Chemistry
The gradual darkening of the ink was due to the oxidation of the iron ions from ferrous (Fe2+) to ferric (Fe3+) state by atmospheric oxygen. (For that reason, the liquid ink had to be stored in a well-stoppered bottle, and often became unusable after a time.) The ferric ions reacted with the tannic acid or some derived compound (possibly gallic acid or pyrogallol) to form a polymeric organometallic compound. The specifics of chemistry of iron gall ink can be found at About.com
While a very effective ink, the formula was less than ideal. Iron gall ink is acidic ranging from roughly equivalent to a lemon to that of a cup of coffee. For this reason some makers of iron gall ink used crushed egg shells (which contain calcium carbonate (CaCO3)) to temper the ink, bringing it closer to a neutral pH value. Depending on the writing surface being used iron gall ink can have unsightly "ghost writing" on the obverse face of the writing surface (most commonly vellum or paper.) Also any excess of ferrous ions remaining in the ink over years, decades, and centuries, could create a rusty halo around the marks and ultimately it might eat holes through the surface it was on.
Paper has its own special problems with iron gall ink. The iron-tannic pigment did not make chemical bonds with the cellulose fibers. The ink stuck firmly to the paper, but largely by mechanical bonding — namely, the dried ink penetrated the spaces between the fibers and, after drying, became entangled in them. The process of decaying the writing surface is accelerated on paper when compared to vellum, doing the damage in decades or years that could take more than a millennium on vellum.
The acidity of iron gall ink is well known but it must also be observed that the case for the acidity of iron gall ink is somewhat overstated. There are several thousands of manuscripts, some of them well over 1,000 years old, with iron gall ink on them that have no damage or degradation whatsoever from the iron gall ink. This understanding however should not be taken to ignore the potential issues of documents that use iron gall ink for the writing.
Read more about this topic: Iron Gall Ink
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