Medieval painters built up a tremendous range of technical resources for obtaining brilliance and permanence. …
In modern practice the solutions [used for oxidizing black ink] used are so pure that when the liquids are first combined no black colour is formed. Indeed, a modern fountain-pen ink would be colourless when new if dye were not added to it to provide some visibility.
— The Materials and Techniques of Medieval Painting by Daniel V. Thompson (Page 81)