Ceramic ware is considered "Vitrified" if it will absorb less than 3 percent of its own weight when boiled in water for five hours and is soaked for an additional 19 hours.
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Porcelain is made with kaolin clay. Kaolin is a white clay that retains it white color when fired. It is fired at temperatures in excess of 1300 C and is more fully vitrified than stoneware. It is usually covered with a clear glaze, which allows the white body to show. The fired color tends to be more of a "cool" white as opposed to China ware that is usually a warmer white color. Porcelain becomes vitrified during the second firing of a two fire process. The second firing of porcelain typically is hotter than that used for China ware. This finish tends to be harder, but more brittle. This higher firing, hard surface can make decorating more difficult than stoneware or China. The composition of the porcelain clays is usually more malleable than the clays used for China ware. This allows for forming of more intricate and detailed shapes.
Stoneware differs from Porcelain not only in color shade, but also in the way it is fired. Where Porcelain becomes vitrified during the second firing (second firing is hotter than first firing), Stoneware is vitrified during a single firing. Generally, the temperature of this firing of Stoneware is a little less than that used for Porcelain. The resulting finish is warmer in color than Porcelain. This finish may make Stoneware slightly easier to decorate than Porcelain. Stoneware can be decorated using Under-Glaze, On-Glaze, or In-Glaze techniques. Under-Glaze decorating is most commonly the preferred method of decoration used for Stoneware.
Bone China is made using a translucent white ceramic clay containing at least 25% bone ash. In England, the percentage of bone ash must be at least 50% for a piece to be considered Bone China. Bone China tends to be slightly translucent in nature and is often used in thin walled pieces, exibiting a delicate, refined look.
Under-Glaze decorating refers to a process where the color is applied directly to the bisque ware or is some cases, greenware. After the color is applied, the ware will either be fired to burn out unwanted volatile materials and then glazed and refired, or may be glazed and fired with no intermediate "Sintering" fire. This process of Under-glazing is used primarily on China, not Porcelain. The resultant decoration is protected from abrasion and wear under the clear glaze finish.
On-Glaze decorating, as it implies, refers to a process whereby color is applied over the finished, fired, glazed surface of a ceramic object. The piece is then fired, but at a low enough temperature to melt and fuse the decorating enamels to the outer surface of the glaze. This "low" firing temperature allows for the use of brighter decorating colors, including metallics such as Gold and Platinum. These metallics and brighter colors cannot be achieved at higher firing temperatures as the pigments will not withstand the greater heat. The disadvantage of this process is the decoration is exposed and subject to abrasion, attack by detergents, and may be more susceptible to leaching of potentially toxic materials (if present) in the decorating enamels.
The California state government in 1986 passed legislation that is intended to warn consumers in the state of the possibility of exposure to toxic chemicals. Officially known as the Safe Drinking Water and Toxic Enforcement Act of 1986, it is better known by its original name of Proposition 65.
Proposition 65 requires the State to publish a list of chemicals known to cause cancer, birth defects, or other reproductive harm. Currently, there are around 750 chemicals listed, with lead and cadmium being the chemicals of concern to decorators.
Proposition 65 requires businesses to notify Californians about significant amounts of these listed chemicals in the products they purchase, in their homes or workplaces, or that are released into the environment. Proposition 65 specifically lays out the required testing method, limits for lead and cadmium leaching, and the warning requirements for articles that exceed the limits.
The test method specified for leaching lead and cadmium from ceramic dinnerware, is ASTM C 738 (AOAC 973.32). Limits for warning purposes are:
Flatware - 0.226 ppm Pb / 3.164 ppm Cd
Small Hollow ware - 0.100 ppm Pb / 0.322 ppm Cd
Large Hollow ware - 0.100 ppm Pb / 0.084 ppm Cd
Cups and Mugs - 0.100 ppm Pb
Ceramic ware does not need warnings if the leaching test results are below the specified levels shown above. In addition, Federal limits still apply to ceramic dinnerware regarding allowable lead and cadmium leaching.
The information provided here is a very brief overview of Proposition 65, and is not meant to answer all questions regarding this law. Any decorators whose products may end up in California (even if not originally sold there), need to be aware of this law, and its ramifications for them, to avoid potential costly problems.
To review the warning requirements, or to view the regulation in its entirety, please refer to the California Office of Environmental Health Hazard Assessment web site at www.oehha.ca.gov