Why do you think the covers of ancient books in China are often blue?

The use of blue (specifically indigo) is related to the dyeing process level at that time. It has little to do with content expression, and the so-called "color symbol" inference is far-fetched. Think about the pattern of porcelain? ) Plant dyes are also the mainstream of ancient dyeing technology in China. General plant dyes are also used to dye cloth and paper, and the most common dyeing method is to use Polygonum blue to make navy blue or indigo. Because the glue used for book binding (that is, chemical synthetic glue) is modern. Thread-fitting appeared in the mid-Ming Dynasty, but earlier bag-fitting, butterfly-fitting and album-fitting (spinning and folding are all considered as their excesses) were the mainstream. What you see is linear, which is not only easier to preserve, but also related to the substantial increase in the number of books printed in the Ming and Qing Dynasties. Other common ones are: madder and safflower; Huang Huai, Turmeric, Gardenia and Cortex Phellodendri; Galla chinensis, hematoxylin, etc. The primary colors of China, composed of cyan, red, yellow, white and black, are called "five colors", which are the basis of Chinese traditional color system. "In the end, blue mining will not be profitable. -The Book of Songs Xiaoya Picked Blue and Picked Blue and Dyed Grass. -"Blue" mentioned in Shuowen refers to Liaolan. Thread-fitting didn't appear very early. The use of so-called "glue" (according to the understanding of paste medicine) is actually earlier, of course, not the modern kind. Modern glue and over-acidified paper make the life of modern books completely incomparable with that of ancient books. Before online packaging appeared, probably several kinds of adhesives were used. Scroll packaging: the main binding form in the 3- 10 century. Based on the paper book theory, from the existing Tibetan scriptures, some papers printed with lines are glued one by one; Cyclone suit: According to the understanding of Mr. Huang Yongnian's edition, it is very close to folding suit. Folding: Folding the long connected scrolls by a certain width, also called Brahma Book. Paste is needed for bonding.