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Don't preheat the oven; you want your plates to slowly warm along with the oven. Introducing room temperature plates to a very hot oven can make them crack or shatter completely. You can put your plates on a baking pan or directly on the oven racks. The plates will have warmed up to this temperature along with the oven. This allows the paint to "cure," or set into the ceramic. If your ceramic paint instructions suggest a different temperature or baking time, follow that instead. Once the oven temperature has reached 325F (163C), leave it on for 40 minutes. You can set a timer on your oven, phone, or a watch to keep track of the time. Most ovens will beep when the chosen temperature has been reached, so set a timer at that point. You want the temperature of your plates to slowly cool down along with the oven. If you handle the plates too early, you can break them. Cooling times depend on your oven but wait at least 2 hours before checking the temperature. Be patient and don't touch the plates while they're still cooling in the oven. Since your oven should be completely cooled, the plates should be cool to the touch as well. You can use oven mitts to remove your plates to be safe. Put your plates in a place where they won't be disturbed by children or pets. Let them rest for at least 3 days before you use them or wash them. Hand wash them for the first time. While washing them, you can check your painted designs and make sure the plates were not damaged while baking in the oven.
Place your plates in a cold oven. Let your oven reach 325F (163C) while the plates are inside. Leave your oven on for 40 minutes to let the plates cure. Turn off your oven to let your plates cool. Remove your plates from the oven once the oven is cooled. Allow the plates to rest for at least 3 days before using or washing them.