Iron Gall Ink

The traditional type of ink for archival documents, used by registrars for recording things like births, deaths and marriages.

Iron gall inks are more risky than most other inks, with particles that could clog your pen's feed, and the ink itself can be acidic, harming steel nibs. Most modern formulations are fairly weak, made to be safer to use. These are still best handled with a bit more care than most standard inks.

Diamine's Registrar's Ink is the real deal, though, made for registrars who have to do things the traditional way - take extra care using this in a fountain pen, or stick to dip pens if you want to be safe.