Watercolour paint is made by mixing pigments with a binder, usually gum arabic, and then applying it with water to a support such as vellum (fine animal skin) or paper. The water evaporates and the binder fixes the pigment to the support. Watercolour was used long before the development in the 1750s of the British watercolour tradition. In mediaeval times, artists illustrated the vellum pages of hand-written books with brightly coloured paintings in watercolour. When the invention of printing in the late 15th century affected demand for such expensive books, some artists experimented with painting separate works of art. The separate portrait miniature was one such development.