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But even commercial glazes aren’t completely problem-free. According to Tami Archer, vice president of Mid-South Ceramic Supply Co. in Nashville, Tenn., knowing a few glaze basics and working closely with the glaze supplier can help ensure a successful glazing experience with commercial glazes.
Common Glazing ProblemsSpecific Gravity. Perhaps the most common misconception when using commercial glazes is that all commercial glazes are alike. “To check the specific gravity or water-to-glaze ratio of their glazes, many potters will simply stick their hand in a bucket of glaze and assume that if it pulls away from their fingernails it will work great on their ware,” Archer says. “However, this is not the case with all glazes. It’s best to check with the supplier to determine how much water is required to achieve the optimum specific gravity for a particular glaze. With Mid-South Ceramic’s Opulence™ glazes, for instance, the optimum specific gravity of 1.50 can be obtained by using around 0.9 parts by weight of water to 1 part by weight of glaze. Other glazes may require a higher or lower water-to-glaze ratio.”
If the specific gravity of the glaze is too high, the glaze will be too thick and will not paint, pour or dip easily. Additionally, glaze usage will be higher than necessary, and deep pinholes may form during firing.
If the specific gravity is too low, the glaze will be too thin. The solid and liquid portions of the glaze will separate easily, and the glaze will require frequent re-mixing. More coats of glaze than usual will be needed to obtain normal thickness, requiring extra time and effort to glaze the piece.
Specific gravity can be determined by weighing 100 cc of glaze in a graduated cylinder. For example, if 100 cc of glaze weighs 154 grams, its specific gravity is 1.54.
Firing Temperature. Making sure that both the bisque and glaze are fired hot enough is another common challenge. Many people think that if they’re using a programmable kiln, the temperature readout is telling them all they need to know. However, “it is always advisable to use pyrometric cones to ensure that your kiln is at the right temperature and is uniform throughout,” Archer says.
An underfired glossy glaze will lack gloss, look dull or appear semi-matte, while an underfired matte glaze will lack its characteristic satin sheen. Worse, underfired glazes on pottery meant for use with food will be unsanitary, as bits of food particles can become trapped inside the pores. The best way to ensure that a pot is properly fired is through an “ink test”—i.e., a drop of ink placed on the ware should be easily wiped off with a damp rag. Ink that is difficult or impossible to remove generally indicates an underfired glaze.
Sometimes it can be difficult to determine whether underfiring is a result of kiln temperature or the glaze formulation. “If you are familiar with a commercial glaze and have had good results with it in the past, an underfired look is almost certainly due to a kiln problem,” Archer says. “If you are unfamiliar with a commercial glaze, cones should be used to ensure that the pottery is fired to the cone called for by the glaze supplier.”
Bisque Condition. Another common problem is wiping the bisque too soon to remove dust and loose bisque particles before glazing. If the bisque isn’t wiped just prior to glazing, the glaze may crawl or develop deep pinholes as a result of dust that may have settled on the bisque during storage.
Glaze Fit. Commercial glazes are formulated to fit a wide variety of clay bodies, but they won’t fit every clay body. A poor glaze fit can lead to unintentional crazing, which resembles a spider web of cracks.
“When this happens, the first thing the potter should do is fire the glaze to a half cone or full cone higher. If the crazing still occurs, then the mismatch between the glaze and the body is severe, and the potter must switch to another body or to another brand of glaze,” Archer says.
Application Method. With some commercial glazes, the glaze viscosity (thickness) may need to be chemically adjusted for the specific application method—whether brushing, dipping or pouring. Other glazes, however, are available that can be used off-the-shelf with virtually any application method.
Shelf Life. A glaze that has outlived its shelf life may become very thick and viscous due to the gelling of soluble frit and raw materials, or the glaze may begin to discolor and produce a foul odor due to bacteria formation. Some commercial glazes are made with very insoluble frits and raw materials to prolong their shelf life. It’s best to check with the glaze supplier to determine the recommended shelf life for a given glaze. No glaze has an infinite shelf life.
Glazes and DesignDesign effects and colors are two ways in which professional potters can differentiate their products from others on the market. Different glazing effects can be achieved with “overlapping,” in which a solid color and a reactive glaze are overlapped on the piece. Often this results in a look where the glaze appears to be mottled or “running.” Other glaze effects can be achieved by mixing two or more commercial colors together thoroughly to create a new, unique color, or by mixing two or more commercial colors together incompletely—for example, mixing white with a little green and black stirred in slightly can create a marble effect.
Glaze color is also important—in fact, color is the most important aspect of any product for the home. “If a product is the wrong color for the customer, he or she will not buy it, no matter how good it may be in other ways,” says Archer. “Professional ceramists who want to sell their products need to be aware of color fashion for the home.”
One way to stay on top of color trends is through information from organizations such as The Color Marketing Group (CMG) of Alexandria, Va., which predicts color fashion trends. Many home furnishing suppliers such as fabric makers, paint suppliers and wallpaper manufacturers design their products to follow these trends. This year’s colors, for instance, are moving away from the neon purples and greens and are now back to some of the earthy “pottery” colors. Some commercial glaze colors* are designed to closely match or coordinate with current CMG colors for the interior home, ensuring that glazed pieces will be popular in today’s market.
Achieving Quality ResultsA glaze can often make or break the final product. While using commercial glazes can eliminate a lot of the guesswork from the glazing process, potters should still be aware of certain variables. The specific gravity of the glaze, the firing temperature required to achieve the desired color or effect, the bisque condition, the interaction between the glaze and the ceramic body, and the desired application method can all affect which type of glaze you choose and how it appears on the final product.
However, unlike homemade glazes, commercial glazes provide the flexibility to quickly and easily change the production process without affecting the quality of the final product. Any potter who wants to adjust his or her firing temperature or change the glaze effect or color of a piece can easily find a commercial glaze that meets their new requirements. And if problems do occur, the glaze supplier is only a phone call away.
For More InformationFor more information about glazes, contact Mid-South Ceramic Supply Co., 1230 4th Ave., N., Nashville, TN 37208; (615) 242-0300; fax (615) 244-3191; e-mail firstname.lastname@example.org; or visit www.opulenceglaze.com.
*Opulence™ glaze colors from Mid-South Ceramic Supply Co. are designed to intentionally match or nearly match CMG colors for the interior home.
Commercial Glazes and Beyond: The Work Of Susan DeMaySusan DeMay, with the help of several assistants, operates a production pottery business in Smithville, Tenn. She also makes time to do unique art pieces and teach three college classes a year at Vanderbilt University in Nashville.
DeMay uses three forming techniques to make her pieces: throwing, slab-rolling and slip casting. A stoneware clay and a stoneware slip are each mixed at her studio.
But it’s the glazing that makes DeMay’s pieces unique. She uses nearly 30 glazes regularly, and periodically employs another half dozen or so. A combination of commercial and homemade glazes can be found in DeMay’s studio, but she’ll use commercial glazes when she can. “The range and intensity of color are very important to us, as well as a compatibility with other glazes,” DeMay says. “We like to use commercial glazes because they come premixed and ready to use, and we know that the colors will be consistent. With homemade glazes, you don’t always have that guarantee.”
DeMay’s patterns are inspired by nature, and her ideas frequently come from personal experience and the artist’s own point of view. She strives to use a rich and full color palette, as well as color balance and harmony. The different surfaces of glazes add an additional interest to the selection—satin mattes; stony mattes; and smooth, high-gloss glazes, along with their respective transparencies and opacities. Fluidity also plays an important role—runniness is part of the image’s character on vertical or sloped surfaces, and one glaze is often swirled and bled into another on horizontal planes.
The overlap between two and sometimes three glazes also creates new and exciting effects. DeMay has explored just about every possible overlap, and exploited her favorites. After nearly two decades of working and reworking her selection, DeMay has committed to memory the immense repertoire of glaze characteristics and qualities.
DeMay’s art pottery is both regionally and nationally renowned. It can be seen in the nation’s capitol and in numerous museum shops in the southeast.