This paper investigates the origin, fluxes, and transformation of carbon compounds within black crusts on the stone walls of the Tower of London. The crusts were analyzed for elemental and organic carbon, including the water soluble fraction. Elemental carbon and low solubility compounds such as oxalates appeared to be conserved because of long residence times. Conversely, more soluble ions, like chloride and formate would be removed from the layers relatively quickly by rainfall. At higher organic carbon concentrations acetic acid may be produced within the crusts from biological transformations. Currently, traffic sources contribute to increasingly organic rich crusts. The deposition of elemental carbon to buildings darkens surfaces and has important aesthetic implications. The increased organic content may have further aesthetic consequence by changing the color of buildings to warmer tones, particularly browns and yellows. Management of historic buildings requires us to recognize the shift away from simple gypsum crusts to those richer in organic materials.