Somehow I’ve gotten on a kick of watching art conservation videos on YouTube. They’re fascinating.
This one used a laser so I figured it was my duty to report on it:
I hope, and am fairly confident, that the laser was not taking off what appeared to be a layer of the surface ! To my eyes, there appeared to be a definite difference in the height of the surface, the clean layer seeming to be about 0.5mm lower than the dirt !
John 
Only half a millimeter of dirt? Must have been somewhere really clean 