Stone monuments are detrimentally affected by acid rain and pollutants.  Organic particulates, soot, algae, and fungal species all attach themselves to the stone's surface in one fashion or another and begin to chemically transform the surface.  Rain then washes away loose grains of stone; in undercut areas where water cannot easily reach, the agents of deterioration build up and hide the carved surfaces.  Additionally, adjacent materials such as bronze or iron can leave runoff stains deep in the interstices of the stone. Cleaning, through chemical and/or mechanical means, restores the stone to its former state and allows intricate details to reemerge.
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