A natural, blue dye extracted from certain lichens, such as Variolaria Roccella tinctorum or V. Lecanora tartarea. Litmus is water soluble and may have been occasionally used as a watercolour pigment. In alkaline solutions (above pH 8.3) litmus turns blue and in acid solutions (below pH 4.5) it is red. The neutral tint is violet. It is listed among the additional pigments used for colouring stucco marble in the nineteenth century. Synonyms: lichen blue; lakemus. Wittenburg (1999); Museum of Fine Arts, Boston (2000). [J. Cassar, R. de Angelis]