I'm not sure if we should rely on this information about IR and iron gall inks. For starters, they appear to have analyzed relatively fresh ink, as it is stated in the supplementary PDF from the article:
Also, it's not hard to find completely different reports from papers studying actual medieval manuscripts. There is a whole section titled "3.3 Visualizing iron-gall ink underdrawing" in the following paper: You are not allowed to view links. Register or Login to view.
In which it is stated:
Then it references some articles, but I think they are paywalled.
Quote:IR reflectance properties of iron gall ink (red) beneath a cadmium yellow paint layer
[23] (p. 57). The ink was produced according to recipe 208 and 211 from the Liber
illuministarum [24]
Also, it's not hard to find completely different reports from papers studying actual medieval manuscripts. There is a whole section titled "3.3 Visualizing iron-gall ink underdrawing" in the following paper: You are not allowed to view links. Register or Login to view.
In which it is stated:
Quote:Given that the texts of medieval manuscripts often were written in iron-gall ink, it is not surprising that underdrawings of manuscript illuminations were drawn in this medium as well. However, the detection of iron-gall ink can be difficult, as it is not readily visualized by IR imaging due to its low absorption in the IR.
Then it references some articles, but I think they are paywalled.