The vast variety of clays available through Hammill & Gillespie can appear confusing to new customers. The reason is that the different ceramic processes require different clay characteristics that in turn specify different clay products. The following TechBrief summarizes the characteristics required for some of the principle ceramic forming processes. Our sales staff and technical director will be please to work with you to select a clay that is perfect for your application.
Throwing Clays
In throwing, we make extreme demands on the plasticity of clay. A really good throwing clay should not take on water readily while being worked, and should stand well and hold its shape even when soft and thin in section. While it is possible to throw simple forms from very granular and nonplastic clays, the complete range of possible thrown shapes demands a dense, highly plastic, and cohesive clay, with just enough rough material in it to furnish a slight "tooth" to aid the clay in standing while wet and soft at the end of the throwing operation. In throwing clays, we must be prepared to accept a high shrinkage and some tendency toward warping, and the ware must, as a consequence, be carefully managed in drying and firing.
As a general rule, throwing bodies are made with as small a percentage as possible of nonplastic substances in them, such as flint and feldspar. A single, naturally occurring clay may be an excellent one for throwing, and such clays are not unusual, especially in earthenware types of clay. If such natural bodies are unavailable, a combination of several plastic clays of various kinds may do as well. Ball clay is almost always used to increase plasticity. When the proportion of ball clay rises to about 30 percent, however, trouble with shrinkage and drying may begin, and the body may become unpleasantly sticky.
Adding flux to a throwing body is something of a problem, because nonplastic powders--such as feldspar, talc, or frit--may seriously decrease the plasticity. If a dark-burning clay is desired, this difficulty may be overcome by adding a red clay of low fusibility and high plasticity, which acts as a flux.
Although throwing clay must be plastic and dense, every thrower discovers that a clay which is too smooth and fine-grained throughout will not stand up in large or tall forms. For making such forms, and especially for pieces which are to be more than about 12 inches high, a clay with some "tooth" to it is necessary. Grog or coarse clay in a throwing body seems to give it the necessary bones or structure to make it stand up. It has been found that grog which has been sized to pass the 30-mesh screen and to stay on the 80-mesh screen is most suitable for throwing clay. If the grog is too fine, it decreases plasticity and makes the clay wet too rapidly; if, on the other hand, the grog is too large in grain size, it makes the clay excessively abrasive to the hands. A rough fire clay or ground flint clay may be added to throwing bodies in place of, or in addition to, grog. In general, about 8 to 10 percent of granular material will greatly improve the performance of a throwing clay. If more than 10 percent of grog is present, it may be found that the clay wets too rapidly during throwing and is insufficiently plastic.
A potter can make only so large or tall a pot with any given clay. Each clay body will reach a point where it will not stand in a higher cylinder, but will slump down. No matter how skillful the thrower is, he cannot achieve a taller piece than his material will allow in structural strength. It is for this reason that the skilled thrower is very much concerned with the composition of his clay. He wants a material which will allow him achieve the full range of form and scale which his skill permits. And, once accustomed to one certain clay, the potter may find it difficult to throw well with any other. It is true that many beginning pottery students struggle along with clay mixtures that the most skilled thrower would find impossible to manage, and in ignorance they tend to blame themselves rather than the material for their failures. The right clay body for wheel work is very important, and a large part of the creative pleasure in throwing pottery lies in the possession of a dense, fat, well-aged, and responsive clay.
Bentonite may be added to a clay to improve its workability on the wheel. Bentonite swells and forms a gel when wet, and the presence of a small amount of it in a clay body will greatly increase plasticity. If more than about two percent of bentonite is used, however, the clay may become excessively sticky and be difficult to wedge. Too much bentonite also may cause drying problems.
Modeling Clays
Making clay mixtures which will be good for modeling is a relatively simple problem. Since modeled objects such as sculptures, tiles, architectural pieces, or large built-up pots are usually rather thick, a clay is required which will dry out rapidly and safely with little danger of cracking. The clay must also fire safely, especially during the initial stages of heating when water is being driven from the clay. A large amount of grog brings about these necessary properties in a modeling clay. Twenty to 80 percent of grog is the usual amount. The grog may be coarse or smooth, depending on the textural effect desired, or a coarse fire clay may be used instead of, or in addition to, grog. Some coarse fire clays by themselves make splendid modeling clays. Some processes, such as building intricate shapes in coils or ropes of clay, or the making of extreme shapes may call for considerable plasticity as well as for coarse texture. Mixtures of very plastic clay, such as ball clay, with coarse material, such as fire clay and grog, may give the right degree of plasticity without unduly increasing shrinkage or making drying and firing difficult. A small percentage of bentonite, up to 2 or 3 percent, has been found useful in giving added cohesiveness and "stand" to a modeling clay. Modeling clay which is too smooth and greasy can be very unpleasant to work with, and even a small amount of experience will make the modeler appreciate a good, rough-textured, plastic clay which can be finished off either smoothly by pressing or burnishing-in the grog or roughly by scraping the surface.
Lightweight clay bodies may be made by mixing combustible aggregates with the plastic clay. These burn out in firing, leaving pits or voids in the fired structure. Sawdust, coffee grounds, ground-up fruit pits, or the like can be used. In the plastic state such organic additions to the clay act somewhat in the manner of grog, adding roughness and facilitating drying. During the firing complete combustion of the aggregate occurs, usually without difficulty if a normal firing cycle is followed. Lightweight bodies have been found useful for large pieces and for architectural elements where weight is a factor.
Pearlite is sometimes used to make a lightweight body. This mineral, a volcanic ash, is very light. It is added to the body as one would add a grog.
Casting Clays
A clay body which will cast well must be designed with the physical nature of casting slips taken into account. The process of casting requires a fluid suspension of clay in water, which will flow readily but which will not settle in the molds. The clay slip must bout smoothly from the mold, leaving a surface which is free from lumps or roughness. Furthermore, pieces which are east must not wet the mold unduly, must release themselves from the mold upon drying and must not have an excessive shrinkage or warpage.
An ordinary mixture of clay and water will not cast well in a plaster mold. For one thing, a great deal of water is required to make clay flow as a liquid suspension. Usually it takes about as much water as clay, by weight, to make a slip. Such a slip, although it will flow, has the serious disadvantage of a tendency to settle, leaving water at the top and a heavy sludge at the bottom. Also when such a slip is drained out of the mold, it will leave a roughness and lumpiness where the excess clay has drained away. Upon drying, the piece is very apt to stick to the mold in spots and to shrink and warp excessively because of the high water content of the clay.
Casting, then, would not be a practical way of making pots unless there were some way of cutting down on the amount of water required to make a fluid slip. The process that achieves this result is known as deflocculation. When clay and water are mixed together to form a slip, they are said to be in a flocculent condition. That is, the minute grains of clay are gathered together in clumps or "flocks," and each grain of clay, instead of floating separately by itself in the water and thus flowing easily over and around its neighboring particles, is drawn into a globule of many particles. These "flocks" or clumps of clay grains require a lot of water, relatively, to make them flow.
Bodies for Jiggering and Pressing
If objects are to be made on the jigger wheel, the clay body must be of medium plasticity and must dry with minimum shrinkage and warpage. Pieces are usually left on the mold after jiggering, and the drying shape must accommodate itself to a new position on the mold as it dries and shrinks to a smaller size. This requires a clay of some toughness, yet one which does not shrink excessively. A blend between the ball clays on the one hand and the more non-plastic kaolins, stoneware clays, or earthenware clays on the other will give the desired properties for any given process or system of drying. There is no way of predicting beforehand exactly what any given combination of clays will do when subjected to a particular process, such as jiggering. In arriving at a suitable jiggering clay, there is no substitute for trial-and-error testing.
Bodies which are to be pressed from plastic clay may be considerably less plastic than those bodies which are to be formed by jiggering. In pressing, the clay must be soft enough to flow into the cavity of the mold while under pressure, but the plasticity of the clay is not much of a factor except insofar as plasticity and strength are related. Pressed ware is commonly handled immediately after pressing and must be strong enough to retain its shape.
Earthenware
Earthenware is usually fired at temperatures below cone 6, and the fired clay remains somewhat porous and open in structure. The vast majority of the world's pottery has been earthenware because of the wide prevalence of earthenware clay and the relative ease of reaching, in the kiln, the temperatures necessary to fire it.
Earthenware has a soft tactile quality and a feeling of lightness quite different from denser forms of pottery. Although more fragile than stoneware or porcelain, it is not so brittle. The color range of earthenware encompasses a beautiful palette which includes various warm grays as well as red, orange, buff, yellow, and brown. These body colors combined with the brilliant colors of low-fired glazes make earthenware an ideal medium for color expression.
Earthenware bodies are usually made up of common red- or buff-burning clays, with only enough other materials added to achieve good working and firing properties. In almost every part of the world there are clays readily available which serve quite well for making earthenware, and the abundance of such clays partly accounts for the fact that man has depended everywhere upon.
Stoneware
Vitreous, gray, buff, or brown ware, fired in the range of cone 6 to cone 14, may be considered stoneware. The name comes, of course, from the dense, hard, impervious character of the body.
In some ways the formulation of stoneware bodies. is simpler than earthenware, because the higher heat makes less flux' necessary. As in the case of earthenware, the best solution is to find a good stoneware clay which, by itself, comes Hear to answering the needs of plasticity, fired density, and color. Many fine stoneware clays are available, particularly in the central and eastern parts of this country. They may be used straight, or altered slightly for more plasticity or for a desired change of color, texture, or glaze fit. A good natural stoneware clay should be plastic enough for throwing; fire to a tan, gray, or light brown color; and be fairly dense at cone 6. The relatively higher heat of stoneware firing makes possible the use of feldspar as the principal body flux. Feldspar is an ideal body flux because it has a long firing range, is cheap, and presents no difficulties or hazards.
For texture, grog and fire clays may be used. Some fire clays are actually quite similar to stoneware clay, although they may have a higher firing range and be coarser and less plastic. For rougher types of bodies combinations of stoneware clays and fire clays are ideal. Common sand is sometimes used instead of grog, but if more than about 10 percent is added dunting may occur.
When no natural stoneware clay is available, adequate bodies for high firing may be made up from kaolin, ball clay, feldspar, and flint, with iron oxide or red clay added for color. Such bodies, however, may lack the plastic quality of a natural stoneware body.
Fired stoneware should have an absorption of 3 percent or less. At its best it is dense and impervious and has a rich earthy color and texture.
Porcelain
Porcelain is a vitreous whiteware of more or less translucency fired to cone 9 or more. It is made by combining whiteburning clays with feldspars and flint. The relatively high heat acting upon the fluxes in the clay results in a dense, impervious body which approaches glass.
Compounding porcelain bodies is actually quite simple since the ingredients are few and the best proportioning of the ingredients is well established in practice. The main~difliculty is that, if only pure and white-burning clays such as the kaolins are used, the clay body is so non-plastic that it is hard to make anything out of it even by casting.
The proportion of 5 parts of clay, 3 parts of feldspar, and 2 parts of flint may be taken as the starting point in formulating porcelain bodies. The clay must be divided between kaolin and ball clay to make the body workable. If extreme whiteness is not desired, more ball clay may be used, and the body as a result will "pot" with less difficulty. Usually, in the interests of uniformity in case slight changes occur in the composition of the clays, several kaolins are used. If a soft feldspar such as nepheline syenite is used, less of it will be needed to bring about translucency. The flint content may vary, but if it exceeds about 25 percent, Bunting may result. The proportioning of the materials depends, of course, on the exact firing temperature, and a body designed for firing at cone 14 will have a good deal less feldspar in it than one designed for cone 9.
Some kaolins, particularly those from certain parts of Florida, are relatively plastic, and the use of these clays helps in the formulation of porcelain bodies which are white and yet reasonably workable. The exceptionally iron-free kaolins from England and from Georgia in the United States tend to be quite nonplastic. Since some ball clay is necessary, the selection of the kind is critical, since ball clays vary a great deal, both in plasticity and in iron content. English ball clay is perhaps the best for porcelain since it is both highly plastic and relatively iron-free. English ball clay contains considerable carbon, which must be screened out before the clay is used.
Bodies which are used commercially for the production of vitreous china are essentially the same as porcelain bodies. In the manufacture of china the body of the ware is matured in a first firing, without glazier In this bisque firing, since the ware is not glazed, it can be supported by special refractory setters, or by nesting in silica sand, and this prevents much of the warping or deformation which would otherwise occur. The ware is glazed in a second firing at about cone 4, usually with a lead boro-silicate glaze. In true porcelain, as distinguished from china, the body and the glaze are both matured in a high firing. This means that the ware must survive the high fire without support from refractory setters, and one of the difficulties of making porcelain is that the body, as it nears maturity, becomes so soft and grasslike that warping and slumping are very apt to occur.
Careful design of the shapes and careful setting and firing will get around most of the difficulty, but losses in porcelain making are inevitably high. The advantage of true porcelain is that not only is the body vitreous and translucent, but the glaze is very hard, scratchproof, and lustrous, and the contact between glaze and body is so intimate and so indistinct that the glaze, instead of appearing as a glassy coating, appears as an integral part of the body.
Although porcelain can be made to cast and jigger well, it is very difficult to achieve a porcelain body which will be good for throwing. This is due to the inherently nonplastic character of white clays. If sufficient ball clay is put in the body to make a highly plastic clay such as is needed for wheel work, the fired result will be cream color or gray rather than white. Bentonite also, when it is used to increase plasticity, makes the body gray.
References:
Rhodes, Daniel, Clay and Glazes for the Potter, Chilton Book Company, Radnor, PA, pp 19-24 (1996).
Van Olphen, H., Clay Colloid Chemistry, John Wiley, New York, (1974)