I had no trouble at all cleaning Platinum Chou Kuro ink out of my #fountainpen (ystudio brassing, fine). It was in there for almost a month, with light daily use.
I was surprised that Chou Kuro smears a bit under a highlighter (on Leuchtturm 120 paper). Debating whether to refill with it or go a round with Carbon Black to compare.
@johns I mean, you do you, but everyone knows that *Real* Small Furry Creatures From Alpha Centauri make their own ink.