The Royal Horticultural Society Diary 2013 focuses on works of art done on vellum. Before paper was invented vellum was the commonest surface used for writing or painting on. Vellum - essentially the skin of an animal that is treated until it becomes paper thin - was expensive to produce and so was reserved for use and commissions by the very wealthy - often royalty. Paintings on vellum have a luminosity and richness that is quite unlike paper. The diary features works of art from the RHS collection, which date mainly from the 18th century. The collection includes images from a book of botanical paintings belonging to the Empress Josephine, as well as others by professional artists and wealthy amateurs.