A pair of hand carved sandstone lions produced in Qin Dynasty lie in the front of the government site of Chongqing city covered by a layer of black crust with missing parts, and cocked or curly edges. To explore the structural and compositional properties of the black crusts and reveal its influence on the surface of stone sculpture, scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray fluorescence spectrometer (XRF), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectrum (XPS) were applied to analyze the samples of black crust and stone base. The conclusion indicated that the black crust was originated from the weathering of prior protective coating including three chief layers: the undercoat, the intermediate coat (mixed by CaSO4 and ZnS) and topcoat observed by elemental Mapping. Among them, the black crust was chiefly attributed to a discolored process from white basic lead carbonate ［(2PbCO3·Pb(OH)2)］ to black galena (PbS), and the carbonization of organic material in the protective coating. The strong —OH peak identified by both FTIR and XPS analysis on the black crust demonstrated an increasing tendency of —OH in the carbonization of the protective coating which made it more hydrophilic and easy to swell when it is damp and shrink when it is dry, so as to cause a large area of peeling, cocking and curling. Most importantly, water would be detained in those spaces between black crusts and the surface of sandstone which could accumulate the corrosion resulting in some damages on the carved sculpture. For example, a high concentration of anhydrite (CaSO4) was detected by SEM/EDAX and XRD in the back of black crust and the surface of sandstone, which can be transformed into gypsum by hydration with volume expansion causing the pulverization of sandstone. Therefore, when encountered with the aged protective coating on the outdoor stone cultural relics, it is important to remove them timely avoiding the further corrosion in the micro spaces between black crusts and the below sandstone.