The glaze is gone from my porcelain crowns and they have dulled. How can I restore them to white?

The glaze doesn't make your crowns whiter, just a more shiny or polished look. Many times the underlying porcelain can be polished to give a "glaze-like" appearance without using glaze at all. Labs can do this, sometimes dentists can do this on veneers or crowns already cemented/bonded into place.The reason whitening toothpaste is ill advised, is because they are abrasive. Imagine using Comet on your porcelain sink. Over the years it wouldn't be shiny any more. Same with using abrasive toothpastes.I am not at all certain that Supersmile toothpaste will whiten your crowns at all. They will remove any surface staining, but I doubt if it will actually make your porcelain whiter.