I'd usually think that iron gall ink was outside my era, but the ink appears in the Dead Sea Scrolls and the Gospel of Judas.  (Indeed the Gospel of Judas turns out to be important for the history of inks, as a transition between earlier lamp-black based inks and later tannic acid based ones.)  The earlier carbon based inks have held up well (unless manhandled), later inks have a reputation of being less reliable over centuries.

Three of the most important questions to ask about a new discovery is 1) whether it could have been made at the time it purports to be from 2) whether it has aged as it should 3) whether its contents reflect the correct slice of the past.  However, new discoveries always tell us new things about the past — that iron gall ink ink is correct for the era, for example — and there can be a fine line between being too conservative and rejecting something as a fake* and being too eager to accept a discovery as authentic.  The exact age of some things (the fine details of handwriting, the permutations of the development of a particular theological idea) may always be disputed by experts; this is why lab work is so important.

(That said, the Dead Sea Scrolls were first identified by students who had little but their educations to work with:  I always hope mine would be equal to such a harrowing pop quiz.)

*  If the holder of the Shapira fragments would come forward, it would be good to give M. H. Shapira the opportunity to clear his name.

-Kushana

P.S. I would like to thank CMeisenzahl and JMW19 of the Fountain Pen Network for these links.