Also there is a paper that discusses comparing two types of iron gall ink by whether they remain high contrast or become faint in IR: You are not allowed to view links. Register or Login to view.
Is says things like "The difference in the appearance of the inks results from their composition – greyish ink probably has more iron (vitriol).", which to me sounds as if iron-rich iron gall inks become fainter in IR?
EDIT: the bottom line for me, there are different compositions of iron gall inks and they don't all behave the same, so I don't think we can exclude iron gall ink just using a single reference in one article.
Is says things like "The difference in the appearance of the inks results from their composition – greyish ink probably has more iron (vitriol).", which to me sounds as if iron-rich iron gall inks become fainter in IR?
EDIT: the bottom line for me, there are different compositions of iron gall inks and they don't all behave the same, so I don't think we can exclude iron gall ink just using a single reference in one article.