No - most fountain pen inks are water-based/water-soluble and the rich, vivid colours available within the PILOT iroshizuku range could not be achieved with a waterproof formula. The advantages of water-soluble inks are that they are more well-behaved and require less maintenance when used in fountain pens. With the pigments or iron gall particles that are used in waterproof inks, there is a risk of clogging or damaging your fountain pen, especially if left in it for a long period of time. Another enjoyable aspect of using water-soluble inks, especially when sketching and painting, is the ability ‘pull colour’ from the ink like you would with watercolour paints. Further dimension can be added to a sketch when using a wet paintbrush.