Platinum Classic: Citrus Black Ink

Platinum Classic Citrus Black is a permanent dark yellow-brown ink that darkens as it dries. This ink is waterproof, lightfast, flows well, and dries well.

Platinum Citrus Black is a modern iron gall ink, which means it’s permanent (and becomes even more permanent as it darkens with time). If you leave this to dry in cheap steel nibs, it may corrode them (iron gall inks are acidic), but it’s perfectly safe for normal use.

Bottle and Plume take:

This is one of my favorite inks - it feels like writing with invisible ink that magically darkens a few seconds after going on the page. It's stunningly beautiful, incredibly well-behaved even on the cheapest paper, and permanent after a bit of time has passed (the yellow will wash off, but the dark grey-brown-ness from the iron gall chemically changing the paper stays). Writes a bit dry, like iron gall.