POTTERY PRODUCTION PRACTICES: Putting the Fire in Clay

September 1, 2007
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Understanding the characteristics of fireclays and how they are mined and processed can offer valuable information regarding their use in any ceramic project.  

As the term implies, fireclays are primarily associated with fire or heat. While the refractory nature of the clay becomes evident when used, fireclays can also contribute particle size variation to a clay body to enhance its forming characteristics.

As a type of clay, fireclays offer potters the most cause for concern when used as part of a clay body formula, and they require vigilant inspection and care when introduced. While other components of the clay body have their own potential for producing imperfections in the forming, drying, glazing or firing stages, fireclays are statistically more likely to be the origin of faults as opposed to ball clays, stoneware clays, bentonites, earthenware clays, feldspars, flint, talc or any of the other raw materials that can compose clay body formulas. Understanding the characteristics of each fireclay and how they are all mined and processed can offer valuable information regarding which to use in any ceramic project.

Origins

The geologic formation of fireclays depended on rock from the earth's crust being weathered by wind, rain, heat, cold and abrasion. The raw materials were then transported to acidic waters containing roots and vegetation, resulting in finely divided minerals containing silicates of alumina, among other materials. After the Glacial Period (800-600 million years ago), the waters receded, leaving sediment and silt ground up by the receding glaciers.

Streams moved the remaining fine particle material to other locations, resulting in different types of fireclay depending on the local geology. Almost all fireclays are formed by their movement along streams, eventually forming into beds. Clays of this type contain impurities gathered on their travels, which can contribute to their plastic qualities, fired color, particle size, organic content and refractory nature.

Consequently, the American Ceramic Society defines fireclay as a "sedimentary clay of low flux content," which enables it to withstand high temperatures before deforming. Generally, fireclays can have a pyrometric cone equivalent (PCE) from cone 31 (3022°F) to cone 36 (3268°F). The pyrometric cone equivalent is a classification of the softening point of a material, in this case fireclay.

To determine PCE, the fireclay is shaped into a "cone" and placed in a cone holder with a series of known Orton (www.ortonceramic.com) cones close to the temperature where the fireclay is projected to soften. The cone holder is placed in a specially designed furnace and the temperature is raised on a schedule until the fireclay cone bends over and the tip touches the cone holder. At this point, the sample is removed, and the cone in question is matched with the "standard" cone and given a PCE value.

Mining Operations

Christy Minerals offers an example of one type of clay mining operation. The mine is located approximately 50 miles west of St. Louis, Mo., and uses selective open pit excavation to extract Hawthorn Bond fireclay. The top layer of overburden is taken away and the clay seam exposed. After carefully removing the clay, it is stockpiled and weathered for 12-18 months to cause it to break down.

The natural freeze/thaw conditions, plus rain and sun, hydrate and dehydrate the clay platelets, which ultimately improves consistency and plasticity. The clay is then transported to the plant where it is dried and placed in a covered shed until it is ground to the appropriate mesh size. Several grinding methods can be used depending on the desired grain size and distribution of the product. The material is then tested for particle size distribution and placed in packaging that ranges from 50- and 100-lb paper bags to larger 3500-lb supersacks.

Clay Processing

Raw Clay is rarely taken from the mine site without some form of processing. Over 99% of clays are destined for use by industry and the commercial sector of the economy. Potters represent less than 1% of the total market, and their limited role affects the quality of clays they can purchase. Clays for commercial applications require a degree of uniformity and quality control, but not necessarily the same kind of processing that potters require. Two machines used in the preparation of raw clay are the roller mill and the hammer mill.

In a typical roller mill operation, some of the free moisture is removed from the clay and it is fed between a ring and a series of rolls that spin around the inner diameter of the ring where it is ground and dried. It is then thrown between the ring and rolls by plows. A stream of air lifts the finer clay particles up to a whizzer (several fan blades); the faster the blades spin, the finer the clay particles need to be to pass through the whizzer. Once through the whizzer, the clay/air stream goes to a cyclone collector where the clay is separated from air during the air floating process. The clay then goes to a packer where it is bagged and ready for shipment. Greenstripe is an example of a roller-milled, air-floated fireclay.

In a hammer mill, crushed clay that is 2 in. and smaller enters from above. High rpm hammers rotate and pulverize the clay, which is pushed through screens at the bottom. The screen size determines the mesh size of the clay and the size of possible contaminates processed with the clay. Hawthorn Bond is an example of a hammer-milled fireclay.

As opposed to the process of screening clay to remove comparatively outsized coarse clumps of clay and large-size contaminates from a given amount of clay, the platelet size of clay is best observed on a microscopic level. Platelet size distribution simply means how many small, medium or large platelets are present in a given amount of clay. The distribution of platelet sizes also affects the texture (or "tooth") of the moist clay during forming operations.

The shape of the platelets, their direction in relation to each other, and the colloidal water adhesion forces that bind them together also play a part in determining the handling characteristics of the moist clay. Platelet size distribution can influence plastic qualities, shrinkage rate, forming potential and drying characteristics. A greater percentage of larger platelets will result in a fireclay that is less plastic with decreased ability to bend when moist. It will shrink less and dry faster than a fireclay with an increased amount of small platelets.

Fireclays having a finer platelet size will shrink more and have greater plasticity when moist. They will take longer to dry and require more extensive water films around the increased amount of platelets in the fireclay to achieve plasticity. However, most clay body formulas contain other types of clays, feldspars, flint and grog, which can be mitigating factors to the above-mentioned fireclay platelet size characteristics.

Figure 1. Raw fireclay nodules caught on a 30 mesh screen. Particles can range in size from 2 to 6 mm.

The mesh size of the screen is a factor in determining the amount of contaminants that are allowed to pass through with the fireclay (see Figure 1). A large 20-mesh screen can allow greater quantities of "tramp" material or contaminants (e.g., twigs, stones, coal, or processing debris such as bolts, wire, and metal parts) to enter the final bag of clay compared to fireclay passing through a smaller 50-mesh screen.

Air floated fireclays, such as Greenstripe 200 mesh, which is naturally finer than clays like Hawthorn Bond, offer lower defect rates from contaminants but also change the "clay body feel" when moist. The higher percentage of fine mesh fireclay used in a clay body formula results in less "tooth" and a gummier feel to the moist clay. Such clay bodies do not stand up on the wheel and feel overly soft. They can take on more water in forming operations and deform more easily due to each small clay platelet being surrounded by water. While these are subjective factors for each potter, there is a recognizable difference when a coarse fireclay like Hawthorn Bond 35 mesh is used in place of the finer Greenstripe 200 mesh as a component in a clay body formula.

Fireclays with comparatively large mesh sizes (28, 35 or 40) can have periodic runs of irregular contaminants. This happens when rips in the screen occur or operator error allows large particles to enter the clay batch. Simply stated, the larger the contaminant size, the greater the chance of it causing a problem in the forming, drying and firing stages. Using a finer mesh fireclay is one option that can diminish the problem, but, again, the moist clay can lack "tooth" or stand-up ability in forming operations. If a coarse mesh fireclay is used, splitting the total fireclay requirement 50/50 with another fireclay of similar or finer mesh can reduce the risk of a bad batch of fireclay disrupting the clay body.

As with any screening procedure, whether it's a 35 mesh or 200 mesh fireclay, some residual material will get through the screen. Clay is not a round or symmetrical material, but has an aspect ratio making one dimension larger than the other. In some cases, depending on how the clay particle hits the screen, it may pass through or be retained and removed. If screened again, the off-size material could be removed.

In most instances, however, the material is not screened again and is simply packaged for shipping. Unfortunately, potters require a greater degree of quality control for their fireclays than processors typically offer. As in any economic profit motive activity, some fireclay processors are more in tune with serving the larger industrial market than the smaller pottery production market.

Several ceramic supply companies have taken it upon themselves to screen fireclays. While the additional cost might be a few cents more per pound, the extra expense can be worth the potential failure of the clay body. Sheffield Pottery, Inc. (www.sheffield-pottery.com) is one ceramic supplier that can screen fireclays or any other material used by potters. Typically, a fireclay is passed through a 30-mesh screen to remove contaminants, though other mesh screening sizes are available.

In addition, the clay can also be moved through a tube containing a powerful rare earth magnet that removes ferrous iron contaminants. The clay will still produce an iron spot pattern when fired, but large particles of iron will not be present. Both systems bring an added degree of processing necessary for clays used by potters.

Economic Considerations

Today's potters find themselves in a difficult economic situation. In a perfect world, they could dictate the exact specifications of the materials required in their craft. However, the extra processing of clays and other raw materials would generate a higher, possibly prohibitive, cost. Individual potters are often forced to accept material as is, which in many instances means clays with particle size variations, chemical composition deviations and tramp material contamination. Inconsistent raw materials and the potter's inability to change this economic fact are recurrent areas contributing to the loss of product.

While ceramic suppliers, as a general policy, have limited liability when it comes to the clay they sell, it is the potters who suffer a loss as their profit margins are tight. It is not unusual for a potter to lose an entire kiln load of pottery due to a raw material causing a defect. A ceramic supplier will replace "bad" clay, but the potter will not be compensated for time and labor spent in the production of defective pots made with substandard clay.

As a general rule, the potter will not be remunerated for the damage caused to the kiln, kiln shelves, adjacent pottery in the kiln or kiln furniture caused by a defective clay or raw material. In many instances, the potter must prove that the defect originated in the raw material and not their own forming or firing procedures.

Figure 2. Particles of iron in fireclay can cause large blemishes in pottery. The defect is more pronounced in reduction-fired ware since the iron fluxes more than in an oxidation atmosphere.

Contaminants

Fireclays are the weakest link in any of the clays that compose clay body formulas. There are several geologic and economic reasons for the presence and continued existence of contaminants found in this necessary but troublesome clay. Fireclays can contain carbonaceous materials like lignite, peat, coal and other associated tramp materials. They can also have variable particle sizes of silica, iron and manganese. At the mine site, even with careful excavation, some of these impurities can find their way into the seams of fireclay.

On a technical level, all of these "bad" qualities can be refined out of the clay. However, as mentioned previously, intense processing is not required on an economic level since the majority of fireclay users (e.g., steel mills, casting foundries and brick manufacturers) use the clay with minimal processing. Since potters represent less than 1/10% of the raw material market in the U.S., it does not make economic sense for mines or raw material processors to refine a product for such a small market share. Unfortunately, this is a common downside for potters who use raw materials specified for industrial requirements.

The major (but certainly not only) contaminants found in some types of fireclays are lime and iron. With iron, the size of the particle is critical as potters want small random brown specks (1/2 to 1 mm in diameter) in the fired clay body. However, large nodules of iron (4 to 6 mm in diameter) in the fired body can cause outsized brown blemishes on pottery surfaces (see Figure 2).

Figure 3. Large (2 to 3 mm) particles of manganese form concave defects (white boxes) in a reduction-fired c/9 (2300°F) clay body.

Fireclays may also contain aggregate lumps of manganese, which can cause brown/black concave or convex defects in fired clay surfaces (see Figure 3). In functional pottery, these disruptions can trap food or liquid, causing unsanitary conditions. The same, but more pronounced, defect occurs when manganese decomposes between 1112 and 1976°F in a reduction kiln atmosphere. The manganese nodules are unaffected by reduction, but melt in irregular "spit out" patterns on the clay body's surface.

Figure 4. Lime pop caused by the expansion of lime in the fireclay component of the clay body.

Lime can cause clay body defects depending on the particle size (see Figure 4). Nodules of lime can expand in the fired clay body as it takes on atmospheric moisture, and the resulting half-moon-shaped crack caused in the clay body disrupts the surface. The same material in powder form does not exert the considerable internal forces of expansion when in contact with moisture.

Figure 5. Glaze blisters are sharp-edged craters caused by carbonaceous material in the fireclay exiting the clay body as a gas at high temperature.

Fireclays mined near the coal fields of Pennsylvania and Illinois are laid down next to seams of coal and lignite (an intermediate carbon-based material between peat and bituminous coal). If not removed by a clean-firing oxidation kiln atmosphere, such contamination can cause black coring/bloating in clay bodies or blistering in glazes due to incomplete combustion of carbonaceous material in the fireclay exiting as a gas through the glaze layer (see Figure 5).

Figure 6. Sharp edge cooling crack in ware caused by cristobalite inversion in the kiln or oven. The larger part of the crack (right) is where the crack started to form.

Aside from tramp material, large silica particles can be associated with coarse (35, 40 and 50 mesh) fireclays that can also cause defects in a clay body. Since potters often prefer coarse material with tooth or grit, fireclays will include coarse particles of silica by design. Both the fine and coarse particles have the potential to form cristobalite as the clay body is heated past cone 8 (2280°F). Excessive cristobalite buildup in the clay body is often due to the kiln stalling or taking an excessively long time to reach temperature past cone 8.

By physical nature, the larger particles of silica in fireclay that convert to cristobalite will produce a larger expansion/contraction, thus exerting a greater pressure on the body during firing and cooling than small particles. However, all alumino-silicate materials (e.g., talc, feldspars and other clays) will contain some level of free silica that can also be subject to the same transformation to cristobalite and the eventual cooling inversion.

If considerable amounts are present in the clay body, an inversion of high-to-low cristobalite takes place around 392°F that can cause a pinging noise in the pots as they cool in the kiln.1 Seemingly intact functional pottery with a buildup of cristobalite can also crack later when heated in a hot kitchen oven due to cristobalite inversion during a similar cooling range (see Figure 6). Cracks can occur anywhere on the pot and have a sharp edge, almost like the pottery has been hit by a hammer.

Pottery Applications

Fireclays are refractory, relatively coarse particle size, low shrinkage clays that are mainly employed as a component in medium- (2232°F cone 6) to high-temperature (2300°F cone 9) clay body formulas. However, they can be used in low-fire (1828°F cone 06) clay body formulas to contribute a coarse particle size and tooth to the forming qualities of the moist clay.

Fireclays can constitute 5 to 30% of a clay body, depending on the firing temperature, forming method, fired color and intended function(s).

Potters use fireclays in diverse ceramic products like floor and wall tiles, functional pottery, and sculptures. The amount and type of fireclay in a clay body formula also depends in part on the cost of bringing the fireclay to the point of manufacture, whether by a ceramic supplier or individual potter. The actual cost of shipping fireclay or any clay is often as expensive as the cost of the material. This economic consideration is apparent when ceramic suppliers stock raw clay and use clays in their moist clay body formulas based, in part, on the lowest shipping cost.

Material Availability

Frequently, the decision to stock one clay over another is decided by transportation costs. It is unusual for West Coast ceramic suppliers to use Midwest fireclays in their stock clay body formulas when West Coast fireclays are readily available and offer lower transportation costs. However, some large West Coast suppliers can make the capital investment to also stock Midwest fireclays as part of their inventory. East Coast ceramic suppliers are also reluctant to carry West Coast mined clays for the same reason.

Economic reasons, not geologic depletion, can dictate a clay or raw materials' disappearance from the pottery market. Some discontinued materials do live on in ceramic supply houses' inventories or individual potters' storage bins, but they are not currently being mined. Before developing any clay body or glaze formula, always inquire if the material is still in production. In some instances, potters have used a clay, feldspar or other ceramic raw material found in their studio only to discover when it is time to reorder that the material is no longer in production.

Once a fireclay is removed from production, it is highly unusual for it to be brought back on the market. A little research will save much time and effort. Some fireclays that are no longer in production are P.B.X. fireclay, North American fireclay, and Pine Lake fireclay. Green Ribbon fireclay and G.S. Blend fireclays were blended by Laguna Clay Co. from Lincoln 60 and Irish Hill Cream fireclays when Greenstripe fireclay was temporarily unavailable.

For medium- to high-temperature clay body formulas (cone 6 and above), the total lack of a fireclay component can affect the drying qualities, handling characteristics, fired color and fired maturity of the clay. Keep in mind that porcelain clay body formulas traditionally do not contain fireclays when high temperature, whiteness and translucency are desired, but in a great percentage of high-temperature stoneware clay body formulas, fireclays are a viable addition to the formula.

Author's Acknowledgements

Shane Bower of Christy Minerals LLC supplied information on the mining and processing of fireclays. Jon Pacini, clay manager, and Jon Brooks, president, of Laguna Clay Co. were most informative and helpful in contributing information on West Coast fireclays. Jim Kassebaum, general manager of Laguna, supplied samples of fireclays for evaluation. Eric Struck, technical service director at Industrial Minerals Co., supplied detailed information on West Coast fireclays and processing operations. Dave Jenkins, president of Ione Minerals, Inc., supplied technical information on the mining of Greenstripe fireclay. Mort Gensberg, Seaway Shipping & Trading, directed my research into West Coast fireclay mines. John Cowen, president of Sheffield Pottery, supplied images and descriptions of the clay screening process. John Benedict, clay manager at Sheffield Pottery, screened samples of fireclay for the article. Jim Keating, technical manager of Gladding McBean, supplied information on Lincoln fireclays.

SIDEBAR: Fireclays Commonly Used by Potters

Lincoln Fireclay
The Gladding McBean (www.gladdingmcbean.com) plant in Lincoln, Calif., has been in operation since 1875, and the Lincoln Clay Products plant, a division of Gladding, has been in operation since 1898. The mine produces Lincoln 60, 8, and 9 fireclays, which are hammer milled. The sedimentary clays are formed by the erosion of the Sierra Mountains in California and are selectively open pit mined. Scrapers peel off overburden soil, exposing seams of clay 2 to 5 ft thick. It is then processed and shipped in 50 and 100 lbs bags and bulk packaging ranging from 20 to 34 mesh. All of the Lincoln fireclays have excellent drying properties with minimum cracking and warping. Technical data sheets are available.

Lincoln Fireclay 60 fires to a buff/cream color and is frequently used in pottery production. Its name is derived from the one to six layers of clay that form the deposit. Over 30,000 tons per year are shipped to pottery manufacturers and ceramic suppliers. It is hammer milled and has coarse iron specks (PCE 28-29).

Lincoln Fireclay 8 has a higher percentage of iron than Lincoln Fireclay 60 and fires to an orange color. It has good plasticity and excellent strength (PCE 28-29).

Lincoln Fireclay 9 fires to a buff/cream color and has granules of siderite (iron carbonate and iron sulfate). It has a low iron content and exhibits good plasticity and excellent strength (PCE 30-31).

IMCO
IMCO 400 is Lincoln 60 fireclay mined by Gladding McBean that is then roller milled and air floated by Industrial Minerals Co. (www.clayimco.com). Industrial Minerals sells 400 to 500 tons per year each of IMCO 400, IMCO 800, Howard Clay and Irish Hill Cream Clay. IMCO 400 has greater strength and fires to a buff/cream lighter color than IMCO 800, both of which are mined in layers. Technical data sheets are available.

IMCO 800 is a smoother fireclay than IMCO 400 and has a yellow-fired color. It is a finer ground fireclay than Lincoln 8, cutting down on the size of contaminants and iron specking.

Howard Clay is a roller-milled, air-floated fireclay mined by Industrial Minerals. It has a lighter fired color than IMCO 400 or IMCO 800.

Irish Hill Cream is a roller-milled, air-floated fireclay mined by Industrial Minerals. It has good green strength and is not as refractory as Howard Clay, but is darker in its fired color.

Greenstripe
The Ione Minerals (www.ioneminerals.com) plant has been in continuous operation since its construction by Gladding McBean in the early 1950s. Fireclays have been mined in the Ione basin of Northern California on and off since the mid 1800s. The Ione basin is about 20 miles long and one to four miles wide, along the western Sierra Nevada foothills.3 The company has eight mine sites over 21,000 acres, where it solar-dries and processes its fireclays and kaolins.

The mine produces Greenstripe fireclay, a very plastic, 200 mesh, air-floated fireclay that features comparatively high shrinkage and excellent green strength. It fires to a light buff color (PCE 31-32). Technical data sheets are available.

Hawthorn Bond
Christy Minerals LLC (www.christyco.com) mines Hawthorn Bond fireclay and sells 80% of its production of fireclays to the pottery market and the art clay industry, which manufactures architectural ceramics. Missouri fireclays, such as Hawthorn Bond and A.P. Green (mainly processed for the brick and refractory industries), take longer than West Coast fireclays to absorb water in the clay mixing process due to their harder granular structure. This characteristic can result in clay bodies with a Missouri fireclay component becoming stiffer after the initial mixing process as water is eventually absorbed into each clay platelet.

Missouri fireclays may also be associated with seams of lime and gypsum that can have nodules from 1/4 to 4 in. in diameter, causing lime pop in the bisque and fired ware if not screened properly in the processing stage. The fireclay can also have nodules of silica disrupting a clay body surface (PCE 31-32). Technical data sheets are available.

SIDEBAR: Clay Body Formulas

Many low-, medium- and high-fire clay body formulas utilize a fireclay component, and it is useful to understand the reasons. It is also important to note that fireclays act in conjunction with other clays and raw materials in a clay body formula.

Listed here are examples of clay body formulas that can be used in oxidation or reduction kiln atmospheres. They can be used on the potter's wheel or in hand building. The primary functions of each fireclay in the formula are described. As with any clay body or glaze, it is always best to test the formula before committing to a large quantity.

Low-Fire Cone 06 #1 (white fired color)
Hawthorn Bond fireclay 50 mesh: 20
Thomas ball clay: 40
Talc: 40

Hawthorn Bond fireclay contributes tooth and a coarse particle size to the clay body. Without the fireclay component, the moist clay may feel gummy and soft when used on the potter's wheel or in hand building operations. As a general rule, particle size differentiation from the use of dissimilar particle size clays is preferred, as it contributes to a mechanical interlocking of clay platelets in the forming stages.

Medium-Fire Cone 6 #2 (light brown in reduction atmosphere/off white color in oxidation atmosphere)
Greenstripe fireclay: 16
EPK kaolin: 16
Goldart stoneware clay: 23
Flint 200x: 10
Nepheline syenite 270x: 20
Kentucky ball clay OM #4: 15
Silica sand 85x: 6%

Greenstripe fireclay contributes a refractory clay component that enables the clay body to withstand the higher temperatures reached in the stoneware firing range. The fireclay also acts to reduce dry and fired shrinkage, as well as lessen or eliminate warping. The variation in particle size of the fireclay aids in forming operations.

High-Fire Cone 9 #3 (medium brown in reduction atmosphere/cream color in oxidation atmosphere)
IMCO 400 fireclay: 20
Goldart stoneware clay: 40
Newman Red stoneware clay: 10
Tennessee #9 ball clay: 10
Custer feldspar: 12
Flint 200x: 8
Grog 48/f: 5%

The addition of IMCO 400 fireclay contributes refractory strength and reduces dry and fired shrinkage. The fireclay also lessens any warping potential in the drying and firing stages. The variation in particle size aids in forming operations.

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