form a crust on top, cover the top of the container with waxed paper before fixing the lid in place.

Wedging

All clay, whether purchased ready-mixed or dug directly from the ground, must be wedged before it is used for modeling. Wedging is a process of cutting and slamming the clay to get it into good working condition and to remove air bubbles from it. Air bubbles inside clay would explode during the firing.

A wire fastened to two pieces of wood that serve as handles can be used to cut the clay. Take a ball of clay and cut it cleanly in half with the wire. Throw one half of the cut clay down on a plaster slab or wooden table top, then slam the other half down on top of the first half. This must be done with enough force to wedge the two halves into one. Scoop up the wedged clay and repeat the process until the clay has been cut and wedged together at least twenty times or until the cut edge shows no air bubbles. The clay is ready to use when it is even in texture and free of air pockets or lumps.

If you plan to do much work with clay, it would be advisable to make yourself a wedging board. To do this, find or make a shallow wooden box, fasten an upright post at the back and fill the box to the brim with plaster. This will form your working surface. Stretch a piece of piano wire or any other strong, thin wire from the top of the post down to the front of the box. The wire must be as tight as possible to cut the clay easily and for this purpose you should use a turn-buckle, available at any hardware store.

Slip

Clay in liquid form is called slip. It is used for casting in a plaster mold. Slip, however, is not merely clay and water. When enough water is added to clay to make a slip that will pour easily, too great a shrinkage results in the cast piece The problem is to make a casting slip with less water but with the same fluidity required for pouring. To solve this problem an alkaline substance, such as sodium silicate, is added. This separates the clay into finer particles that are held in suspension, and a liquid clay is formed without using too much water.

This process is called deflocculation. While you can make your own casting slip, deflocculation is a tricky business and it is usually much safer to buy the prepared slip. Or, better still (and cheaper), purchase the casting body in powdered form and mix it with water. The amount of water to be added and the directions for mixing are given by the manufacturer.

Buying clay

All the clays and clay bodies mentioned can be purchased from your ceramic dealer in plastic form for modeling, in liquid form for casting or in powdered form to mix for either modeling or casting. It is cheaper, although sometimes not as convenient, to buy your clay in powdered form. Aside from the fact that it is less expensive, dry clay is easier to store and, too, if you plan to make any clay bodies of your own, you must have the clay in powdered form for weighing with other dry ingredients.

When buying your clay be sure to keep in mind the purpose for which it is intended. That is, do not buy stoneware or porcelain unless you have access to a kiln that will reach the required high temperature. If you intend to model your clay, buy a clay with enough plasticity to render it workable; do not try to harden a casting slip and use it for modeling because slip contains ingredients necessary for casting but not always desirable for modeling. Instead, ask your dealer for the same clay in plastic form.

Digging your own clay

All the clays mentioned above may be purchased from a ceramic dealer or, if you have a strong arm and a great deal of time on your hands, you may dig them yourself. Frankly, digging is not recommended for any but the most enthusiastic ceramist. There are so many prepared clays available to the ceramist that digging seems as pointless as advising a modern-day painter to grind his own colors and weave his own canvas. An important point to consider, if you are seriously eying the shovel, is that much more is involved than the actual digging.

Any clay that has been dug from the ground requires cleaning. It must first be dried, then pounded and broken up, made into liquid form and screened. In addition, natural clay must be tested as to its characteristics. Is it plastic? What is its firing range? Is it porous? All these properties must be tested before the clay can be used. You must judge for yourself whether this amount of work is desirable, keeping in mind the fact that prepared clays are not expensive, especially if purchased in powdered form, and that they have been cleaned and properly classified as to their specific characteristics.

Clay bodies

The distinction between clay and clay bodies is one which seems confusing to newcomers in the ceramic field, but in reality, it is a simple one. Clay is that material which comes directly from the ground and, as such, is difficult to work with. Some natural clays are not plastic enough to be workable; others are too plastic and do not hold up well in firing; still others will mature at such high temperatures as to be impractical for ceramic use. To combat these difficulties and make life a little easier for the ceramist, clay bodies are manufactured.

A clay body is simply a blend of various clays, combined with other ingredients where necessary. A clay that is too plastic may be added to a non-plastic clay, another clay may be added for its color, grog used for texture, or glass cullet (ground glass) added to provide a lower maturing point. In this way a clay body is produced with the particular qualities desired. The general types of clay bodies used for ceramic ware are these:

Earthenware-A low firing (approximately 1700 -2000 ) body that is comparatively soft and porous and will not hold liquid unless glazed. Its color is buff, red or white. Earthenware is the most widely used of the clay bodies and will probably be used for most of your ceramic ware. Stoneware A high firing (approximately 2300 -2600 ) hard, vitreous body which is able to hold liquid when left unglazed. Stoneware bodies usually fire to shades of gray or brown and often contain fire clay or grog which produces a rough texture. Porcelain A body that requires the highest fire of all ceramic artware (approximately 2300 -2800 ). It is a hard, vitreous body that fires to a translucent white.

Working with clay

It is not necessary to possess an extensive knowledge of chemistry for ceramic work and this book will provide only such technical information as may be of practical value to the ceramist. We are’ not concerned here with chemical symbols and formulas. Our only interest in chemistry is in those areas where it serves as a basis for a better understanding of ceramics. For this reason, since anyone working in ceramics will necessarily be working with clay in one form or another, it would be well to learn something of the general composition of clay before attempting to discuss ways and means of making ceramics.

Composition of clay

In brief, clay is a natural product of the earth, formed by the decomposition of rock. While the earth’s crust contains many chemical elements, only a few are important here. The main element is silicon, which makes up more than half of the earth’s rocky crust, and the element of next importance is aluminum. These elements, when found in combination with oxygen, are called silica and alumina. Pure clay is composed of one part alumina, two parts silica and chemically combined water. It is not essential to know these terms but it may prove helpful.

There are many kinds of clay. In fact, no two clays dug from the earth will be exactly the same. However, all clay has two intrinsic properties: when moist it is plastic (which means that it can be modeled into shapes without breaking), and when exposed to extreme heat it will become hard, losing this plastic quality forever.

Kinds of clay

It is difficult to work with natural clays as they are, but it may be of value to know a little about them if you want to make your own clay bodies. While there are hundreds

of kinds of natural clay, only the most common types which concern the ceramist are listed here.

Kaolin-A white clay with a coarse texture. It is difficult to work with and matures (hardens) at very high temperatures.

Ball Clay-This clay fires white, has an extremely fine texture and is highly plastic. It is used mainly in porcelain and white-ware bodies where workability is needed.

Fire Clay-A dark colored, rough textured clay that will stand high temperatures but is not plastic. It is usually used in stoneware bodies and for firebrick, furnace linings, etc.

Stoneware Clay-A smooth, plastic clay that fires to a light buff color and will stand high temperatures.

Slip Clay-A low firing clay used as a glaze on a higher firing clay.

While ceramic art is one of the oldest forms of expression known to man, its magic, mystery and excitement keep ceramics a constantly new and challenging adventure. Few other mediums embody so completely the thrill of the unknown that fascinates each student, artist or hobbyist who enters the field of ceramics. The magic that lures the ceramist is the timeless magic of earth and fire. The mystery that intrigues him is the process by which dull mud can be transformed into a hard, jewel-like object.

To discover the secrets of clay, fire, color and glazes, to experiment with them, regulate and control them, to create from his knowledge a thing of utility and beauty- this is the goal of the ceramist. The unpredictable factor that is always present in ceramics only stimulates his imagination and adds excitement to an already absorbing medium. There are no hard and fast rules. Anything goes-if it works!

The clay with which ceramics are fashioned can be modeled, pressed, stamped, rolled or carved. It can be thrown on a wheel or made liquid and poured into molds. It can be textured, painted with contrasting clay, colored with brilliant glazes, decorated with designs or rubbed with stains. Its finished surface can be shiny or dull, rough or smooth, bright in color or subdued. It can sparkle with metallic luster or glow with natural earth color. The range of creative possibilities is endless, the only limitations being those imposed by the skill and imagination of the ceramist.

Ceramics can be an art, a business, a hobby, a therapy, a classroom project or anything one might care to make of it. Whatever the individual approach, it is the author’s hope that this book may serve as inspiration to all who wish to explore the magic that is ceramics.