Sailor SOUBOKU is a new blue-black pigment-based ink offering a deeper blue color than SEIBOKU (more of a traditional blue-black color) and has the same waterproof, permanent, and lightfast quality that Sailor pigmented inks are known for. Sailor SOUBOKU is safe to use in all fountain pens, but it is recommended to flush your pen thoroughly and regularly. Sailor SOUBOKU is offered in a new square-shaped 50mL glass bottle and a box of 12 cartridges.