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Unfortunately, the tooth whitening products that are available and that we as dentists use in the mouth to lighten the color of teeth only work on natural tooth enamel.
Porcelain crowns will not react with the active ingredient in the whitening products. The porcelain in your caps is essentially colored glass, has no pores and is sealed on the outside with an impervious glaze. Some superficial staining can occur on porcelain caps but this easily comes off when your dentist cleans your teeth.
The only way to change the color of your caps is to have them replaced with new, lighter colored ones.
I hope you find this information helpful. If you have any concerns regarding this issue that I have not addressed, please let me know and I will get back to you with more information.
George McKee DDS