From the manufacturer:
Winsor & Newton Calligraphy Inks offer the widest colour range available for use with a Dip pen or Fountain pen. All the inks have been formulated with carefully chosen pigments to ensure maximum brilliance of colour. Single pigments have been used wherever possible to make clean colour mixtures. The formulations of the Inks are non waterproof ensuring they are non clogging and have good flow characteristics when used in a Fountain or Dip pen.