An obsolete lake pigment made from the juice of buckthorn berries (primarily Rhamnus catharticus), mordanted on alum. The primary colourant in R.catharticus is kaempferol, but berries from other buckthorn plants may contain rhamnetin or quercetin. Sap green is a fugitive dark, yellowish green colour that was used in watercolour paints in the 17th and 18th centuries. It is listed as one of the additional pigments (?) used for colouring stucco marble in the nineteenth century. Synonyms: bladder green; iris green; verd vessie. Wittenburg (1999); Museum of Fine Arts, Boston (2000). [J. Cassar, R. de Angelis]