Clay bodies are usually designed around plastic materials, which are the component clays, and non-plastic materials, such as feldspars, flint, grog or refractory calcines. We use ball clays to impart plasticity, and fireclays and stoneware-type clays to add both plasticity and strength. Kaolins can be used to impart whiteness, refractoriness and plasticity. Red clays can add color or texture, and alter the vitrification temperature if desired. We add flint, feldspar or other fluxes and materials, such as pyrophyllite, talc and frits, to improve vitrification and glaze fit. We also take into consideration the forming method, particle size and chemical analysis of each component clay. Finally, we add non-clay materials to impart color and/or strength and texture, or to improve plasticity and workability. This total blend forms a synergy, with each of the individual clays and components working together to form a balanced clay body that fits our requirements.
The forming method and firing temperature determine the ratio of plastic to non-plastic materials for a given body, and it is this ratio that makes for a successful clay body formula. While plastic bodies, such as those used in hand-throwing, jiggering and casting, have different ratios at each temperature range, casting bodies have essentially the same ratio regardless of the temperature range. By paying careful attention to the relationships between the component clays, as well as the amounts of plastic and non-plastic materials, a successful composition can be achieved for a particular application.
Once we understand some of these basic concepts, we can assume a greater degree of control and responsibility over our work. We know what goes into the clay body and in what proportions. We know what each clay contributes to the whole mix. We know exactly the amount of flux needed—whether feldspar, frit or talc—and we also know the amount of glass former (flint) required. We also know the amounts of refractory calcines and pyrophyllite that are necessary, as well as any coloring materials that might be required. Finally, we know what ratios of these materials are needed to achieve a vitrified clay body with specific absorption and shrinkage characteristics. Understanding these principles allows us to change or alter a formula to meet our own individual and particular needs. It also enables us to solve any number of clay-related problems that are more than likely to happen at some point in our clay working careers.
So what do we need to know to create the ideal clay body?
This temperature range includes white-firing and terra cotta bodies. It is not unusual to find ratios of 50 parts plastic materials to 50 parts non-plastic materials. Ratios of 60:40 also work, and there are many exceptions.
The selection of fluxes and glass formers is of primary importance for these bodies. The main flux at this range is talc, but auxiliary fluxes such as frit, nepheline syenite, Plastic Vitrox/PV Clay or red clay are also used. Ball clays are the prevalent clays used at this range for white-burning earthenwares, but many compositions contain other clays, such as fireclays and kaolins, and some even contain small quantities of stoneware clays. Red clays can also be included to create terra cotta bodies at this temperature range.
The major problem at this temperature range is getting the plastic materials to form a glassy matrix with fluxes that are poor melters at this temperature. Flint won’t form a usable glass, and no other flux besides frit will help melt the flint at such a low temperature. Frits can be used in either white or red bodies but lead to other problems at this temperature range, the least of which is their expense. If a high soda frit is used, some gelling can occur in the body since sodium-based materials act as deflocculants. Another issue at this temperature range is bisque firing hot enough to burn off the many organics present in the ball clays. This is referred to as loss on ignition (LOI) and will result in a slight change in the weight of the clay. If the offending organics and carbonaceous materials are not successfully burned off, defects such as pinholes usually show up on the glazed surface.
To successfully remove these materials and prevent glaze defects, the clay body should be bisque-fired at a higher temperature than it is glazed. Cone 04 or 03 is usually the temperature of choice, but firing these types of bodies must be done carefully. The hotter the body is fired, the more its absorbency decreases. This can result in a bisque body that will not absorb the glaze in the necessary thickness to achieve the proper color. Adding a body frit can help the body achieve vitrification at a lower temperature but will render it even tighter. It can be difficult to find a bisque firing temperature that is low enough to allow these bodies to remain somewhat porous, yet high enough to prevent defects resulting from trapped organic materials.
Terra cotta clay bodies in this temperature range have the added problem of the excellent fluxing nature of the red clay component. For example, Cedar Heights Redart provides a distinctive, beautiful red color but also adds to the vitrification of the body. In large amounts, it can severely lower the body’s melting point so that it is fully vitrified in the bisque stage. It can also lead to black coring and bloating in fuel-fired kilns.
For these reasons, anyone working with these types of bodies must be able to stay within a very narrow firing range. The body must be formulated carefully, and kilns must fire precisely.
In general, these bodies are poor candidates for functional ware. They are often soft, punky (not fired to maturity), under-vitrified and prone to delayed moisture crazing. Clear glazes used on these bodies are primarily frit-based and contain no other glass formers, so the resulting surface is not very durable. However, most commercial glaze and under-glaze color systems fit these types of bodies quite well, so they might be an excellent choice for ware that is primarily decorative.
This temperature range enables the use of a broad range of fluxes, such as potash and soda feldspars. Flint can be used as a glass former, and pyrophyllite can be added as a non-plastic component. The ratios for plastic to non-plastic components can be 50:50 or 60:40. Buff or darker bodies can exist in ratios of 70:30 and even 80:20. Using soda spars, talc or nepheline syenite can help achieve the proper vitrification. Ball clays in conjunction with some stoneware-type clays or fireclays can also be used very successfully. Use of Cedar Heights Redart and other red clays in sufficient quantities will aid in maturing the fired clay body due to their high iron content. This temperature range is also excellent for producing majolica-type surfaces on terra cotta clay bodies. These bodies will be durable and strong, and will not have any of the problems that are associated with their lower-fired plastic earthenware counterparts.
Proper glaze fit is much easier to achieve in this temperature range due to the use of feldspars, flint and pyrophyllite in the body. Mid-range “whiteware” or porcelain-type bodies can be achieved with the proper selection of white burning ball clays, such as Old Hickory TI-21, and kaolins, such as 6 Tile™, Pioneer® or EPK. However, white bodies in this range must contain more flux, as the kaolins are quite refractory.
If compounded correctly, clay bodies fired at this temperature can be as strong as or even stronger than their high-fired counterparts, with the added benefit of requiring less energy during firing.
Plastic High Fired Bodies, Cone 8-10
This is an attractive working temperature for many potters since it produces work that is highly vitrified in both porcelain and stoneware formulations. Just about all clays are usable at this firing temperature. Ratios need to be no lower than 70 parts plastics to 30 parts non-plastic but can go higher. Again, however, there are many exceptions to this rule.
For buff to dark-colored firing bodies, lower amounts of red clays are advisable in reduction atmospheres to avoid black coring and bloating. Used in small amounts, high iron bearing clays such as Redart, Newman Red, C-Red, Imco 500, Lizella and Occmulgee are excellent additions to darken the color of the body.
Fireclays and stoneware-type clays are also widely used. Some materials include Greenstripe 200 mesh fireclay, Hawthorn Bond 50 mesh fireclay, Cedar Heights Goldart, Roseville™ or Salt Lick. Kentucky Stone is another satisfactory clay material to include. White to off-white firing ball clays, such as Old Hickory TI-21, Foundry Hill Creme and Tennessee #10, are equally important for plasticity. These clays also have a lower iron content than Old Mine #4 ball clay, which has changed considerably since the mine source of the clay has changed. A variety of kaolins, such as 6 Tile, EPK or Pioneer, are also excellent additions to a stoneware plastic body. Many other clays can also be used.
Porcelain clay bodies can use domestic or imported kaolins. Flint and feldspar can promote a translucent body if they are present in high amounts and if the body is thin enough. However, ball clays added to porcelain formulas will compromise the whiteness and translucency because of the iron and other materials present. A variety of light firing ball clays, such as Old Hickory TI-21, are available and can be added to porcelain mixtures. A standard four-part porcelain body consisting of equal parts of kaolin, ball clay, flint and feldspar is a mainstay for many potters. It has a firing range of cone 6-10 and can also be used as a casting body when deflocculated properly.
Casting Bodies, Cone 06-10
Regardless of the temperature range, casting bodies require a plastics-to-non-plastics ratio of 50:50. This ratio can deviate slightly, but not by much. While casting bodies use a variety of both plastic and non-plastic clays, the resulting body is quite non-plastic and cannot easily be worked on a potter’s wheel, even if it is mixed to a throwing consistency.
The firing range and vitrification of casting bodies is determined by their non-plastic material components. Earthenware casting bodies use talc as the primary flux. Mid-range bodies can use talc, nepheline syenite and soda feldspars, such as Kona F-4. Flint is highly desirable, as is a small amount of pyrophyllite. At the stoneware and porcelain range, Custer Feldspar can be used, as well as some of the previously mentioned fluxes. Again, both flint and pyrophyllite are highly beneficial.
The casting rate is the amount of time the liquid slip must remain in the mold before it is poured out. This is determined not only by specific gravity and viscosity, but, most importantly, by having a balance of coarse and fine particle materials in the clay body. Some kaolins and ball clays, such as Velvacast® kaolin and Old Hickory FC-340 ball clay, are specifically blended to the optimum particle size for slip casting. By combining these coarse particle sized clays with finer particle sized clays, the casting rate can be successfully controlled.
If you use a pre-mixed clay body, it is incumbent upon you, the user, to ask the right questions of your supplier. How does the company test its clay bodies? When were the last tests run? How does the company make sure that no offending materials are present in larger mesh clays? If the clay body contains fireclay, what mesh is it, and is the fireclay screened before it is added to the body?
If your supplier cannot answer these and other basic questions, you might need to look for one that can—or consider formulating your own clay body.