Get the latest tech news
Making iron gall ink from oak galls
Adventures in programming, hardware, DSP, and chiptune
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). 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. In my case, I wasn't keen on learning to sanitize and prepare bird feathers, nor spending even more money on bamboo nibs for a casual project (I had already purchased iron sulfate and gum arabic).
Or read this on Hacker News