Platinum Black Carbon ink in cartridge form, 4 pieces, for use only in Platinum pens (proprietary cartridge). A waterproof, pigmented black ink (as opposed to dye-based) that is highly regarded amongst fountain pen enthusiasts and artists.
This ink is safe for fountain pens, but it does carry some caveats as a waterproof, highly pigmented ink. To prevent clogging, it is best to use this ink in a pen you write with fairly regularly - or flush the pen more often than normal. Do not allow this ink to sit in a pen for extended periods of time.