iron gall ink

Iron gall ink is a type of ink historically used for writing, with its origins tracing back centuries. It is made from a mixture of a soluble iron salt, typically ferrous sulfate, and an extract of tannin. When first applied to paper, the ink appears bluish-black, but it darkens over time and becomes insoluble in water, giving it permanence. Modern blue-black inks are based on this formulation, often with added dyes to make the writing more legible initially. While it was a common writing ink for many centuries and used in early drawings, its color can turn a dull brown and it may tend to disintegrate over time.12