Ceramic Decorating / Columns

Ceramic Decorating: Reducing Heavy-Metal Leaching

July 20, 2000
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Most tableware manufacturers use unleaded glazes when producing tableware patterns, but design requirements occasionally mandate the use of decorative borosilicate enamels containing some heavy metals. When using such colors, tableware decorators must consider the ability of a heavy metal to leach—or migrate—from the tableware surface to food or beverages.

Decorators, independent testing laboratories and regulatory authorities follow highly accurate testing protocols to measure minute levels of leaching in parts-per-million (ppm). The Society of Glass and Ceramic Decorators (SGCD) advises tableware manufacturers to test ware for leaching when heavy-metal borosilicate enamels are used, since such tests enable companies to insure regulatory compliance and market their products with confidence.

Materials Selection

Many companies choose to minimize leaching concerns by simply using non-heavy-metal borosilicate enamels whenever possible. In addition, many drinkware decorators maintain policies against any use of heavy-metal borosilicate enamels in the lip and rim area, which includes the rim and the top 20 mm of the outside of a drinking vessel. They instead use either precious metals or unleaded enamels to achieve design goals.

But even when working with heavy-metal borosilicate enamels, glass and ceramic decorators can still take several steps to minimize leaching problems. Materials selection is critical, and decorators should work with color or decal suppliers to determine whether a color or decal might pose a leaching problem after it is applied to glass or ceramicware. While leaching from finished ware is the primary concern, efforts to minimize leaching begin with an understanding of an enamel’s properties when used in the direct decorating process or when used to print ceramic decals.

Decorators often ask color suppliers to conduct sample tests before a color is selected and delivered to the decorating plant. Wayne Zitkus, now plant manager for Syracuse China, Syracuse, N.Y., and formerly technical manager for Libbey Inc., Toledo, Ohio, reports that Libbey developed its own supplier requirements to insure that all color shipments conform to company standards. Libbey’s color procurement policy addresses leaching from a decorated glass surface with results measured in micrograms of release per square centimeter.

By establishing similar requirements, Zitkus notes that companies can greatly enhance their ability to consistently produce ware that will easily meet FDA, California and other standards. Zitkus adds, however, that Libbey as a rule does not use heavy-metal borosilicate enamels in the lip and rim area of glassware, and this enables the company to avoid lip and rim leaching problems altogether.

Surface Coverage

If a decorator chooses to use heavy-metal borosilicate enamels, the pattern and overall coverage of such enamels is critical. When leaching tests are conducted to measure the overall leaching from an entire surface, the size of the area decorated with heavy-metal borosilicate enamels is important.

For example, a dinner plate that is completely covered with such an enamel would likely generate higher leaching results than a similar plate where such colors constituted only a small portion of the overall pattern. Tableware designers should therefore consider enamel options before specifying color selections to minimize potential problems.

Firing Adjustments

Decorators can also adjust firing cycles to further reduce heavy metal leaching from foodware. As demonstrated in tests conducted by Libbey, even minor adjustments to firing temperature curves can significantly reduce leaching levels from ware decorated with heavy-metal borosilicate enamels.

Libbey made minor firing cycle adjustments to its first temperature curve by slowing the lehr belt and increasing the temperature by 56°C in the zone located prior to the firing zone. In the second adapted firing curve, sample shell tumblers remained in the firing zone for 13.9 minutes (23% longer than in the original curve).

Identical ware was fired under both conditions. By following the second curve, ware was produced with a 17.6% lower lead release value and a 7.1% lower cadmium release value than ware fired under the original firing conditions.

Zitkus notes that these simple time and temperature changes resulted in dramatic reductions in lead and cadmium release without reducing manufacturing productivity. He adds that “a company should experiment with lehr curves using its own firing equipment and the enamels that it uses to decorate glass or ceramicware” to find the most advantageous conditions for its applications.

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