Platinum Carbon Black ink is a carbon-based ink that is made for use in fountain pens. This ink is essentially India ink without the shellac or other adhesive. It addresses the issue that many fountain pen artists face, of needing a waterproof ink that is safe for use in fountain pens.
As I previously mentioned, Noodler's makes lines of "waterproof" and "bulletproof" fountain pen inks that are waterproof in the sense that they cannot be washed away with water. However, they are not necessarily waterproof in the artist's meaning; the inks sometimes can be smeared with water applications. So, the inks are not as useful to those who like to sketch in fountain pen and then watercolor those sketches, for example. Platinum Carbon Black ink is waterproof in the artist's meaning; the ink does not smear in water (or alcohol, which is a terrific bonus).
Though the ink is intended for use in fountain pens, I recommend being careful in selecting which fountain pen to use. The ink is still carbon-based, meaning that it contains suspended carbon particles. Though these particles are very fine, they can eventually build up in the narrow feeds of most fountain pens.
I have used Platinum Carbon Black ink in Kaweco Sport fountain pens, Kaweco Sport ink roller pens, the Kuretake brush pen, and the Platinum Carbon brush pen - soft (one of the two pens for which this ink is specifically intended, the other being the Platinum Carbon Desk fountain pen, a fountain pen with a broader feed than usual). The ink rollers and brush pens handle the ink beautifully; these pens have fairly open and broad feeds compared to a fountain pen. In the fountain pens, however, the ink tends to dry on the nib and clog fast. Dipping the pen's nib in water usually gets it restarted, but I recommend not letting the ink sit more than a few days in a fountain pen, fully cleaning the fountain pen between each refill, and designating this ink to only a few fountain pens that you are willing to risk clogging badly, just in case the clog becomes irreparable.
All that said, this is an enjoyable ink to use. I have used it for both sketching in ink and inking planned drawings. It erases more than an India ink would, so I sometimes re-ink lines after erasing away the pencils. The ink is waterproof and alcohol-proof, so I have been able to use it in pictures that I water color or color with alcohol-based markers without smearing.
Platinum Carbon Black ink, being an import and a very niche product, is much more expensive than other India inks or fountain pen inks. So, I do not make it an all-purpose replacement for either, instead using it for specific purposes.
BUY AGAIN? Yes.