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.
It is used by conservators to test manuscripts and documents without damaging them because it is non-bleeding. Gall-10rar
The (sometimes referred to as iron gall test paper) is a specially designed, non-bleeding test paper used in conservation to detect iron (II) ions in hydrophilic substrates. Key details about this paper include: The test paper is moistened and applied to the ink