Archive for October, 2005

In order to produce several identical pieces, the ceramist uses a plaster mold. This mold is a hollow plaster form made over the original model. Into this, the liquid clay (slip) is poured or plastic clay is pressed to form each new piece. A plaster mold can be made over models of clay, plaster, glass, metal or almost any hard, non-porous material.

TYPES OF MOLDS

The simplest type of mold is a press mold into which plastic clay is pressed to form the piece. Press molds are usually small in size (for buttons, jewelry, small tiles, etc.) and only one side of the piece is molded. The exposed side becomes the back of the finished piece.

A drain mold is used for hollow ware such as jars, cups, vases, etc. Slip is poured into the drain mold and allowed to stand until the desired thickness of the walls is obtained. Then the excess slip is poured out. The hollow piece remains in the mold until it hardens enough to remove.

With a solid cast mold the thickness of the wall of the piece is determined by the mold itself, in contrast to the drain mold where the thickness is determined by the length of time the slip is allowed to stay in the mold. This type of mold enables the ceramist to cast shallow, thin-walled pieces, such as plates, shallow bowls, tiles, cup handles, etc. It is generally a two-piece mold and is made in the same manner as a two-piece drain mold (see page 22).

Firing schedule

Whether you are glaze firing or bisque firing, it is a good practice to start your firing slowly. There are two reasons for this: to allow the chemical moisture to escape, and to eliminate the stress and strain on your kiln and ware caused by rapid heating.

All electric kilns, except the very small test kilns, have a switch marked Low, Medium and High. Start your kiln on low heat, leaving the door open slightly to permit a path of escape for the moisture. In about thirty minutes, turn the switch to medium, still keeping the door ajar. When another thirty minutes have elapsed, close the door and turn the switch to high. One way to make certain all moisture has disappeared is to hold a glazed and fired piece in front of the door opening. Any moisture will condense on the glazed surface. If the piece remains dry, it is safe to close the door.

If your kiln is equipped with a pyrometer, you will know that this “water smoking” period is over when the temperature reaches about 500 -600 .The time of firing will vary according to the size of your kiln. Naturally, a large kiln will require a much longer period of time to reach the desired temperature than a smaller kiln. As soon as you have acquired some firing experience, you will be able to approximate the length of time closely enough to dispense with running back and forth to the kiln every five minutes for two or three hours.

Cooling

After the kiln has been shut off it must be allowed to cool gradually before the door can be opened. Again, the length of this cooling period depends upon the size of your kiln. A safe rule to follow here is to allow twice the amount of time for cooling as it took to reach the desired temperature. Thus, if the kiln has taken four hours to fire, it should take eight hours to cool. Even when the door has been opened, you cannot simply reach in and haul out the ware. This is one of the most difficult aspects of ceramic work-waiting to see your masterpieces come out of the kiln. But, do not remove any ware unless you can touch it with your bare hands. Any ware that is too hot to handle has not cooled sufficiently and may crack or shatter if it is removed at this point. Patience is a highly desirable quality for the ceramist and, sadly enough, one which few of us possess. Although we all know better, we continually ignore the rules, yank out the too hot ware-and usually end up with the disastrous results we deserve. It is hoped that the reader can exercise greater wisdom!

Preparing the ware for firing

The most important point to remember when preparing to fire is that all ware, whether green or glazed, must be absolutely dry before it is placed in the kiln. To be on the safe side, allow several days for drying. If the ware is not thoroughly dry, it may crack or explode during firing. Other than this precaution, there is little in the way of preparation for bisque firing.

Glaze firing requires a bit more preparation. Each glazed piece should be “dry footed” before it is placed in the kiln. This is simply a means of removing glaze from the bottom rim of the piece by rubbing the rim with sandpaper, although you may want to leave the rims of dinner plates glazed since an unglazed rim will absorb dirt in time. When you stilt such a piece, use a stilt with wire prongs which permit less contact with the ware than clay prongs.

Stacking the kiln

Stacking or loading the kiln for firing is essentially a matter of care and sensible planning. Naturally, it is desirable to put as much ware as possible in your kiln, but don’t overdo it because you must allow enough room for the air to circulate evenly throughout the firing chamber.

When loading the kiln with green ware, place your heavier and thicker ware on the bottom. This is the coolest area in the kiln and heavier ware cannot tolerate the rapid temperature change as well as thinner ware. You may stack smaller pieces of green ware on larger pieces since they will not stick together unless glazed, but they must never touch the electric elements. Be careful, when piling pieces on top of one another, to watch the weight distribution. Do not place too much weight on any one piece or you may wind up with warped or cracked ware. Use another shelf instead of trying to stack too many pieces together.

If you are firing two pieces that are to fit together, such as a box and lid, fire them together in their correct positions. Sculptured pieces should be fired separately or at least on a separate shelf since they have a greater tendency to explode, thereby ruining everything else in the kiln. In stacking the kiln for glaze firing you cannot place your pieces on top of one another. Glazed pieces cannot touch each other, any part of the kiln or adjacent green ware, or they will stick fast. Glazed ware must be held up by stilts or other supports, depending upon the size and shape of the piece, so that it does not stick to the shelf. You may fire glazed and unglazed ware at the same time, provided they both have the same maturing temperature and do not touch each other.

Illustration 16
A front loading kiln is being stacked for gold firing. The pieces cannot touch each other or the kiln itself.

Kiln furniture

In order to fire your ceramic pieces, a certain amount of kiln furniture is required. Kiln furniture is a term applied to the various shelves, posts and stilts necessary for supporting the ware. You will need at least one removable shelf and four posts to support the shelf, although it is better to have several shelves and perhaps a dozen posts in sets of three different heights.

To protect your shelves from any glaze that may fall on them, paint the shelves with a fairly thick coat of kiln wash before each glaze firing. The wash can be bought in prepared form or you can make your own by mixing one part china clay with one part flint plus enough water to make a creamy liquid. The coating of kiln wash will enable you to remove your piece from the kiln without its sticking to the shelf if excess glaze has run off. Any bits of glaze remaining on the shelf should be chipped off after each firing.

Illustration 15
These are different kinds of kiln furniture.

The kiln supports you will have most occasion to use for glazed ware are stilts which hold up the glazed piece on three clay or wire prongs. It is well to have on hand a large supply in assorted sizes. Glazed beads can be strung on wires, several plates can be fired together in a special plate rack and there are a number of supports for ceramic jewelry or other small pieces that do not have a flat base.

Cones and pyrometers

Because of the extreme heat reached in a ceramic kiln, the temperature cannot be gauged by an ordinary thermometer. One method of determining the maturing temperature of ceramic ware is by means of pyrometric cones. These cones are small pyramid-shaped pieces of clay that melt at predetermined temperatures and are numbered according to their melting temperatures. The cone is embedded in a small piece of clay and placed in the kiln so that it can be seen through the peephole. (The cone should not be in a straight up-and-down position, but should be leaning slightly to one side.) When the cone starts to melt and bends to about a 45 angle, it means that the temperature denoted by the number on the cone has been reached.

It is usually a wise precaution to place three cones of different numbers in the kiln. Stand them in a row, arranging them in this manner: place the cone denoting the desired

Illustration 13
Cones are placed at a slight angle in a lump of clay.

temperature in the middle; at its right place a cone of the next lower temperature and at its left place a cone of the next higher temperature. For example, if you are planning to fire the kiln at cone 06 (1860 ), this cone would be placed in the middle, with cone 07 (1815 ) to the right and cone 05 (1900 ) to the left. When the first cone (07) bends, it is a warning that the middle cone (06) is about to go over. As soon as the tip of cone 06 bends to an angle of about 45 , turn the kiln off. If the last cone (05) is still standing, you know for certain that you have not overfired the kiln, but if this cone has completely melted you can be sure that the ware or glaze has been fired beyond its maturing temperature.

Cones do not actually measure the exact temperature of the kiln. Rather, they indicate when the ware or glaze has matured. The rate of firing will affect the cones. If the firing is slow, the cone will bend at a slightly lower temperature than if the firing is rapid, in which case the cone will bend at a slightly higher temperature.

For an accurate measurement of the exact temperature inside the kiln, a pyrometer with a thermocouple is used. The thermocouple is a metal tip consisting of two dissimilar metals welded together. When the metal tip is subjected to the heat in the kiln it generates a tiny current of electricity which is measured and translated on the pyrometer into degrees of heat. This, of course, eliminates the need for cones since the correct temperature is recorded on the pyrometer dial.

Another convenient device for the kiln operator is an automatic cutoff. Anyone who has ever sat up half the night waiting for a kiln to go off will appreciate the advantages of a device that shuts the kiln off auto-

Illustration 14
A pyrometer measures kiln temperature accurately.

matically at the desired temperature. There are several kinds of automatic cutoffs on the market and your ceramic dealer will be able to advise you about the most suitable type for your kiln and pocketbook.

Kilns

There are various kinds of kilns available for ceramic firing, but the most satisfactory for the average ceramist is the electric kiln. An electric kiln has several advantages. It is easier to install, cleaner and far more dependable than any other type of kiln. The only disadvantage, as far as the studio ceramist is concerned, is that most of the studio-size kilns contain electric elements made of

Illustration 12
This large top loading kiln is easy to load and see into.

nichrome wire which melts at temperatures higher than 2000 . This means that only earthenware or other comparatively low firing ware can be fired in them. However, for the ceramist desiring to fire stoneware or porcelain, there are electric elements available which will reach temperatures of about 2600 . Electric kilns may be of the front loading type, which means that the door opens from the front of the kiln, or they may be top loading kilns with the door opening from the top. Either type is satisfactory and selection is largely a matter of personal preference. A top loading kiln is easier to load and permits a better view inside, but with a fairly large kiln the door of the top loader may be extremely heavy to lift and you may find it necessary to attach a weighted pulley to it.

A front loading kiln is sometimes more awkward to load and harder to see into, but it has one big advantage. If you plan to include copper enameling in your ceramic activities, it is essential that you use a front loading kiln. Enamels are flash fired, which necessitates quick removal of the enameled piece from the kiln at temperatures of about 1400 -1500 , and you may get badly burned if you attempt such a removal from a top loading kiln. Therefore, if copper enameling has a place in your future plans (and you do not intend to buy a large kiln, which is somewhat impractical for copper enameling), consider buying a front loading kiln that can be used for both ceramics and enameling.

In addition to electric kilns, oil or gas kilns are used, but they are primarily for firing on a mass production scale because the cost of operating an electric kiln large enough to be of any use to a ceramic factory is far too high.

All the kilns mentioned are periodic kilns, kilns that operate on a complete cycle from cold to warm, hot, cooling and to cold again, Large industrial plants, however, use tunnel kilns which maintain constant heat sections through which the ware travels slowly on moving cars. A tunnel kiln is obviously not the most practical kiln for the studio ceramist, but it is interesting to see so, if you have the chance, visit a ceramic plant and watch one in operation.

Glazes, too, have their own maturing temperatures, meaning the temperature at which the glaze will melt and harden into a glass-like covering over the clay. If your glaze and clay have the same maturing temperature, then only one firing is necessary. The green ware may be decorated, glazed and then fired, allowing the glaze and clay to mature in one operation.

If your glaze and clay mature at different temperatures, you will have to resort to a separate firing for each. But, a word of warning: when the clay and glaze do not have the same maturing temperatures, the glaze may not fit the body of the clay properly, with unfortunate results.

If, for example, the clay should shrink more than the glaze, “shivering” may result, in which case entire sections of glaze will chip or “shiver” off the ware. On the other hand, if the glaze shrinks more than the clay, the finished piece may “craze,” a condition in which the glaze pulls away, leaving small cracks of the clay body exposed. To be on the safe side, use a glaze and clay that mature at the same temperature.

Incorrect firing of glazes will also cause trouble. If a glaze made to be fired at 1940 is fired instead at 1875 , the glaze may melt but not fit the body as well as if it had been fired at its correct maturing temperature. It may change color completely or it may become dull rather than shiny. We will cover this subject more fully in subsequent chapters, but, generally speaking, it is recommended that you always fire your glazes at their correct temperatures unless you are deliberately experimenting.

After your clay piece has been formed it cannot be considered a finished piece yet, for clay must be fired if it is to be of any permanent value. Correct firing is as important to the success of your ceramic work as forming the piece, but it is often treated with a casualness that results in much needlessly ruined ware. If you understand the phenomenal change that occurs in clay when it is heated, you can readily see the importance of learning to control the fire properly.

Effects of heat on clay

Exposing clay to extreme heat produces a change in the molecular formation, causing a material which was formerly soft, plastic and easily molded by hand to become hard and brittle for all time. The fired clay can be ground and reduced again to its initial fine powdered form, but science has yet to find a way to restore the original plasticity. Once the clay has been subjected to intense heat its plastic quality is gone, never to return again. Clay frequently changes color when fired and, if the heat is high enough, will even become waterproof. Since clay reacts as it does to heat, firing must be regarded as a major factor in the making of ceramics.

Each clay has its own maturing temperature, that is, the temperature at which the clay reaches maximum hardness and density before melting and changing shape. When we say that earthenware fires at 1875 or stoneware fires at 2600 (all temperatures, mentioned in this book are Fahrenheit), we mean that those clays mature at those temperatures. Naturally, it is necessary to know the maturing temperature of your clay before you attempt to fire it.

Clay in its raw state is called green ware. Clay that has been fired but not glazed is referred to as bisque ware. Some ceramists bisque fire their pieces before decorating or -glazing them. However, this is not really necessary unless the piece is a small, fragile figurine, in which case bisque firing insures safer handling of the piece. Otherwise, there is usually no reason for bisque firing green ware as a preliminary step to decorating.

Grog

To prevent squashing, warping and cracking of a large sculptured piece during firing or drying, you can add grog to the clay before wedging. Grog is simply clay which has already been fired, ground up and screened. When added to the clay it provides openings through which moisture can escape. The grade and quantity of grog used depends upon the size of the piece you plan to construct, since a larger piece requires more and coarser grog than a smaller piece. For most pieces a medium grog (30-60 mesh) is used but for very large pieces a coarser grog (20-40 mesh) should be used.
The best way to add grog to your clay is by embedding it in the clay with a rolling pin. This is done before the clay is wedged.

Illustration 11
This sculptured tobacco jar of red clay would be a handsome object for any pipe smoker’s
desk. As you form your piece, be sure to turn it often to see all angles.

Divide your mass of clay into sections small enough to be wedged easily and roll out a section to a thickness of about ^4 of an inch. Sprinkle grog over the surface and press it into the clay with a rolling pin. To make sure that the grog is evenly distributed throughout the clay, fold the piece of clay in half and roll it out again, repeating the process until the grog is completely embedded in the clay. Then you are ready to proceed with the wedging.

Finishing

When you have completed your clay piece, whether it is a bowl, box or sculptured figure, you may want to smooth the surface, although in many cases this detracts from the hand-formed appearance that is so attractive in a ceramic piece. If, however, you want a smoother surface you may sponge the dry piece with a damp sponge or smooth it with sandpaper. If you sand the surface be sure to remove all the excess clay dust with a damp sponge; otherwise, when the piece is glazed the glaze may not adhere properly to the surface. The top and bottom of the piece may be made level by grinding them in a circular motion on a piece of sandpaper fastened to a board. Here again, remember to sponge off the loose dust particles.

Potter’s wheel

A potter’s wheel is used to form cylindrical clay objects. It consists, basically, of a flat disk, much like a large banding wheel, which is put in motion by the potter himself, or it may be motor driven. While the wheel is turning, the potter throws a lump of clay onto the center of the wheel. Using both hands to push and pull the clay as it whirls on the wheel, the potter forms his vase or bowl. The success of a thrown piece depends on the skill with which the potter is able to coordinate the speed of the wheel with the wall thickness and size of the piece. Throwing on a wheel is a special skill requiring practice and practice and more practice. A thorough description of the procedure is impossible here. If you are interested in throwing, try to watch a potter working at his wheel so you can see this most exciting and exacting skill.

Sculpture

The true test of your artistic craftsmanship in the medium of clay will come when you make your first piece of ceramic sculpture. The first thing to consider is the most suitable type of clay for modeling the piece. It is necessary to have a clay that is plastic enough to be worked with ease, yet, particularly if the piece is large, firm enough not to squash under its own weight. Most modeling clays are firm enough for smallish pieces (6 inches or so) but with larger pieces it is very important to check the firmness of the clay. Consult your dealer about the best type of clay for your purpose, remembering also to keep in mind the firing range of your kiln.

As to the color of the clay, this is mainly a matter of personal preference. There are a number of red and buff clays that are suitable for sculptured pieces or, if you plan to use brilliant colors as a decoration, there are many white clay bodies available that can be fired in almost any kiln. The sculptor’s main tools are, of course, his hands, but there are a few tools that are a convenient supplement. A wire tool is useful for gouging and a hardwood tool or two can be used for pressing on lumps of clay or smoothing. Start with these basic tools and add to the collection gradually as your needs dictate.

Illustration 10
Sculptor’s modeling tools are often helpful.

In constructing a piece of sculpture, concern yourself primarily with the general shape detail can come later. You should work in much the same manner as a painter who first establishes the broad masses and values before he even thinks of details. Make your general shape larger than the desired finished size to allow for shrinkage and for the carving off that will be done in forming the piece. (Check with your dealer about the amount of shrinkage you can expect in your piece, since clays differ in shrinkage percentages.) As you work, turn the piece around often so that you can view it from all angles. Remember that, unlike the painter, you are working in three dimensions.

When your piece is completed and still in a leather hard (partially dried) stage it must be hollowed out. A solid lump of clay with no openings through which moisture can escape will be certain to break apart when fired. If the piece has a wide enough base, hollow it out from underneath. Otherwise you will have to cut the piece in half and scoop out the inside with a hole extending through the base. Leave a wall about V2 to % of an inch thick, depending upon the size of the piece. A larger piece should have thicker walls than a smaller piece. After the inside has been scooped out, join the halves together again with slip. If you use enough grog (see below) in your clay, it may not be necessary to hollow out the piece.