Success in ceramics depends on your using the right clay in the right way.

Clay is among the most abundant of earth's natural resources, as well as one of the cheapest and most useful.There are many different kinds of clays, natural and synthetic, used for different purposes in ceramic work.Before you begin shaping your first piece of pottery, you should know certain facts about the properties of the various clays you will use.To begin with, you should always make sure that you have chosen the right clay for each project.Needless to say, it is quite discouraging to find a piece ruined in the kiln because you used the wrong kind of clay.

Some purists suggest that you dig and prepare your own clay.Although this might be fascinating fun, it certainly is not for the beginner.Also, it is unnecessary work, since you can buy prepared clay for only a few cents a pound.If you plan to use the clay for modeling, you can purchase it already mixed and seasoned, ready for use.If you plan to use the clay for casting, that also can be bought already prepared, but since it is liquid and sold by weight, it may seem extravagant to pay for so much water.

Modeling clay is called plastic; clay used for casting, or pouring into molds, is re­ferred to as slip.How you plan to use the clay will determine which you purchase, although one type can actually serve both purposes, the main difference being in the mixing and preparation.

When you buy clay, always be sure to ask the dealer about its firing range.Some clays do not mature unless they are fired at very high temperatures, while others will disintegrate if fired too high.The same dealer who sells you the clay can also ad­vise you about which types of glaze to use on it.

Rarely is clay found completely free of impurities.The most common of these are sand, lime carbonate, iron sulphide and organic matter.These impurities cause variations in the color and texture of the clay.Most pure clays lack many desirable properties which have to be supplied by adding ingredients to them.If you found a pure clay plastic enough to shape into a piece of pottery, you would probably be dismayed to find it shrink excessively un­der fire, causing the model to warp and crack.

Two minerals which must be added to pure clay are flint and feldspar.Flint, which is silica, is employed to control shrinkage and to open the clay body when fired, permitting moisture and gases to escape.Feldspar makes the clay fuse prop­erly at the desired temperature.

Talc is also added to many clays, espe­cially if they are to be used for casting figurines.This is done to prevent shrink­age and also to lessen the tendency to craze.(Crazing refers to the tiny cracks which appear in a glaze when it does not fit the clay.)

To lower the maturing point of a clay, ground glass is sometimes mixed into the body.

To strengthen a clay, or to reduce its tendency to crack, a grog may be added.Grog is composed of fired clay which has been ground into small particles.Sand or powdered quartz can also be utilized effec­tively as clay strengtheners.

Nowadays the hobbyist need not be bothered with the complicated details of preparing his own clays.Most dealers stock a large variety, either ready-mixed to a plastic consistency or ready for the hobby­ist to mix with water and wedge to the consistency he requires.

Best known of the commercial grades of clay are ball clay, china clay and fire clay.

Ball clay is highly plastic and can easily be shaped by hand.It fuses at a fairly high temperature and is used in the manufac­ture of all kinds of pottery, chinaware and stoneware.England, Kentucky, Tennessee and Florida supply some of the finest ball clays.

China clay, otherwise known as kaolin, is the purest known form of clay.High-quality china clay is abundant in England and is exported extensively.In this coun­try, Georgia is the prime source of supply.Kaolin is the only type of raw, pure clay the hobbyist should purchase for starting.This is because it has so many uses.To mention only a few, kaolin is utilized in making most glazes; it can be used in mak­ing underglazes; it is an ingredient of kiln-wash.

Fire clay has a very high vitrification, or hardening, point.This clay is valuable in the construction of kilns and the manu­facture of refractory tiles, bricks, and other objects which must be able to withstand extremely high temperatures.

For the purposes of the hobbyist, it might be advantageous to classify clays according to their ultimate purposes.

First, there is pottery clay.Probably the two kinds most used are gray stoneware and terra-cotta clay.Both will produce a good plastic, workable pottery clay.The stoneware clay fires a light buff and per­mits the use of lighter and more brilliant colors in glazing.The terra-cotta clay fires a reddish-brown and consequently darkens all glaze colors.This should be considered when choosing clay for a project.If both types of pottery clay can be obtained, they will increase the possibilities available in decoration and colors.Terra cotta is espe­cially suited to sculpture, and is best left unglazed as the warm color and matte sur­face produce an effective appearance.

Next is sculpture clay, particularly de­sirable for this purpose because it is stronger, more rigid, and can take more stress and strain than other clays.It con­tains an addition of grog.Sculpture clay can be bought prepared or made by mix­ing grog—about 20% by weight—with either of the pottery clays.

Finally, there is jewelry clay.The re­quirements for a jewelry clay are quite different from those for pottery or sculp­ture clay.As the object to be made is smaller in volume, the details must be finer and sharper, the colors on the finished piece should be more vivid, and the firing temperature need not be as high.There­fore, a fine-grained white clay, with a high percentage of flint, is recommended be­cause it gives brilliance to the glaze colors and fires at a low temperature.

Preparing and Storing Clay

Whenever possible, it is simpler to buy clay in a moist state, but it can be pur­chased in dry powder form and mixed as needed.This will require several days, de­pending on the atmosphere and the tem­perature of the room in which the clay is mixed.

To prepare clay from the dry powder: The powder should be spread in a sink, tub or flat pan and mixed with water to the consistency of thick molasses.All lumps should be removed by stirring.Allow water to evaporate until the clay reaches a slushy state, then spread it on plaster bats to dry.Turn and press the mass before a crust develops on either side.When it passes beyond the sticky state to a soft firmness, remove it from the bats, wedge and store it.

Clay is easier and better to use when it has aged in the damp state.This means leaving it in a damp place—a box or crock —for a period of a week to two or three months, the longer the better.

So long as the clay is moist, it is very plastic and easy to work.If it is pushed, it moves readily and stays in the new shape.If a piece of clay is added to the original piece, it is only necessary to lay it firmly in place and work it into the mass with a finger or tool.But once the clay has hard­ened, it is practically impossible to add moist clay to it.

Clay shrinks as it dries.Thus, when moist clay is added to the semi-dry body, the moist clay shrinks more, and the un­equal shrinkage causes it to crack and peel.Good modeling clay normally shrinks about one inch to every 12 inches during the processes of drying and firing.Half the shrinkage takes place while the completed model is drying, the other half takes place in the kiln.

Slip, or liquefied clay, to be used for casting objects in molds and for modeling and decorative purposes, can also be pur­chased in prepared form.Basically it is clay and water with some other chemicals added to produce certain definite effects.

The chemicals used in slip are called deflocculants.The purpose of a "defloc" is to form a liquid clay without using too much water.In plain language, a defloccu-lant is a "water wetter." That is, a small amount of liquid is made to perform the function of a larger quantity.The principle involved is that an alkaline water can hold more clay than an acid one.It separates the clay into finer particles so that they are held in suspension or a colloidal state.

Castings can be made of slips that do not contain a defloc.But it takes far longer for the mold to absorb the extra water, and the results are rarely as satisfactory.Spe­cial casting bodies may be purchased in powder form.The amount of water to add and the directions for mixing are given by the manufacturer.Common deflocculants are potassium carbonate, sodium gallate, sodium tannate, and a combination of so­dium carbonate and sodium silicate (waterglass).

If you prefer making your own casting slip, rather than buying it ready mixed, take about 10 pounds of powdered clay and add five pints of water and one teaspoon of potassium carbonate, or any other de-flocculant.Mix well and let stand over­night.Stir for a few minutes each day for several days.Your greatest plague will be air bubbles, which must be removed from the slip by stirring and banging.After the slip has been seasoned for one week, strain through a 60- or 80-mesh sieve.A good slip has the consistency of honey.

Plain water-and-clay slips can be used as an adhesive material for joining two pieces of plastic or leather-hard clay.In performing such an operation, it is advis­able to make your slip of the same ma­terial as your ware.Otherwise the junc­ture may be visible after drying and finish­ing.If, for example, you use a red slip for mending a white clay body, the repair would mar the beauty of the completed piece.

Here is the way you get sizable quantities of clay.Contained in this is clay in plastic bags.

Plastic bags prevent moisture content from escap­ing; keep clay in them.Shown: red and white clay.

For decorating, on the other hand, a con­trasting slip can be used with winning re­sults.Never slip-decorate a piece that has passed its leather-hard stage of drying, however.If the ware to be so decorated is too moist, the slip will tend to soften it.If the ware is dry-hard, you will have diffi­culty getting the slip to adhere properly.More than likely, the slip will chip and peel away.

Plastic clay should be kept in a container with a lid.Stoneware jars, garbage cans and laundry tubs are adaptable for this purpose.For hand-shaped pottery, the clay must be kept quite soft, not sticky, and firm, not shapeless.For sculpture, it should be stiffer so that it will not slump or sink or lose its shape.

While working on an object, the unused portion of the wedged clay should be kept under a damp cloth, or it will become too hard.If the clay is too moist, it can be rolled or wedged on a dry table or dry plaster bat until it is of the right consistency.If it is too stiff, it can be rolled or wedged on a damp surface until it is right.

To increase plasticity, you may add one of the following: acetic acid, ball clay, ben-tonite, dextrin, glycerine, tannic acid.

To increase the strength of clay, you may use: fire clay, flint, grog, lignin extract.(Note: Grog increases strength in dry form.The other materials weaken clay in the unfired state but increase strength after firing.)

When clay is very stiff it may be re­claimed by leaving it on a moist plaster bat, covered with a damp cloth.Clay that is bone-dry must be broken and mashed and worked like dry powder clay.It is far bet­ter to err on the side of keeping clay too wet than too dry.

Slip should always be stored in dust tight containers.Some slips have a tend­ency to form a skin or crust on the top.This can partly be prevented by covering the slip pail or jar with wax paper and string.

Use Clay Properly

Craftsmanship is all-important in any part of the ceramic process.The materials can be dirty and messy—it is wise to wear an apron in the workshop—and unless a strict control is maintained over them, the results may be discouraging.Rules often limit the imagination and creativity of the artist, but here are some suggestions which will help insure good results:

1. Avoid sharp, angular edges or corners.They create strain and cracking.Clay adapts itself more readily to rounded edges and soft curves.Also, after firing, an edge that is too sharp can be dangerous—like a chipped glass, it is hard enough to lacerate the skin.
   
   
2. To add soft to harder clay in a project which cannot be completed in one sitting, slowly soften the hard clay with a damp cloth or sponge.Too rapid absorption of water by clay ■will cause cracks.Apply a coat of slip before adding new, soft clay.
   
   
3. To mend cracks on still-moist clay bodies, probe to the bottom of the crack and fill it with clay of the same consistency.Repeat if the crack reappears.Before al­ lowing it to dry, a piece should be carefully inspected and all cracks repaired.If the clay is hard, the problem is much more difficult; more time and patience are re­quired.First, moisten the area very slowly, probe to the bottom of the crack, and care­ fully fill it with a solution of slip or clay mixed with 50% by volume of grog.
   
   
4. Lightly sponge the surface of a piece, after it is shaped to completion, to remove rough and sharp edges.Do not, however, expect sponging to cover up poor work.

Slip (liquefied clay) is available in one-gallon containers, comes in many colors and varieties.

Dogs cast in white and red clay slip illustrate how initial clay color can be planned for effect.

Maibleized effiects shown below were very easy to produce.Objects are white with marbleizing of blown, red, blue, green, black, yellow, etc.To achieve them, just add powdered engobes in the desired color to your slip, prior to pouring it in the mold.Don't mix the color in: it will mix in itself as it pours, and with better results.

1. If a project is to be worked on over a long period of time, it must be kept damp between operations.A damp closet may be used.This is a specially constructed cup­ board which is lined with zinc.An old ice­ box, a tin box or a butter tub with plaster cast across the top may also serve as stor­age vessels.The piece should be wrapped in a damp cloth, dampness depending on shape, size, sturdiness and condition of clay.Wrap the cloth around the object, to re­duce weight on the clay, and close to the piece, to reduce evaporation.
   
   
2. Before a piece of sculpture is per­mitted to dry, it may need to be hollowed.If it is more than IV2 inches thick at any point, clay should be removed from the center.A half-inch wall of clay should be maintained.Hollowing allows the clay to dry with greater speed and with less stress and strain, and may open up air pockets which would cause disaster during the firing process.
   
   
3. How long a piece requires to dry de­pends entirely on its shape, thickness, and fragility.Slow drying under any circum­ stances is essential.Rapid drying causes a dry crust to develop on the surface, through which moisture cannot escape, and warp­ing develops.Cracks may also appear at the points of greatest strain.A piece should be allowed to dry in its damp box until the shiny moisture disappears.Then it may be removed and left at room temperature for a day or so, after which it can be placed over a radiator or in a hot box where some means of heating is employed.Pieces must be completely dry before firing.Moisture turns to steam in the kiln and exerts tre­mendous pressure in escaping.This almost always results in the piece exploding.

To test dryness: In cold weather, place the bottom against the window pane for a few seconds.Look for condensation which will appear on the window if the piece is still too moist.There is no reliable dryness test for hot weather.

Finally, the novice ceramist should be familiar with a modeling material called plasteline, or plastecine.In essence, plaste­line is comprised of ball clay, lanolin and glycerine.Since the clay flour is mixed with an oil base rather than water, plaste­line is very plastic and will not dry, shrink or harden.It finds its greatest use for sculpting prototypes of castware.Unless the artist wishes to make reproductions of his sculpture, he should model directly in clay.Plasteline cannot be fired.A mold must be made of the plasteline figure and slip poured into it.When the mold is com­pleted, the plasteline can be mashed up and used again.•

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