It identifies the presence of iron (II) ions (ferrous ions). These ions are responsible for iron gall ink corrosion , a destructive process that causes ink to eat through paper (oxidative degradation of cellulose).
The test paper is moistened and applied to the ink. If iron (II) ions are present, the test paper changes color (turns a red/purple complex) due to the indicator bathophenanthroline reacting with the iron. Gall-10rar
It is used by conservators to test manuscripts and documents without damaging them because it is non-bleeding. It identifies the presence of iron (II) ions (ferrous ions)