 The ink that was used in medieval manuscripts was almost invariably made from oak gauze. These are produced when the oak gauze wasp lays its egg on an oak tree and instead of an acorn being formed it forms these hard round balls which are called oak gauze. They're really tough. To make the ink they need to be smashed with a hammer to make these fragments like this. These are then put in a jar and rain water added on top to cover them and left just on the window sill for a few days. Gradually the tannic acid is leached out of the oak gauze and this is the crucial thing in the ink. The water gradually turns into pale brown which over a couple of days turns into a darker brown. Then there needs to be some sort of glue adhesive added to it to make sure that the ink sticks to the surface of the skin and in this case usually gum arabic is used. These are these rather yellowy amber colored lumps which are dissolved in water to make a sticky solution and also copperase is added. Copperase is what ferro sulfate was called in medieval times and this gradually turns that pale brown into a darker brown and eventually into a dense black. When in use on exposure to air it turns into that really deep rich black that we're used to in looking at medieval manuscripts.