The VMS ink is NOT iron-gall

[Jorge_Stolfi]

The third (publicly available) specifically talks about 840-1000nm light and demonstrates how one type of iron gall ink remains fully visible, while the other becomes faint (though still visible).

I still haven't got the two other papers you cited.  Maybe tomorrow.

But on that Fig.5 of that last paper we can see two inks, one still quite dark while the other is faint.

I propose that the authors made the same mistake that McCrone did: they determined that both inks contained iron, and, since they were both black and on vellum, they assumed that both were IGI.

In an attempt to explain why some of that black ink faded in the infrared, they hypothesized that it was IGI of inferior quality.  They also speculated that it could be "cheaper" IGI using copper or zinc instead of iron.  But they did not really determined that; and anyway those metals cannot replace iron in IGI, because the polymerization of the latter happens as iron changes from the +2 to +3 state -- whereas copper and zinc are +2 only, and cannot change to +3.
 
Well, I propose that the ink that faded on Fig.5 simply was not IGI.  

The reason people used IGI was, again, because it was waterproof and rubbing-proof.  But it came in only one color -- black.  

Therefore, any inks or paints of other colors that one sees on velum are not IGI, but watercolors (tempera, gouache): suspensions of solid mineral or lampblack particles in water, with a binder that would hold glue the particles to the vellum when dry.  That incluedes the red ink used for highlighting and titles in many manuscripts of the time.  But the adhesion of binder to vellum is by weak (Van der Waals) bonds, not the strong (covalent) bonds that tannin makes with proteins; and all binders that could be used at the time could be softened by water and degraded by humidty.  

Therefore, any colored inks or paints would not be waterproof or rubbing-proof.

But then, if the book or document was to have writing in colors other than black, there would have been no point in using IGI for the black text.  

But then, on such books the Scribe may as well have used a mineral or lampblack tempera color also for the black text.  Which would be much easier to prepare than IGI, and would last indefinitely.

Therefore, when reading any paper (like that one) about properties of IGI on medieval manuscripts, one must read carefully to see whether they have actually determined that the ink is IGI, or just assumed that it was.

Apart from that, note that the paper I cited was specifically concerned with distinguishing IGI from ocher-based watercolor ink in drawings under paint.  Not just in visualizing ink under paint.  So, if indeed there is a contradiction with those other papers you cited (and I still don't think there is), it is my experts disagreeing with your experts -- not the experts disagreeing with me.  Another instance of Forte's Law...

All the best, --stolfi

PS. Another puzzling detail in that article is that, on Fig.7, they indicate the sampled areas as being the black heads of three notes; but the element composition shows as much mercury (Hg) as iron (Fe).  I can't imagine why a black ink would contain mercury.  I suspect that in fact the measurement was made on the note before the area "1" and/or the one between "2" and "3", which are crossed by red staff lines.  And cinnabar (mercury sulfide) was a common red pigment.