Yang Xi
Although no autograph copies of Yang Xi's famous calligraphy have survived, one well-documented stone rubbing version of the revealed "Huangting jing" (黃庭經, Yellow Court Scripture) has been given special attention. The Song dynasty connoisseur and critic Mi Fu (1051-1107) analyzed four manuscripts of the "Huangting jing", and said the best one was written on a silk scroll with a provenance that he traced back to the early 8th century. Mi disagreed with a former owner Tao Gu (陶穀, 903-970) who said that Wang Xizhi wrote the manuscript, and concluded it was a superb example of calligraphy from the Six Dynasties period. The Ming dynasty calligrapher and painter Dong Qichang (1555-1636) was so impressed by the Mi Fu manuscript that he made a copy, recommend it as the best model for studying "kaishu" script, and included it as the first example in his classic "Xihong tang fatie" (戲鴻堂法帖, Calligraphy Compendium of the Hall of the Playful Goose). Dong's colophon attributed the calligraphy to Yang Xi himself and described it as "the traces of a holy immortal" (Ledderose 1984: 262-263, who calls Dong's attribution to Yang "extremely optimistic").
Iron gall ink
The earliest recipes for oak gall ink come from Pliny the Elder, and are vague at best. Many famous and important manuscripts have been written using ferrous oak gall ink, including the Codex Sinaiticus, the oldest, most complete Bible currently known to exist, thought to be written in the middle of the fourth century. Due to the ease of making iron gall ink and its quality of permanence and water resistance this ink became the favored one for scribes in the European corridor as well as around the Mediterranean Sea. Surviving manuscripts from the Middle Ages as well as the Renaissance bear this out as the vast majority are written using iron gall ink, the balance being written using lamp black or carbon black inks. Many drawings by Leonardo da Vinci were made with iron gall ink. Laws were enacted in Great Britain and France specifying the content of iron gall ink for all royal and legal records to ensure permanence in this time period as well.