Waterproof Fountain Pen Ink

A couple of notes here - some 'waterproof' inks can still be washed out somewhat with water, but they should leave at least a legible mark behind.

And while these should all be safe to use in fountain pens, some extra care is a good idea - don't let these inks dry out in a pen, they're much safer used regularly, and flushed out well if you're not going to use the pen for a while.

Diamine's Registrar's Ink especially, requires some care, and can clog pens, use at your own risk! But it's also a very unusual ink to find these days - a real archival ink as used by registrars for many, many years.