i just went to the dentist today to put on 6 crowns on front teeth. My dentist used temporary cement and said he can send them back the crowns for all types of adjustments, including making them a shade or 2 whiter if i desire. I am thinking about doing this, but it doesn't make sense to me that a crown can go to a lab and come back whiter, how do the labs accomplish this on something that is supposed to be color stable?