As reported by the Uppsala University Library:
The manuscript dates from the 14th century and it belonged to the monastic library at Vadstena Convent after its purchase in Konstanz in 1417. ..Top photo: the black threads of the medieval repair have disintegrated; modern stabilization with a silk gauze.
Some time after the book was copied, the holes and tears have been mended artistically with silk of various colours, mainly in blanket stitch as used in embroidery...
During the project the black thread was subjected to colour analysis to find out which colouring agents had been used. The results showed that the black dye had been made with iron sulphate and tannin, which implies that the thread was dyed with iron filings and various plants that contain the tanning agent tannin e.g. oak apple and sumac leaves. The whole dyeing process is very acidic and if the wrong proportions of tannin and iron salt are used, sulphuric acid forms considerably accelerating the natural decomposition of the thread or material.
Lower photo: An intact medieval repair.
Via Erik Kwakkel and allhomosapienswelcome.