This article was originally in the February 1999 issue of Ceramics Monthly.
At some point when mixing your own clay body or when ordering a custom blend from a ceramics supplier, you’ll find that one of the ingredients is no longer available. As I’ve said before, its not a question of if this will happen, but when. Knowing which material to use as a replacement is important. Keep in mind that each contributes its own set of qualities and acts in a specific way in conjunction with other materials. The fired color, shrinkage, absorption and forming characteristics of the clay body are dependent on this combination. Gaining a better understanding of the various raw materials will offer flexibility when the time comes to make substitution decisions.
Economics plays a significant role in the availability of raw materials. The large industrial users of clays, feldspars, talcs, etc., dictate the quality and availability of materials that potters use in clay bodies and glazes. Often when a raw material is no longer available, it remains in abundant supply but is simply not economically viable to mine and distribute. Potters typically constitute less than 0.01% of the producers market.An example of this economic reality is Burns Brick Company, which mined Ocmulgee clay for brick plants. It started as a small family-run business, selling its clay to potters as a sideline. At some point, a large corporation purchased the company and determined that processing, bagging and shipping clay to potters was not profitable enough, so the decision was made to stop production of Ocmulgee clay for sale to potters and ceramics supply companies.
On a more positive note, the major purchasers set the parameters for the raw materials used in their particular products, thus almost guaranteeing uniformity. For example, Edgar Plastic Kaolin (EPK), a secondary clay (transported by wind or water from its original site of formation) that has been mined in Putnam County, Florida, since 1892, is used in the manufacture of spark plugs. The spark plug company requires an exact chemical composition, and demands consistency with every load delivered to the factory. Whenever possible, potters should take advantage of such stable situations in choosing materials that are “guaranteed” by industry to perform with proven consistency.
Foretelling the future of clays used by industry is impossible, but potters can “tag along” to some extent, incorporating these “guaranteed” clays whenever they can in their clay bodies. When developing a clay body, be sure to contact the mines and raw material processors to determine how stable supplies will be in the future. Often, ceramics suppliers cannot comment accurately on availability, as they are only a link in the chain of supply. It is up to the potter to investigate which industries are currently using the material, then to determine the likelihood for that material remaining in production.
Raw material “drift” can also bring about the need for substitution. Many factors can contribute to a reliable clay turning into something different. Occasionally, the changes occur almost imperceptibly, but after a while the total of small changes can push the clay into a problem-producing state. At this point, substituting a new material for the one that has become unsatisfactory for the clay body will be necessary.
Just because the name stays the same on the outside of a bag of clay does not mean the clay inside the bag stays the same. An example of the same name clay with a different material in the bag is Kentucky-Tennessee Clay Company’s Old Mine 4 ball clay. In the mid 1970s, OM 4 was mined from a single pit of clay. It was then decided the overall consistency of the clay would be improved by blending several individual clays to duplicate the characteristics of the single pit OM 4. Blending clays has proven to produce a more reliable product, while extending the life of useful deposits. Most clay mines are now using the blending process to produce ball clays.
While it should be noted that the blended OM 4 has greater uniformity, it is also true that the blended OM 4 is different from the original single-pit deposit of OM 4. Potters who used the original OM 4 have reported a difference in the handling qualities of the clay. In a sense, by using a leveling-out process in blending clays, the mine has decreased the risk of getting a bad batch of clay. Of course, this has also eliminated the possibility of getting a truly outstanding single-pit batch of clay.
Potters should not decide to find a substitute for a raw material because of cost. Frequently, this is false economy, as the substantial cost in producing any pottery or sculpture is labor, not the cost of raw materials. If you are ever tempted to use a less expensive clay that doesn’t work quite as well, remember it can be very costly if the substitute material significantly changes the clay body’s performance. Reliable, consistent materials that produce good results should be the goal, not saving an extra penny per pound on the cost of materials. The same principle applies to the use of any clay body additives, such as Additive A, Macaloid, nylon fibers. If any additive results in a better clay body with fewer defects, it is well worth the extra cost. In practical terms, the price per pound of clay can increase if it helps cut down on defects, while using a less expensive material can be very expensive in the time and effort required to produce a failed piece.
Often, the need for a substitution comes about only because a clay body has to be mixed immediately and the required material is not in the studio. Potters may not have control in regard to mining economics, but they can exercise some control by keeping a well-stocked studio. Emergency substitutions should only be made when they are truly unavoidable.
If the potter simply wants to experiment with the clay body recipe, several critical factors should be understood before attempting a change. First, mix up only small amounts of the revised recipe and test thoroughly. The experiment could prove to be very expensive, if hundreds of pounds are mixed and the revised recipe does not work. The temptation then is to make a further revision for a better result. Most often, trying to save a mistake will result in more unusable clay taking up room in the studio. It is usually better to start again from a recipe that works.
Once a small batch of revised clay body tests well, mix up another small batch and fire it in other kilns or even the same kiln. In reduction kilns, the intermediate testing phase is especially important, as reduction kiln atmospheres can be inconsistent in themselves. Obviously, experience gained from using the clay body over many kiln firings and with various glazes will be vital in building confidence in the revised recipe. This is also the time to note if any minor adjustments to the recipe are required. The idea is to build up a track record of good results with the revised clay body before committing to it.
How to Make Substitutions
Most clay bodies consist of between three and eight separate materials. The number of different materials is dependent to some extent on the functions the clay body must serve. Porcelains usually have kaolin, feldspar and flint components, while some stonewares can have a fireclay, ball clay, stoneware clay, feldspar, flint, and grog component. When replacing a material in the clay body, it is first necessary to determine the specific group to which the material belongs, then choose another material in that group with the closest match in mesh or particle size.
Particle size matching is most important. An unproductive match can occur when using the same chemical composition material with a radically different material size, as in substituting 325-mesh flint, which is a silica, with 60-mesh silica sand. The larger mesh sand would not melt with the other raw materials, and would also change the handling and throwing characteristics of the clay body. The closer the particle sizes match, the less likely a substitute will change the handling characteristics of the clay body.
Particle size can also affect the degree of melting or vitrification that occurs in the clay body. Smaller particle size materials go into a melt faster and more completely than larger size materials because of their greater surface area exposed to heating.
The smaller particle or platelet size can also make the moist clay body shrink excessively, warp or crack in the drying or firing stages. Moist clay bodies that have large quantities of small particle size materials can feel gummy or soft during forming operations. When substituting materials, keep in mind the particle size distribution (small = ball clays and bentonites; medium = stoneware clays; large = fireclays and grogs) and try not to unbalance the clay body.
The clay component of the recipe can be arranged into classifications. The most common clays found in potters’ clay bodies are fireclays, stoneware clays, ball clays, earthenware clays, kaolins and bentonites. Within each group of clays are many individual products. A partial list of ball clays contains Thomas, XX Sagger, Kentucky OM 4, and Tennessee 9. Within some groups, subgroups exist; for example, EPK (Edgar Plastic Kaolin) is a plastic kaolin, and Kaopaque 20 is a nonplastic kaolin.
The most common mistake happens when choosing a substitute clay from the wrong group of clays. Often, a stoneware clay is confused with a fireclay, the result being the clay body does not have “tooth” or stand- up ability in forming. In high-temperature bodies, the lack of a true fireclay component makes the clay less refractory or heat resistant, which can result in warping, shrinking or bloating during firing.
The fired color of the replacement clay must also be considered. For example, while one ball clay can be substituted for another, choosing the substitute ball clay with approximately the same fired color is important. Both ball clays will be plastic, but a darker firing ball clay can easily change the tint of the fired clay body, a change that is most noticeable in porcelains or light-firing stonewares. As the clay body matures, the dark ball clays iron and manganese component becomes more visible. Conversely, dark-firing clay bodies can accept a wider range of color variations in substitute clays because the replacement clay color is lost in the dark clay body’s fired color.
Most kaolins, ball clays and some stoneware clays that the potter uses for clay bodies are air floated. The process begins when the crude clay, ranging in size from lumps a few inches in diameter to powder, is fed into a dryer. The moisture content of the clay (16% to 20%, depending on the time of year the clay is mined and the characteristics of the individual clay deposit) is reduced to 1% to 3%. The dried clay is then fed into a roller mill or hammer mill. Oversize material is ground again and sand is thrown out. The clay moves by air stream through a spinning blade to sort particle size. The sorted clay is then swept by air into storage silos or rail cars, or bagged for future shipment.
While making the clay easier for the processor to handle, store and ship, air floating also produces a lower moisture content clay. This can result in a longer time for the clay to “age” or achieve its maximum plasticity when water is finally added during the clay- mixing operation. It has been theorized that the excessive drying necessary for air floating decreases the organic content of the clay, and that the lower organic content adversely affects the moist clay’s plasticity; however, Cedar Heights Clay Company, Old Hickory Clay Company, H. C. Spinks Clay Company and Kentucky-Tennessee Clay Company assert that the temperature reached during the air floating process does nothing more than drive off free water from the raw clay, and the process does not lower the organic content of the clay.
The following is a list of raw materials, with explanations of when to use each as a substitute. Not listed are materials that do not have a practical substitution or would involve extensive clay body testing for the purposes of substitution. Groups of materials enclosed in brackets can be substituted for each other in a clay body. For example, [Thomas ball clay and Taylor ball clay] are both light in fired color and can be readily substituted for one another in a clay body.
Ball clays contribute plasticity to the total clay body. Because of their small platelet size, they fill spaces between larger platelet size clays. The smaller platelet size also provides a greater surface area for the water film surrounding each particle. More water film platelets in the clay body increase plasticity in the moist clay.
Each ball clay contains different amounts of naturally occurring metallic oxides, such as iron and manganese, which can affect the fired color; however, most do little, the exceptions being the very few high-iron ball clays. For example, F-2 Ball Clay mined by Old Hickory Clay Company has an iron content of 4.36%, as compared to most ball clays averaging an iron content of 1.2%. The high iron content of F-2 can cause the fired clay to become much darker, depending on how much ball clay is used in the recipe.
Overall, when substituting ball clays, always choose the one with the percentages of iron and manganese closest to the original ball clay. Hand- building and throwing bodies can accept wider variations and still produce a good result, as opposed to slip casting bodies, which are very sensitive to any raw material substitution.
The most widely available off- white-fired-color ball clays are [Tennessee 1 (SPG 1), Tennessee 10, Coppen Light, H. C. Spinks C&C, Old Hickory 5 and Old Hickory 1 Glaze Clay].
The most widely available cream- colored ball clays are [Foundry Hill Cream, 1 Glaze Clay, Jackson, Kentucky OM 4, Kentucky Special, Kentucky Stone, M&D, Thomas, Taylor, XX Sagger, Tennessee 9, Tennessee 5, Spinks HC5, Mississippi M&D, Bell Dark, Gold Label and Bandy Black].
The Chinese and German types of barium carbonate can easily be interchanged in a clay body, provided the particle size is similar. Used in amounts of 0.25% to 2%, based on the dry weight of the clay body, barium carbonate prevents scumming in the fired ware. It reacts with calcium and magnesium soluble salts, changing the salts into calcium carbonate or barium sulfate, which are insoluble and inert. By eliminating soluble salts in the clay body, subsequent firing discoloration is reduced. Some red earthenware clays are more susceptible to soluble salt problems, but even so-called “clean” clays can occasionally have high levels of troublesome salts; consequently, barium carbonate is found in many different clay bodies.
When adding barium carbonate to a clay body, be sure to wear a dust mask (see “Is Barium Carbonate Safe?” in the September 1997 CM). Also, always wash your hands after the clay- mixing operation.
As a group, bentonites are the most plastic of all the clays. They are added to clay bodies to increase the plastic qualities of the moist clay in forming operations. Usually, 1% to 2% bentonite is used, based on the dry weight of the clay batch. Higher percentages can make the moist clay thixotropic (the moist clay body deforms under pressure) and gummy when water is added during handbuilding or throwing operations.
Bentonites can be classified as to fired color and degree of plasticity. The darker-firing (due to organic content and other contamination) bentonites have greater plastic qualities than the cleaner, light-firing bentonites.
Light-firing bentonites include [INERT (325 mesh), Western Bentonite (200 mesh), Bentonite B and Bentolite White GK129].
Ibex-200 is a dark-firing bentonite that can be used in dark clay bodies. If a high-iron or high-manganese bentonite is used in a white-firing clay, it can cause black specking, which becomes increasingly noticeable as the clay body reaches vitrification.
Clay body additives that can be used in place of bentonite are [Epsom salts (magnesium sulfate), Additive A, Macaloid and Veegum T], all of which contribute greater plasticity to the clay body than any of the bentonites (see “Additives for Glazes and Clay Bodies” in the December 1998 CM). It is usually not possible to make a direct substitution of additive for bentonite, though; the appropriate level of additive will require testing.
The major source of flux in clay bodies at stoneware temperatures (Cone 6 to Cone 12) is feldspar. Incorporating the correct amount will result in a hard, vitreous clay body.
Feldspars fall into three basic groups: potassium feldspars, sodium feldspars and lithium feldspars. When making a substitution in a clay body, begin by determining if the original feldspar is potassium, sodium or lithium, then choose the replacement feldspar from within the same group. Chemical analysis sheets are available from the ceramics supplier or the processor of the feldspar. The analysis sheet will show the percentages of each oxide contained within the feldspar. Disregard the silica (Si02) and the alumina (Al203) content, and look for the highest percentage of oxide, which will indicate the feldspar group. For example:
Excluding Si02 and Al203, the K20 or potassium content is the highest oxide percentage in the chemical analysis. Therefore, G-200 is a potassium-based feldspar. The potassium feldspars include [Custer, G-200, K200 and Primas P]; no longer available are [Oxford, Buckingham/261-F, Yankee, A-3/ Elbrock, Maine Spar, Madoc “H,” Godfrey and Clinchfield 202].
The sodium feldspars are [Kona F- 4, Nepheline Syenite (270 mesh), Minex 3, Nepheline Syenite (400 mesh), Minex 4, Calspar, Primas S, NC-4, Unispar 50, C-6 and Minnspar 200]; no longer available are [Eureka, Clinchfield 303, Minpro 4, Bainbridge, 56 Glaze Spar and 54 Glaze Spar].
Lithium feldspars are [spodumene and petalite]; no longer available are [Lepidolite and Lithospar]. The high- iron-content spodumene mined by Foote Mineral Company has been discontinued; it produced bubbling when mixed with the clay body water. Spodumene from Australia is marketed by F&S Alloys and Minerals Corporation; it is low in iron and mixes well in clay bodies without effervescing in water. Spodumene (LM) from Canada is produced by Tannco Company.
Silica is commonly found in clay body recipes as flint. Other materials, including clays, feldspars, talc, pyro- phyllite and wollastonite, can also contribute silica to the clay body.
Flint reduces dry shrinkage and warping; it also promotes glaze fit, as it forms with fluxes and other materials to create a vitrified, dense, nonab- sorbent body. It can be purchased as 400, 325 and 200 mesh, all of which are suitable for use in clay bodies. The larger (200 mesh) size is used whenever mesh is unspecified in the recipe.
Classified as to their plastic and nonplastic qualities when moist, kaolins contribute white firing color to the clay body, providing other clays in the recipe also fire white. Many porcelain clay bodies contain as much as 50% kaolin, so choosing the correct kaolin substitution is critical in producing a successful result. Occasionally, some kaolins cannot be substituted on a direct one-for-one basis.
Mined in England, Grolleg kaolin is a unique clay that is most difficult to substitute, due to its handling and forming characteristics. Using a combination of plastic and nonplastic kaolins is often necessary to duplicate the qualities of Grolleg kaolin; however, blending different clays to achieve a workable substitution often leads to many hours of trial-and-error testing with little if any real gain.
The plastic kaolins are [Edgar Plastic Kaolin (EPK), Grolleg, Stockalite, Kaolex D-6, McNamee, 6-Tile, Pioneer, Laguna 1, Sapphire, Treviscoe, T-7, Standard Porcelain (SP) and Super Standard Porcelain (SSP)].
Nonplastic kaolins are [Kaopaque 20, Ajax P, Delta, SnoCal 707, Kingsley, English China Clay, Georgia Kaolin and Velvacast].
The calcined kaolins [Glomax LL or Ajax-SC] have been heated to remove the chemical water, decreasing the clay shrinkage. To produce calcined kaolin in the studio, fire any kaolin past dull red heat to approximately 1100°F.
Additions of highly refractory, nonplastic, large-platelet-size fireclay to the clay body will decrease shrinkage and warping as the moist clay dries; however, as a group, fireclays are potentially the weakest part of any clay body, due to their variability in chemical composition and/or particle size. At any time, fireclays can include coal, twigs, sand or excessive amounts of silica, all of which can cause problems in the forming or firing stages.
The plastic fireclays include [Imco- 400 (Lincoln 60 fireclay ground finer), Imco-800 and Sutter 200]; Pine Lake is no longer mined.
Medium plastic fireclays are [A. P. Green Missouri (28 mesh), A. P. Green Missouri (20 mesh), Hawthorne Bond (20 mesh), Hawthorne Bond (35 mesh), Lincoln 60 (20 mesh) and Greenstripe]; P.B.X., a high-iron fireclay, is no longer mined.
North American Fireclay (20 mesh) is a nonplastic fireclay.
In low-fire (Cone 06 to 04) clay bodies, talc promotes glaze fit and can represent 50% of the recipe. In high- temperature (Cone 6 to 10) clay bodies, talc provides for a dense, vitreous fired clay. Talc can also affect iron in the clay body, causing a light brown or red tone in high-temperature reduction clays.
Not all talc is the same. Each has its own distinctive characteristics, which can influence plasticity, maturing temperature, fired clay color and glaze fit. On the East Coast, NYTAL HR100 Talc is available; it is comparable to Pioneer-2882 on the West Coast. Westex Talc is a platelike nonfibrous talc that might produce a slight off-white color, as compared with NYTAL HR 100 and Pioneer- 2882. Some other talcs are Sierralite (high alumina content), Soapstone 78SS (dirty for use in glazes), TDM 92 (high organic matter) and Talc 80/ 20 (partly calcined for use in dry- press clay bodies).
As a group of clays, the stonewares are of medium platelet size and fairly plastic when moist. In many high- temperature clay bodies, stoneware clay is the majority of the recipe. The light-tan-firing stoneware clays are [Cedar Heights Goldart (air floated, 200 mesh), Goldart Ceramic Grade Fireclay (not air floated, 50 mesh), Roseville and Yellow Banks 401]. Due to its excellent defloccu- lation properties, Roseville stoneware clay is a better choice for a slip-cast- ing body than the others. Although the Yellow Banks air-floated stoneware matures at a lower temperature than Goldart (Yellow Banks 401 fired at Cone 8 has 1% absorption versus Goldart 4.17% absorption), Yellow Banks is still a good substitute for clay bodies in the Cone 6 to Cone 8 temperature ranges.
High-iron-content stoneware clays are [Newman Red/Neuman Red and C-Red/Carbondale Red]; Lizella stoneware has a lower iron content and yields a lighter-firing clay.
The most common commercially mined earthenware clay is Cedar Heights Redart, a high-iron content, red to brown firing clay. Sheffield Slip Clay, mined in Massachusetts by Sheffield Pottery, is a low-temperature clay that fires a lighter red to brown color than Redart because of its lower iron content; in some recipes, it can substitute for Redart, provided it is not the only iron-bearing clay used. Yellow Banks 101, mined by United Clays of Indiana, also has a lower iron content than Redart, but can be used as a direct substitute. Ranger Red Clay/Ranger Shale, produced by Trinity Ceramics Supply in Texas, is another high-iron, red-firing clay that can be substituted for Redart.
When more “tooth” or stand-up ability is needed in forming operations, grog is usually added to the clay body. It also decreases warping and shrinkage during the drying and firing stages. For every 10% of grog added to a clay body, the firing shrinkage of the body is decreased by approximately 0.75%; however, plasticity is also decreased. Grogs have an irregular particle shape with sharp edges, as compared with some silica sands, which are oblong and have rounded smooth edges. The sharp- edge quality to the grog makes it ideal for locking itself into the clay body during throwing operations.
Several factors are important in choosing a suitable grog substitute. Grogs are produced in various mesh sizes, ranging from powders to large pebbles. Higher mesh numbers indicate a smaller particle size. For example, a 20-mesh grog is pebble size, while a 100-mesh grog is a fine powder. Some have one set of numbers followed by a slash with another set of numbers, indicating the particle size ranges of the grog. For example, grog 20/48 ranges in particle size from 20 mesh to 48 mesh; however, Maryland Refractory 48/f is a 48-mesh grog decreasing in size to a fine powder.
Grog mesh sizes frequently used by potters are [Calamo 80 and lone 406 (65 mesh)], [Calamo 60 and lone 412 (40 mesh)], [HCR HI 48/f and Maryland Refractories 48/f].
Some grogs are processed from used industrial firebrick, which are occasionally contaminated with metallic oxides or other impurities. The amount of contamination and the defect it can produce in the fired clay body is dependent on the original use of the firebrick. Virgin grog, such as Harbison-Walker Refractories Calamo grogs and lone grogs, are made from flint clays that have a low alkali/iron content, high chemical purity and are consistent in chemistry.
Molochite is a white-firing porcelain grog produced in several mesh sizes. Mullite, Cordierite and Kyanite are other nonplastic materials ranging in size from powder to large pebbles that can be added to cut shrinkage and warping. Just remember that increasing the amount of a nonplastic material in the composition of a clay body will correspondingly decrease the clay body’s plasticity.
Metallic Coloring Oxides
To enhance or change the fired color of a clay body, metallic coloring oxides or their carbonate forms can be added; however, results can vary in the shades of color they produce from one batch to the next. Body stains, which contain metallic coloring oxides together with stabilizers, are more expensive, but they do produce consistent colors.
Although a high percentage of oxide or stain can yield intense colors, it can also decrease the moist clay body’s handling qualities. In reduction atmospheres, higher amounts of metallic coloring oxides and/or stains can cause overfluxing and brittleness as well. Before making a coloring oxide substitution, consider the forming method and the kiln atmosphere to be used. Testing a substitute metallic oxide or stain is worth the effort and will prevent costly mistakes.
Iron oxide is the most commonly used clay body metallic coloring oxide. It can produce light to dark browns with an addition of 0.25% to 2%, based on the dry weight of the clay body. [Red iron oxide (approximately 90% Fe203) and Spanish red iron oxide (approximately 81% Fe203)] are almost identical in their effects on clay body color. Red iron oxide also is produced in refined grades, such as [Red NR 4284 and Red iron oxide 2199], both of which are more than 90% Fe203; they will cause the clay body to fire darker in color than the natural red iron oxide ore. Black iron oxide would not be the best choice for a direct substitute for any of the red iron oxides, as it can easily overflux the clay body.
In a sense, frits (which are melted, fast cooled and ground into powders) are manmade feldspars. They can contain soluble oxides in an insoluble form, while also tying up toxic oxides in safe glassy matrixes.
Frits are generally not used as a clay body flux due to their strong and fast melting action, and they are not recommended for clay bodies fired above Cone 02; however, small amounts of frit can be used to create low-temperature vitreous clay bodies. The most severe drawback is the short maturing range; sometimes the clay body can easily be underfired or overfired within a one-cone range.
Some frits are also slightly soluble, and when used in a clay body, they can break down, leaching into the water system of the moist clay body. The clay can then become rock hard or soft like silly putty (thixotropic) when pressure is applied. The most common reason given for not using frits is their expense, but such an assessment is not economically sound if a small frit addition makes the clay body perform as required. Listed in the chart below are the most commonly used frits and their equivalents.
What Every Pottery should Know
The substitution of any material should be the choice of last resort. This is because of the variables involved when substituting one material for another, no matter how closely they appear to match. Nevertheless, that choice will have to be made someday. A thorough knowledge of raw materials is the best insurance to guard against that day when a change becomes necessary.
Knowledge of raw materials should be obtained from several sources. No one source, either from past experience, former teachers, fellow potters, magazines, books or ceramics consultants, will afford you a comprehensive understanding of raw materials. Its up to you to gather and evaluate the information, to get to know the material on a practical “nuts and bolts” level. Sometimes, a material will act differently in the studio than is reported on the written page. It can even behave differently from one bag to the next bag. Only through experience and repeated test firings does a workable knowledge of ceramic materials begin to come together. The combinations of materials and kiln atmospheres can become very complex very fast; however, if the potter starts with a few basic materials and builds upon experiential and empirical knowledge, things will fall into place. Read, listen, try and try again.
The author Ceramics consultant Jeff Zamek resides in Southampton, Massachusetts.
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