I kept Platinum Carbon Black inked up for more than five months, to see how it would clean out of a pen, and the answer is, nice and easy. This is a pigment ink that is a fairly low-maintenance ink. I’m chuffed.
I had it in a Platinum Plaisir. Here’s the pen’s feed after flushing out Carbon Black with water only. Perfectly clean.

This was one of my extended “torture tests” of an ink. I occasionally do these for the blog to check the limits of certain inks, because I like inks that I’m not afraid to keep in a pen for a while.
I have done extended tests on modern iron gall inks here, here and here — just to prove that modern iron galls from reputable makers are safe for extended time periods in pens with stainless steel nibs. Which they are.
But sometimes, you find yourself inadvertently giving a torture test to an ink that can’t handle it. Whoops.
In this case, I purposely chose to torture-test Platinum Carbon Black. It’s my favorite pigment ink and waterproof ink, and I’d noticed that it seemed to clean out pretty easily. I’m always happy to find low-maintenance inks that are waterproof — like the newer Sailor Souboku. I wanted to be sure about Carbon Black, too.
I decided to keep a cartridge of Carbon Black in a Platinum Plaisir I’d bought in mid-April. I used the Plaisir like normal — writing with it every once in a while, and then putting it back in the pen cup, where it might sit unused for days until the next time. The pen always started up and wrote perfectly: that may owe something to Platinum’s “slip and seal” cap design, but Carbon Black is nicely lubricated, too.
I finished Carbon Black on October 7. After more than five months in the pen, Carbon Black cleaned out perfectly, just flushing with water. It didn’t take much longer than cleaning out Waterman Serenity Blue would have — my standard for a low-maintenance ink.
Another photo, this time of the other side of the feed. The feed is clear of ink and unstained.

I ran the section through a cycle in my ultrasonic cleaner to verify: all the Carbon Black was gone.
Now, I can’t pull this particular nib to check if any ink may be trapped under the nib, but I did the next best thing I could think of. I attached a converter, and filled it with Montblanc Golden Yellow ink, which is the lightest ink I have. And then I wrote with that for a few days. The ink flowed normally, the nib wrote normally and the ink writes in its normal light yellow color, with no smears of leftover black ink.

My conclusion is a happy one. Platinum Carbon Black is a waterproof pigment ink that is low-maintenance. I won’t worry about using it in any cartridge-converter pen.