Why are there silver dots on calligraphy paper in Ming and Qing Dynasties?

Mercury point of rice paper. According to the information about the production technology of Xuan paper, we can know that the calligraphy paper used in Ming and Qing Dynasties was Xuan paper, and the water and glue used in the production process of Xuan paper contained trace mercury, which made the paper surface appear silver spots. Xuan paper, a specialty of Jingxian County, Xuancheng City, Anhui Province, is a national geographical indication product. Jingxian county is located in the southern edge of mid-latitude, classified by meteorological indicators, and belongs to the humid climate of north subtropical and subtropical monsoon.