Black Crust-Induced Spalling of Marble: An Multi Analytical Study on the Danbi Stone Carvings

Black crust and spalling are common deterioration phenomena affecting marble relics, yet their correlation remains inadequately understood. Hyperspectral imaging, reflectance spectroscopy, portable X-ray Fluorescence (p-XRF), infrared thermography, Scanning Electron Microscopy coupled with Energy-Dispersive Spectroscopy (SEM-EDS), and microbiological analysis was employed to connect these two types of deterioration on the Danbi stone carving of the Confucian Temple in Beijing. Spectral and thermal analyses reveal that black crust significantly reduces reflectance and increase solar absorption by 27%, resulting in thermal stress. p-XRF and SEM-EDS analyses indicated that black crust is enriched in Fe, Ti, Zn, Pb, As and clay minerals, while spalling areas display increase Ca, reflecting substrate exposure. Microscopy reveals microcracks at the layer–substrate interface. Microbiological analyses identify Cladosporium anthropophilum and Alternaria alternata as contributors to surface-darkening. These multi-scale datasets collectively demonstrate that alterations in surface chemistry and bio-mediated darkening promoting the formation of black crusts, which subsequently induce marble spalling due to solar absorption and thermal stress. These findings clarify the coupled physical–chemical–biological pathways through which black crust accelerates stone spalling.