The key is finding the same spot before and after exposure, or alternatively making sufficient surface measurements to obtain a statistically valid sample of changes to the artifact. Attached is a bibliography of several microscopic applications, not all of them successful.

Thank you for your help; i already done the technique you said in others experiments, like for instance the exposure to salt mist , but in this case it is a matter of wath can i use to clean the surface because it is very dirty by the diesel and gasoline combustion PM. Wath kind of chemical liquid can i use to clean this material from the surface of the stones?