How to remove a porcelain crown that was put on with temporary cement as a try in but is stuck and won't come off?

My dentist put my front six porcelain crowns in with temporary cement so I could try them out. When I went to have them permanently bonded four of them came off easily but two are stuck. Dentist tried many techniques and nothing worked. Now they are scratched and need to be glazed but he wants me to wait and see if they will come off. It has been two months and they are still stuck. What can I do to get them to come off?

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