The ink the Egyptians used was great for papyrus. Not as good for parchment. Writing on parchment calls for a different kind of ink. This new ink needs acid that will bite into the surface while staying nice and black. Not a whole lot of acid—just enough so the ink interacts with the surface of the parchment.
Where do you find acid? Somehow, somebody observed that when wasps lay their eggs on the tree bark, the eggs irritate the tree which reacts by growing a round gall-nut around the eggs to isolate them. If I were a tree I’d do the same thing myself. After the eggs hatch the baby wasps leave behind gall—a bitter, caustic chemical. Ink-makers collected those gall-nuts to make a slightly acidic ink. To get the gall out of the gall-nuts, they steeped them in water for a week.
Back to the beginning of The Western Civ User’s Guide to Reading & Writing.