This smooth-flowing ink is specifically formulated to be safe for all fountain pens. Each bottle contains 50 ml of water-based dye ink. The bottle is cleverly designed with an internal reservoir, allowing you to enjoy every last drop of your fountain pen ink. In normal bottles, as you approach depleting ink reservoirs it gets more and more difficult to suck up ink. Each of these ink bottles easily solves that problem with a simple plastic tube in the bottle. When the ink level gets low, tighten the bottle cap, flip the bottle upside down, and then flip it right side up. Atmospheric pressure causes the ink level in the tube to remain higher than in the rest of the bottle, allowing you to fill your fountain pen with ease.