Could a chemist reproduce inks of medieval quality, then apply them to vellum?

[Jorge_Stolfi]

What exactly are tannins?They are simply tannic acid. Essentially, iron that has been chemically ‘burned’ by tannic acid (or any acid) turns black and can no longer rust. Rust = (iron + oxygen).

A common definition of acid is a molecule that can donate protons -- that is, lose hydrogen atoms to other molecules while retaining their electrons.  Tannins are formally acids, but that is not what makes them the key ingredient of iron-gal ink.  Tannin cannot be replaced by another acid in the formula of iron-gall ink.

Tannins are large multi-branched molecules full of six-sided benzene rings bearing multiple -OH groups.  These rings bind strongly to both proteins and iron ions.  They will indiscriminately bind to protein molecules and tie them into messy tangles that most organisms cannot digest. In particular, they will bind to the molecules that stick out of the cells of the mucosas in one's mouth, including the proteins that detect sugars and other flavors; that is the reason for the adstringent sensation one gets when eating an unripe banana. 

Tannins will also bind to the iron(II) ions of a solution of green vitriol.  The combined molecule iron(II)+tannin absorbs light better than iron(II) alone, and thus the solution turns from very pale green to a darker bluish gray.  In the proportions used for IGI, the complexes of iron(II)+tannin still stay dissolved. 

However, if the solution is exposed to air, the iron(II) ions will react with oxygen to produce iron(III) ions. The iron(III)+tannin molecule absorbs light much more strongly, so the solution will turn intense black.  

Tannin binds much more strongly to iron(III) ions than to iron(II) ions; and moreover the iron(III) catalyzes other reactions of tannin with oxygen, causing the tannin molecules to bind to each other. In the end, both processes result into an insoluble polymer of tannin+ iron.  If that happens while the ink is still in the bottle, this polymer will settle to the bottom and the ink will be ruined.  If it happens after the ink has been applied, the in ink will become a waterproof film chemically attached to the vellum.

Green vitriol contains (negative) sulfate ions besides (positive) iron(II) ions.  When the latter combine with tannin, the sulfate ions combine with hydrogen ions released by the tannin to form sulfuric acid.  That is not a problem when writing on vellum, because the surface is usually impregnated with chalk, that neutralizes the acid. 

On the other hand, if iron-gall ink is used on paper, this acid will remain behind after the water evaporates, and will slowly cause the paper to become brittle and eventually crumble into dust, turning the text into a string of letter-shaped holes.  But this damage takes decades to occur, so scribes who had IGI at hand would use it on paper too, for letters and other documents that were not meant to last that long.
 

This leads me to suspect that the VM drawings, star charts and astrological pages were drawn first.
Because on other pages, the text also gets progressively darker.  Is it a different ink, or was the artist simply impatient?

Again, the ink of the text cannot be iron-gall ink, because it is not waterproof at all.   The faded appearance of the Zodiac pages can be explained by them having been handled much more often than the other pages.

All the best, --stolfi