You have to remember that porcelain is stacked in many layers to acheive the final shade. Now after you've brushed the porcelain thousands of times, and basically etched the porcelain with countless acid assaults via the normal simple sugar fermentation process and from acidic beverage consumption over the years, you'll see some loss of the outer layers of the porcelain and a shift in the color.
Think of it this way, when you adjust the porcelain as your seating a crown, you'll see a change in the color most of the time even if you've just barely removed any glaze. Remember to get a crown that's say A-2 in color, it's not just place opaque porcelain over the coping and then add A-2 upto full contour, your lab tech is building that final A-2 shade with many layers of various shaded porcelains. If you ever get the chance to watch a lab tech either custom stain/tint a crown chairside or watch the ceramist build the crown from opaquer up in the lab DO IT, you'll really get a true appreciation not just for what the lab tech/ceramist does, but also how you as a clinicain can achieve better results, not just for indirect ceramic restorations, but also for your direct composite restorations.
What you'll see, especially with female patients (not a sexist thing but women as a whole are way more esthetically conscious than men with repect to their teeth) is that 10 or so years after placement, women will start inquiring about replacing their crowns (especially premolars forward) due to the color shifting of the porcelain and/or their natural enamel. Men on the other hand tend to be more of the "if it ain't broke don't fix it" mantra 😀 😀
And BTW Bill, I'm on my way out the door in about 10 minutes to goto Foxboro to watch Peyton becoming "Poutin" again! 😀 😀
