Ceramic Industry

PPP: Enhancing Clays with Additives

September 1, 2001
Over the past 40 years, many clay additives and conditioners have been developed to eliminate such time-consuming steps, making pottery production easier and more efficient than ever before.

Wheel-thrown bottles made with a soda fired clay body incorporating calcium lignosulfonate. The pieces were fired to 2300ºF.
At one time, potters “aged” their moist clay in damp cellars or storage containers for years to help achieve the workability they required. Over the past 40 years, however, many clay additives and conditioners have been developed to eliminate such time-consuming steps, making pottery production easier and more efficient than ever before.

Calcium lignosulfonate* is just one in a series of non-traditional methods of increasing the plastic properties of clay. Developed in 1950s to increase the plasticity and green strength of brick,1 calcium lignosulfonate is now used in almost every type of ceramic manufacturing operation. When used in clay bodies, calcium lignosulfonate makes the water “wetter,” allowing less water to be used to make the clay body more plastic. The additive contains a lignin-derived binder that mechanically takes the place of some water in the clay body, causing adhesion of the clay plates. This action reduces the risk of clay body defects, such as excessive shrinkage, warping and cracking during drying and firing.

The increase in plasticity is most noticeable in short or non-plastic clay bodies, such as Raku, sculpture, and jigger- and ram-pressed formulas. The additive can also be used in soda, salt, porcelain, low-fire and tile clay bodies.

Traditionally, ball clays and/or bentonites have been used to increase plasticity, but both types of clay need large amounts of water to make them plastic. Additionally, excessive amounts of ball clay can make the body feel gummy and soft when moist, causing problems in the forming stages. The moist clay body can also exhibit thixotropy or a “jello-like” quality when forms are pulled up on a pottery wheel. Calcium lignosulfonate can replace all or part of the ball clay/bentonite component in clay body formulas, giving potters greater flexibility to choose non-plastic clays in their formulas while also decreasing the total amount of water required.

Calcium lignosulfonate also substantially increases the green strength (e.g., pots that are formed but are not dry) and dry strength (e.g., pots that have been formed and dried) of clay bodies, reducing chipping and damage caused by handling the ware. Increasing the durability of unfired ware is especially crucial for large ceramic forms that must be moved around in the studio or loaded onto a kiln.

Calcium Lignosulfonate as a Lubricant

In addition to having a positive effect on the clay body for the forming and handling processes, calcium lignosulfonate can also help lower energy costs and increase equipment life in the mixing and extrusion stage. As a soluble, natural polymer, the additive imparts a higher charge density to the particle surface of the clay. It absorbs into the clay platelet, causing a negative charge that allows the clay platelets to slide past each other in the clay/water structure. This produces a clay mix that offers less resistance to pug mill and mixer parts. The decreased resistance lowers energy costs and extends the life of the mixer and pug mill. The lower moist clay resistance and lubrication also help make extrusion faster and easier for extruded clay body formulas.

Wheel-thrown stoneware clay containing calcium lignosulfonate. The piece is 16 in. tall and was fired to 2300ºF.

Types of Calcium Lignosulfonate

In many clay bodies, calcium sulfate and magnesium sulfate travel to the body surface upon drying, causing a white “fuzzy” residue called scumming. This effect can also appear as a white flaky material after firing, and in some cases may not emerge until days or months later, after the clay is exposed to moisture. Barium carbonate is added to some forms of calcium lignosulfonate in various ratios to neutralize the soluble salts, preventing scumming. Type 1 contains 2 to 4 lbs per 2000-lb-equivalent of barium carbonate; Type 3 contains 5 to 7 lbs per 2000-lb equivalent; and Type 4 contains 8 to 10 lbs per 2000 lb equivalent. The type used depends on the level of neutralization needed. Type 2 does not contain barium carbonate and can be used to add plasticity in applications where scumming is not a problem.

Because they contain barium carbonate, Types 1, 3 and 4 are only available in a liquid form that chemically links the barium to the polymeric structure of the lignosulfonate. This chemical linkage eliminates the hazards that are associated with dry barium powder. (Barium carbonate is toxic if ingested.)

All types of calcium lignosulfonate can be safely used in any clay body, including low-fire red clay bodies. While none of the different types of calcium lignosulfonate are toxic, normal handling precautions should be used when working with any form of the additive.

Mixing the Additive

The most effective method of mixing the additive is to add water to the dry clay mix until the moist clay starts to ball up. At that point, calcium lignosulfonate can be added. Additional water is then added to get the desired moisture content for the forming operation. The additive can also be mixed and dispersed in hot water, and all of this mixture can be used in the clay mixing operation. Complete and thorough mixing of the additive in the clay mixture is crucial to ensure the best performance results.

Every gallon of the liquid form of calcium lignosulfonate contains 5.3 lbs of water and 5.3 lbs of the additive. When using the liquid form, always base the amount of additive used on the dry component weight of the liquid. The additive can be used in clay bodies from 0.25 to 2% of the total dry weight of the clay body formula. In most throwing and handbuilding clay body formulas, an even lower amount can be used—as little as 1⁄16 to 1⁄8% will significantly improve the handling qualities of the clay. Additions of more than 5% will greatly increase green and dry strength, causing the clay to become extremely hard when dry—in some cases, the clay can even be dropped on the floor without breaking. Start by using 0.25%, then increase the amount of additive in 0.25% increments until the desired results are achieved.

Some ceramic suppliers have also begun using calcium lignosulfonate in their clay body formulas, making it even easier for potters to gain the benefits of the additive without having to worry about mix ratios. Ceramic Supply in Lodi, N.J., is just one such company that recently began using the additive in its moist HMI clay body formulas to increase customer satisfaction. “While the additive does add extra cost to the clay bodies, it dramatically increases the handling properties of the product,” said Larry Sussberg, the company’s owner.

Economic Considerations

Of course, calcium lignosulfonate is not the only additive available to potters. However, it is one of the most widely used, and can be considered representative of many other additives available in the industry. Each additive offers a particular benefit or range of benefits to potters—but they also add some expense. Clay body additives can range in price from $2.00 to $6.50/lb, depending on the type of additive and the amount needed to achieve the desired result. In most clay body formulas, the price per pound of additive will increase the price of the clay, as Sussberg noted, but this increase is generally in the range of .01 to .02 cents/lb and is not a significant expense in terms of the overall cost of the formula.

While the extra expense may cause some potters to shy away from using additives, the costs can often be easily recovered through fewer defects and less waste if the right additives are used. One pot or sculpture saved through using an additive can be enough for the additive to pay for itself.

Additives as Problem Solvers

If your production process is experiencing clay body defects, using an additive may be the perfect solution. While additives cannot correct all defects, they can be useful in a number of situations. If you are unsure of how and when to use an additive, contact a supplier and ask for advice. Describe the problem in simple and direct terms—an accurate description of what you are observing is a crucial factor in the recommendation of an additive.

If you want to experiment on your own and are reasonably sure that an additive is the appropriate fix for the problem, start with the lowest amount possible. If no change occurs, try increasing the amount of additive in small increments until you achieve the results you are looking for. If the additive does not produce the anticipated results, you may be using an incorrect amount or the wrong additive for your formula, and more testing may be required.

Today’s potters can choose from a variety of additives that are potent, reliable and consistent. Clay additives cannot make bad pots better or sculpture more beautiful, but they can give the potter a tool to solve specific production problems.

Acknowledgements

The author would like to thank Scott McBride at LignoTech USA for information and technical assistance over the years. His concern for accuracy and the time he spent providing information about calcium lignosulfonate are greatly appreciated.

For More Information

For more information about calcium lignosulfonate, contact LignoTech USA, Inc., 67 Depot St., Suite 203, Ellijay, GA 30540; (706) 635-5251. For more information about clay body formulas containing calcium lignosulfonate, contact Ceramic Supply of NY/NJ, 7 Route 46 West, Lodi, NJ 07644; (973) 340-3005.

*Lignosulfonate is sold commercially as Additive-A®, a registered trademark of LignoTech USA, a division of Borregaard LignoTech.