A coin that has been treated in some way that changes its general appearance cannot be graded numerically and is eligible for Details Grading only. Most such surface alterations are performed through the application of chemicals, fillers or heat.
ARTIFICIAL TONING refers to the process whereby patina is imparted to a coin in an accelerated reaction process using chemicals and / or heat. Artificial toning may be removable with proper conservation by NCS but, since it often hides improper cleaning or other surface issues, the coin may yet require Details Grading.
Artificial toning can be extremely difficult to detect, and even harder to explain once discovered. Perhaps the best explanation is that it just does not appear natural.