Iron gall is the ink that most of the documents still legible after four hundred years were written in. It works chemically rather than by dyeing: iron salts react with tannic acid and with the cellulose of the paper itself, so the mark becomes part of the sheet. It goes down a pale blue-grey, darkens visibly within an hour, and settles to a dense near-black at about 48 hours. It is genuinely waterproof once cured.
Modern iron gall is far gentler than the historical article but it is still mildly acidic, and it is the one ink in this shop that needs discipline. Flush the pen every two weeks. Do not leave it sitting in a pen for a month. Do not use it in a vintage gold nib or in anything with brass internals you cannot reach. In a steel nib with a flushable feed it is completely safe.
It is at its best in a fine or extra fine, where the slow oxidation shows as a line that gets darker while you read it back. 50 ml faceted glass.
