By filling the cast iron pot with soapy water and warming the pot on the stove, the metal of the pot should expand enough to allow the plate to be removed, without harming either piece.
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Placing it in the refrigerator for an hour and then heating the pot may help it release the plate. After warming the pot, you should be able to place the pot in you sink upside down and the plate should fall out. The solution is that metal will expand when heated. The plate will not expand as much as the metal in the pot will. THis will allow the plate to be removed.
Yes I would recommend keeping the soapy water in the pot. Actually bringing the water to a boil may help release the grip. Just be very careful when turning the hot pot over in the sink. The steam can burn you. .
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