Discusses the role of oil-fired carbonaceous particles in gypsum formation on urban limestone. Describes: the calcite and gypsum crust which grows on limestone as sulphation proceeds; gypsum crusts found on sandstone, silicatic stone and bronze surfaces. Hypothesizes that carbonaceous particles catalyse the sulphation process and thus stone decay. Considers that the course of stone decay in venice supports this view. Describes a mathematical model simulating the impact of atmospheric factors on this mechanism. Includes optical microscopy photograph of crust and a micrograph of an oil-fired carbonaceous particle. References.