Quote Originally Posted by amuderick View Post
iron gallate and iron tannate pigments are very lightfast and permanent.
Iron gallate was known to the ancient Greeks, who used it as ink. Writings in 'iron ink' are still readable after thousands of years.

Gallic acid's known mild reducing action made it a candidate for a developer at the beginning of silver-halide photography.

I guess the winner for image permanence would be cyanotype on papyrus or scraped hide.