Okay, please poke holes in my understanding and correct me. Toning of silver coins is most often because of sulfur. Sulfur and silver combine rather easily to form silver sulfide; that's the black we've used to seeing on silver. When a silver dip is used to remove the toning it removes the silver sulfide. And that's where the problem is. Note I said "it removes the silver sulfide", NOT "it removes the sulfide". So some of the silver at the surface of a coin is removed. And the removed silver comes from the microscopic flow lines formed when a coin is struck. It's those microscopic flow lines that gives a coin with original surfaces its hourglass reflectivity. That's why a coin dulls down after being dipped. The pros that have practiced a lot use a quick dip in a diluted solution to clean their coins. Then a total bath in distilled water to stop the cleaning process. Many times that was undetectable by the TPGs (or ignored in the case of many 19th century coins since a high percentage have an old cleaning that has retoned naturally). But with the new PCGS system I'm not sure that a light dip is undetectable.