indigo

Evidence

"Indigo greatly improves the stability of the ink by forming a strong complex with ferrous and ferric ions, subsequently removing these damaging ions from the paper or parchment (Larkworthy & Nyholm 1959)."

Remarks

Indigo is added to enhance the blue-black hue of iron gall ink and serves a critical stabilization role by complexing with free iron ions that would otherwise cause oxidative damage to the substrate (p. 20-21).