Making iron gall ink from oak galls
Back in May, I was taking a walk around the neighborhood when I came across an oak tree with dozens of galls hanging from the stem. At first I thought these were seed pods and didn't even recognize the tree as an oak, but PlantNet (and later Seek) informed me it was a valley oak. At this point I realized the pulpy brown masses hanging from the stems were oak galls, and I had the idea to make my own iron gall ink like people did in centuries past.
Oak trees form galls (such as oak apples and marbles) when certain wasps deposit their eggs in oak trees (often in leaf buds). The larvae produce chemicals which turn the oak bud into a swollen ball filled with tannin-rich pulp, which protects the growing insect from predators like birds (not always successfully). After the insect emerges, the galls dry out into a spongy mass which hangs on the tree or falls to the ground.
To make ink, people would collect galls (before or after the insect exits), crush them into a fine powder, and soak it in solvents like water, beer, or wine to dissolve the tannins and convert it into gallic acid. Afterwards, people would combine the tannin liquid with an iron sulfate solution to create iron-tannin compounds with a blue-black color, which can be used as ink (usually with gum arabic added to make the liquid flow better). Traveling Scriptorium has a page with details on historical iron gall ink production.
