Porcelain's glassy finish comes from the firing process in which it is baked at high temperatures. Whether a porcelain plate is safe to use in a microwave depends on its decoration.
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During its creation, porcelain is heated to 2,650 degrees Fahrenheit, much higher than any temperature a microwave oven is going to reach. As a result, porcelain itself can easily resist the heat of a microwave without being damaged. Most porcelain dishes carry a microwave-safe label from their manufacturers.
Although porcelain itself is perfectly safe in a microwave, not all porcelain plates are safe to use in one. Some porcelain plates have additional decoration that might be damaged by being microwaved. If a porcelain plate has metallic decoration of any kind, you should not use it in a microwave. Gold or silver decorations will be damaged by the microwave and the metal can cause sparks that may lead to a fire.