Specially formulated to be non-clogging in technical pens, Koh-I-Noor Ultradraw Waterproof Ink is waterproof, permanent, fadeless, and acid-free. It has a longer open-pen time than most inks, yet dries quickly on the drawing surface. Ultradraw is a dense black india ink, yet it erases easily. Use Ultradraw for airbrush, dip pens, and calligraphy pens, as well as technical pens. It comes in a ¾ oz (22 ml) plastic bottle with a filler spout, and is recommended for use on paper.