Hard paste porcelain is also known as true porcelain. Out of all the wide varieties of ceramics, true porcelain is the hardest, the most durable—which is one of the reasons porcelain insulators largely replaced glass insulators, beginning in the nineteenth century. True porcelain was first created in China, and to this day that country is the source of most superior porcelain products, such as electrical porcelain insulators. In fact, the world’s largest manufacturer of electrical porcelain insulators is located in Dalian, China. Probably during the Tang Dynasty (618-907) hard paste porcelain was created from the ceramic known as stoneware. Stoneware is material made when clay is fired at high heat—1200 degrees C to 1315 degrees C (2192 F to 2399 F.) Stoneware is non-porous, impervious to liquids even when not glazed. Sometime during the Tang Dynasty, the Chinese mixed kaolin—a pure white clay—with petuntse, a type of feldspar found only in China, and began to make porcelain products. Since kaolin is extremely resistant to high temperatures, they could fire the mixture of kaolin and petuntse at heats between 1250 degrees C to 1450 degrees C (2280 F to 2640F.) At those extreme temperatures, the petuntse melts into a non-porous natural glass and fuses with the kaolin. The body and glaze become one solid material. When you break apart a piece of true porcelain products, it’s impossible to differentiate between the two materials used. In addition to its amazing hardness and strength, true porcelain is well known for resisting electrical conductivity, which makes it the ideal material with which to manufacture insulators and transformer bushings. Porcelain insulators were first used in the 1850’s for telegraph insulators. Although at that time they were not in common use. Glass insulators were still preferred. For one reason, they were cheaper than porcelain insulators. In addition, people thought the clear glass insulators would discourage insects from building nests in or around them. However, the need for electrical power distribution in the 1880’s demanded insulators that were much larger and stronger than the glass insulators currently in use. Insulators had to be manufactured that could resist conductivity for lines carrying tens of thousands of volts. As the demand for porcelain insulators rose, some companies produced them using dry press porcelain. Dry press is easier to manufacture than wet press porcelain. It’s made by pressing mostly dry, granular porcelain into molds. However this method of producing porcelain insulators leaves tiny cracks and holes in the clay, rendering such insulators risky for distributing high voltages of electricity. In the late Nineteenth Century, manufacturers increasingly turned to wet press porcelain for use in electrical porcelain insulators. Wet press porcelain is made by mixing it while wet, making sure to remove all air from the clay before shaping it in molds and firing. It lacks the tiny holes found in dry press porcelain and is therefore the best material to use in porcelain insulators. This was dramatically demonstrated in 1896 when electrical porcelain insulators were chosen for use on the first line to distribute power from Niagara Falls. Various companies submitted samples of their electrical porcelain insulators in a competition to win the contract to supply porcelain insulators for the Niagara-Buffalo transmission line. All the insulators submitted were made from dry process porcelain, except one—the electrical porcelain insulator made by the Imperial Porcelain company. All the insulators were tested in brine at 40,000 volts, and only the one made by Imperial—the wet process electrical porcelain insulator—passed the test.
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A brief overview of the history of the manufacture of porcelain, and why it is the best material to use in insulators.
Mark Meister, Meister International, has more than 29 years of experience in the Electric Power Industry.
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