f17r

[Jorge_Stolfi]

I'm not sure if the disappearance of the ink in IR means that it's transparent or that it just absorbs at similar rate as the parchment, compared to some of the paint which looks really high contrast black. I took the far IR image (940nm) and tried to apply correction to it to increase the contrast at the levels of black corresponding to the text. A lot of the text is still visible, so the ink is not totally transparent in IR.

When the article says "ochre [inks] already become transparent at 850 nm", I understand that means "noticeably transparent compared to what they look like at shorter wavelengths, and by 1200 nm, where iron-gall is still black, ochre inks are completely gone.  

If you look at other IR wavelengths, you will see that the transition is not abrupt.  The ink fades gradually as the wavelength increases.

So I don't think that the fact that some glyphs still very faintly visible at 940 nm contradicts that article.  In any case, the ink is definitely not iron-gall.

The brown pigment of the ink is probably not ocher, which is a brownish yellow orange. It is more likely to be sienna or umber, which are mineral mixtures similar to ocher but with some amount of other black iron oxides or of manganese dioxide, which makes them darker brown.  Manganese dioxide may well be more opaque than ocher beyond 850 nm; if so, that may be what keeps the text still barely visible at 940.

All the best, --stolfi