Winsor and Newton Granulation Medium is available in 75ml bottles from Pegasus Art.
Granulation Medium gives a mottled or granular appearance to colours which usually give a smooth wash, such as Winsor Red or Winsor Blue.
By adding granulations Medium to colours that already granulate, such as Viridian or French Ultramarine, the effect is further enhanced.
Granulation is popular wherever you want to add interest or dimension to otherwise flat areas.