How to identify ancient copybooks?

Shangyu Post is a letter from Wang Xizhi, a calligrapher in the Eastern Jin Dynasty, and the original has been lost. Only the handed down book has been treasured by people, with the collection seal of Evonne (Song Huizong) in the Song Dynasty and the collection seal of Wang Fu in the Ming Dynasty. It is generally believed that this copy is a copy of the Tang Dynasty, but there are different opinions, because Shangyu's post was engraved in the Song Dynasty, and the font of the block copy is inconsistent with that of the copy. Therefore, the identification of replicas has been pending and controversial.

In the past, through the comparative analysis of calligraphy styles, especially comparing Shangyu Tie with other paintings and calligraphy collected by Evonne, cultural relics workers found that the seal and stamp format of Evonne's collection were the same. Because this seal is clear and there is no special difficulty in identification, there is no doubt that this replica existed in Song Huizong before. However, the exact date of the copy has not yet been determined. You know, it took seven or eight hundred years from the Eastern Jin Dynasty to Song Huizong!

What do we do? The cultural relics staff found that there was a seal on the replica, but it was completely unrecognizable, so that it overlapped with another small seal in its position. It seems that to find out the date of Shangyu Post Station, the almost invisible Athena Chu is promising.

In order to find this seal, cultural relics workers used soft X-ray photography, so that an "internal contract seal" on the lower left of Shangyu Post was clearly displayed. "Neiyue Seal" is a collection seal of the late Tang Dynasty (Li Yu). In the existing calligraphy and painting in China, its true face has never been seen, and the printed text is only scattered in relevant records. Now, we have found it in Shangyu Post, which not only makes us see the true meaning of the seal collected by the royal family in the Southern Tang Dynasty for the first time, but also provides important clues for the identification of Shangyu Post. It points out to people that the manuscripts were hidden in the court of the Southern Tang Dynasty, much earlier than the Northern Song Dynasty, so Shangyu Post was basically a manuscript of the Tang Dynasty.

Soft X-ray technology is a new technology developed in 1980s. The wavelength of the so-called soft X-ray is longer than the usual X-ray, which is about 0.6×10-10 ~ 0.9×10 meter. The shorter the wavelength of X-ray, the stronger the penetrating power. Soft X-rays have a long wavelength and cannot penetrate heavy metals, but easily penetrate nonmetals, light metals, animals and plants and human soft tissues with low atomic numbers. Although the "internal contract seal" on Shangyu's post has been blurred for a long time, there will always be some inkpad (mercury oxide) infiltrating into the paper layer when it is covered on paper. Therefore, according to the principle that soft X-rays can penetrate paper but not heavy metal mercury, it is not difficult to understand why the original fuzzy seal can be clearly displayed in the taken photos.