This is a question that I encounter frequently in my work with historic documents. Everything from land indentures to family bibles other documents contain iron gall ink. The recipe is said to have originated with Pliny the Elder, an Ancient Greek philosopher. The ingredients required to make iron gall ink are simple: oak galls, ferrous sulfate and gum arabic.
I was recently discussing this recipe with a friend, and she happened to have observed oak galls for the first time in her yard, where she took some photos. She was kind enough to share her photos, which you can see in this article. Oak galls are created when the gall wasp lays an egg on an oak leaf. The oak leaf then exudes tannic and gallic acids to create the oak gall. This is what an oak gall looks like:
If left to its own devices, the gall wasp larvae will mature, and then escape through a hole in the gall, leaving the gall formation behind.
To make iron gall ink, the oak galls are collected, dried and crushed. They are then soaked overnight, and cooked with ferrous sulfate. Gum arabic may be added to adjust the flow quality and behavior & look of the ink.
This recipe was commonly used throughout Western civilization, including Europe and the Colonial & Civil War era United States. The ingredients were easy to procure, and the recipe was common knowledge for those who were literate.
It is worth noting that high levels of copper and gum arabic are associated with iron gall ink corrosion as they age.
Many thanks to Cheri Rieman, who was kind enough to contribute her photos for this article!