Paintings are cleaned by
carefully removing disfiguring
material from the original paint.
soot and smoke, grime, dirt, mud, liquids, foodstuff, markings, graffiti, aged varnish,
mismatched overpaint, water damaged varnish.
Most discoloring material can be delicately and safely lifted from the surface, with little or no effect on the original paint.
more on transforming effects of cleaning.