<b>Discovery of a mineral film as a protective layer of sandstone</b>

Despite experiencing thousands of years of wind and rain, some stone carvings have remained remarkably well-preserved. Understanding the information recorded is of great significance for reconstructing the level of anthropogenic pollution in the environment in the past. Here, we observed a black to reddish-brown, relatively dense mineral film with a thickness of approximately 150 μm on the surface of sandstone.