Ceramic Industry

POTTERY PRODUCTION PRACTICES: Stoneware Clay Body Formulas Part 3

February 2, 2006
Clay body additives and the right mixing techniques can be used to accelerate the aging process and achieve an optimum level of plasticity. When creating a stoneware clay body formula, one of the most important characteristics is plasticity. The unique attributes displayed by clay/water structures contribute to the plastic qualities of moist clay.

A stoneware vase.


Plasticity

Clay body additives and the right mixing techniques can be used to accelerate the aging process and achieve an optimum level of plasticity.

When creating a stoneware clay body formula, one of the most important characteristics is plasticity. The unique attributes displayed by clay/water structures contribute to the plastic qualities of moist clay.

Under magnification, clay looks like many plate structures, which makes for an ideal surface when the clay is brought into contact with water. An example of this type of attraction can be shown when paper is torn into small pieces. The paper represents a flat, high-surface area of dry clay, which does not bond or stick together. Add just enough water to soak each piece of paper (representing the clay platelet/water attraction process), and the pieces of paper cling together, allowing the combined mass to be bent or shaped. The dry pieces of paper essentially become "plastic" when they are brought in contact with water, similar to the reaction that occurs with clay.

Clay platelets and water attract and join in part by the colloidal action of water attaching to the clay and to itself. The water's surface tension binds together the clay platelets to create a flexible mass. However, when clay is first mixed, not all platelets are immediately permeated with water. When dry platelets touch other dry platelets within the moist clay mix, the clay exhibits a low level of plasticity. As the clay ages, water migrates to each platelet, resulting in greater plasticity.

The delayed reaction in achieving optimum plasticity is one reason Japanese potters once gathered, mixed and stored ("aged") moist clay for future generations of potters. While such traditional methods are no longer used, other techniques are available to increase a clay body's working qualities. The two primary factors that determine a moist clay's plasticity are the clay body formula and the amount of time the body spends in the moist state. Different clay body additives and the right mixing techniques can be used to accelerate the process.

Additive Solutions

One of the most common methods used to enhance the natural plasticity of clay/water structures is adding materials such as bentonites, ball clays, organic agents, Epsom salts and lignosulfonates to the formulation.

Bentonites

Bentonites are an extremely plastic group of clays that can enhance the working qualities of the clay body. They have small platelet structures that physically touch larger platelet structure clays in the moist mix. The small platelet size of bentonites also increases the water film bonding of the entire clay water structure. However, the amount of bentonite added should not exceed 2% of the clay body formula, as it can cause the moist clay to become "gummy" when used on a potter's wheel or in other forming operations.

Ball Clays

Ball clays are also composed of small platelet structures that increase the surface areas touching other clays and raw materials in the clay mix, thereby increasing plasticity. Ball clays also increase the colloidal film action of the clay body. However, amounts of more than 25% of the formula can cause excessive dry shrinkage and warping. High percentages of ball clay can also cause the moist clay to take on water at a faster rate during the throwing process, resulting in the clay slumping and sagging during the last stages of forming on the wheel, and can cause thin areas such as an excessively trimmed pot bottom to crack during drying.

A stoneware pot

Organic Agents

Mold growth in the moist clay can increase plasticity since it increases the binding action or attraction of the clay platelets. When mixing a 100-lb batch of clay formula, add 1/2 cup of beer, coffee or apple cider vinegar, or 3 oz of yeast, to the water before adding the water to the clay. All of these agents will start mold growth in the clay/water mixture. Moist clay kept in a warm, dark place for several days can also be the perfect medium to grow mold.

Different types of mold grow under various conditions of heat, moisture content, atmosphere and time. The type of mold growth cannot be predicted; however, green mold seems to offer the best plastic additions to moist clay, while black mold growth is often difficult to blend into the clay body. Black mold can also create a gas as it decomposes in the moist clay structure, which can cause problems in the forming or firing stages. Stoneware clay bodies seem to successfully accept wider variations of mold growth than porcelain clay bodies. Always mix or wedge clay before beginning any forming process to thoroughly blend any mold into the clay body. (Potters who are sensitive to mold should not pursue this method of increasing plasticity.)

Epsom Salts

Adding about 5 oz of Epsom salts (magnesium sulfate) per 100 lbs of dry clay formula (approximately 0.3% of the formula) can increase plasticity on another level. Epsom salts increase the attraction of clay platelets in the moist clay state and cause the clay to become flocculated. Clay platelets are then drawn together, similar to the way in which the north and south poles of a magnet are attracted and bind together. The overall effect is a tight, plastic clay body with good throwing properties. Each clay body will require a slightly different percentage of Epsom salts due to the individual materials in the formula. Too high a percentage of Epsom salts can cause salt migration to the drying clay surface, which can result in blistering and carbon being trapped in the clay. Adding the Epsom salts to the water and then adding that mixture to the clay mixing operation can help ensure that the salts are dispersed evenly throughout the batch.

A stoneware cup with a matte green glaze.

Lignosulfonates

Adding lignosulfonates such as Additive A1 (a proprietary blend of water-soluble lignosulfonates and organic/inorganic chemicals) in amounts of 0.06 to 0.75% of the dry weight of the clay can increase the moist clay's plasticity without changing its fired shrinkage, absorption or clay color. Lignosulfonates are produced in several versions, some of which contain barium in a safe, non-toxic form. In the barium versions, lignosulfonate can eliminate the scumming that is common in clay bodies where soluble salts are present.2 Lignosulfonates can increase plasticity and green strength in a clay body without causing excessive shrinkage rates in the drying and firing stages, as encountered with excessive additions of ball clay.

The Right Mixing Techniques

Traditionally, hand mixing or blending of clays and other ceramic raw materials was the only method available to achieve a clay with a workable consistency. While hand mixing will still accomplish the goal, clay mixing machines and pug mills are labor- and time-saving necessities when greater quantities of uniform consistency moist clay are required. Often the moist clay is blended and compacted much better by a machine than could ever be accomplished by mixing the dense, heavy clay by hand. Whatever method is used, the goal is to get each clay platelet surrounded by a film of water with all the other raw materials in the clay body blended into a uniform mass to ensure the best plasticity.

Mixing and Pug Milling

The combination of a clay mixer and pug mill is the most efficient and popular method of mixing clay. Ceramic supply companies use both machines in the production of stock and private clay body formulas. In most machines, the dry clay and water are first blended in the mixer, and then the moist clay is placed in the pug mill and compressed by a mechanical screw. The clay then goes through a chamber where the air is removed and the clay is extruded out of the pug mill nozzle in a usable condition. Some newer systems combine the mixing and pugging operations in one unit.

The clay compaction achieved through pugging can create a denser fired clay body-pug milled formulas average 0.25-0.50% less in fired absorption than the same formula not pugged. This is because compacted clay platelets fuse faster and more completely during firing than non-compacted platelets.

Wedging

Further increases in flexibility and consistency are often achieved by wedging the clay before any forming operation. One reason is that wedging can eliminate any weaknesses in the pugged clay. During pugging, fine particles within the moist clay are rubbed by the pug mill auger blades and separated from coarse particles in the clay, creating two separate clay bodies or shear zones within the clay mass. When the clay is turned or screwed through the machine, the fine particle clays move up against the coarse particle clays in a concentric ring pattern. When the clay is extruded, the two different clay bodies will create weak spots or shear lines that can show up as cracks in the ware. These cracks, when they do appear (typically during the drying stage), will look very much like the circular pattern of a sliced "jelly roll" cake.

Since the extruded clay is moist and compacted, the cracks might not be visible when the slab of clay is in its plastic shipping bag. To discover if a pug milled clay has such a weak area, slice 2 in. from the extruded face of the moist clay and let this slab sit in water overnight. If the clay shows "jelly roll" cracking patterns the next day, it must be wedged thoroughly before attempting to make wide-based forms such as plates, which are more likely to fail due to their greater horizontal surface area.

Wedging can also correct any platelet disruption in the clay body that was caused by the pug mill extrusion, and it can help potters determine the clay's moisture content and make adjustments as needed. For these reasons, it is highly recommended that all clay bodies be wedged by hand before moving on to the forming operations.

Filter Pressing

In filter press operations, the dry clay body formula is mixed with excess water to form a liquid slurry. The slurry is then pumped into a series of absorbent leaf-shaped bags; as the bags are compressed, the excess water is pressed from the liquid clay. The "leaves" of moist clay can then go on to the pug mill for further mixing and de-airing. One advantage of filter pressing is that each clay platelet is surrounded by water in the slurry stage, and the water-soaking period produces a clay with greater plasticity compared to other mixing methods. However, filter pressing is labor-intensive and is not frequently used in pottery studios due to its excessive expense.

Whatever the mixing method, the best results are obtained when the moist clay is allowed to "rest" or "age" for several days before any forming operation. After mixing, the moist clay is pliable and plastic and can be bent into shapes. However, each clay platelet is not thoroughly wet (except filter pressed clays) and will have a minimal amount of plasticity. A few days or weeks later, the clay platelets in that same moist clay will be fully saturated and surrounded with water, causing the body to be highly plastic and easy to form.

The Perfect Formula

Is there a perfect clay body formula that can accomplish miracles and overcome poor forming techniques or improper firing cycles? Of course not. Potters attempting to use any clay body formula should begin with a small test batch before getting out the mixer and making a ton. No shortcut exists for making good products. Start by reading and obtaining other potters' knowledge to aid in the search for your own clay body formula, and be prepared to test and experiment. The magic formula is in the potter, not the clay.

Editor's note: All images in this article are courtesy of Jeff Zamek. Part 1 of this three-part series, "The Basics," appeared in the Summer 2006 (June) edition of Pottery Production Practices, and Part 2, "The Perfect Body," appeared in the Fall 2006 (October) edition.

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