Damage or protection? The role of smoked crust on sandstones from Yungang Grottoes

Abstract Black crusts, such as dust deposit and smoked smudge, are the main pollutants in Yungang Grottoes, the world famous Buddhist heritage site. It is always considered that the black crust should be removed during the conservation process of stones. In this paper, the investigation of the role of smoked crust on the Yungang sandstones was performed to verify if the smoked layer on the stone surface would be harmful or protective. The results of SO 2 corrosion experiments with and without water both show that the samples covered with smoked layer can play a protective role on the sandstones, which means that the smoked layer on the stones can prevent the attack of SO 2 and water, the major deterioration agents in Yungang area. The multi-factor deterioration cycle experiments, including salt crystallization, freeze–thaw damage, wet–dry cycle and acid corrosion, also indicate that the smoked samples are less deteriorated and salt crystallization is the most severe damage to the sandstones.