CERAMIC PROCESSES AND MATERIALS-AN INTRODUCTION

Before attempting any of the projects in this book, it is advisable to have a general orien­tation to ceramic methods and materials. Read this introduction through—it will give you such a background. Only basic tools and materials are recommended, and elementary processes dis­cussed, in order not to confuse the beginner with too much information at the outset.

You will notice that each project is set up with a list of materials and tools needed, followed by a full description of the process. Read this first, before beginning to work. Accompanying you will find photographs of each major step in the process with descriptive captions. You can follow these as you proceed.

BASIC TOOLS NEEDED

The tools illustrated (Figure 1) are adequate for the beginner and will serve for all the projects.

#1 Wedging wire. This tool is made by at­taching two pieces of ½" dowel to the ends of a 12" copper wire, about 18 gauge. It is used to cut blocks of clay in two in the wedging process (see Figure 2). The dowels are used as handles, but they also make the wire easy to find.

#2 Knife. A knife is needed for all trimming and cutting processes, but especially for trim­ming tiles. Any knife with a short blade such as a paring or Sloyd knife will do.

#3 Metal pallets. These come in a variety of shapes and sizes with a plain cutting edge or a toothed edge. Pallets are used for smoothing or scraping the surface of clay. The toothed edge scrapes the high places or bumps quickly and easily, but the surface is finally smoothed with a plain cutting edge. Often the plain cutting-edged pallet alone is enough to smooth the surface. Two or three metal pallets will cover most needs.

#4 Rubber pallet. A rubber pallet is used to clean glaze or slip from the mortars or pans in which they have been mixed. The pallet not only cleans the container, but saves the slip or glaze.

#5 Pointed tool. A pointed tool is needed for a variety of processes, such as incising designs on pottery, or doing sgraffito decoration (scratch­ing designs through slip into the surface of pot­tery). A %" dowel about 6" long, with a nail inserted in one end, makes an excellent tool.

#6 Elephant ear sponge. This is a flat, thin, fine-grained, natural

 sponge, similar in shape to an elephant's ear, from which it gets its name. It is used for finishing surfaces and edges of pot­tery and sculpture. Cellulose complexion sponges can also be used.

#7 Brushes. Several soft, hair brushes are needed for applying slip and glaze, and for mending. These can be of average quality, such as imitation camel's hair. Nos. 3, 5, 8, and 10 will serve all purposes.

#8 Syringe. A hand irrigating syringe with hard rubber nozzle and a bulb can be purchased in any drug store. It is used for slip decoration (see Project XV).

#9 Wood modeling sticks. Choose a number of modeling sticks of a variety of shapes for the modeling, working, and finishing of clay. Sticks about 6" to 8" long with thin, flat blades are preferable to thick, large ones. The blade ends are used for cutting, smoothing, and shaping surfaces and for welding fine lines. The round ends are used for welding coils together and for general modeling and shaping.

#10 Wire loop tools. Several loop tools with tempered metal loops are necessary for smooth­ing the surfaces of pottery and sculpture, espe­cially for cutting down high places, for hollow­ing out sculpture before firing, and for carving and sgraffito work in the decorative processes (see Project XV). Select three or more tools of sizes varying from 8" to 10" long, and with vari­ous shaped loops.

#11 Tiles or plaster bats. Tiles or plaster bats are needed for supporting or for keeping objects moist while working on them. Unglazed biscuit tiles, 4" x 4" and 6" x 6", are adequate for most projects. They may be purchased from a ceramic supply house. Plaster bats can be made (see Project XVIII). They can be used for any size, and especially for work requiring a base larger than 6" x 6".

#12 Wedging table. If any considerable amount of clay work is to be done, as for ex­ample in class or group work, a wedging table is essential. This is a table about 2' deep and 3' long with a cement or plaster top. (The top can be made by first constructing a box about 6" deep with a solid wood bottom. Half fill the box with gravel or pebbles and pour cement or plaster into it until it is even with the top.) A wedging wire is arranged by erecting a 1" x 2" post at the back of the table. This should project about 2' above the table top and extend down as far as necessary to be well supported. It should be anchored to the table with screws and bolts. A steel piano wire is stretched from the top of the 1" x 2" post to the front of the table (see Sketch 1). A 2" turnbuckle, set near the top, will help to make the wire taut. (In using the wedging table the clay is cut in two by push­ing it against the wire and slamming the two pieces on top of each other.)

Figure 1—Basic tools needed

#1   Wedging wire
#2 Knife
#3  Metal pallets
#4 Rubber pallet
#5 Pointed tool
#6 Elephant ear sponge
#7 Brushes
#8 Syringe
#9 Wood modeling sticks
#10 Wire loop tools

STAPLES

Pottery clay. There are two kinds—gray stone­ware and terra cotta clay—which will produce a good plastic, workable pottery clay. If both can be procured, they will increase the variety of possibilities available in decoration and col­ors. The stoneware clay fires a light buff and permits the use of lighter and brighter colors in glazing. The terra cotta clay fires a reddish-brown and consequently darkens all glaze colors and this should be taken into account when choosing clay for a project. It is especially suited to sculpture, and is best left unglazed as the warm color and mat surface produce an effective appearance. Fifty pounds of moist clay of either kind, or twenty-five pounds of each, will be am­ple for one person for the projects suggested in this book.

Sketch 1-Wedging table

Sculpture clay. This clay is particularly desir­able for making sculpture because it is stronger, more rigid, and can take more strain and stress than the other clays. It contains an addition of grog (burnt fireclay ground up). Sculpture clay can be bought prepared, or made by mixing grog—about 20% by weight—with either one of the pottery clays. Grog can be procured from a ceramic supply house in buff or terra cotta. Twenty-five pounds of moist sculpture clay is ample for one person for the projects suggested in this book.

KINDS OF CLAY USED

There are many kinds of clay, natural and syn­thetic, used for different purposes, in ceramic work. Four varieties are recommended. Each is prepared for a special type of construction, but either the gray or terra cotta pottery clay can be used for all of the projects. Any of the clays described here may be purchased from the sources named on page 19, but the beginner should in­vestigate his own local sources as clays vary ac­cording to geographical location and one may find an especially fine clay in his own community.

Jewelry clay. The requirements for a jewelry clay are quite different from those for pottery or sculpture clay. As the objects to be made are smaller in volume, the detail must be finer and sharper, the colors on the finished piece should be more brilliant, 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 brilliancy to the glaze colors and fires at a low temperature. Two pounds of moist jewelry clay is ample for one person for the projects suggested in this book, but it is usually not sold in less than five-pound lots. The extra can be used for casting and for white slip in decoration.

Figure 2—Wedging clay—cutting the mass with a wedging wire.

Figure 3—Wedging clay—the two pieces are slammed together with the cut edges facing in opposite directions.

PREPARING THE CLAY

Whenever possible, it is simpler to buy clay in a moist state, but it can be purchased in a dry powder form and mixed as needed. This will require several days, depending on the atmos­phere and 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 until it is smooth. Allow water to evaporate until 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 (see paragraph below for wedg­ing). Clay is easier and better to use when it has "aged" in the damp state. This means leaving it in a damp place (box or crock) for a period of a week to two or three months, the longer, the better (see How to Care for Clay, right).

WEDGING. Each time clay is used—before be­ginning any of the projects, or when resuming work if a piece is put away before completion— the clay must be wedged. The purpose of wedg­ing is to remove all air pockets and holes which would cause a piece to explode in the firing; to eliminate foreign matter such as nails, cloth, and leaves; and to secure an even consistency. Never use unwedged clay.

The clay is patted or rolled into a flat rectangu­lar mass, cut in two with a wire or knife, and the two pieces slammed together on the table with the cut edges facing in opposite directions (see Figures 2 and 3). It is fhen rolled and kneaded like dough, patted into a rectangular mass, cut in two, and slammed together with the cut ends facing in opposite directions. This operation is repeated about twenty times. If you have a wedging table, the clay is cut in two by pushing it against the wire.

HOW TO CARE FOR CLAY

Clay must be kept in a container with a lid. Stoneware jars (five- to ten-gallon size), garbage cans, and laundry tubs are adaptable. A wet cloth over the clay will help maintain the right consistency. Consistency varies according to the use to which the clay will be put. For pottery, the clay must be 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 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 (a table moistened with a wet cloth) until it is right. When clay is very stiff it may be reclaimed by leaving it on a moist plaster bat/(a bat soaked in water for about five minutes), covered with a damp cloth. Clay that is bone dry must be broken and mashed, and worked like dry powder clay (see Preparing the Clay, left).

CARE OF CLAY WHILE MAKING AN OBJECT. Craftsmanship cannot be emphasized too strongly in dealing with any part of the ce­ramic process. The materials in themselves are dirty arid messy, 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 the following sugges­tions will help to insure good results:

#1 On the whole, avoid sharp, angular edges or corners. They create strain and cracking. Clay adapts itself more readily to rounded edges and soft curves.

#2 To add soft to harder clay in a project that 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 vio­lent cracks. Soft clay cannot be added to truly dry clay. When the surface of the hard clay becomes moist and slightly pliable, roughen or score with a stick or tool and apply a coat of slip before adding new, soft clay.

#3 Mending cracks. If the clay still contains some moisture and is partially plastic, probe to the bottom of the crack and fill it with clay of the same consistency. Repeat if the crack reappears. Cracks should be avoided by good construction and craftsmanship during the process of making the piece. Before allowing it to dry, a piece should be carefully checked, and any and all cracks repaired. If the clay is hard, the problem is far more difficult and more time and patience are required. First moisten the area very slowly, probe to the bottom of the crack, and slowly fill it with a solution of slip or clay mixed with 50% by volume of grog.

#4 The surface of clay may be sponged lightly on completion of a project to remove rough and sharp edges and to smooth the surface. Do not expect sponging to cover or eradicate poor con­struction or craftsmanship.

#5 If a project must be worked upon over a long period of time, it should be kept damp between operations. A damp-closet (a specially constructed cupboard which is zinc lined), an old ice box, a tin box, or a butter tub with plaster cast in the top may serve as a storage space. The piece should be wrapped in a damp cloth, dampness depending on shape, size, sturdiness or lack of it, and condition of clay. Wrap cloth around, to reduce weight on the clay, and close to the piece, to reduce evapora­tion.

#6 Before a piece of sculpture is allowed to dry, it may need to be hollowed. If it is more than 1½" thick at any point, clay should be removed from the center. A ½" 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 have caused disaster during the firing process.

■0-7 Drying. The speed with which a piece Is allowed to dry depends entirely on its shape, thickness, and fragility. Slow drying under any circumstances is essential. Rapid drying causes a dry crust to develop on the surface, through which the moisture cannot escape, and warp­ing develops. Cracks may appear at the points of greatest strain. A piece should be allowed to dry in its damp-box until the shiny moisture has disappeared. 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. To test dryness: In cold weather, place bottom against window pane for a few seconds. Look for condensation which will appear on window. There is no reliable test for dryness in hot weather.

PREPARATION OF SLIP FOR CASTING IN PIECE MOLDS

Weigh 1000-5000 grams of dry powdered clay, either terra cotta or stoneware, depending on the size of the object to be cast and the number of reproductions desired, and place it in a pail or dish pan. The amount of water required de­pends on the nature of the clay being used. Gen­erally speaking 300-500 cc. of water would be used to each 1000 grams of dry clay. Mix water and clay thoroughly and strain mixture through a 36 mesh sieve. (You can secure such a sieve from your ceramic supply house.) The addition of .4% sodium silicate (water glass) at this point will make the slip thinner and easier to pour, re­duce the amount of water necessary, and thereby the degree of shrinkage of the finished piece. The slip should be kept in a container with a lid and stirred thoroughly before each use.

If dry powdered clay and other equipment are not available, slip can still be made satisfactorily, but in a far more primitive and less controlled way. Fill a pail half-full of water. Drop into it either hard scrap clay, or moist plastic clay broken into small pieces no larger than an egg, until 1" to 2" of water remains on top. Allow to set one day or longer, stirring occasionally. When clay has disintegrated and the mixture has the consistency of slush, add water until it will pour in a thick stream, then strain through a sieve and add sodium silicate in small quantities (teaspoonful) until slip thins to molasses consistency. Store in airtight container.

SURFACE DECORATION

The surface decoration may result from the natural texture and color of the clay used, as in the case of terra cotta clay in sculpture. It may also result from any one, or a combination of decorative processes. Incising or carving the surface, or painting designs with colored slips are simple means of decoration. In addition to these, however, clay work, especially pottery, may receive a coating of color such as a clay slip or glaze.

MATERIALS USED FOR SURFACE DECORATION

CLAY SLIPS, ENGOBES. These terms are used interchangeably in ceramic books and catalogs to describe the material and the application— in liquid form to the surface of an unfired clay object—of colored clays, either in their natural state or where a color agent has been added. The color agents may be natural earth or syn­thetic colors, stains, metallic oxides.

In this book you will find suggested the use of slips made from the two pottery clays (gray and terra cotta) and also slips or engobes which may be purchased in a variety of colors from a ce­ramic supply house. To avoid difficulties, such as peeling and cracking due to different rates of expansion and contraction, it is recommended that you buy colored clay slips designed for your specific pottery clays. They will come in dry pow­der form; a pound each of blue, yellow, green, pink, and black will be ample for one person for all projects. Gray and terra cotta slips may be made from the pottery clays in a manner similar to that described on page 13 under Preparation of Slip for Casting. White may be obtained by making slip from the jewelry clay; to assure an opaque covering, add 10% by weight of tin oxide. You may improvise your own colored slips by adding small quantities of color agents to the gray pottery or white jewelry clays. This is not recommended unless you already have some familiarity with ceramic materials and are pre­paring slips in large quantities.

Most of the decoration recommended in this book is based on slip decoration. Clay slips, when used for decoration, may be an all-over coat of one color through which a design may be cut, or may be one or more colors applied to the object by painting or other means. The dry colored clays should be mixed with water to the consistency of thick cream and placed in covered glass jars. The exact consistency desired is de­pendent upon the amount of moisture present in the clay object (thick on wet clay, thinner on nearly dry clay), and the technique used in ap­plication of the slip. The slips should be applied evenly and to the approximate thickness of X»" if a solid color is desired, otherwise there will be probability of uneven color after the glaze firing. If a glaze is put over this slip, it will probably produce an uneven color after it is fired if the slip is uneven in thickness. If slip is applied too thickly, it will tend to crack and break away from the surface of the object. For best results, apply slip when clay is leather-hard, or about half-way dry. Pieces may be left with the slip coat only, but sometimes a transparent glaze is put over it (see Glazes, below).

GLAZES. Glazes consist of materials which, when subjected to intense heat in a kiln (oven), melt and form a thin glass coating on the surface of the clay. Glazes serve two major functions-utilitarian and aesthetic. In pottery the glaze gives a smooth surface, prevents food and water being absorbed by the clay, and is more easily cleaned. A glaze also gives a more pleasing color and texture than the clay and adds greatly to the beauty of the piece. Glazes are not neces­sary for practical purposes in sculpture, but some­times they increase the excitement and interest of an object.

The choice of glazes in general, or of a glaze for a particular object, requires considerable knowl­edge, sensitivity, and experience. For the begin­ner, commercial glazes which have been care­fully checked for satisfactory results are sug­gested. If possible, one should secure both clays and glazes from the same source of supply be­cause glazes are designed for specific clays and firing temperatures. Commercial glazes come in powder form. Glazes should be mixed with water. Usually 65 cc. of water to 100 grams of glaze will give the consistency of heavy cream. The consistency needed varies according to the in­gredients of the glaze, the porosity of the fired clay piece to which the glaze is applied, and the method of application. Experimentation with combinations and variations will provide interest­ing results. Eventually the beginner will reach a point where he will be eager to design his own glazes. Information may be obtained from books, but better still, if the student wishes to go into glazing extensively, he should take a course with a professional potter who can guide him and interpret the results.

Transparent and Opaque Glazes. In selecting commercial glazes, the beginner will need a variety of colors in both transparent and opaque glazes. One-pound lots will be sufficient for ex­perimentation, then those which are found more desirable may be ordered in larger quantity. You may adhere to one firing temperature—Cone 06 or 1860° F., for both biscuit and glaze firing— or you may choose a glaze that fuses at a lower temperature than that necessary for biscuit fir­ing. ("Biscuit" refers to pieces fired once with­out glaze.)

Transparent glazes should be applied over all objects in which clay slips have been used so that the decoration may show through. The transparent glaze may be clear, without any color, or may have small percentages of color agents. Opaque glazes are applied on pieces which have no slip decoration, where it is de­sirable to have a solid color, or to cover the color of the clay itself. Opaque glazes are some­times called enamels in a catalog.

In addition to transparency and opacity, glazes vary in texture. They may be glassy and shiny, or mat, dull, or waxy. If you wish to change the texture of a shiny glaze to a mat glaze, you can try adding varying amounts of one of the fol­lowing materials: bone ash 0-5%; titanium oxide 0-5%; zinc oxide 0-10%; rutile 0-10%. Also glazes vary in type, based on their pri­mary ingredient. Lead glazes are most gener­ally used by beginners because they are easy to apply, fire at a low temperature—Cone 06, 1860° F.—and form a durable surface. They offer the common range of colors, but have a yellowish tinge and are not as brilliant as boracic or alkaline glazes which offer such colors as tur­quoise, Persian blue, and cherry red. The latter are, however, more difficult to apply. Lead glazes may be purchased from any ceramic supply house, but as houses name or number their glazes differently, one should order them ac­cording to the description or sample shown in the catalog of a specific ceramic firm.

In making jewelry or tiles, a boracic or alkaline glaze, firing at a still lower temperature—Cone 015, 1430° F.—may be used. More brilliant col­ors, including red, can be achieved with these and the glaze is durable enough where food and water are not in contact with the glaze surface.

CHARACTERISTICS AND USE OF GLAZES. Glazes, in and of themselves, their choice and application, involve a complicated process. There are many variations and their behavior is some­times erratic. The best advice is to restrain your expectations until you see the results. Constantly experiment, and keep careful records of the ex­periments. Be alert to new and different qualities as they appear.

There are several common and recurring defects which you should recognize in glazes:

#1 Crazing. This is the appearance of small cracks throughout the glaze. It is caused by the glaze contracting more than the clay in cooling, and is the main reason for buying from the same source, clays and glazes which have been de­signed for each other. Craze effects are not de­sirable for pottery to be used for food because through the cracks water and food juices seep. However, crazing, called "crackle," is some­times used as a decorative technique for sculp­ture or such objects as cigarette boxes. Oil paint or ink may be rubbed into the surface to accentu­ate the "crackle."

#2 Crawling. The glaze may pull away from the body in small areas and expose the clay. Sometimes retiring to a higher temperature, or applying a second coat of glaze with an addition of a flux (feldspar), and refiring will remedy this.

#3 Flowing. If the glaze is fired beyond the temperature for which it was designed, it may flow off the piece. To stiffen, add low percent­ages of flint or china clay to the glaze.

#4 Peeling. Either the glaze, or possibly both the glaze and slip, may peel or break from the surface of the clay piece. One cause may be a too-thick application of slip, or applying slip on ware that is too dry.

APPLICATION OF GLAZES. Glazes may be applied in a number of ways. The simplest means is painting the glaze on the body of the object with a brush. The largest soft hair brush should be used. Keep the brush full of glaze and dab, rather than stroke, the color onto the sur­face. Glaze may also be poured. Interiors of pottery are always done this way. A quantity— Ms to 14 the volume of the pot—of glaze should be thinned slightly with water and poured into the pot, revolved quickly to the top edge, and any excess poured out. An ample quantity of glaze is needed to cover the exterior by pour­ing. The piece should be stood on chicken wire (1/2" mesh) laid over a basin or pot to catch the glaze. The glaze is then poured over it evenly, and the excess put back into the supply con­tainer. A third method is dipping. This requires a large amount of glaze so that the piece to be glazed can be completely immersed. Glaze may also be sprayed, but this method is not ad­vised here because of the expensive equipment needed

FIRING CLAY AND GLAZES FOR PERMANENCE AND AESTHETIC EFFECT

Clay must be baked in an oven, called a kiln, to make it desirable and permanent. This baking is referred to as firing. Pottery pieces in particular must be fired to be useful. The beginner is ad­vised to take his pieces to a professional potter, a commercial firm, a school, or settlement house where pottery is fired for a fee, unless he is under the direction of a professional ceramic teacher or craftsman. Firing of large pieces requires a kiln of a fairly large size and a variety of accessory equipment which is expensive and takes con­siderable skill to operate. However, the beginner is given the following information about kilns and firing to help him better understand the processes involved in ceramics so that he may design with forethought and plan for the finished product.

In recent years, some experimentation has been made with self-hardening clays and materials which can be "fired" in a kitchen oven, but neither of these is recommended in this book because they have been found impractical and contrary to the best ways of the craft. Clays be­gin to change from their natural state into hard,, permanent form at 750° F. They must be car­ried higher—to various temperatures, depend­ing on the specific clay—to a point where certain particles within the clay melt and bind together the remaining particles. The terra cotta clays suggested here reach this point around 1800°-2000° F.; the gray clays around 2200° F.

1860° F., and is used for biscuit and glaze fir­ings. More brilliant glazes, including red, can be obtained at Cone 015, about 1430° F., and are recommended for jewelry.

FIRING OF CLAY

The clay piece must be absolutely dry when placed in the kiln. It must be heated very slowly at the beginning, and cooled slowly at the end, of the firing process. Jewelry and mold pieces do not require as much time (as little as 1 ½-2 hours) because they have thin walls and are less liable to crack or burst.

Clay that is fired only once is called biscuit ware. Sculpture is often considered finished in the bis­cuit state, particularly when executed in terra cotta or sculpture clays. Pottery, as stated on page 15, must have a glaze covering at least on the inside, to make it usable and easy to clean. Furthermore, the color of gray clay is light buff after its biscuit firing—an innocuous and uninter­esting effect which needs the color and texture of glaze to give it quality.

USE OF THE TEST KILN-AN OVEN FOR EXPERIMENTAL FIRING

Though the beginner is advised against firing his own pieces, an exception can be made in the firing of jewelry, small tiles, and small pieces made from molds. The reason is that test kilns, run by electricity, are fairly inexpensive (from $50 to $100), they can be safely operated in a home, shop, or classroom, and they are easily operated. Because of the rapid rise in the tem­perature in a test kiln (one to two hours for a rise of 1800° F.), only thin objects no more than ¼" in thickness should be fired. Any object thicker than that may explode in the kiln. If the kiln you buy does not have a thermometer, it will be necessary to use cones to gauge the temperature. Such a kiln has a spy-hole (a hole cut in the front of the kiln with a removable plug). The cone should be placed so that it can be seen through the spy-hole.

FIRING OF GLAZES

Glazes are seldom applied to unfired clay; never in the projects suggested here because of the difficulty of application. Decoration and color by means of clay slips may be secured by applying the slips before the first firing.

Glazes are applied when the pieces have been fired once, and require a second firing which does not need to be as high in temperature as the first, although it can be. Glazes are designed for specific temperatures. It is essential to know the exact temperature required for a glaze in firing the glaze kiln. In ceramics, temperatures are ex­pressed in two ways: by degrees, and by cones. Cones are small, elongated triangles of clay and glaze materials, designated by numbers, which represent approximate melting points. There are two which are most commonly used for the type of projects in this book. Cone 06 melts around clay solution. Pins and stilts are used with pot­tery and sculpture that has a broad base, while shelves coated with china clay solution should be used with jewelry, and sculpture that rests on spindle legs. Additional shelves for a kiln may be made from slabs of porous refractory brick sawed and cut to size with an old back-saw or cross-cut saw.

STACKING A KILN. In stacking a kiln of any size, it is advisable not to place the ware too close to the heating elements or walls of the kiln as this may distort the shape of the objects or create glossy spots on the surface. When stacking a biscuit kiln, the pieces may touch or be set inside or on top of each other to use the space as economically as possible. Dis­cretion should be used, however, in not putting a heavy piece on a light piece, thus causing dam­age to the lighter piece.

Much more care is needed when arranging a glaze kiln. Glazed pieces must not touch each other or they will join when the glaze melts. At least Va" should be left between all glazed ob­jects. Furthermore, glazed objects must be set on pins or stilts or on shelves which have been coated with a mixture of a 50-50 flint.

When a firing has reached its maximum tempera­ture, the heat is cut off and the kiln allowed to cool. The time needed depends on the volume of the kiln and the pieces inside. A small test kiln should be given at least a half-hour. Do not let your curiosity cause you to open the kiln too soon or it may result in damage to your pieces, and perhaps injury to yourself.

SUMMARY OF FIRING AND GLAZING

As a summary of the preceding material on firing and glazing, the following generalizations may serve as guide posts, both practical and aesthetic. They should not be regarded as hard and fast rules. This book aims not to limit the imagination of the beginner, but to encourage the applica­tion of his creative powers and of an experi­mental approach.

PRACTICAL CONSIDERATIONS:

#1 Try to procure glazes designed especially for the clays you use.

#2   Fire the clay once before applying glaze.

#3 Be accurate, neat, careful with all parts of the firing, glazing process.

#4 Clay slips, rather than glazes, have been suggested where several colors are desired be­cause they are more easily handled and produce more even results.

#5    Glazes should be used on all pottery—at least on the interior.

#6 Glazes are not essential for sculpture; they sometimes cover and destroy the detail and modeling. The terra cotta clay after biscuit firing is usually an attractive color for sculpture.

#7 Most glazes do not cover the bumpy surface of sculpture clay sufficiently well to merit their use over it.

AESTHETIC CONSIDERATIONS: In choosing a glaze, the following factors must be kept in mind; the combination of the correct ones will give a highly successful result.

#1 The function of the piece: Function varies the intensity or softness of a color and the rough­ness or smoothness in texture which is desirable.

#2 The size of the piece: Generally speaking, large pieces need softer colors, duller texture,-small ones can be brilliant and glossy.

#3 The shape of the piece: The simpler the shape, the more excitement can be allowed the glaze; the more active the shape, the less atten­tion to be drawn to the glaze.

#4 The color of the clay: The terra cotta dark­ens all transparent colors, most opaque glazes will cover it. The gray clay is a light buff after biscuit firing and will not change the lighter colors.

#5 Texture: Texture, of clay and glaze, influ­ences greatly the end result.

#6 Transparency and opacity of a glaze: The choice of transparent or opaque glaze depends on what lies beneath the glaze.

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