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Glazes are continuous coatings that are usually prepared from fused silicate mixtures and fusion-bonded to a ceramic substrate to render the finished product impermeable to liquids and gases, create aesthetics, provide a protective coating, and increase the product's strength. However, for many producers of functional low-fire ceramic and Raku products, glazes do not perform as they should, and instead exhibit crazing (a network of fine, cracked lines in the fired glaze).
Common remedies for crazing focus on improving the compatibility and interaction between the body and glaze through material changes, such as adding silica or low expansion frits, or using a higher expansion ceramic body. Other remedies involve changing the oxide levels of the body or glaze by adding A12O3, adding or substituting low-expansion boron, substituting low-expansion CaO or SrO for high expansion Na2O and K2O, or introducing low-expansion Li2O or MgO. In many cases, these remedies create body-glaze combinations that survive the initial post-firing contraction of the ceramic body in the kiln. However, the long-term test of these solutions is whether the products survive multiple heating/cooling cycles, repeated water immersion and ambient moisture absorption. Water entering the ceramic body from any of these sources over time will cause the body to swell, which stresses the glaze and causes crazing. Often, crazing leads ultimately to failure of the ceramic body, and the commercial value of the piece is reduced or even lost.
Because the water absorption occurs though the unglazed, porous areas on ceramic bodies after firing, improving only the body-glaze compatibility and interaction will not prevent long-term crazing. Instead, the problem must be addressed at its source-by sealing the unglazed feet of the ware.
Recently, a new coating* was developed specifically for this purpose. With a minimal investment in time and materials, pottery producers can significantly increase the durability and value of their finished ware.
*Cerama-Seal TM, manufactured by Elias Ceramics, Salem, Ore.
A Crazing ProblemElias Ceramics, located in Salem, Ore., began producing porcelain Christmas ornaments in 1994 and soon expanded into other product lines, including dog and cat bowls, mugs, saucers and canisters. However, the cost of the materials and energy required to produce porcelain products was extreme, and the company quickly discovered that it could not compete with lower-cost imports. By switching to a low-fire ceramic body, Elias Ceramics was able to cut its costs by 75%, which enabled the small company to become profitable. However, there was a catch.
"Soon after we started selling the low-fire products, our customers began coming to us with complaints about crazing," explains Debessai Elias, the company's owner. "The products were fine when they left our facilities, but they began crazing and cracking as soon as they were put into use. We asked different experts in the business and other ceramic producers, but no one was able to offer a solution."
Elias Ceramics spent the next six years performing extensive research, testing different materials to try to correct the problem. Then, in 2003, the company experienced a breakthrough.
"We developed a coating that, when applied to the foot of functional pottery in the greenware stage and fired at cone 03 or higher, would vitrify with the ceramic body and prevent water penetration. Because the coating seals the ware, it prevents crazing and cracking," says Elias.
Two layers of the coating are applied to the foot ring of the greenware and 1/4-in. beyond using a brush or dip technique. (Flat-bottomed ware, such as mugs or plates, requires only one layer of coating if the flat area will be glazed at the bisque stage, while products that are bisque fired at cone 04 require three layers.) The greenware is then placed upside down in the kiln to prevent the coating from coming into contact with the kiln shelf and is fired in the range of cone 03 to cone 4. The coated bisque can be glazed as usual through dipping or brushing, and any glaze that adheres to the foot ring is simply removed with a wet sponge. (Glaze on the 1/4 in. of coating beyond the foot ring is left intact to provide for a good seal transition between the glaze and coating.) Glazed items are then placed with the foot ring directly on the kiln shelf for the glaze firing at the normal temperature.
The resulting ware is completely sealed and can be used for any functional purpose as long as a food-safe glaze has been used. Coated test pieces have been boiled and pressure-cooked without any adverse effects. They have also been subjected to multiple freeze-thaw cycles, and the results have indicated that the coated ware is durable enough for use in year-round outdoor applications, even in regions with cold winters.
"This coating has enabled us to eliminate our crazing and cracking problems while increasing the durability of our ware," says Elias. "Now we can compete with porcelain producers while maintaining the cost benefits of using low-fire ceramic bodies. We believe this product has benefits in any application where companies are having problems with cracking or crazing due to moisture penetration."
What About Stilts?Glazed ceramic bodies are often placed on metal pegs, or stilts, in an attempt to minimize the unglazed, porous areas that occur in the body during firing. However, stilts corrode or oxidize over time during cyclical kiln firings, then eventually bend or break. This can cause not only the physical loss of the glazed bodies in the kiln charge, but also catastrophic kiln damage if the ware falls onto and/or fuses into the kiln's refractory walls.
Stilts also leave porous pinholes in the fired ceramic body that become entry points for moisture from ambient air and/or cyclical water immersions during use. Some potters cover these pinholes with commercial polymeric coatings to stop water entry, but the coatings quickly wear off the ceramic body during everyday use and/or washing.
The new ceramic coating eliminates the need for stilts on footed products. According to Arber Russell, co-owner of Knots 'n Pots, a wholesale/retail supplier of greenware, bisque and ceramic supplies in Spokane, Wash., this benefit could save a significant amount of damaged ware.
"One of the pieces that we fire quite frequently has always given us problems with splitting and cracking because it had to be bisque-fired on stilts. I put the new coating on the product, fired it without the stilts, and it came out of both the bisque and glaze firing without any breaking, cracking or crazing. I think this could potentially save us a lot of problems," she says.
On flat-bottomed ware that requires stilts during the glaze firing, the stilts will leave little or no marks on the finished product and will not penetrate the ceramic seal.
Functional RakuFor many potters and artists, the term "functional Raku" is an oxymoron. The glazing and firing process that gives Raku its unique aesthetics often causes crazing, relegating the finished pieces to a decorative status.
Eunice Yates, owner of Eunice Ceramics & Porcelain, a paint-your-own ceramic studio in Molalla, Ore., believes that the new coating will change that. "A lot of our students make Raku vases that they would like to use as functional pieces, but we experience a lot of cracking and crazing," she says. "We glazed the inside of a Raku vase with the new coating and left water in the finished vase for a week-and it didn't leak. Ordinarily, we would have had to use a liner of some kind. This will make all the difference in the world with the Raku finish-it's a significant breakthrough."
Increasing SuccessIn today's competitive economic environment, pottery producers and bisque suppliers must continually increase their quality and yields while reducing their production costs. Innovative technologies like the new ceramic coating are making it easier to achieve these goals.
For more information about the new ceramic coating, contact Valley View Ceramic Supply, 15528 S.E. Anderegg Pkwy., Clackamas, OR 97015; (866) 832-5544 or (503) 658-2453; fax (503) 658-8027; or e-mail email@example.com .
Elias Ceramics can be reached at 2306 D Pringle Rd., S.E., Salem, OR 97302; (888) 999-0240 or (503) 378-7520; fax (503) 587-9400; e-mail firstname.lastname@example.org ; or visit http://www.eliasceramics.com .
SIDEBAR: A Simple Crazing Solution1. Apply two layers of the coating to the foot ring at the greenware stage, letting the coating dry between layers. Extend the coating up to 1/4 in. beyond the foot ring.
2. Flat-bottomed greenware, such as mugs and plates, should have only one layer of coating if the flat area is to be glazed at the bisque stage.
3. Place the greenware upside down in the kiln. Do not let the sealed area touch the kiln shelf to avoid contaminating the coating with the kiln wash.
4, 5 and 6. Glaze the bisque as usual through dipping or brushing.
7 and 8. Remove any glaze that adheres to the foot ring with a wet sponge. Glaze on the 1/4 inch beyond the foot ring should be left intact to create a good seal transition between the coating and glaze.
9. Place the glazed items with the foot ring down on the shelf. The coating has vitrified the area like porcelain. Flat-bottomed ware can be glazed and then stilted if desired. Because the coating has vitrified the area, few or no marks will be left on the finished product.
10 and 11. Finished products with the coating.
12. Greenware that is more than 1/3 in. thick should be fired at one cone higher than normal to ensure vitrification.