f17r

[oshfdk]

[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.

But is it possible to determine if they became transparent or just nearly the same shade as the parchment? If you look at the parchment in processed MSI images, it looks nearly the same shade, just because it occupies the largest portion of the image, so when performing level/gamma/white point correction the parchment dominates the image. I'm not sure if absolute levels are preserved in the original TIFFs. Maybe it's not the ink getting lighter, maybe it's the parchment getting darker.