Oak apples - also known as oak galls - ar e the result of parasitic wasps laying eggs in various parts of the oak and the tree responds by producing abnormal growth around the egg and developing larvae which is the gall. These galls have a high tannin content and it is this feature which has led to their use in the manufacture of ink. Its use can be traced back at least to Roman times and was used right up until the 20th century.
Tannin alone produces brown liquid, but with the addition of iron oxide turns deep black I read. Iron oxide can literally be produced by steeping rusty nails in water. So I try it all out. Crushed oak gall inks soaked in water for a couple of weeks - rusty nails the same (crushing not necessary). Filter both liquids and combine. Sure enough I have a strong black liquid. Gum arabic to thicken and bind. Now I have ink. Relatable chemistry.